Partial List of Abstracts, 1999

Anderson, D.E., Striegl, R.G., Stannard, D.I., Michmerhuizen, C.M., McConnaughey, T.A., and LaBaugh, J.W., 1999, Estimating lake atmosphere CO₂ exchange: Limnology and Oceanography, v. 44, n. 4, p. 988-1001.

Lake-atmosphere CO₂ flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake-water CO₂ storage and with flux predictions from boundary-layer and surface-renewal models. Over a 3-yr period, lake-atmosphere exchanges of CO₂ were measured over 5 weeks in spring, summer, and fall. Observed springtime CO₂ efflux was large (2.3-2.7 mmol m^-2s^-1) immediately after lake-thaw. That efflux decreased exponentially with time to less than 0.2 mmol m^-2s^-1 within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO₂ concentrations, lake CO₂ storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (-0.17 mmol m^-2s^-1) to the lake's surface, while late fall flux was trendless and smaller (-0.0021 mmol m^-2s^-1). Large springtime efflux afforded an opportunity to make direct measurement of lake-atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air-water gas-transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake-water storage change in CO₂, agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary-layer and surface-renewal models. Comparison of measured and modeled transfer velocities for CO₂, along with measured and modeled cumulative CO₂ flux, indicates that in most instances the surface-renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary-layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.

Amelung, Falk, Galloway, D.L., Bell, J.W., Zebker, H.A., and Laczniak, R.J., 1999, Sensing the ups and downs of Las Vegas: InSAR reveals structural control of land subsidence and aquifer-system deformation: Geology, v. 27, p. 483-486.

Land subsidence in Las Vegas, Nevada, United States, between April 1992 and December 1997 was measured using spaceborne interferometric synthetic aperture radar. The detailed deformation maps clearly show that the spatial extent of subsidence is controlled by geologic structures (faults) and sediment composition (clay thickness). The maximum detected subsidence during the 5.75 yr period is 19 cm. Comparison with leveling data indicates that the subsidence rates declined during the past decade as a result of rising ground-water levels brought about by a net reduction in ground-water extraction. Temporal analysis also detects seasonal subsidence and uplift patterns, which provide information about the elastic and inelastic properties of the aquifer system and their spatial variability.

Anderman, E.R., and Hill, M.C., 1999, A new multi-stage ground-water transport inverse method, Presentation, evaluation, and implications: Water Resources Research, v. 35, no. 4, p. 1053-1063.

More computationally efficient methods of using concentration data are needed to estimate groundwater flow and transport parameters. This work introduces and evaluates a three-stage nonlinear-regression-based iterative procedure in which trial advective-front locations link decoupled flow and transport models. Method accuracy and efficiency are evaluated by comparing results to those obtained when flow- and transport- model parameters are estimated simultaneously. The new method is evaluated as conclusively as possible by using a simple test case that includes distinct flow and transport parameters, but does not include any approximations that are problem dependent. The test case is analytical; the only flow parameter is a constant velocity, and the transport parameters are longitudinal and transverse dispersivity. Any difficulties detected using the new method in this ideal situation are likely to be exacerbated in practical problems. Monte-Carlo analysis of observation error ensures that no specific error realization obscures the results. Results indicate that, while this, and probably other, multistage methods do not always produce optimal parameter estimates, the computational advantage may make them useful in some circumstances, perhaps as a precursor to using a simultaneous method.

Andrews, E.D., Johnston, C.E., Schmidt, J.C., and Gonzales, M., 1999, Topographic evolution of sand bars, in Webb, R.H., Schmidt, J.C., 1999, topographic evolution of sand bars in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., The controlled flood in the Grand Canyon: American Geophysical Union Monograph 110, p. 117-130.

Sand bars deposited in lateral separation eddies are an essential biological and recreational resource of the Colorado River downstream from Glen Canyon Dam. Since 1986, however, sustained discharges substantially in excess of the power-plant capacity have not occurred, and approximately half of the sand bars that existed in 1986 have been degraded by erosion and encroachment of vegetation. A primary purpose of the 1996 controlled flood release from Glen Canyon Dam was to measure the rate of sand deposition and erosion as well as to observe the adjustment of sand bar topography during a period of sustained high flow. Repeated, detailed bathymetric surveys of 5 eddies were made before, during, and after a flood of 1275 m³/s for 7 days to determine the topographic evolution of sand bars. Two of the eddies are located upstream of the Little Colorado River, the primary source of sand to the Colorado River through Grand Canyon, and 3 eddies are located downstream of the Little Colorado River. The topography of sand bars in all 5 eddies adjusted rapidly during the first several hours of the flood. Sand bars aggraded and degraded by as much as 3.5 m within less than 24 hrs. The general pattern of deposition and erosion observed during the first day persisted to varying degrees throughout the flood, even though a few to several thousand cubic meters of sand were eroded and deposited from one day to the next. The area of sand exposed above the 565 m³/s for 7 water-surface elevation and available for camping, however, increased in all the eddies studied. Subaqueous mass failures of over steepened portions of the sand bar appeared to occur in all eddies.

Bargar, J.R., Reitmeyer, R., and Davis, J.A., 1999, Spectroscopic confirmation of uranium (VI)-carbonato adsorption complexes on hematite: Environmental Science and Technology, v. 33, p. 2481-2484.

Evaluating societal risks posed by uranium contamination from wasted management facilities, mining sites, and heavy industry requires knowledge about uranium transport in groundwater, often the most significant pathway of exposure to humans. It has been proposed that uranium mobility in aquifers may be controlled by adsorption of U(VI)- carbonato complexes on oxide minerals. The existence of such complexes has not been demonstrated, and little is known about their compositions and reaction stoichiometries. We have used attenuated total reflectance Fourier transform infrared (ATR-FTIR) and extended X-ray absorption fine structure (EXAFS) spectroscopies to probe the existence, structures and compositions of ºFeOsurface- U(VI)-carbonato complexes on hematite throughout the pH range of uranyl uptake under conditions relevant to aquifers. U(VI)-carbonato complexes were found to be the predominant adsorbed U(VI) species at all pH values examined, a much wider pH range than previously postulated based on analogy to aqueous U(VI)-carbonato complexes, which are trace constituents at pH < 6. This result indicates the inadequacy of the common modeling assumption that the compositions and predominance of adsorbed species can be inferred from aqueous species. By extension, adsorbed carbonato complexes may be of major importance to the groundwater transport of similar actinide contaminants such as neptunium and plutonium.

The computer program WTAQ calculates hydraulic-head drawdowns in a confined or water-table aquifer that result from pumping at a well of finite of infinitesimal diameter. The program is based on an analytical model of axial-symmetric ground-water flow in a homogenous and anisotropic aquifer. The program allows for well-bore storage and well-bore skin at the pumped well and for delayed drawdown response at an observation well; by including these factors, it is possible to accurately evaluate the specific storage of a water-table aquifer from early-rime drawdown data in observation wells and piezometers. For water-table aquifers, the program allows for either delayed or instantaneous drainage from the unsaturated zone. WTAQ calculates dimensionless or dimensional theoretical drawdowns that can be used with measured drawdowns at observation points to estimate the hydraulic properties of confined and water-table aquifers. Three sample problems illustrate use of WTAQ for estimating horizontal and vertical hydraulic conductivity, specific storage, and specific yield of a water-table aquifer by type-curve methods and by and automatic parameter-estimation method.

An important gap in the misunderstanding of the hydrology of the Middle Rio Grand Basin, central New Mexico, is the rate at which water from the Rio Grande recharges the Santa Fe Group aquifer system. Several methodologies - including use of the Glover-Balmer equation, food pulses, and channel permeameters - have been applied to this problem in the Middle Rio Grande Basin. In the work presented here, ground-water temperature profiles and ground-water levels beneath the Rio Grande were measured and numerically simulated at four sites. The direction and rate of vertical ground-water flux between the river and underlying aquifer was simulated and the effective vertical hydraulic conductivity of the sediments underlying the river was estimated through model calibration.

Seven sets of nested piezometers were installed during July and August 1996 at four sites along the Rio Grande in the Albuquerque area, though only four of the piezometer nests were simulated. In downstream order, these four sites are (1) the Bernalillo site, upstream from the New Mexico State Highway 44 bridge in Bernanlillo (piezometer nest BRN02); (2) the Corrales site, upstream from the Rio Rancho sewage treatment plant in Rio Rancho (COR01); (3) the Paseo del Norte site, upstream from the Paseo del Norte bridge in Albuquerque (PDN01); and (4) the Rio Bravo site, upstream from the Rio Bravo bridge in Albuquerque (RDR01). All piezometers were completed in the inner-valley alluvium of the Santa Fe Group aquifer system. Ground-water levels and temperatures were measured in the four piezometer nests a total of seven times in the 24-month period from September 1996 through August 1998.

The flux between the surface- and ground-water systems at each of the field sites was quantified by one-dimensional numerical simulation of the water and heat exchange in the subsurface using the heat and water transport model VS2DH. Model calibration was aided by the use of PEST, a model independent computer program that uses nonlinear parameter estimation.

Mean vertical hydraulic conductivities were estimated by model calibration and range from 1.5 x 10^-5 to 5.8 x 10^-6 meters per second (m/s). Mean simulated vertical ground-water flux for the BRN02 piezometer nest is 3.30 x 10^-7 m/s; for the COR01 piezometer nest is 3.58 x 10^-7 m/s; for the PDN01 piezometer nest is 4.22 x 10^-7 m/s; and for the RBR01 piezometer nest is 2.05 x 10^-7 m/s. Comparison of the simulated vertical fluxes and vertical hydraulic conductivities derived from this study with values from other studies in the Middle Rio Grande Basin indicate agreement between 1 and 3.5 orders of magnitude for hydraulic conductivity and within 1 order of magnitude for vertical flux.

Bekins, B.A., Baehr, A.L., Cozzarelli, I.M., Essaid, H.I., Haack, S.K., Harvey, R.W., Shaprio, A.M., Smith, J.A., Smith, R.L., 1999, Capabilities and challenges of natural attenuation in the subsurface: Lessons from the U.S. Geological Survey Toxics Substances Hydrology Program, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - A Proceeding of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3--Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 37-56

Bekins, B.A., Cozzarelli, I.M., Godsy, E.M., Warren, Ean, Tuccillo, M.E., Essaid, H.I., and Paganelli, V.V., 1999, Chemical and physical controls on microbial populations in the Bemidji Toxics Site crude-oil plume, in Morganwalp, D.W. and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3--Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 133-142.

Bekins, B.A., Godsy, E.M, and Warren, E., 1999, Distribution of microbial physiologic types in an aquifer contaminated by crude oil: Microbial Ecology, v. 36, p. 263-275.

High-resolution (decadal) records of climate change from the Owens, Mono, and Pyramid Lake basins of California and Nevada indicate that millennial-scale oscillations in climate of the Great Basin occurred between 52.6 and 9.2 14C ka. Climate records from the Owens and Pyramid Lake basins indicate that most, but not all, glacier advances (stades) between 52.6 and ~15.0 14C (~60.0 to ~14.0 14C ka), stadial/interstadial oscillations were recorded in Owens and Pyramid Lake sediments by the negative response of phytoplankton productivity to the influx of glacially derived silicates. During glacier advances, rock flour diluted the TOC fraction of lake sediments and introduction of glacially derived suspended sediment also increased the turbidity of lake water, decreasing light penetration and photosynthetic production of organic carbon. It is not possible to correlate objectively peaks in the Owens and Pyramid Lake TOC records (interstades) with Dansgaard-Oeschger interstades in the GISP2 ice-core d¹⁸O record given uncertainties in age control and difference in the shapes of the OL90, PLC92, and GISP2 records. In the North Atlantic region, some climate records have clearly defined variability/cyclicity with periodicities of 102 to 103 yr; these records are correlatable over several thousand km. In the Great Basin, climate proxies also have clearly defined variability with similar time constants, but the distance over which this variability can be correlated remains unknown. Globally, there may be minimal spatial scales (domains) within which climate varies coherently on centennial and millennial scales, but it is likely that the sizes of these domains vary with geographic setting and time. A more comprehensive understanding of the mechanisms of climate forcing and the physical linkages between climate forcing and system response is needed in order to predict the spatial scale(s) over which climate varies coherently.

The d¹³C values of individual trihalomethanes (THM) formed on reaction of chlorine with dissolved organic carbon (DOC) leached from maize (corn; Zea maize L.) and Scirpus acutus (an aquatic bulrush), and with DOC extracted from agricultural drainage waters were determined using purge and trap introduction into a gas chromatograph-combustion-isotope ratio monitoring mass spectrometer. We observed a 16.8^0/00 difference between the d¹³C values of THM produced from the maize and Scirpus leachates, similar to the isotopic difference between the whole plant materials. Both maize and Scirpus formed THM 12^0/00 lower in ¹³C than whole plant material. We suggest that the low value of the THM relative to the whole plant material is evidence of distinct pools of THM-forming DOC, representing different biochemical types or chemical structures, and possessing different environmental reactivity.

Humic extracts of waters draining an agricultural field containing Scirpus peat soils and planted with maize formed THM with isotopic values intermediate between those of maize and Scirpus leachates, indicating maize may contribute significantly to the THM-forming DOC. The difference between the d¹³C values of the whole isolate and that of the THM it yielded was 3.9^0/00, however, suggesting diagenesis plays a role in determining the d¹³C value of THM-forming DOC in the drainage waters, and precluding the direct use of isotopic mixing models to quantitatively attribute sources.

Technical and practical aspects of applying geostatistics are developed for individuals involved in investigations at hazardous-, toxic-, and radioactive-waste sites. Important geostatistical concepts, such as variograms and ordinary, universal, and indicator kriging, are described in general terms for introductory purposes and in more detail for practical applications. Variogram modeling using measured ground-water elevation data is described in detail to illustrate principles of stationarity, anisotropy, transformations, and cross validation. Several examples of kriging applications are described using ground-water-level elevations, bedrock elevations, and ground-water-quality data. A review of contemporary literature and selected public domain software associated with geo statistics also is provided, as is a discussion of alternative methods for spatial modeling, including inverse distance weighting, triangulation, splines, trend-surface analysis, and simulation.

The issues of development of alpine areas and the possible influences of climate change in the Rocky Mountain region have both increased the focus of understanding processes controlling the water quality of mountain streams. This report presents 10 years of water quality and stream flow data from two headwater basins in Summit County, Colorado. The upper reach of the Snake River is acidic and metal-enriched from the natural and anthropogenic weathering of pyrite in the watershed, whereas the water quality of Deer Creek is pristine with a circumneutral pH. The Snake River and Deer Creek watersheds have been sites of extensive research for the past 15 years, and the data in this report have been used in these interpretative studies. The data sets are one of the longest water quality records for streams in the Upper Colorado Rivers basin and provide a description of how water quality has varied with differences between years in snowpack and other climatic parameters.

Dissolved organic carbon (DOC) from terrestrial sources forms the major component of the annual carbon budget in many headwater streams. In high-elevation catchments in the Rocky Mountains, DOC originates in the upper soil horizons and is flushed to the stream primarily during spring snowmelt. To identify controls on the size of the mobile soil DOC pool available to be transported during the annual melt event, we measured soil DOC production across a range of vegetation communities and soul types together with catchment DOC export in paired watersheds in Summit County, Colorado. Both surface water DOC concentrations and watershed DOC export were lower in areas where pyrite weathering resulted in lower soil pH. Similarly, the amount of DOC leached from organic soils was significantly smaller (p<0.01) at sites having low soil pH. Scaling point source measurements of DOC production and leaching to the two basins and assuming only vegetated areas contribute to DOC production, we calculated that the amount of mobile DOC available to be leached to surface water during melt was 20.3 g C m^-2 in the circumneutral basin and 17.8 g C m^-2 in the catchment characterized by pyrite weathering. The significant (r² = 0.91 and p < 0.05), linear relationship between over-winter CO2 flux and the amount of DOC leached from upper soil horizons during snowmelt suggests that the mechanism for the difference in production of mobile DOC was heterotrophic processing of soil carbon in snow-covered soil. Furthermore, this strong relationship between over-winter heterotrophic activity and the size of the mobile DOC pool present in a range of soil and vegetation types provides a likely mechanism for explaining the interannual variability of DOC export observed in high-elevation catchments.

Seven nested headwater catchments (8 to 161 ha) were monitored during five summer rain events to evaluate storm runoff components and the effect of catchment size on water sources. Two-component isotopic hydrograph separation showed that event-water contributions near the time of peakflow ranged from 49% to 62% in the 7 catchments during the highest intensity event. The proportion of event water in stormflow was greater than could be accounted for by direct precipitation onto saturated areas. DOC concentrations in stormflow were strongly correlated with stream ¹⁸O composition. Bivariate mixing diagrams indicated that the large event water contributions were likely derived from flow through the soil O-horizon. Results from two-tracer, three-component hydrograph separations showed that the throughfall and O-horizon soil-water components together could account for the estimated contributions of event water to stormflow. End-member mixing analysis confirmed these results. Estimated event-water contributions were inversely related to catchment size, but the relation was significant for only the event with greatest rainfall intensity. Our results suggest that perched, shallow subsurface flow provides a substantial contribution to summer stormflow in these small catchments, but the relative contribution of this component decreases with catchment size.

Brown, G.K., MacCarthy, P., and Leenheer, J.A., 1999, Simultaneous determination of Ca, Cu, Ni, Zn, and Cd binding strengths with fulvic acid fractions by Schubert's method, Analytica Chimica Acta, v. 402, 169-181.

The equilibrium binding of Ca²⁺, Ni²⁺, Cd²⁺, Cu²⁺ and Zn²⁺ with unfractionated Suwannee river fulvic acid (SRFA) and an enhanced metal binding subfraction of SRFA was measured using Schubert's ion-exchange method at pH 6.0 and at an ionic strength (m) of 0.1 (NaNO₃). The fractionation and subfractionation were directed towards obtaining an isolate with an elevated metal binding capacity or binding strength as estimated by Cu²⁺ potentiometry (ISE). Fractions were obtained by stepwise eluting an XAD-8 column loaded with SRFA with water eluents of pH 1.0 to pH 12.0. Subfractions were obtained by loading the fraction eluted from XAD-8 at pH 5.0 onto a silica gel column and eluting with solvents of increasing polarity. Schuberts ion exchange method was rigorously tested by measuring simultaneously the conditional stability constants (K) of citric acid complexed with the five metals at pH 3.5 and 6.0. The log K of SRFA with Ca2+, Ni2+, Cd2+, Cu2+ and Zn2+ determined simultaneously at pH 6.0 follow the sequence of Cu²⁺ > Cd²⁺ > Ni²⁺ > Zn²⁺ > Ca²⁺ while all log K values increased for the enhanced metal binding subfraction and followed a different sequence of Cu²⁺ > Cd²⁺ > Ca²⁺ > Ni²⁺ > Zn²⁺. Both fulvic acid samples and citric acid exhibited a 1:1 metal to ligand stochiometry under the relatively low metal loading conditions used here. Quantitative ¹³C nuclear magnetic resonance spectroscopy showed increases in aromaticity and ketone content and decreases in aliphatic carbon for the elevated metal binding fraction while the carboxyl carbon, and elemental nitrogen, phosphorus, and sulfur content did not change. The more polar, elevated metal binding fraction did show a significant increase in molecular weight over the unfractionated SRFA.

Sr isotopes can be used in granitoid catchment studies to partition base cautions to mineral weathering and atmospheric sources it the ⁸⁷Sr/⁸⁶Sr ratio provided by each source can be quantified. Although the ⁸⁷Sr/⁸⁶Sr ratio of the atmospheric component is relatively easy to quantify, the ⁸⁷Sr/⁸⁶Sr ration of the weathering component is more difficult to quantify due mainly to incongruent weathering. To determine the ⁸⁷Sr/⁸⁶Sr ratio of the weathering component, we propose a graphical method in which the array of Nb/Sr and ⁸⁷Sr/⁸⁶Sr ratios of weathering residues in a rock or soil profile is regressed to the Nb-free component. We apply the technique to weathering profiles at a catchment in the Georgia (USA) Piedmont, and show that the calculated ⁸⁷Sr/⁸⁶Sr ratio of the weathering component is that of plagiocase in the bedrock weathering environment, and on average that of stream baseflow in the saprolite and colluvium weathering environments.

From July 1994 through May 1997, the U.S. Geological Survey in cooperation with the Department of Energy, sampled 86 wells completed in the Snake River Plain aquifer at and near the Idaho National Engineering and Environmental Laboratory (INEEL). The wells were sampled for a variety of constituents including one- and two-carbon halocarbons. Concentrations of dichlorodifluoromethane (CFC-12), trichlorofluoromethane (CFC-11), and trichlorotrifluoromethane (CFC-113) were determined. The samples for halocarbon analysis were collected in 62-milliliter flame sealed borosilicate glass ampoules in the field. The data will be used to evaluate the ages of ground waters at INEEL. The ages of the ground water will be used to determine recharge rates, residence time, and travel time of water in the Snake River Plain aquifer in and near INEEL. The chromatograms of 139 ground waters are presented showing a large number of halomethanes, haloethanes, and haloethenes present in the ground waters underlying the INEEL. The chromatograms can be used to qualitatively evaluate a large number of contaminants at parts per trillion to parts per billion concentrations. The data can be used to study temporal and spatial distribution of contaminants in the Snake River Plain aquifer. Representative compressed chromatograms for all ground waters sampled in this study are available on two 3.5-inch high density computer disks. The data and the program required to decompress the data can be obtained from the U.S. Geological Survey office at Idaho Falls, Idaho. Sulfur hexafluoride (SF6) concentrations were measured in selected wells to determine the feasibility of using this environmental tracer as an age dating tool of ground water. Concentrations of dissolved nitrogen, argon, carbon dioxide, oxygen, and methane were measured in 79 ground waters. Concentrations of dissolved permanent gases are tabulated and will be used to evaluate the temperature of recharge of ground water in and near the INEEL.

Metal concentrations of the soluble fraction of the cytoplasm (cytosol) and the whole body were determined in the caddisfly Hydropsyche spp. (Trichoptera). Metal accumulation in the cytosol and the whole body were compared in samples collected along 380 kms of a contamination gradient in the Clark Fork river in four consecutive years (1992-1995), and from a contaminated tributary (Flint Creek). Samples from the contaminated sites were compared to an uncontaminated tributary (Blackfoot River). Relations between cytosolic metal concentration and cytosolic protein (used as a general biomarker of protein metabolism) also were examined in 1994 and 1995. Relative to whole body concentrations, cytosolic metal concentrations varied among metals and years. Spatial patterns in whole body and cytosolic Cd, Cu and Pb concentrations were qualitatively similar each year, and these concentrations generally corresponded to contamination levels measured in bed sediments. The proportions of metals recovered in the cytosol of ranged from 12 to 64% for Cd and Cu and from 2 to 38% for Pb. Zinc in the whole body also was consistent with contamination levels, but cytosolic Zn concentrations increased only at the highest whole body Zn concentrations. As a result, the proportion of Zn recovered in the cytosol ranged from 16 to 63% and tended to be inversely related to whole body Zn concentrations. The proportions of cytosolic metals varied significantly among years and, as a result, interannual differences in metal concentrations were greater in the cytosol than in the whole body. The results demonstrated that Hydropsyche in the river were chronically exposed to biologically available metals. Some features of this exposure were not evident from whole body concentrations. In general, protein levels did not correspond to cytosolic metal concentrations. A variety of environmental factors could interact with metal exposures to produce complex responses in protein metabolism. Systematic study will be necessary to differentiate the effects of multiple environmental stressors on organisms living in contaminated ecosystems.

Campbell Rehmann, L.L., Welty, C., and Harvey, R.W., 1999, Stochastic analysis of virus transport in aquifers: Water Resources Research, v. 35, p. 1987-2006.

A large-scale model of virus transport in aquifers is derived using spectral perturbation analysis. The effects of spatial variability in aquifer hydraulic conductivity and virus transport (attachment, detachment, and inactivation) parameters on large-scale virus transport are evaluated. A stochastic mean model of virus transport is developed by linking a simple system of local-scale free-virus transport and attached-virus conservation equations from the current literature with a random-field representation of aquifer and virus transport properties. The resultant mean equations for free and attached viruses are found to differ considerably from the local-scale equations on which they are based and include effects such as a free-virus effective velocity that is a function of aquifer heterogeneity as well as virus transport parameters. Stochastic mean free-virus breakthrough curves are compared with local model output in order to observe the effects of spatial variability on mean one-dimensional virus transport in three-dimensionally heterogeneous porous media. Significant findings from this theoretical analysis include the following: (1) Stochastic model breakthrough occurs earlier than local model breakthrough, and this effect is most pronounced for the least conductive aquifers studied. (2) A high degree of aquifer heterogeneity can lead to virus breakthrough actually preceding that of a conservative tracer. (3) As the mean hydraulic conductivity is increased, the mean model shows less sensitivity to the variance of the natural-logarithm hydraulic conductivity and mean virus diameter. (4) Incorporation of a heterogeneous colloid filtration term results in higher predicted concentrations than a simple first-order adsorption term for a given mean attachment rate. (5) Incorporation of aquifer heterogeneity leads to a greater range of virus diameters for which significant breakthrough occurs. (6) The mean model is more sensitive to the inactivation rate of viruses associated with solid surfaces than to the inactivation rate of viruses in solution.

Carmody, R.W., and Seal II, R.R., 1999, Evaluation of that sulfur isotopic composition and homogeneity of the Soufre de Lacq reference material: Chemical Geology, v. 153, p. 289-295.

Sulfur isotopic analysis of the elemental sulfur reference material Soufre de Lacq, prepared as silver sulfide by chromous chloride reduction and as copper sulfide by sealed-tube synthesis, indicates that Soufre de Lacq is isotopically homogeneous across different size fractions to within analytical uncertainty (±0.15 parts per mil). The sulfur isotopic composition of aliquots of Soufre de Lacq prepared by these two techniques are identical to within analytical uncertainty. The mean sulfur isotopic composition for Soufre de Lacq prepared as silver sulfide and copper sulfide (relative to VCDT) is +16.20 ± 0.15 parts per mil (1sigma ).

Carter, V., Rybicki, N.B., Landwehr, J.M., Reel, J.T., and Ruhl, H.A., 1999, Summary of correlations among seasonal water quality, discharge, weather, and coverage by submersed aquatic vegetation in the tidal Potomac River and Potomac Estuary, 1983-96: U.S. Geological Survey Open-File Report 98-657, 81 p.

Carter, V., Ruhl, H.A., Rybicki, N.B., Reel, J.T., and Gammon, Patricia T., 1999, Vegetative resistance to flow in south Florida: Summary of vegetation sampling at sites NESRS3 and P33, Shark River Slough, April, 1996: U.S. Geological Survey Open-File Report 99-187, 73 p.

Carter, V., Ruhl, H.A., Rybicki, N.B., Reel, J.,T., and Gammon, Patricia T., 1999, Vegetative resistance to flow in south Florida: Summary of vegetation sampling at sites NESRS3 and P33, Shark River Slough, November, 1996: U.S. Geological Survey Open-File Report 99-218, 89 p.

Cayan, D.R., Redmond, K.T, and Riddle, L.G., 1999, ENSO and hydrologic extremes in the Western United States: J. Clim., v. 12, p. 2881-2893.

Frequency distributions of daily precipitation in winter and daily stream flow from late winter to early summer, at several hundred sites in the western United States, exhibit strong and systematic responses to the two phases of ENSO. Most of the stream flows considered are driven by snowmelt. The Southern Oscillation index (SOI) is used as the ENSO phase indicator. Both modest (median) and larger (90th percentile) events were considered. In years with negative SOI values (El Niño), days with high daily precipitation and stream flow are more frequent than average over the Southwest and less frequent over the Northwest. During years with positive SOI values (La Niña), a nearly opposite pattern is seen. A more pronounced increase is seen in the number of days exceeding climatological 90th percentile values than in the number exceeding climatological 50th percentile values, for both precipitation and stream flow. Stream flow responses to ENSO extremes are accentuated over precipitation responses. Evidence suggests that the mechanism for this amplification involves ENSO-phase differences in the persistence and duration of wet episodes, affecting the efficiency of the process by which precipitation is converted to runoff. The SOI leads the precipitation events by several months, and hydrologic lags (mostly through snowmelt) delay the stream flow response by several more months. The combined 6-12-month predictive aspect of this relationship should be of significant benefit in responding to flood (or drought) risk and in improving overall water management in the western states.

Lava Falls Rapid, which was created and is maintained by debris flows from Prospect Canyon, is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Debris flows enter the Colorado River at tributary junctures, creating rapids. The frequency of debris flows is an important consideration when management of regulated rivers involves maintenance of channel morphology. We used cosmogonic ³He, ¹⁴C, and historical photographs to date 12 late Holocene and historic debris flows from Prospect Canyon. The highest and oldest deposits from debris flows on the debris fan yielded a ³He date of about 3 ka, which indicates predominately late Holocene aggradation of one of the largest debris fans in Grand Canyon. The deposit, which has a 25-m escarpment caused by river reworking, crossed the Colorado River and raised its base level by 30 m for an indeterminate although likely short period. We mapped depositional surfaces of 11 debris flows that occurred after 3 ka. Two deposits inset against the highest deposit yielded ³He ages of about 2.2 ka, and at least two others followed shortly afterwards. At least one of these debris flows also dammed the Colorado River. The most recent prehistoric debris flow occurred no more than 0.5 ka. The largest historic debris flow, which constricted the river by 80%, occurred in 1939. Five other debris flows occurred after 1939; these debris flows constricted the Colorado River by 35-80%. Assuming the depositional volumes of late Holocene debris flows can be modeled using a lognormal distribution, we calculated recurrence intervals of 15 to more than 2000 years for debris flows from Prospect Canyon.

Recently, methods have been developed to analyze NO₃^- for d¹⁵N and d¹⁸O, improving our ability to identify NO₃^- sources and transformations. However, none of the existing methods are suited for waters with low NO₃^- concentrations (0.7-10 mM). We describe an improved method for collecting and recovering NO₃^- on exchange columns. To overcome the lengthy collection loading times imposed by the large sample volumes (7-70 L), the sample was prefiltered (0.45 mm) with a large surface area filter. Switching to AG2X anion resin and using a coarser mesh size (100-200) than previous methods also enhanced sample flow. Placement of a cation column in front of the anion column minimized clogging of the anion column by dissolved organic carbon (DOC) accumulation. This also served to minimize transfer of unwanted oxygen atoms from DOC to the ¹⁸O portion of the NO₃^- sample, thereby contaminating the sample and shifting d¹⁸O. The cat-AG2X method is suited for on-site sample collection, making it possible to collect and recover NO₃^- from low ionic strength waters with modest DOC concentrations (80-800 mM), relieves the investigator of transporting large volumes of water back to the laboratory, and offers a means of sampling rain, snow, snowmelt, and stream samples from access-limited sites.

We tested the accuracy of 95% individual prediction intervals for hydraulic heads, streamflow gains, and effective transmissivities computed by groundwater models of two Danish aquifers. To compute the intervals, we assumed that each predicted value can be written as the sum of a computed dependent variable and a random error. Testing was accomplished by using a cross-validation method and by using new field measurements of hydraulic heads and transmissivities that were not used to develop or calibrate the models. The tested null hypotheses are that the coverage probability of the prediction intervals is not significantly smaller than the assumed probability (95%) and that each tail probability is not significantly different from the assumed probability (23%). In all cases tested, these hypotheses were accepted at the 5% level of significance. We therefore conclude that for the groundwater models of two real aquifers the individual prediction intervals appear to be accurate.

The fact that dependent variables of groundwater models are generally nonlinear functions of model parameters is shown to be a potentially significant factor in calculating accurate confidence intervals for both model parameters and functions of the parameters, such as the values of dependent variables calculated by the model. The Lagrangian method of Vecchia and Cooley [Vecchia, A.V., and Cooley, R.L., 1987, Water Resources Research, v. 23(7), p. 1237-1250] was used to calculate nonlinear Scheffe-type confidence intervals for the parameters and the simulated heads of a steady-state groundwater flow model covering 450 km² of a leaky aquifer. The nonlinear confidence intervals are compared to corresponding linear intervals. As suggested by the significant nonlinearity of the regression model, linear confidence intervals are often not accurate. The commonly made assumption that widths of linear confidence intervals always underestimate the actual (nonlinear) widths was not correct. Results show that nonlinear effects can cause the nonlinear intervals to be asymmetric and either larger or smaller than the linear approximations. Prior information on transmissivities helps reduce the size of the confidence intervals, with the most notable effects occurring for the parameters on which there is prior information and for head values in parameter zones for which there is prior information on the parameters.

Anthropogenic nutrient enrichment of the coastal zone is now a well-established fact. However, there is still uncertainty about the mechanisms through which nutrient enrichment can disrupt biological communities and ecosystem processes in the coastal zone. For example, while some estuaries exhibit classic symptoms of acute eutrophication, including enhanced production of algal biomass, other nutrient-rich estuaries maintain low algal biomass and primary production. This implies that large differences exist among coastal ecosystems in the rates and patterns of nutrient assimilation and cycling. Part of this variability comes from differences among ecosystems in the other resource that can limit algal growth and production - the light energy required for photosynthesis. Complete understanding of the eutrophication process requires consideration of the interacting effects of light and nutrients, including the role of light availability as a regulator of the expression of eutrophication. A simple index of the relative strength of light and nutrient limitation of algal growth can be derived from models that describe growth rate as a function of these resources. This index can then be used as one diagnostic to classify the sensitivity of coastal ecosystems to the harmful effects of eutrophication. Here I illustrate the application of this diagnostic with light and nutrient measurements made in three California estuaries and two Dutch estuaries.

Periphyton samples from the Big Cypress National Preserve were analyzed for concentrations of copper, lead, zinc, mercury, and methylmercury. Concentrations of organic carbon, inorganic carbon, nitrogen, and phosphorus in periphyton samples also were determined. The samples were extracted with sodium acetate solution at a pH of 5.5 to determine exchangeable and carbonate phase metal concentrations in periphyton. Total metal concentrations in the periphyton were directly related to the degree of calcite saturation in the water column. Exchangeable and carbonate phase metal concentrations were directly related to the percent inorganic carbon in the samples. A connection between geochemistry of trace metals and calcite precipitation and dissolution is suggested.

Cozzarelli, I.M., Baedecker, M.J., Eganhouse, R.P., Tuccillo, M.E., Aiken, G.R., Bekins, B.A., and Jaeschke, J.B., 1999, Long-term geochemical evolution of the Bemidji Toxics site crude-oil plume, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3 - Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 123-132.

Cozzarelli, I.M., Herman, J.S., Baedecker, M.J., and Fischer, J.M., 1999, Geochemical heterogeneity of a gasoline-contaminated aquifer: Journal of Contaminant Hydrology, v. 40, p. 261-284.

The scale of biogeochemical reactions was studied in a physically and chemically heterogeneous surficial Coastal Plain aquifer contaminated by a gasoline spill. The physical heterogeneity of the aquifer is manifested in two hydrologic units, a shallow local aquifer of perched water and a regional sandy aquifer. Over the studied vertical interval of 21.3 ft (6.5 m), concentrations of reactive species varied by orders of magnitude, and the impact of biodegradation was expressed to widely varying degrees. A thin (3 ft thick) section of the perched-water zone was the most contaminated; total aromatic hydrocarbons were as high as 19.4 mg/l. Hydrocarbons were degraded by microbially mediated reactions that varied over short vertical distances and time. Anaerobic processes dominated within the low-permeability clay unit, whereas in the more permeable sandy layers nitrate reduction and aerobic degradation occurred. Hydrocarbons were more persistent over time in the low-permeability layer due to the limited availability of electron acceptors for degradation. The microbial degradation of hydrocarbons was linked to sulfate and iron reduction in the clay unit and led to alterations in the aquifer solids; electron microscopy revealed the presence of FeS minerals encrusting primary aquifer grains. High concentrations of Fe²⁺ in groundwater, up to 34.5 mg/l, persist in kinetic disequilibrium in the presence of elevated H₂S levels of 1.0 mg/l. Assessment of aquifer heterogeneities and groundwater contamination was possible due to sample discrimination at a scale of approximately 2 ft (similar to 0.6 m), a much finer resolution than is attempted in many remedial investigations of polluted aquifers. The information obtained in this type of study is essential to the development of models capable of estimating the fate of hydrocarbons at a site scale.

A regional-scale, steady-state, saturated-zone ground-water flow model was constructed to evaluate potential regional ground-water flow in the vicinity of Yucca Mountain, Nevada. The model was limited to three layers in an effort to evaluate the characteristics governing large-scale subsurface flow. Geoscientific information systems (GSIS) were used to characterize the complex surface and subsurface hydrogeologic conditions of the area, and this characterization was used to construct likely conceptual models of the flow system. Subsurface properties in this system vary dramatically, producing high contrasts and abrupt contacts. This characteristic, combined with the large scale of the model, make zonation the logical choice for representing the hydraulic-conductivity distribution. Different conceptual models were evaluated using sensitivity analysis and were tested by using nonlinear regression to determine parameter values that are optimal, in that they provide the best match between the measured and simulated heads and flows. The different conceptual models were judged based both on the fit achieved to measured heads and spring flows, and the plausibility of the optimal parameter values. One of the conceptual models considered appears to represent the system most realistically. Any apparent model error is probably caused by the coarse vertical and horizontal discretization.

Dean, W.E., 1999, The carbon cycle and biogeochemcial dynamics in lake sediments: Journal of Paleolimnology, v., 21, p. 375-393.

The concentrations of organic carbon (OC) and CaCO₃ in lake sediments are often inversely related. This relation occurs in surface sediments from different locations in the same lake, surface sediments from different lakes, and with depth in Holocene sediments. Where data on accumulation rates are available, the relation holds for organic carbon and CaCO₃ accumulation rates as well. An increase of several percent OC is accompanied by a decrease of several tens of percent CaCO₃ indicating that the inverse relation is not due to simple dilution of one component by another. It appears from core data that once the OC concentration in the sediments becomes greater than about 12%, the CO₂ produced by decomposition of that OC and production of organic acids lowers the pH of anoxic pore waters enough to dissolve any CaCO₃ that reaches the sediment-water interface. In a lake with a seasonally anoxic hypolimnion, processes in the water column also can produce an inverse relation between OC and CaCO₃ over time. If productivity of the lake increases, the rain rate of OC from the epilimnion increases. Biogenic removal of CO₂ and accompanying increase in pH also may increase the production of CaCO₃. However, the decomposition of organic matter in the hypolimnion will decrease the pH of the hypolimnion causing greater dissolution of CaCO₃ and therefore a decrease in the rain rate of CaCO₃ to the sediment-water interface.

A photolinear analysis of the Eldridge-Wilde well field, Pinellas County, Florida, was conducted to identify zones of fracture concentration. Photolinear fracture traces and lineaments correspond to vertical zones of fracture concentration. Forty-three photolinears, ranging in length from 269 meters to 2.53 kilometers were mapped on four aerial photographs. The dominant azimuthal orientation of the photolinears is in the range of 41 degrees to 50 degrees. The relation between well yield and well location relative to fracture trace location was investigated. Of the forty-eight production wells in the field study area for which specific yield data were available, seven are located on mapped fracture traces, one of which is on a fracture trace intersection. Boxplots of specific yields of wells "on" and "off" fracture traces show that the median productivity of the "on" group, 308 gallons per minute per foot of drawdown. However, nonparametric statistical analysis could not reject the null hypothesis that both samples were from the same statistical population. Application of this photolinear analysis to any subsequent water supply investigations would require supplementary inquiries, including field checking of the mapped photolinears and test drilling. In the course of the investigation, a list of sixteen hydrogeologic variables that can influence porosity and permeability at the Eldridge-Wilde well field was developed.

Dodge, K.A., Hornberger, M.I., and Bouse, R.M., 1999, Water quality, bed sediment and biological data (October 1997 through September 1998) and statistical summaries of data for streams in the Upper Clark Fork Basin, Montana: U.S. Geological Survey Open File Report 99-251.

Water, bed sediment, and biota were sampled in streams from Butte to below Missoula as part of a program to characterize aquatic resources in the upper Clark Fork basin of western Montana. Sampling stations were located on the Clark Fork and major tributaries. Water-quality data were obtained periodically at 15 stations during October 1997 through September 1998 (water year 1998). Data for 15 bed-sediment and 15 biological stations were obtained in August 1998. The primary constituents analyzed were trace elements associated with tailings from historical mining and smelting activities. Water-quality data include concentrations of selected major ions, trace elements, and suspended sediment in stream samples. Daily values of streamflow, suspended-sediment concentration, and suspended-sediment discharge are given for three stations. Bed-sediment data include trace-element concentrations in the fine-grained and bulk fractions. Biological data include trace-element concentrations in whole-body tissue of aquatic benthic insects. Quality-assurance data are reported for analytical results of water, bed sediment, and biota. Statistical summaries of water-quality, bed-sediment, and biological data are provided for the period of record at each station since 1985.

Eganhouse, R.P., Matthews, L.L., Cozzarelli, I.M., and Scholl, M.A., 1999, Evidence for natural attenuation of volatile organic compounds in the leachate plume of the Norman, Oklahoma landfill, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3 - Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 531-540.

Everett, C.R., Chin, Y., and Aiken, G.R., 1999, High pressure size exclusion chromatography analysis of dissolved organic matter isolated by tagential flow ultrafiltration: Limnology and Oceanography, v. 44, p. 1316-1322.

A 1,000-Dalton tangential-flow ultrafiltration (TFUF) membrane was used to isolate dissolved organic matter (DOM) from several freshwater environments. The TFUF unit used in this study was able to completely retain a polystyrene sulfonate 1,800-Dalton standard. Unaltered and TFUF-fractionated DOM molecular weights were assayed by high-pressure size exclusion chromatography (HPSEC). The weight-averaged molecular weights of the retentates were larger than those of the raw water samples, whereas the filtrates were all significantly smaller and approximately the same size or smaller than the manufacturer-specified pore size of the membrane. Moreover, at 280 nm the molar absorptivity of the DOM retained by the ultrafilter is significantly larger than the material in the filtrate. This observation suggests that most of the chromophoric components are associated with the higher molecular weight fraction of the DOM pool. Multivalent metals in the aqueous matrix also affected the molecular weights of the DOM molecules. Typically, proton-exchanged DOM retentates were smaller than untreated samples. This TFUF system appears to be an effective means of isolating aquatic DOM by size, but the ultimate size of the retentates may be affected by the presence of metals and by configurational properties unique to the DOM phase.

Performance Assessment (PA) is the use of mathematical models to simulate the long-term behavior of engineered and geologic barriers in a nuclear waste repository; methods of uncertainty analysis are used to assess effects of parametric and conceptual uncertainties associated with the model system upon the uncertainty in outcomes of the simulation. PA is required by the U.S. Environmental Protection Agency as part of its certification process for geologic repositories for nuclear waste. This paper is a dialogue to explore the value and limitations of PA. Two 'skeptics' acknowledge the utility of PA in organizing the scientific investigations that are necessary for confident siting and licensing of a repository; however, they maintain that the PA process is, at least as it is currently implemented, is an essentially unscientific process with shortcomings that may provide results of limited use in evaluating actual effects on public health and safety. Conceptual uncertainties in a PA analysis can be so great that results can be confidently applied only over short time ranges, the antithesis of the purpose behind long-term, geologic disposal. Two 'proponents' of PA agree that performance assessment is unscientific, but only in the sense that PA is an engineering analysis that uses existing scientific knowledge to support public policy decisions, rather than an investigation intended to increase fundamental knowledge of nature; PA has different goals and constraints than a typical scientific study. The 'proponents' describe an ideal, six-step process for conducting generalized PA, here called probabilistic systems analysis (PSA); they note that virtually all scientific content of a PA is introduced during the model-building steps of a PSA; they contend that a PA based on simple but scientifically acceptable mathematical models can provide useful and objective input to regulatory decision makers. The value of the results of any PA must lie between these two views and will depend on the level of knowledge of the site, the degree to which models capture actual physical and chemical processes, the time over which extrapolations are made, and the proper evaluation of health risks attending implementation of the repository. The challenge is in evaluating whether the quality of the PA matches the needs of decision makers charged with protecting the health and safety of the public.

Simmonsite, Na₂LiAlF₆, a new mineral of pegmatitic-hydrothermal origin, occurs in a late-stage breccia pipe structure that cuts the Zapot amazonite-topaz-zinnwaldite pegmatite located in the Gillis Range, Mineral Co., Nevada, U.S.A. The mineral is intimately intergrown with cryolite, cryolithionite and trace elpasolite. A secondary assemblage of other alumino-fluoride minerals and a second generation of cryolithionite has formed from the primary assemblage. The mineral is monoclinic, P2₁ or P2₁/m, a = 7.5006(6) Å, b = 7.474(1) Å, c = 7.503(1) Å, b= 90.847(9)^o, V = 420.6(1) ų, Z = 4. The four strongest diffraction maxima [d (Å), hkl, I/I₁₀₀ are (4.33, 111 and 111, 100); (1.877, 400 and 004, 90); (2.25, 131, 113, 131 and 311, 70); and (2.65, 220, 202, 022, 60). Simmonsite is pale buff cream with white streak, somewhat greasy, translucent to transparent, Mohs hardness of 2.5-3, no distinct cleavage, subconchoidal fracture, no parting, not extremely brittle, D_m is 3.05(2) g/cm³, and D_c is 3.06(1) g/cm³. The mineral is biaxial, very nearly isotropic, N is 1.359(1) for l = 589 nm, and birefringence is 0.0009. Electron microprobe analyses gave (wt%) Na = 23.4, Al = 13.9, Al = 13.9, F = 58.6, Li = 3.56 (calculated), with a total of 99.46. The empirical formula (based on 6 F atoms) is N_1.98Li_1.00Al_1.00F ₆. The crystal structure was not solved, presumably because of unit-cell scale twinning, but similarities to the perovskite-type structure exist. The mineral is named for William B. Simmons, Professor of Mineralogy and Petrology, University of New Orleans, New Orleans.

Fram, M.S., Fujii, J.L., Weishaar, B.A., Bergamaschi, B.A., and Aiken, G.R., 1999, How DOC composition may explain poor correlation between specific trihalomethane formation potential and specific UV absorbance, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 2 - Contamination of hydrologic systems and related ecosystems: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, p 423-430.

Sediment chronologies based on radioisotope depth profiles were developed at two sites in the San Francisco Bay estuary to provide a framework for interpreting historical trends in organic compound and metal contaminant inputs. At Richardson Bay near the estuary mouth, sediments are highly mixed by biological and/or physical processes. Excess ²³⁴Th penetration ranged from 2 to more than 10 cm at eight coring sites, yielding surface sediment mixing coefficients ranging from 12 to 170 cm²/year. At the site chosen for contaminant analyses, excess ²¹⁰Pb activity was essentially constant over the upper 25 cm of the core with an exponential decrease below to the supported activity between 70 and 90 cm. Both ¹³⁷Cs and ^239,240Pu penetrated to 57-cm depth and have broad subsurface maxima between 33 and 41 cm. The best fit of the excess ²¹⁰Pb profile to a steady state sediment accumulation and mixing model yielded an accumulation rate of 0.825 g/cm²/year (0.89 cm/year at sediment surface), surface mixing coefficient of 71 cm²/year, and 33-cm mixed zone with a half-Gaussian depth dependence parameter of 9 cm. Simulations of ¹³⁷Cs and ^239,240Pu profiles using these parameters successfully predicted the maximum depth of penetration and the depth of maximum ¹³⁷Cs and ^239,240Pu activity. Profiles of successive 1-year hypothetical contaminant pulses were generated using this parameter set to determine the age distribution of sediments at any depth horizon. Because of mixing, sediment particles with a wide range of deposition dates occur at each depth. A sediment chronology was derived from this age distribution to assign the minimum age of deposition and a date of maximum deposition to a depth horizon. The minimum age of sediments in a given horizon is used to estimate the date of first appearance of a contaminant from its maximum depth of penetration. The date of maximum deposition is used to estimate the peak year of input for a contaminant from the depth interval with the highest concentration of that contaminant. Because of the extensive mixing, sediment-bound constituents are rapidly diluted with older material after deposition. In addition, contaminants persist in the mixed zone for many years after deposition. More than 75 years are required to bury 90% of a deposited contaminant below the mixed zone. Reconstructing contaminant inputs is limited to changes occurring on a 20-year time scale. In contrast, mixing is much lower relative to accumulation at a site in San Pablo Bay. Instead, periods of rapid deposition and/or erosion occurred as indicated by frequent sand-silt laminae in the X-radiograph. ¹³⁷Cs, ^239,240Pu , and excess ²¹⁰Pb activity all penetrated to about 120 cm. The distinct maxima in the fallout radionuclides at 105-110 cm yielded overall linear sedimentation rates of 3.9 to 4.1 cm/year, which are comparable to a rate of 4.5 plus or minus 1.5 cm/year derived from the excess ²¹⁰Pb profile.

Sea level, velocity, temperature, and salinity (conductivity and temperature) data collected in Suisun Bay, California, from December 11, 1992, through May 31, 1993, by the U.S. Geological Survey are documented in this report. Sea-level data were collected at four locations and temperature and salinity data were collected at seven locations. Velocity data were collected at three locations using acoustic Doppler current profilers and at four other locations using point velocity meters. Sea-level and velocity data are presented in three forms (1) harmonic analysis results, (2) time-series plots (sea level, current speed, and current direction versus time), and (3) time-series plots of the low-pass filtered data. Temperature and salinity data are presented as plots of raw and low-pass filtered time series.

The velocity and salinity data collected during this study document a period when the residual current patterns and salt field were significantly altered by large Delta outflow (three peaks in excess of 2,000 cubic meters per second). Residual current profiles were consistently seaward with magnitudes that fluctuated primarily in concert with Delta outflow and secondarily with the spring-neap tide cycle. The freshwater inputs advected salinity seaward of Suisun Bay for most of this study. Except for a 10-day period at the beginning of the study, dynamically significant salinities (>2) were seaward of Suisun Bay, which resulted in little of no gravitational circulation transport.

Yucca Mountain straddles the west boundary of the Nevada Test Site in an arid, remote, and thinly populated region of southwestern Nevada. It is the potential site of a monitored geologic repository for the nation's commercial and military spent nuclear fuel, high-level radioactive waste derived from reprocessing of uranium and plutonium, surplus plutonium, and other nuclear-weapons materials. The fundamental rationale for a geologic repository for radioactive materials is to securely isolate them from the environment and its occupants to the greatest extent possible.

Hoch, A.R., Reddy, M.M., and Drever, J.I., 1999, Importance of mechanical disaggregation in chemical weathering in a cold alpine environment, San Juan Mountains, Colorado: Geological Society of American Bullentin, February 1999, v. III, no. 2, p. 304-314.

Weathering of welded tuff near the summit of Snowshoe Mountain (3660 m) in southwestern Colorado was studied by analyzing infiltrating waters in the soil and associated solid phases. Infiltrating waters exhibit anomalously high potassium to silica ratios resulting from dissolution of a potassium-rich glass that occurs as a trace phase in the rock. In laboratory experiments using rock from the field site, initial dissolution generated potassium-rich solutions similar to those observed in the field. The anomalous potassium release decreased over time (about 1 month), after which the dominant cation was calcium, with a much lower potassium to silica ratio. The anomalous potassium concentrations observed in the infiltrating soil solutions result from weathering of freshly exposed rock surfaces. Continual mechanical disaggregation of the rock due to segregation freezing exposes fresh glass to weathering and thus maintains the source of potassium for the infiltrating water. The ongoing process of creation of fresh surfaces by physical processes is an important influence on the composition of infiltrating waters in the vadose zone.

Although waste water discharge into San Francisco Bay has increased since the 1950's, this has been accompanied by investments in advanced treatment. Since 1976, near-monthly samples of sediments and the deposit feeding clam, Macoma balthica, have been collected near the Palo Alto Regional Water Quality Control Plant (PARWQCP) to determine how changes in waste water discharge affected trends in silver, copper, and zinc concentrations. Long-term reductions of metals in M. balthica and sediments were evident as waste water treatment improved at PARWQCP. Mean annual silver concentrations in M. balthica were 106 mg/g in 1978, 55.4 mg/g in 1987 and 3.6 mg/g in 1997. These declines coincided with improved treatment processes implemented by PARWQCP. Mean annual concentrations of copper in M. balthica declined from a maximum of 287 mg/g in 1980 to the minimum of 24 mg/g in 1991. Temporal changes in zinc concentrations were nearly bimodal, with the highest concentrations occurring during the years of high precipitation. Copper bioaccumulation was strongly correlated with copper loads from the PARWQCP until 1990, suggesting that effluents from the treatment plant were the primary source of copper to M. balthica during this period. Copper loadings from the PARWQCP continued to decline steadily after 1990, but copper in M. balthica continued to increase to a high of 71 mg/g in 1996, showing no significant correlation to copper loadings. Thus, as the localized sources of copper decreased, inputs from outside sources (for example, urban runoff), became more important in controlling metal bioaccumulation. Stream flow and precipitation were used as surrogate measures for metal loads entering into the Bay and show a strong correlation to copper bioaccumulation from 1989-97. The high concentrations of metals in the mudflat may have had and adverse affect on the resident population of organisms as measured by the low number of reproductively active individuals present between 1974-1983 (<20 percent). As metal concentrations began to decrease, 70-100 percent of the population were reproductively active. Reproductive patterns typical of less impacted sites in the Bay were not consistently observed at the Palo Alto mudflat until 1989, corresponding to mean annual copper concentrations of 35 mg/g in tissues and mean annual silver concentrations of 11 mg/g.

Concentrations of Ag, Al, Cr, Cu, Fe, Hg, Mn, Ni, Pb, V and Zn were determined in six sediment cores from San Francisco Bay (SFB) and one sediment core in Tomales Bay (TB), a reference estuary. SFB cores were collected from between the head of the estuary and its mouth (Grizzly Bay, GB; San Pablo Bay, SP; Central Bay, CB; Richardson Bay, RB, respectively) and ranged in length from 150 to 250 cm. Concentrations of Cr, V and Ni are greater than mean crustal content in SFB and TB sediments, and greater than found in many other coastal sediments. However, erosion of ultramafic rock formations in the watershed appears to be the predominant source. Baseline concentrations of other metals were determined from horizons deposited before sediments were influenced by human activities and by comparing concentrations to those in TB. Baseline concentrations of Cu co-varied with Al in the SFB sediments and ranged from 23.7 ± 1.2 µg/g to 41.4 ± 2.4 µg/g. Baseline concentrations of other metals were less variable: Ag, 0.09 ± 0.02 µg/g; Pb, 5.2 ± 0.7 µg/g; Hg, 0.06 ± 0.01 µg/g; Zn, 78 ± 7 µg/g. The earliest anthropogenic influence on metal concentrations appeared as Hg contamination (0.3-0.4 µg/g) in sediments deposited at SP between 1850 and 1880, apparently associated with debris from hydraulic gold mining

Hostettler, F.D., Kvenvoiden, K.A., Rosenbauer, R.J., and Short, J.W., Aspects of the Exxon Valdez oil spill-a forensic study and a toxics controversy, 1999, in Morganwalp, D.W. and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Charleston, S. Carolina. March 8-12, 1999: U.S. Geological Survey Water-Resources Investigations Report 99-4018B, v. 2, p. 135-144.

San Francisco Bay is one of the world's largest urbanized estuarine systems. Its water and sediment receive organic input from a wide variety of sources; much of this organic material is anthropogenically derived. To document the spatial and historical record of the organic contaminant input, surficial sediment from 17 sites throughout San Francisco Bay and sediment cores from two locations--Richardson Bay and San Pablo Bay--were analyzed for biomarker constituents. Biomarkers, that is, `molecular fossils', primarily hopanes, steranes, and n-alkanes, provide information on anthropogenic contamination, especially that related to petrogenic sources, as well as on recent input of biogenic material. The biomarker parameters from the surficial sediment and the upper horizons of the cores show a dominance of anthropogenic input, whereas the biomarker profiles at the lower horizons of the cores indicate primarily biogenic input. In the Richardson Bay core the gradual upcore transition from lower maturity background organics to a dominance of anthropogenic contamination occurred about 70-100 years ago and corresponds to the industrial development of the San Francisco Bay area. In San Pablo Bay, the transition was very abrupt, reflecting the complex depositional history of the area. This sharp transition, perhaps indicating a depositional hiatus or erosional period, dated at pre-1952, is clearly visible. Below, the hiatus the biomarker parameters are immature; above, they are mature and show an anthropogenic overlay. Higher concentrations of terrigenous n-alkanes in the upper horizons in this core are indicative of an increase in terrigenous organic matter input in San Pablo Bay, possibly a result of water diversion projects and changes in the fresh water flow into the Bay from the Delta. Alternatively, it could reflect a dilution of organic material in the lower core sections with hydraulic mining debris.

Geochemical correlation and differentiation of hydrocarbons from crude oils and coals is difficult. The complex mixture of the hydrocarbon constituents and the dynamic nature of these constituents in the environment as they weather contribute to this difficulty. A new parameter, the polycyclic aromatic hydrocarbon (PAH) refractory index, is defined here to help in this correlation. The PAH refractory index is a ratio of two of the most refractory constituents of most crude oils, namely triaromatic steranes and monomethylchrysenes. These are among the most persistent compounds in oil after deposition in the environment and thus retain reliably the signature of the original petroleum input. This index is utilized in Prince William Sound (PWS) to differentiate three different oils, as well as to provide evidence that coal, not oil, is the dominant source of the PAHs which are prominent constituents of marine sediments from PWS and the Gulf of Alaska.

The decrease in permeability (k) of the continental crust with depth (z), as constrained by geothermal data and calculated fluid flux during metamorphism, is given by log k = -14 - 3.2 log z, where k is in meters squared and z is in kilometers. At moderate to great crustal depths (> approximately 5 km), this curve is defined mainly by data from prograde metamorphic systems, and is thus applicable to orogenic belts where the crust is being thickened and/or heated; lower permeabilities may occur in stable cratonic regions. This k-z relation implies that typical metamorphic fluid flux values of approximately 10^-11 m/s are consistent with fluid pressures significantly above hydrostatic values. The k-z curve also predicts that metamorphic CO₂ flux from large orogens may be sufficient to cause significant climatic effects, if retrograde carbonation reactions are minimal, and suggests a significant capacity for diffuse degassing of Earth (10¹⁵ -10¹⁶ g/yr) in tectonically active regions.

Selenium stable isotope ratio measurements should serve as indicators of sources and biogeochemical transformations of Se. We report measurements of Se isotope fractionation during selenate reduction, selenite sorption, oxidation of reduced Se in soils, and Se volatilization by algae and soil samples. These results, combined with previous work with Se isotopes, indicate that reduction of soluble oxyanions is the dominant cause of Se isotope fractionation. Accordingly, Se isotope ratios should be useful as indicators of oxyanion reduction, which can transform mobile species to forms that are less mobile and less bioavailable. Additional investigations of Se isotope fractionation are needed to confirm this preliminary assessment.

We have developed a new method for measurement of natural Se isotope ratio variation which requires less than 500 ng Se per analysis and yields ±0.2^0/00 precision on ⁸⁰Se/⁷⁶Se. A double isotope spike technique corrects for isotopic fractionation during sample preparation and mass spectrometry. The small minimum sample size is important, as Se concentrations are often below 1 ppm in solids and 1 µg/L in fluids. The Se purification process is rapid and compatible with various sample matrices, including acidic rock or sediment digests.

Joye, S.B., Connell, T.L., Miller, L.G., Jellison, R., and Oremland, R.S., 1999, Oxidation of ammonia and methane in an alkaline, saline lake: Limnol. Oceanogr., v. 44, p. 178-188.

The oxidation of ammonia (NH₃) and methane (CH₄) was investigated in an alkaline saline lake, Mono Lake, California (USA). Ammonia oxidation was examined in April and July 1995 by comparing dark ¹⁴CO₂ fixation rates in the presence or absence of methyl fluoride (MeF), an inhibitor of NH₃ oxidation. Ammonia oxidizer-mediated dark ¹⁴CO₂ fixation rates were similar in surface (5-7m) and oxycline (11-15 m) waters, ranging between 70-340 and 89-186 nM d^-1, respectively, or 1-7% of primary production by phytoplankton. Ammonia oxidation rates ranged between 580-2,380 nM d^-1 in surface waters and 732-1,548 nM d^-1 in oxycline waters. Methane oxidation was examined using a 14CH₄ tracer technique in July 1994, April 1995, and July 1995. Methane oxidation rates were consistently higher in July, and rates in oxycline and anaerobic bottom waters (0.5-37 and 7-48 nM d^-1, respectively) were 10-fold higher than those in aerobic surface waters (0.04-3.8 nM d^-1). The majority of CH₄ oxidation, in terms of integrated activity, occurred within anoxic bottom waters. Measured oxidation rates and water column concentrations were used to estimate the biological turnover times of NH₃ and CH₄. The NH₃ pool turns over rapidly, on time scales of 0.8 d in surface waters and 10 d within the oxycline, while CH₄ is cycled on 10³-d time scales in surface waters and 10²-d time scales within oxycline and bottom waters. Our data suggest an important role for NH₃ oxidation in alkaline, saline lakes since the process converts volatile NH₃ to soluble NO₂^-, thereby reducing loss via lake-atmosphere exchange and maintaining nitrogen in a form that is readily available to phytoplankton.

Kerkhof, L., Voytek, M.A., Sherrell, R., Mille, D., and Schofield, O., 1999, Variability in bacterial community structure during upwelling in the coastal ocean: Hydrobiologia, v. 401, p. 139-148.

Kharaka, Y.K., Thordsen, J.J., Evans, W.C., 1999, Crustal fluids: CO2 of mantle and crustal origins in the San Andreas fault system, California, in Armannsson, H., ed., Proceedings 5th International Symposium on the Geochemistry of Earth Surface (GES-5): Rotterdam, A.A. Balkema, p. 515-518.

Chemical and isotopic analyses of water and gases were determined from 44 thermal and saline springs and wells located near the San Andreas fault system to investigate the role of fluids in the dynamics of this major plate-bounding fault. Results indicate that the waters are mainly of meteoric origin, with shallow to moderate circulation depths (down to 6 km). The chemical compositions of water and gases are highly variable, controlled mainly by the enclosing rock types. However, compositions and isotope abundances of noble gases and d¹³C values of HCO₃ indicate a significant (up to 50%) mantle component for volatiles. Because upper mantle gases are dominated by CO₂ (C/³He ~ 1010), model calculations yield high CO₂ flux values of 0.001-1 kg CO2/(km²-s). The highest CO₂ fluxes are from fluids in the Franciscan assemblage, where C and He isotopes indicate a dominantly crustal source for CO₂. Presently we are measuring the surficial and dissolved CO₂ fluxes and C isotopes over two large drainage areas where data from springs are available. Preliminary results yield total (uncorrected for soil CO₂) values ~0.001-1 kg CO₂/(km²-s). Numerical simulations indicate that the CO₂ flux values, extrapolated to seismogenic depths, are sufficient to generate lithostatic fluid pressures, and thus explain the mechanical weakness of this fault. Furthermore, the model times required to increase fluid pressures to lithostatic values are comparable to those of earthquake cycles.

Kile, D.E., and Eberl, D.D., 1999, Crystal growth mechanisms in miarolitic cavities in the Lake George ring complex, Colorado: American Mineralogist, v. 84, p. 718-724.

The Crystal Peak area of the Pikes Peak batholith, near Lake George in central Colorado, is world-renowned for its crystals of amazonite (the blue-green variety of microcline) and smoky quartz. Such crystals, collected from individual microlitic pegmatites, have a remarkably small variation in crystal size within each pegmatite, and the shapes of plots of their crystal size distributions (CSDs) are invariably lognormal or close to lognormal in all cases. There observations are explained by a crystal growth mechanism that was governed initially by surface-controlled kinetics, during which crystals tended to grow larger in proportion to their size, thereby establishing lognormal CSDs. Surface-controlled growth was followed by longer periods of supply controlled growth, during which growth rate was predominantly size-independent, consequently preserving the lognormal shapes of the CSDs and the small size variation. The change from surface- to supply controlled growth kinetics may have resulted from an increasing demand for nutrients that exceeded diffusion limitations of the system. The proposed model for crystal growth in this locality appears to be common in the geologic record, and can be used with other information, such as isotopic data, to deduce physico-chemical conditions during crystal formation.

Kile, D.E., Wershaw, R.L., and Chiou, C.T., 1999, Correlation of soil and sediment organic matter polarity to aqueous sorption of nonionic compounds: Environmental Science and Technology, v. 33, p. 2053-2056.

Polarities of the soil/sediment organic matter (SOM) in 19 soil and 9 freshwater sediment samples were determined from solid-state ¹³C-CP/MAS NMR spectra and compared with published partition coefficients (K_oc) of carbon tetrachloride (CT) from aqueous solution. Nondestructive analysis of whole samples by solid-state NMR permits a direct assessment of the polarity of SOM that is not possible by elemental analysis. The percent of organic carbon associated with polar functional groups was estimated from the combined fraction of carbohydrate and carboxylamide-ester carbons. A plot of the measured partition coefficients (K_oc) of carbon tetrachloride (CT) vs. percent polar organic carbon (POC) shows distinctly different populations of soils and sediments as well as a roughly inverse trend among the soil/sediment populations. Plots of K_oc values for CT against other structural group carbon fractions did not yield distinct populations. The results indicate that the polarity of SOM is a significant factor in accounting for differences in K_oc between the organic matter in soils and sediments. The alternate direct correlation of the sum of aliphatic and aromatic structural carbons with K_oc illustrates the influence of nonpolar hydrocarbon on solute partition interaction. Additional elemental analysis data of selected samples further substantiate the effect of the organic matter polarity on the partition efficiency of nonpolar solutes. The separation between soil and sediment samples based on percent POC reflects definite differences of the properties of soil and sediment organic matters that are attributable to diagenesis.

Dissolved sulfide concentrations in the water column and in sediment pore waters were measured by square-wave voltammetry (nanomolar detection limit) during three cruises to the Santa Barbara Basin in February 1995, November-December 1995, and April 1997. In the water column, sulfide concentrations measured outside the basin averaged 3 ± 1 nM (n = 28) in the 0 to 600 m depth range. Inside the basin, dissolved sulfides increased to reach values of up to 15 nM at depths >400 m. A suite of box cores and multicores collected at four sites along the northeastern flank of the basin showed considerable range in surficial (<0.5 cm) pore-water sulfide concentrations: <0.008, 0.01, 0.02, to as much as 0.4 mM at the 340, 430, 550, and 590 m sites, respectively. At a core depth of 10 cm, however, pore-water sulfides exhibited and even wider range: 0.005, 0.05, 0.1, and 100 mM at the same sites, respectively. The sulfide flux into the deep basin, estimated from water-column profiles during three cruises, suggests a fairly consistent input of 100-300 nmole m^-2 h^-1. In contrast, sulfide fluxes estimated from pore-water sulfide gradients at the sediment water interface were much more variable (-4 to 13,000 nmole m^-2 h^-1). Dissolved silicate profiles show clear indications of irrigation at shallow sites (340 and 430 m) in comparison to deeper basin sites (550 and 590 m) with low (<10 mM) bottom-water dissolved-oxygen concentrations. Pore-water profiles indicate ammonia generation at all sites, but particularly at the deep-basin 590 m site with concentrations increasing with sediment depth to >400mM at 10 cm. Decreases in water-column nitrate below the sill depth indicate nitrate consumption (-55 to -137 mmole m^-2 h^-1) similar to nearby Santa Monica Basin. Peaks in pore-water iron concentrations were generally observed between 2 and 5 cm depth with shallowest peaks at the 590 m site. These observations, including observations of the benthic microfauna, suggest that the extent to which the sulfide flux, sustained by elevated pore-water concentrations, reaches the water column may be modulated by the abundance of sulfide-oxidizing bacteria in addition to iron redox and precipitation reactions.

Landa, E.R. , 1999, Transferability of soil cleanup standards in remedial actions associated with technologically enhanced naturally occurring radioactive materials: Geochemical perspectives: International Atomic Energy Agency Symposium on Restoration of Environments with Radioactive Residues, Nov.29- Dec. 3, 1999; Arlington, VA., Contributed papers, IAEA-SM-359, p. 101-104.

During the 1980s, many new molecular biology techniques were developed, providing new capabilities for studying the genetics and activities of organisms. Biologists and ecologists saw the promise that these techniques held for studying different aspects of organisms, both in culture and in the natural environment. In less than a decade, these techniques were adopted by a large number of researchers studying many types of organisms in diverse environments. Much of the molecular-level information acquired has been used to address questions of evolution, biogeography, population structure and biodiversity. At this juncture, molecular ecologists are poised to contribute to the study of fundamental characteristics underlying aquatic community structure. The goal of this overview is to assess where we have been, where we are now and what the future holds for revealing the basis of community structure and function with molecular-level information.

Landa, E.R., Beals, D.M., Halverson, J.E., Michel, R.L., Cefus, G.R. 1999. Tritium and plutonium in waters from the Bering and Chukchi Seas: Health Physics, v. 77, p. 668-676.

During the summer of 1993, seawater in the Bering and Chukchi Seas was sampled on a joint Russian-American cruise [BERPAC] of the RV Okean to determine concentrations of tritium, ²³⁹Pu and ²⁴⁰Pu. Concentrations of tritium were determined by electrolytic enrichment and liquid scintillation counting. Tritium levels ranged up to 420 mBq L^-1 and showed no evidence of inputs other than those attributed to atmospheric nuclear weapons testing. Plutonium was recovered from water samples by ferric hydroxide precipitation, and concentrations were determined by thermal ionization mass spectrometry. ^239+240Pu concentrations ranged from <1 to 5.5 µBq L^-1. These concentrations are lower than those measured in water samples from other parts of the ocean during the mid-1960's to the late 1980's. The ²⁴⁰Pu:²³⁹Pu ratios, although associated with large uncertainties, suggest that most of the plutonium is derived from world-wide fallout. As points of comparison, the highest concentrations of tritium and plutonium observed here were about five orders of magnitude lower than the maximum permissible concentrations allowed in water released to the off-site environs from licensed nuclear facilities in the United States. This study and others sponsored by the International Atomic Energy Agency and the Office of Naval Research's Arctic Nuclear Waste Assessment Program are providing data for the assessment of potential radiological impacts in the Arctic regions associated with nuclear waste disposal by the former Soviet Union.

Landwehr, J.M., Reel, J.T., Rybicki, N.B., Ruhl, H.A., and Carter, V., 1999, Chesapeake Bay habitat criteria scores and the distribution and abundance of submersed aquatic vegetation in the tidal Potomac River and Potomac Estuary, 1983-1997: U.S. Geological Survey Open-File Report 99-219, 34 p.

Legates, D.R., and McCabe, G.J., 1999, Evaluating the "Goodness-of-Fit" of Hydrologic and Hydroclimatic Models: Water Resources Research v. 35, p. 233-241.

Correlation and correlation-based measures (e.g., the coefficient of determination) have been widely used to evaluate the "goodness-of-fit" of hydrologic and hydroclimatic models. These measures are oversensitive to extreme values (outliers) and are insensitive to additive and proportional differences between model predictions and observations. Because of these limitations, correlation-based measures can indicate that a model is a good predictor, even when it is not. In this paper, useful alternative goodness-of-fit or relative error measures (including the coefficient of efficiency and the index of agreement) that overcome many of the limitations of correlation-based measures are discussed. Modifications to these statistics to aid in interpretation are presented. It is concluded that correlation and correlation-based measures should not be used to assess the goodness-of-fit of a hydrologic or hydroclimatic model and that additional evaluation measures (such as summary statistics and absolute error measures) should supplement model evaluation tools.

The formation and spatial distribution of phytoplankton blooms in estuaries are controlled by (1) local mechanisms, which determine the production-loss balance for a water column at a particular spatial location (i.e. control if a bloom is possible), and (2) transport-related mechanisms, which govern biomass distribution (i.e. control if and where a bloom actually occurs). In this study, the first of a 2-paper series, we use a depth-averaged numerical model as a theoretical tool to describe how interacting local conditions (water column height, light availability, benthic grazing) influence the local balance between phytoplankton sources and sinks. We also explore trends in the spatial variability of the production-loss balance across the topographic gradients between deep channels and lateral shoals which are characteristic of shallow estuaries. For example, under conditions of high turbidity and slow benthic grazing the highest rates of phytoplankton population growth are found in the shallowest regions. On the other hand, with low turbidity and rapid benthic grazing the highest growth rates occur in the deeper areas. We also explore the effects of semidiurnal tidal variation in water column height, as well as spring-neap variability. Local population growth in the shallowest regions is very sensitive to tidal-scale shallowing and deepening of the water column, especially in the presence of benthic grazing. A spring-neap signal in population growth rate is also prominent in the shallow areas. Population growth in deeper regions is less sensitive to temporal variations in tidal elevation. These results show that both shallow and deep regions of estuaries can act as sources or sinks for phytoplankton biomass, depending on the local conditions of mean water column height, tidal amplitude, light-limited growth rate, and consumption by grazers.

The development and distribution of phytoplankton blooms in estuaries are functions of both local conditions (i.e. the production-loss balance for a water column at a particular spatial location) and large-scale horizontal transport. In this study, the second of a 2-paper series, we use a depth-averaged hydrodynamic-biological model to identify transport-related mechanisms impacting phytoplankton biomass accumulation and distribution on a system level. We chose South San Francisco Bay as a model domain, since its combination of a deep channel surrounded by broad shoals is typical of drowned-river estuaries. Five general mechanisms involving interaction of horizontal transport with variability in local conditions are discussed. Residual (on the order of days to weeks) transport mechanisms affecting bloom development and location include residence time/export, import, and the role of deep channel regions as conduits for mass transport. Interactions occurring on tidal time scales, i.e. on the order of hours) include the phasing of lateral oscillatory tidal flow relative to temporal changes in local net phytoplankton growth rates, as well as lateral sloshing of shoal-derived biomass into deep channel regions during ebb and back into shallow regions during flood tide. Based on these results, we conclude that: (1) while local conditions control whether a bloom is possible, the combination of transport and spatial-temporal variability in local conditions determines if and where a bloom will actually occur; (2) tidal-time-scale physical-biological interactions provide important mechanisms for bloom development and evolution. As a result of both subtidal and tidal-time-scale transport processes, peak biomass may not be observed where local conditions are most favorable to phytoplankton production, and inherently unproductive areas may be regions of high biomass accumulation.

Mandernack, K.W., Bazylinski, D.A., Shanks, W.C. and Bullen, T.D., 1999, Oxygen and iron isotope studies of magnetite produced by magnetotactic bacteria: Science, v. 285, p. 1892-1896.

A series of carefully controlled laboratory studies was carried out to investigate oxygen and iron isotope fractionation during the intracellular production of magnetite (Fe₃O₄) by two different species of magnetotactic bacteria at temperatures between 4° and 35°C under microaerobic and anaerobic conditions. No detectable fractionation of iron isotopes in the bacterial magnetites was observed. However, oxygen isotope measurements indicated a temperature-dependent fractionation for Fe₃O₄ and water that is consistent with that observed for Fe₃O₄ produced extracellularly by thermophilic Fe³⁺-reducing bacteria. These results contrast with established fractionation curves estimated from either high-temperature experiments or theoretical calculations. With the fractionation curve established in this report, oxygen-18 isotope values of bacterial Fe₃O₄ may be useful in paleoenvironmental studies for determining the oxygen-18 isotope values of formation waters and for inferring paleotemperatures.

In the upper crust, where hydraulic gradients are typically <1 MPa km-1, advective heat transport is often effective for permeabilities k ³ 10-16 m ² and advective mass (solute) transport for k ³ 10-20 m². Regional-scale analyses of coupled groundwater flow and heat transport in the upper crust typically infer permeabilities in the range of 10-17 to 10-14 m², so that heat advection is sometimes significant analyses of metamorphic systems suggest that a geochemically significant level of permeability can exist to the base of the crust. In active metamorphic systems in the mid to lower crust, where vertical hydraulic gradients are likely >10 MPa km-1, the mean permeabilities required to accommodate the estimated metamorphic fluid fluxes decrease from ~10-16 m² to ~10-18 m² between 5- and 12-km depth. Below ~12 km, which broadly corresponds to the brittle-plastic transition, mean k is effectively independent of depth at ~10-18.5± 1 m². Consideration of the permeability values inferred from thermal modeling and metamorphic fluxes suggests a quasi-exponential decay of permeability with depth of log k » -3.2 log z – 14, where k is in meters squared and z is in kilometers. At mid to lower crustal depths this curve lies just below the threshold value for significant advection of heat. Such threshold value for significant advection of heat may represent an optimum for metamorphism, allowing the maximum transport of fluid and solute mass that is possible without advective cooling.

Mariner, R.H. and Lowenstern, J.B., 1999, The geochemistry of water from springs, wells, and snowpack on and adjacent to Medicine Lake volcano, Northern California: Transactions of the 1999 Geothermal Resources Council Annual Meeting, v. 23, p. 319-326.

Chemical analyses of waters from cold springs and wells of the Medicine Lake volcano and surrounding region indicate small chloride anomalies that may be due to water-rock interaction or limited mixing with high-temperature geothermal fluids. The Fall River Springs (FRS) with a combined discharge of approximately 37 m³/s, show a negative correlation between chloride (Cl) and temperature, implying that the Cl is not derived from a high-temperature geothermal fluid. The high discharge from the FRS indicates recharge over a large geographic region. Chemical and isotopic variations in the FRS show that they contain a mixture of three distinct waters. The isotopic composition of recharge on and adjacent to the volcano are estimated from the isotopic composition of snow and precipitation amounts adjusted for evapotranspiration. Enough recharge of the required isotopic composition (-100 per mil delta D) is available from a combination of the Medicine Lake caldera, the Fall River basin and the Long Bell basin to support the slightly warmer components of the FRS (32 m³/s). The cold-dilute part of the FRS (approximately 5 m³/s) may recharge in the Bear Creek basin or at lower elevations in the Fall River basin.

Marinsky, J.A., Mathuthu, A., Ephraim, J.H., and Reddy, M.M., 1999,Calcium ion binding to a soil fulvic acid using a Donnan potential: Radiochimica Acta, v. 84, p. 205-211.

Calcium ion binding to a soil fulvic acid (Armadale Bh Horizon) was evaluated over a range of calcium ion concentrations, from pH 3.8 to 7.3, using potentiometric titrations and calcium ion electrode measurements. Fulvic acid concentration was constant (100 milligrams per liter) and calcium ion concentration varied up to 8 x 10^-4 moles per liter. Experiments discussed here included: (1) titrations of fulvic acid-calcium ion containing solutions with sodium hydroxide; and (2) titrations of fully neutralized fulvic acid with calcium chloride solutions. Apparent binding constants (expressed as the logarithm of the value, log b_app) vary with solution pH, calcium ion concentration, degree of acid dissociation, and ionic strength (from log b_app = 2.5 to 3.9) and are similar to those reported by others. Fulvic acid charge, and the associated Donnan Potential, influences calcium ion-fulvic acid ion pair formation. A Donnan Potential correction term allowed calculation of intrinsic binding constants vary from 1.2 to 2.5 (the average value is about log b = 1.6) and are similar to, but somewhat higher than, stability constants for calcium ion-carboxylic acid monodentate complexes.

Marzolf, G.R., Bowser, C.J., Hart, R.J., Stephens, D.W., and Vernieu, W.S., 1999, Photosynthetic and respiratory processes: an open approach, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., the 1996 controlled flood in Grand Canyon: American Geophysical Union, monograph no. 110, p. 205-216.

The investigation examined open-stream methods for detection of photosynthetically driven (light dependent) chemical change, and subsequently provided an estimate of flood effects on the photosynthetic community in the 25-km reach of the Colorado River below Glen Canyon Dam. Observations reported here confirm that the dynamics of oxygen concentration and pH are correlated because photosynthesis and respiration in the benthic community cause the diel patterns. Observations at the constant low flows immediately before and after the controlled flood permitted measurement of changes in stream chemistry caused by biological activity and provided a test of the hypothesis that plants were scoured from the channel by the flood. The diel amplitudes of oxygen concentration and pH change were decreased after the flood as the biomass was scoured. Patterns of oxygen production and carbon dioxide removal varied along the 25-km reach.

Marzolf, G.R., Jackson, W., and Randle, R., 1999, Flood releases from dams as management tools: interactions between science and management, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., the 1996 controlled flood in Grand Canyon: American Geophysical Union Monograph no. 110.

The linkage between management and science is sometimes strained. Management must be broadly attentive to the perceived desires of various resource users, while scientists are usually forced on natural phenomena, how they are controlled, and how perturbation effects change. These different perspectives commonly result in mismatching objectives and misunderstandings. The 1996 controlled flood represents a productive convergence of river science and dam management stimulated by the process of writing an environmental impact statement. The EIS task focused on effects of the dam operations on the river. The understanding and prediction of the effects required by the EIS was underpinned by scientific inquiry coordinated by the Bureau of Reclamation's Glen Canyon Environmental Studies (GCES) program. The controlled flood itself was a demonstration of a management tool while serving also as a manipulative experiment to test theoretical ideas about how the river works. It became clear that management's role involves the definition of the problems and goals and science's role in development of objective knowledge about natural phenomena.

Changing patterns of correlations between the historical average June-November Southern Oscillation Index (SOI) and October-March precipitation totals for 84 climate divisions in the western US indicate a large amount of variability in SOI/precipitation relations on decadal time scales. Correlations of western US precipitation with SOI and other indices of tropical El Niño-Southern Oscillation (ENSO) processes were much weaker from 1920 to 1950 than during recent decades. This variability in teleconnections is associated with the character of tropical air-sea interactions as indexed by the number of out-of-phase SOI/tropical sea surface temperature (SST) episodes, and with decadal variability in the North Pacific Ocean as indexed by the Pacific Decadal Oscillation (PDO). Decadal modes of tropical and North Pacific Ocean climate variability are important indicators of periods when ENSO indices, like SOI, can be used as reliable predictors of winter precipitation in the US.

April 1 snowpack accumulations measured at 311 snow courses in the western United States (U.S.) are grouped using a correlation-based cluster analysis. A conceptual snow accumulation and melt model and monthly temperature and precipitation for each cluster are used to estimate cluster-average April 1 snowpack. The conceptual snow model is subsequently used to estimate future snowpack by using changes in monthly temperature and precipitation simulated by the Canadian Centre for Climate Modeling and Analysis (CCC) and the Hadley Centre for Climate Prediction and Research (HADLEY) general circulation models (GCMs). Results for the CCC model indicate that although winter precipitation is estimated to increase in the future, increases in temperatures will result in large decreases in April 1 snowpack for the entire western US. Results for the HADLEY model also indicate large decreases in April 1 snowpack for most of the western US, but the decreases are not as severe as those estimated using the CCC simulations. Although snowpack conditions are estimated to decrease for most areas of the western US, both GCMs estimate a general increase in winter precipitation toward the latter half of the next century. Thus, water quantity may be increased in the western US; however, the timing of runoff will be altered because precipitation will more frequently occur as rain rather than as snow.

McGlynn, B., McDonnell, J.J., Shanley, J., Kendall, C., 1999, Riparian zone flowpath dynamics: Journal of Hydrology, v. 222, p. 75-82.

The hydrology of the near-stream riparian zone in upland humid catchments is poorly understood. We examined the spatial and temporal aspects of riparian flowpaths during snowmelt in a headwater catchment within the Sleepers River catchment in northern Vermont. A transect of 15 piezometers was sampled for Ca, Si, DOC, other major cations, and d¹⁸O. Daily piezometric head values reflected variations in the stream hydrograph induced by melt and rainfall. The riparian zone exhibited strong upward discharge gradients. An impeding layer was identified between the till and surficial organic soil. Water solute concentrations increased toward the stream throughout the melt. Ca concentrations increased with depth and DOC concentrations decreased with depth. The concentrations of Ca in all piezometers were lower during active snowmelt than during post-melt low flow. Ca data suggest snowmelt infiltration to depth; however, only upslope piezometers exhibited snowmelt infiltration and consequent low d¹⁸O values, while d¹⁸O values varied less than 0.5ppt in the deep riparian piezometers throughout the study period. Ca and d¹⁸O values in upslope piezometers during low streamflow were comparable to Ca and d¹⁸O in riparian piezometers during high streamflow. The upland water Ca and d¹⁸O may explain the deep riparian Ca dilution and consistent d¹⁸O composition. The temporal pattern in Ca and d¹⁸O indicate that upland water moves to the stream via a lateral displacement mechanism that is enhanced by the presence of distinct soil/textural layers. Snowmelt thus initiates the flux of pre-melt, low Ca upland water to depth in the riparian zone, but itself does not appear at depth in the riparian zone during spring melt. This is despite the coincident response of upland groundwater and stream discharge.

Vertical profiles of streamwise velocity measured over bed forms are commonly used to deduce boundary shear stress for the purpose of estimating sediment transport. These profiles may be derived locally or from some sort of spatial average. Arguments for using the latter procedure are based on the assumption that spatial averaging of the momentum equation effectively removes local accelerations from the problem. Using analogies based on steady, uniform flows, it has been argued that the spatially averaged velocity profiles are approximately logarithmic and can be used to infer values of boundary shear stress. This technique of using logarithmic profiles is investigated using detailed laboratory measurements of flow structure and boundary shear stress over fixed two-dimensional bed forms. Spatial averages over the length of the bed form of mean velocity measurements at constant distances from the mean bed elevation yield vertical profiles that are highly logarithmic even though the effect of the bottom topography is observed throughout the water column. However, logarithmic fits of these averaged profiles do not yield accurate estimates of the measured total boundary shear stress.

To estimate bed-load sediment transport rates in flows over bed forms such as ripples and dunes, spatially averaged velocity profiles are frequently used to predict mean boundary shear stress. However, such averaging obscures the complex, nonlinear interaction of wake decay, boundary-layer development, and topographically induced acceleration downstream of flow separation and often leads to inaccurate estimates of boundary stress, particularly skin friction, which is critically important in predicting bed-load transport rates. This paper presents an alternative methodology for predicting skin friction over 2D bed forms. The approach is based on combining the equations describing the mechanics of the internal boundary layer with semiempirical structure functions to predict the velocity at the crest of a bedform, where the flow is most similar to a uniform boundary layer. Significantly, the methodology is directed toward making specific predictions only at the bed-form crest, and as a result it avoids the difficulty and questionable validity of spatial averaging. The model provides an accurate estimate of the skin friction at the crest where transport rates are highest. Simple geometric constraints can be used to derive the mean transport rates as long as bed load is dominant.

McMahon, P.B., Bohlke, J.K., and Bruce, B.W., 1999, Denitrification in marine shales in northeastern Colorado, USA: Water Resources Research, v. 35, p. 1629-1642.

Parts of the South Platte River alluvial aquifer in northeastern Colorado are underlain by the Pierre Shale, a marine deposit of Late Cretaceous age that is <1000 m thick. Ground water in the aquifer is contaminated with NO₃, and the shale contains abundant potential electron donors for denitrification in the forms of organic carbon and sulfide minerals. Nested piezometers were sampled, pore water was squeezed from cores of shale, and an injection test was conducted to determine if denitrification in the shale was a sink for alluvial NO₃^- and to measure denitrification rates in the shale. Measured values of NO₃^-, N₂, NH₄⁺, d¹⁵N[NO₃^-], d¹⁵N[N₂], and d¹⁵N[NH₄⁺] in the alluvial and shale pore water indicated that denitrification in the shale was a sink for alluvial NO₃^-. Chemical gradients, reaction rate constants, and hydraulic head data indicated that denitrification in the shale was limited by the slow rate of NO₃^- transport (possibly by diffusion) into the shale. The apparent in situ first-order rate constant for denitrification in the shale based on diffusion calculations was of the order of 0.04-0.4 yr^-1, whereas the potential rate constant in the shale based on injection tests was of the order 60 yr^-1. Chemical data and mass balance calculations indicate that organic carbon was the primary electron donor for denitrification in the shale during the injection test, and ferrous iron was a minor electron donor in the process. Flux calculations for the conditions encountered at the site indicate that denitrification in the shale could remove only a small fraction of the annual agricultural NO₃^- input to the alluvial aquifer. However, the relatively large potential first-order rate constant for denitrification in the shale indicated that the percentage of NO₃^- uptake by the shale could be considerably larger in areas where NO₃^- is transported more rapidly into the shale by advection.

Meier, M., Namjesnik-Dejanovic, K., Maurice, P., Chin, Y.P., and Aiken, G.R., 1999, Fractionation of aquatic natural organic matter upon sorption to goethite and kaolinite: Chemical Geology, v. 157, p. 275-284.

Natural organic matter (NOM) consists of a complex mixture of organic molecules; previous studies have suggested that preferential sorption of higher molecular weight, more hydrophobic, and more aromatic components may lead to fractionation of the NOM pool upon passage through porous media. Our work expands upon previous studies by quantifying the change in solution-phase weight average molecular weight (M_w) upon sorption of bulk (rather than isolated) surface water NOM from the Suwannee River (SR) and the Great Dismal Swamp (GDS) to goethite and kaolinite at different sorption densities and at pH 4, 22 ^oC. High pressure size exclusion chromatography (HPSEC) was used to quantify changes in M_w upon sorption, and molar absorptivities at l = 280 nm were used to approximate changes in solution NOM aromaticity. Two SR water samples were used, with M_w = 2320 and 2200 Da; a single GDS sample was used, with M_w = 1890 Da. The SR NOM was slightly more hydrophobic and aromatic. These differences were reflected in greater sorption of SR NOM than GDS NOM. Both surface water NOMs showed a much greater affinity for goethite than for kaolinite. HPSEC analysis of the NOM remaining in solution after 24 h reaction time with goethite revealed that the largest changes in solution phase M_ws (decreases by 900-1700 Da) occurred at relatively low equilibrium sorbate concentrations (approximately 5-20 mg C/L); the decrease in solution M_w suggested that reactive surface sites were occupied disproportionately by large and intermediate size NOM moieties. At higher equilibrium NOM concentrations (>20 mg C/L), as percent adsorption decreased, M_w in solution was similar to original samples. A smaller decrease in solution NOM M_w (300-500 Da at 10-20 mg C/L approximately 100 Da at >20 mg) also occurred upon sorption to kaolinite. Overall, our results showed that factors (as related to NOM composition, clay mineral surface properties, and position along the sorption isotherm) which promote a higher percent sorption lead to the most pronounced decreases in solution M_w.

The ontogeny of five flood-plain segments is described for a period of 18 yr following a major flood in 1978 on the Powder River in southeastern Montana. The flood plains developed on relatively elevated sand and gravel deposits left within the channel by the 1978 flood. In cross section, the flood plains resemble benches with well-developed natural levees. Flood-plain growth occurred as sediment was draped onto preexisting surfaces in layers of sand and mud a few centimeters to decimeters thick, resulting in some lateral, but mostly vertical accretion. Annual and biannual measurements indicated that, as the flood-plain segments grew upward, the annual rate of vertical accretion decreased as the partial duration recurrence interval for the threshold or bankfull discharge increased from 0.16 to 1.3 yr. It is clear that a constant recurrence interval for overbank flow cannot be meaningfully assigned to this type of flood-plain ontogeny. These flood plains did not grow on migrating point bars, and vertical accretion at least initially occurred within the channel, rather than across the valley flat during extensive overbank flows. Sediments of these flood plains define narrow, elongated stratigraphic units that border the active channel and onlap older flood-plain deposits. These characteristics are considerably different from those of many facies models for meandering river deposits. Facies similar to those described in this paper are likely to be preserved, thereby providing important evidence in the geologic record for episodes of periodic channel expansion by ancient rivers.

Morel-Seytoux, H.J., and Nimmo, J.R., 1999, Soil water retention and maximum capillary drive from saturation to oven dryness: Water Resources Research, v. 35, p. 2031-2041.

This paper provides an alternative method to describe the water retention curve over a range of water contents from saturation to oven dryness. It makes two modifications to the standard Brooks and Corey [1964] (B-C) description, one at each end of the suction range. One expression proposed by Rossi and Nimmo [1994] is used in the high-suction range to a zero residual water content. (This Rossi-Nimmo modification to the Brooks-Corey model provides a more realistic description of the retention curve at low water contents.) Near zero suction the second modification eliminates the region where there is a change in suction the second modification eliminates the region where there is a change in suction with no change in water content. Tests on seven soil data sets, using three distinct analytical expressions for the high-, medium-, and low-suction ranges, show that the experimental water retention curves are well fitted by this composite procedure. The high-suction range of saturation contributes little to the maximum capillary drive, defined with a good approximation for a soil water and air system as H_cM = ò ₀^¥ k_rwdh_c, where k_rw is relative permeability (or conductivity) to water and h_c is capillary suction, a positive quantity in unsaturated soils. As a result, the modification suggested to describe the high-suction range does not significantly affect the equivalence between Brooks-Corey (B-C) and van Genuchten [1980] parameters presented earlier. However, the shape of the retention curve near ?natural saturation? has a significant impact on the value of the capillary drive. The estimate using the Brooks-Corey power law, extended to zero suction, will exceed that obtained with the new procedure by 25 to 30%. It is not possible to tell which procedure is appropriate. Tests on another data set, for which relative conductivity data are available, support the view of the authors that measurements of a retention curve coupled with a speculative curve of relative permeability as from a capillary model are not sufficient to accurately determine the (maximum) capillary drive. The capillary drive is a dynamic scalar, whereas the retention curve is of a static character. Only measurements of infiltration rates with time can determine the capillary drive with precision for a given soil.

Morin, R.H., and Flamand, R., 1999, Analysis of stress-induced oval fractures in a borehole at Deep Sea Drilling Project Site 504, eastern equatorial Pacific: Journal of Geophysical Research, v. 104, n. B2, p. 2767-2775.

Deep Sea Drilling Project (DSDP) Hole 504B is located in the eastern equatorial Pacific Ocean and extends to a total depth of 2111 m beneath the seafloor (mbsf). Several acoustic televiewer logs have been obtained in this well during successive stages of drilling, and the resulting digital images have revealed numerous oval-shaped fractures seemingly etched into the borehole wall. A theoretical examination of these stress-induced features identifies a unique and ephemeral set of stress distributions and magnitudes that are necessary for their production. Consequently, the ovals provide a basis for quantifying the magnitudes and orientations of the maximum and minimum horizontal principal stresses, SH and Sh, at this site. Vertical, truncated breakouts and horizontal tensile fractures define the spatial boundaries of the ovals. Explicit criteria for their occurrence are combined with estimates for various physical properties of the rock to yield a range of possible values for the horizontal principal stresses. The conspicuous oval geometry is completed by a curved fracture that joins the vertical and horizontal components. Its degree of curvature is delineated by the modified Griffith failure criterion and is directly related to the principal stress difference (S_H - S_h). Matching a series of type curves corresponding to specific values for (S_H - S_h) with the actual undistorted well bore images allows the magnitude of the stress difference to be further constrained. With a value for (S_H - S_h) of 45 ± 5 MPa the individual magnitudes of SH and Sh are determined more precisely. Final estimates for the horizontal principal stresses in DSDP Hole 504B at a depth of 1200 mbsf are S_H is greater than or equal to 141 MPa and less than or equal to 149 MPa and S_h is greater than or equal to 91 MPa and less than or equal to 109 MPa. Stress magnitudes derived from this approach rely heavily upon the values of a variety of physical properties, and complementary laboratory measurements performed on relevant rock samples provide critical information. Uncertainties in estimating these properties translate into less precise predictions of principal stresses.

Morin, R.H., Senior, L.A., and Decker, E.R., 1999, Hydrogeologic structure of the Newark Basin, Pennsylvania, from borehole geophysical logs, in Amadei, B., Kranz, R.L., Scott, G.A., and Smeallie, P.H., eds., Rock Mechanics for Industry--Proceedings of the 37th U.S. Rock Mechanics Symposium, June 6-9, 1999: Rotterdam, Netherlands, A.A. Balkema, v. 2, p. 941-948.

The Brunswick and underlying Lockatong Formations are comprised of lithified Mesozoic sediments that constitute part of the Newark Basin in southeastern Pennsylvania (USA). These fractured rocks form an important regional aquifer that consists of gradational sequences of shale, siltstone, and sandstone, with fluid transport occurring primarily in fractures. An extensive suite of geophysical logs was obtained in seven wells located at the borough of Lansdale, PA in order to characterize the areal hydrogeologic system. Analyses of these data indicate that the aquifer can be separated into two distinct structural domains which may, in turn, reflect different mechanical responses to basin extension: (1) In the shallow zone above 125 m, the dominant fracture population consists of gently dipping bedding-plane partings that strike N46^oE and dip to the NW at about 11^o. Fluid flow is concentrated within the upper 80 m and transmissivities rapidly diminish in magnitude with depth. (2) The zone below 125 m marks the appearance of numerous steeply dipping fractures that are orthogonal to the bedding plane features, striking subparallel at N227^oE but dipping sharply to the SE at 77^o. This secondary set of fractures is associated with a reasonably thick (approximately equal to 60 m) high-resistivity, low-transmissivity sandstone/siltstone unit that is abruptly terminated by a thin shale bed at a depth of 190 m. This lower contact effectively delineates the aquifer?s vertical extent and the observed lack of hydraulic productivity with increasing depth may be associated with the gradational transition into the Lockatong Formation.

Nimmo, J. R., Shakofsky, S. M., Kaminsky, J. F., Lords, G. S., 1999, Laboratory and field hydrologic characterization of the shallow subsurface at an Idaho National Engineering and Environmental Laboratory waste-disposal site: U.S. Geological Survey Water-Resources Investigations Report 99-4263.

The characterization of waste-disposal sites requires knowledge of unsaturated flow, normally obtained using formulations based on Darcy's law of Richards' equation, for which two hydraulic properties of the porous media, the unsaturated hydraulic conductivity, and the soil-water retention relation, must be determined. The extensive modification of the shallow subsurface at a waste-burial site can alter these properties and hence the unsaturated hydrology. These properties and their alteration should be accounted for in planning and constructing a waste facility. Our study assesses, through comparison with actual flow behavior, (1) the value of several types of standard unsaturated hydraulic property measurements for characterizing a waste-disposal site at the Idaho National Engineering and Environmental Laboratory and (2) the effect of landfill-construction disturbance on unsaturated-zone flow at this site. The site has a simulated waste trench constructed by excavating and replacing the soil to a depth of several meters. We measured hydraulic and other properties of the unsaturated medium by various techniques, including laboratory measurements on bulk soil and on minimally disrupted core samples, and field experiments. Among these techniques are high-resolution water-retention measurements capable of showing minor structural differences with submersible pressure outflow cells, and a field instantaneous profile experiment involving 24 hours of flood infiltration followed by redistribution. We performed measurements by identical procedures on the simulated waste trench and on nearby undisturbed soil. The laboratory and field methods have considerable overlap in terms of the properties measured, especially the unsaturated hydraulic properties. The field methods include direct observation of changing conditions within the unsaturated zone, thus providing information about the behavior of water flow in addition to the property measurements.

The results permit comparisons in at least three ways: measured soil properties of undisturbed versus waste-trench soil, laboratory versus field measurement of properties, and measured versus observed flow behavior. The general character of the hydraulic properties is similar between the undisturbed and waste-trench soil. There are tendencies in the slopes of hydraulic conductivity and water retention curves that are consistent with a reduction in breadth of the pore-size distribution caused by waste-trench construction. Property measurements from the laboratory substantially agree with those from the field. The observed flow behavior in the unsaturated zone shows marked differences in the undisturbed versus waste-trench soil that are not predicted from the property measurements by means of a one-dimensional Richards' equation model. Richards' equation is a better approximation in the disturbed rather than in the undisturbed medium. Layering and preferential flow are both major influences in the undisturbed medium, and are much less significant in the simulated waste trench. In the waste trench as opposed to the undisturbed soil, initial infiltration is slower, but the lack of significant layering permits water to move more freely to depths below the zone of evapotranspiration. Although laboratory and field methods gave consistent hydraulic properties, the hydrologic phenomena of main interest required direct observation of water flow in the field.

Nishri, A., Brenner, I.B., Hall, G.E., and Taylor, H.E., 1999, Temporal variations in dissolved selenium in Lake Kinneret (Israel): Aquatic Sciences, v. 61, no. 3, p. 215-233.

Selenium is an essential micronutrient for the growth of the dinoflagellate Peridinium gatunense that dominates the spring algal bloom in Lake Kinneret. The relationship between the levels of dissolved selenium species and the occurance of algal blooms in this lake was studied. During algal blooms of P. gatunense in spring and of the blue-green Aphanizomenon ovalisporum in fall (in 1994) the concentration of epilimnetic dissolved organic Se (Se_org) increased whereas that of selenite (Se^IV) decreased, to levels below the limit of detection: 5 ng/1. The disappearance of Se^IV during these blooms is attributed to algal uptake and it is suggested that the growth of both algae may have depended on Se_org regeneration. A budget performed for selenate (Se^VI) suggests that this species is also consumed by algae but to a lesser extent than Se^IV (in 1994 similar to 40% of the epilimnetic load). During the stratification period the hypolimnion of Lake Kinneret becomes anoxic, with high levels of dissolved sulfide. The affects of this environment on the distribution of Se oxy-anions, selenite (Se^IV) and selenate(Se^VI), were also studied.

The purpose of this study was to examine distributions of organic C, total N, and total P in carbonate sediments from sites of continuous and known sedimentation rate ²¹⁰Pb and ¹³⁷Cs dated), in eastern and central Florida Bay. These sediments provide a record of historical changes in the C, N, and P load to the eastern and central bay. Analyses were conducted on sediments from cores collected at five sites, and on buried seagrass fragments at two sites. At three of the sites, sediments from seagrass-covered and adjacent barren areas were examined to determine differences in sedimentary geochemistry. Stable isotope analyses (d¹³C and d¹⁵N) of sedimentary organic C and total N and of buried seagrass fragments were also carried out at two sites to examine possible changes in nutrient sources to the estuary. Results were consistent with recent increases in N and P in eastern Florida Bay, beginning in the early to mid 1980's. The timing of the increase in nutrient load observed in the sediment data directly preceded the first observations of massive microalgal blooms and seagrass dieoff in Florida Bay in 1987. The observed nutrification was greater for P than N, and was most pronounced at the most northeasterly site sampled (Pass Key). Isotope data (d¹⁵N) suggested that an increase in algal production accompanied the increase in N load at the Pass Key site. A long record of organic C, total N, and total P distributions from Whipray Basin in central Florida Bay showed historical peaks (mid 1700's and late 1800's) in organic C and total N, but not total P; these enrichments were nearly equivalent to recent inputs to the estuary. Barren areas were observed to have generally lower concentrations of organic C, total N, and total P in near surface sediments compared to seagrass-covered areas, but had generally similar concentrations in deeper sediments. This suggested that barren areas adjacent to seagrass-covered sites were places where relict sediment was physically transported and covered seagrass beds. This dataset provides an historical view of changes in nutrient inputs to Florida Bay, and baseline information needed for nutrient modeling of the bay.

Paillet, F.L, 1999, Characterizing fractured-zone flow using numerical flow-log models, in Amadei, B., Kranz, R.L., Scott, G.A., and Smeallie, P.H., eds., Rock mechanics for industry--Proceedings of the 37th U.S. Rock Mechanics Symposium, June 6-9, 1999: Rotterdam, Netherlands, A.A., Balkema, v. 2, p. 819-826.

Water-quality monitoring and hydraulic testing in fractured bedrock aquifers involves two important tasks: 1) identifying the hydraulically active fractures intersecting the borehole, and 2) inferring how the specific entry or exit ports in the borehole wall are connected to large-scale flow paths in the region surrounding the borehole. Effective characterization of fractured bedrock flow results when hydraulically active fractures and fracture zones are first identified using flow logs, and then the hydraulic properties of these active zones are given by subsequent hydraulic tests. A more difficult technical problem is relating the hydraulic properties of the few specific fractures that serve as borehole entry ports to the large-scale hydraulic properties of the surrounding rock mass. This problem is addressed through a generalized borehole flow model inversion formulated so that the boundary conditions at the outer edge of the boundary layer can be inferred from the properties of measured borehole flow.

Pendall, E., Betancourt, J.L., and Leavitt, S.W., 1999, Paleoclimatic significance of dD and d¹³C in pinyon pine needles from packrat middens spanning ther last 40,000 years: Paleogeography, Paleoclimatology, and Paleoecology, v. 147, p. 53-72.

We compared two approaches to interpreting dD of cellulose nitrate in piñon pine needles (Pinus edulis) preserved in packrat middens from central New Mexico, USA. One approach was based on linear regression between modern dD values and climate parameters, and the other on a deterministic isotope model, modified from Craig and Gordon's terminal lake evaporation model that assumes steady-state conditions and constant isotope effects. One such effect, the net biochemical fractionation factor, was determined for a new species, piñon pine. Regressions showed that dD values in cellulose nitrate from annual cohorts of needles (1989-1996) were strongly correlated with growing season (May-August) precipitation amount, and d¹³C values in the same samples were correlated with June relative humidity. The deterministic model reconstructed dD values of meteoric water used by plants after constraining relative humidity effects with d¹³C values; growing season temperatures were estimated via modern correlations with dD values of meteoric water. Variations of this modeling approach have been applied to tree-ring cellulose before, but not to macrofossil cellulose, and comparisons to empirical relationships have not been provided. Results from fossil piñon needles spanning the last ~40,000 years showed no significant trend in dD values of cellulose nitrate, suggesting either no change in the amount of summer precipitation (based on the transfer function) or dD values of meteoric water or temperature (based on the deterministic model). However, there were significant differences in d¹³C values, and therefore relative humidity, between Pleistocene and Holocene.

Pereira, W.E., Wade, T.L., Hostettler, F.D., and Parchaso, F., 1999, Accumulation of butyltins in sediments and lipid tissues of the Asian clam, Potamocorbula amunensis near the Mare Island Naval Shipyard, San Francisco Bay, California: Marine Pollution Bulletin, v. 38, p. 1005-1010.

Studies of butyltin compounds in soil, benthic sediments and the Asian clam Potamocorbula amurensis were conducted at the former Mare Island Naval Shipyard, and nearby Mare Island and Carquinez Straits in San Francisco Bay, California. Soils from a sandblast abrasives dump site at the shipyard contained low concentrations of mono-, di- and tributyltin (0.3-52 ng/g, total butyltin). Similarly, concentrations of total butyltin in benthic sediments from nearby Mare Island and Carquinez Straits ranged from 1.3 to 8.1 ng/g. In contrast, clams accumulated much greater concentrations (152-307 ng/g, total butyltin). Tributyltin (TBT) and dibutyltin (DBT) made up from 54-85% to 15-46%, respectively, of the total butyltin body burden of the clams. Biota Sediment Accumulation Factors (BSAFs) for butyltins in Potamocorbula were in reasonable agreement with literature values; they are greater than those of neutral hydrophobic compounds, suggesting that partitioning and binding processes may be involved in bioaccumulation. Tributyltin is a potent endocrine disrupting chemical. There is potential for long-term chronic effects of TBT in San Francisco Bay.

Dated sediment cores collected from Richardson and San Pablo Bays in San Francisco Bay were used to reconstruct a history of polycyclic aromatic hydrocarbon (PAH) contamination. The sedimentary record of PAHs in Richardson Bay shows that anthropogenic inputs have increased since the turn of the century, presumably as a result of increasing urbanization and industrialization around the Bay Area. Concentrations range from about 0.04-6.3 µg/g. The dominant origin of the PAHs contributing to this modern contamination is from combustion processes. Depth profiles in San Pablo Bay indicate higher concentrations of PAHs since the 1950s than during the late 1800s, also presumably resulting from an increase in urbanization and industrialization. Total PAHs in San Pablo Bay range from about 0.04-1.3 µg/g. The ratios of methylphenanthrenes /phenanthrene and (methylfluoranthenes + methylpyrenes)/fluoranthene were sensitive indicators of anthropogenic influences in the estuary. Variations in the ratio of 1,7-dimethylphenanthrene /2,6-dimethylphenanthrene indicate a gradual replacement of wood by fossil-fuel as the main combustion source of PAHs in San Francisco Bay sediments. The profile of perylene may be an indicator of eroding peat from marshlands.

Pizzuto, J.E., Webb, R.H., Griffiths, P.G., Elliott, J.G., and Melis, T.S., 1999, Entrainment and transport of cobbles and boulders from debris fans, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., The controlled flood in Grand Canyon: Scientific experiment and management demonstration: Washington, D.C. American Geophysical Union, Geophysical Monograph 110, p. 53-70.

Observation of marked cobbles and boulders during the 1996 controlled flood document the flood's effectiveness in reworking debris fans along the Colorado River in Grand Canyon. In addition, the hydraulics associated with initial sediment motion were defined on debris fans at 127.6 Mile and Lava Falls Rapid. At 127.6 Mile, peak Shields stresses, scaled by the mean particle size, varied from 0.06 to 0.24 and exceeded empirical entrainment threshold values by a factor of tow. At Lava Falls Rapid, peak Shields stresses exceeded 0.3. Rocks tagged with radio transmitters at Lava Falls moved at Shields stresses that varied by more than an order of magnitude because of the unique shape, orientation, and topographic setting of each particle. Critical Shields stresses for the tagged particles are quantified using a new method for computing the ration of threshold drag force to submerged weight that accounts for varying particle orientation, geometry, and packing. Theoretical computations of critical Shields stresses are well-correlated with critical Shields stresses estimated from field measurements of the local depth-slope product. Particles with a b-axis diameter as large as 2 m were displaced during the flood, and the tagged particles at Lava Falls Rapid moved an average distance of 230 m.

Calcite crystal growth was studied in the laboratory by determining the weight gain of single crystals of Iceland spar in Ca-HCO₃ solutions at 34 degrees C and CO₂ partial pressures of 0.101, 0.0156 and 0.00102 atm, and calcite saturation indices of 0.2 to 1.9. At constant CO₂ partial pressure, P_CO2 and temperature, the rate of crystal growth varied linearly with the activity product of Ca²⁺ and HCO₃^- in solution. At constant saturation state, growth rate was nearly independent of P_CO2 between 0.00102 and 0.0156 atm, but increased at the highest P_CO2. The maximum calcite growth rate was 25,000 mm/ka at Si_c=1.6 and P_CO2 of 0.101 atm. Growth rate decreased with decreasing saturation state, but rates less than 100 mm/ka could not be reliably measured by the laboratory weight-gain method. Near-equilibrium laboratory rates of calcite dissolution and crystal growth in ?clean? Ca-HCO₃ solutions decrease abruptly, and remain very small over the range of Si_c=0±0.2.

Poeter, E.P., and Hill, M.C., 1999, UCODE, A computer code for universal inverse modeling: Computer in geosciences, v. 25, n. 4, p. 457-462.

This article presents the U.S. Geological Survey computer program UCODE, which was developed in collaboration with the US Army Corps of Engineers Waterways Experiment Station and the International Ground Water Modeling Center of the Colorado School of Mines. UCODE performs inverse modeling, posed as a parameter-estimation problem, using nonlinear regression. Any application model of set of models can be used; the only requirement is that the have numerical (ASCH or text only) input and output files and that the numbers in these files have sufficient significant digits. Application models can include preprocessors and postprocessors as well as models related to the processed of interest (physical, chemical and so on), making UCODE extremely powerful for model calibration. Estimated parameters can be defined flexibly with user-specified functions. Observations to be matched in the regression can be any quantity for which a simulated equivalent value can be produced, thus simulated equivalent values are calculated using values that appear in the application model output files and can be manipulated with additive and multiplicative functions, if necessary. Prior, or direct, information on estimated parameters also can be included in the regression. The nonlinear regression problem is solved by minimizing a weighted least-squares objective function with respect to the parameter values using a modified Gauss-Newton method. Sensitivities needed for the method are calculated approximately by forward or central differences and problems and solutions related to this approximation are discussed. Statistics are calculated and printed for use in (1) diagnosing inadequate data or identifying parameters that probably cannot be estimated with the available data, (2) evaluating estimated parameter values, (3) evaluating the model representation of the actual processes and (4) quantifying the uncertainty of model simulated values. UCODE is intended for use on any computer operating system: it consists of algorithms programmed in perl, a freeware language designed for text manipulation and Fortran90, which efficiently performs numerical calculations.

This report describes the development, application, and evaluation of two methods for estimating the magnitude and frequency of floods at ungaged, unregulated, rural basins in North Carolina--(1) the regional regression method and (2) the region-of-influence method. A comparison of these two methods, based on their predictive ability and ease of application, also is presented. In order to compare the two methods on an equal basis, each method was applied to the same available data. the regional regression and region-of-influence methods of estimation were applied to the current data base of 317 sites with at least 10 years of unregulated peak-flow record and evaluated.

Data from ice 3590 meters below Vostok Station indicate that the ice was accreted from liquid water associated with Lake Vostok. Microbes were observed at concentrations ranging from 2.8 x 10³ to 3.6 x 10⁴ cells per milliliter; no biological incorporation of selected organic substrates or bicarbonate was detected. Bacterial 16S ribosomal DNA genes revealed low diversity in the gene population. The phylotypes were closely related to extant members of the alpha- and beta-Proteobacteria and the Actinomycetes. Extrapolation of the data from accretion ice to Lake Vostok implies that Lake Vostok may support a microbial population, despite more than 10⁶ years of isolation from the atmosphere.

Ravichandran, M., Aiken, G., Ryan, J.N., and Reddy, M.M., 1999, Inhibition of precipitation and aggregation of metacinnabar (mercuric sulfide) by dissolved organic matter isolated from the Florida Everglades: Environmental Science & Technology, v. 33, n. 9, p. 1418-1423.

Precipitation and aggregation of metacinnabar (black HgS) was inhibited in the presence of low concentrations ( greater than or equal to 3 mg C/L) of humic fractions of dissolved organic matter (DOM) isolated from the Florida Everglades. At low Hg concentrations (less than or equal to 5 x 10^-8 M), DOM prevented the precipitation of metacinnabar. At moderate Hg concentrations (5 x 10^-5 M), DOM inhibited the aggregation of colloidal metacinnabar (Hg passed through a 0.1 µm filter but was removed by centrifugation). At Hg concentrations greater than 5 x 10^-4 M, mercury formed solid metacinnabar particles that were removed from solution by a 0.1 µm filter. Organic matter rich in aromatic moieties was preferentially removed with the solid. Hydrophobic organic acids (humic and fulvic acids) inhibited aggregation better than hydrophilic organic acids. The presence of chloride, acetate, salicylate, EDTA, and cysteine did not inhibit the precipitation or aggregation of metacinnabar. Calcium enhanced metacinnabar aggregation even in the presence of DOM, but the magnitude of the effect was dependent on the concentrations of DOM, Hg, and Ca. Inhibition of metacinnabar precipitation appears to be a result of strong DOM-Hg binding. Prevention of aggregation of colloidal particles appears to be caused by adsorption of DOM and electrostatic repulsion.

Reynolds, R.L., Rosenbaum, J.G., Van Metre, P.C, Tuttle, M., Callender, E., and Goldin, A., 1999, Greigite (Fe₃S₄) as an indicator of drought - The 1912-1994 sediment magnetic record from White Rock Lake, Dallas, Texas, USA: Journal of Paleolimnology, v. 21, p. 193-206

Combined magnetic and geochemical studies were conducted on sediments from White Rock Lake, a reservoir in suburban Dallas (USA), to investigate how land use has affected sediment and water quality since the reservoir was filled in 1912. The chronology of a 167-cm-long core is constrained by the recognition of the pre-reservoir surface and by ¹³⁷Cs results. In the reservoir sediments, magnetic susceptibility (MS) and isothermal remanent magnetization (IRM) are largely carried by detrital titanomagnetite that originally formed in igneous rocks. Titanomagnetite and associated hematite are the dominant iron oxides in a sample from the surficial deposit in the watershed but are absent in the underlying Austin Chalk. Therefore, these minerals were transported by wind into the watershed. After about 1960, systematic decreases in Ti, Fe, and Al suggest diminished input of detrital Fe-Ti oxides from the surficial deposits. MS and IRM remain constant over this interval, however, implying compensation by an increase in strongly magnetic material derived from human activity. Anthropogenic magnetite in rust and ferrite spherules (from fly ash?) are more common in sediment deposited after about 1970 than before and may account for the constant magnetization despite the implied decrease in detrital Fe-Ti oxides.

Ritson, P., Bouse, R.M., Luoma, S.N., and Flegal, A.R., 1999, Identification of historical and contemporary sources of lead contamination to San Francisco Bay employing stable lead isotopes: Marine Chemistry, v. 64, p. 71-84.

Variations in stable lead isotopic composition (²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb) in three sediment cores from the San Francisco Bay estuary document temporal changes in sources of lead during the past two centuries. Sediment, with lead from natural geologic sources, and relatively homogenous lead isotopic compositions are overlain by sediments whose isotopic compositions indicate change in the sources of lead associated with anthropogenic modification of the estuary. The first perturbations of lead isotopic composition in the cores occur in the late 1800s concordant with the beginning of industrialization around the estuary. Large isotopic shifts, toward the lower ²⁰⁶Pb/²⁰⁷Pb, occur after the turn of the century in both Richardson and San Pablo Bays. A similar relationship among lead isotopic compositions and lead concentrations in both Bays suggest contamination from the same source (a lead smelter). The uppermost sediments (post 1980) of all cores also have a relatively homogenous lead isotopic composition distinct from pre-anthropogenic and recent aerosol signatures. Lead isotopic compositions of leachates from fourteen surface sediments and five marsh samples from the estuary were also analyzed. These analyses suggest that the lead isotopic signature identified in the upper horizons of the cores is spatially homogenous among recently deposited sediments throughout the estuary. Current aerosol lead isotopic compositions [Smith, D.R., Niemeyer, S., Flegal, A.R., 1992. Lead sources to California sea otters: industrial inputs circumvent natural lead biodepletion mechanisms. Environmental Research 57, 163-175] are distinct from the isotopic compositions of the surface sediments, suggesting that the major source of lead is cycling of historically contaminated sediments back through the water column. Both the upper core sediments and surface sediments apparently derive their lead predominantly from sources internal to the estuary. These results support the idea that geochemical cycling of lead between sediments and water accounts for persistently elevated lead concentrations in the water column despite 10-fold reduction of external source inputs to San Francisco Bay [Flegal, A.R., Rivera-Duarte, I., Ritson, P.I., Scelfo, G., Smith, G.J., Gordon, M., Sanudo-Wilhelmy, S.A., 1996. Metal contamination in San Francisco Waters: historic perturbations, contemporary concentrations, and future considerations in San Francisco Bay. In: Hollobaugh, J.T. (Ed.), The Ecosystem. AAAS, pp. 173-188].

Runoff of road salt from an interstate highway in New Hampshire has led to contamination of a lake and a stream that flows into the lake, in spite of the construction of a diversion berm to divert road salt runoff out of the lake drainage basin. Chloride concentration in the stream has increased by over an order of magnitude during the 23 yr since the highway was opened, and chloride concentration in the lake has tripled. Road salt moves to the lake primarily via the contaminated stream, which provides 53% of all the chloride to the lake and only 3% of the total streamflow to the lake. The stream receives discharge of salty water from leakage through the diversion berm. Uncontaminated ground water dilutes the stream downstream of the berm. However, reversals of gradient during summer months, likely caused by transpiration from deciduous trees, result in flow of contaminated stream water into the adjacent ground water along the lowest 40-m reach of the stream. This contaminated ground water then discharges into the lake along a 70-m-wide segment of lake shore. Road salt is pervasive in the bedrock between the highway and the lake, but was not detected at all of the wells in the glacial overburden. Of the 500 m of shoreline that could receive discharge of saly ground water directly from the highway, only a 50-m-long segment appears to be contaminated.

Rostad, C.E., Pereira, W.E., and Leiker, T.J., 1999, Distribution and transport of selected anthropogenic lipophilic organic compounds associated with Mississippi River suspended sediment, 1989-1990: Archives of Environmental Contamination and Toxicology, v. 36, p. 248-255.

In the first study on this scale, distribution and transport of selected hydrophobic halogenated organic compounds associated with suspended sediment from the lower Mississippi River and its principal tributaries were determined during two spring and two summer cruises. Lipophilic organic compounds identified on the suspended sediment included hexachlorobenzene, pentachlorobenzene, pentachloroanisole, dacthal, chlordane (cis- and trans-), nonachlor (trans-), chlorthalonil, and penta-, hexa-, hepta-, and octachlorobiphenyls. Most of these compounds come from nonpoint sources. Mass loadings of most of the compounds increased from upstream to downstream on the main stem of the Mississippi River. Of the tributaries studied, the Ohio River had the most significant effect on contaminant loads. Suspended sediment transport to the Gulf of Mexico of the most abundant, widely distributed compound class, PCBs, was estimated at 6,750 kg per year.

Ruhl, H.A., Rybicki, N.B., Reel, J.T., and Carter, V., 1999, Distribution and abundance of submersed aquatic vegetation in the tidal Potomac River and upper Potomac Estuary, Maryland, Virginia, and the District of Columbia, 1993-1998: U.S. Geological Survey Open-File Report 99-233, 39 p.

The U.S. Geological Survey determined submersed aquatic vegetation and abundance in the tidal Potomac River and upper Potomac Estuary annually from 1993-1998. Observations were made on the location and abundance of submersed aquatic vegetation and the relative coverage of species present at the end of the growing season. The survey area extended from Chain Bridge in Washington, D.C. to Maryland Point, or as far downstream as the Route 301 Bridge depending on annual research needs. A set of 1:24,000-scale maps was created in the field and laboratory using aerial photographs and shoreline survey data collected over a period of several weeks. A set of smaller scale maps was also created to summarize the distribution and abundance of submersed aquatic vegetation and Hydrilla verticillata in the tidal Potomac River and upper Potomac estuary.

A model for trace metals that considers in-stream transport, metal oxide precipitation-dissolution, and pH-dependent sorption is presented. Linkage between a surface complexation submodel and the stream transport equations provides a framework for modeling sorption onto static and/or dynamic surfaces. A static surface (e.g., an iron-oxide-coated streambed) is defined as a surface with a temporally constant solid concentration. Limited contact between solutes in the water column and the static surface is considered using a pseudokinetic approach. A dynamic surface (e.g., freshly precipitated metal oxides) has a temporally variable solid concentration and is in equilibrium with the water column. Transport and deposition of solute mass sorbed to the dynamic surface is represented in the stream transport equations that include precipitate settling. The model is applied to a pH-modification experiment in an acid mine drainage stream. Dissolved copper concentrations were depressed for a 3 hour period in response to the experimentally elevated pH. After passage of the pH front, copper was desorbed, and dissolved concentrations returned to ambient levels. Copper sorption is modeled by considering sorption to aged hydrous ferric oxide (HFO) on the streambed (static surface) and freshly precipitated HFO in the water column (dynamic surface). Comparison of parameter estimates with reported values suggests that naturally formed iron oxides may be more effective than synthetic oxides used in laboratory studies. The model's ability to simulate pH, metal oxide precipitation-dissolution, and pH-dependent sorption provides a means of evaluating the complex interactions between trace metal chemistry and hydrologic transport at the field scale.

Bacteriophage PRD1 and silica colloids were co-injected into sewage-contaminated and uncontaminated zones of an iron oxide-coated sand aquifer on Cape Cod, MA, and their transport was monitored over distances up to 6 m in three arrays. After deposition, the attached PRD1 and silica colloids were mobilized by three different chemical perturbations (elevated pH, anionic surfactant, and reductant). PRD1 and silica colloids experienced less attenuation in the contaminated zone where adsorbed organic matter and phosphate may be hindering attachment of PRD1 and silica colloids to the iron oxide coatings. The PRD1 collision efficiencies agree well with collision efficiencies predicted by assuming favorable PRD1 deposition on iron oxide coatings for which the surface area coverage was measured by microprobe analysis of sediment thin sections. zeta potentials of the PRD1, silica colloids, and aquifer grains corroborated the transport results, indicating that electrostatic forces dominated the attachment of PRD1 and silica colloids. Elevated pH was the chemical perturbation most effective at mobilizing the attached PRD1 and silica colloids. Elevated surfactant concentration mobilized the attached PRD1 and silica colloids more effectively in the contaminated zone than in the uncontaminated zone.

Rybicki, N.B., Ruhl, H.A., Reel, J.T., and Carter, V., 1999, A survey of potential dredge disposal sites in the tidal Potomac River and upper Potomac Estuary between Washington, D.C. and the Route 301 Bridge: U.S. Geological Survey Open-File Report 99-234, 10 p.

Several potential sites for the disposal of material dredged from the tidal Potomac River are identified. Five land sites, seven shallow-water sites, and four deep-water sites are identified. The shallow-water sites are further identified as being suitable for placement of the dredge material to create wetlands, create or restore islands, or nourish beaches.

Rybicki, N.B., Reel, J., Ruhl, H.A., Gammon, P.T., Carter V., and Lee, J.K., 1999, Biomass and Vegetative Characteristics of Sawgrass Grown in a Tilting Flume as Part of a Study of Vegetative Resistance to Flow, U.S. Geological Survey Open- File Report 99-230.

The U.S. Geological Survey is studying vegetative resistance to flow in the south Florida Everglades as part of a multidisciplinary effort to restore the South Florida Ecosystem. In order to test the flow resistance of sawgrass, one of the dominant species in the Everglades, uniform, dense stands of sawgrass were grown in a tilting flume at Stennis Space Center, Mississippi. Depth of water in the flume was controlled by adding or removing metal plates at the downstream end of the flume. A series of experiments were conducted at various flow depths, and the velocity, flow depth, and water-surface slope were measured. During each set of experiments, the sawgrass was sampled in layers from the sediment water interface for vegetative characteristics, biomass, and leaf area index. The results of the vegetation sampling are summarized in a series of tables.

Faults at convergent plate boundaries are important conduits for fluid escape, and recent evidence suggests that fluid expulsion along them is both transient and heterogeneous. For the Nankai and Barbados convergent margins, we have used numerical models to investigate the long-term partitioning of expelled fluids between diffuse flow and flow along connected high-permeability fault conduits. For a simple case of spatial heterogeneity, we estimated the extent of high-permeability conduits necessary to maintain a balance between incoming and expelled fluids. For the case of transient expulsion, we constrained the duration of elevated permeability required to balance the fluid budgets. Comparison of modeled and observed geochemical profiles suggests that the initiation of connected flow conduits is delayed with respect to the time of accretion into each accretionary complex and may be related to burial below a critical depth, either where the overlying wedge is sufficiently thick to prevent fluid escape to the sea floor or where sediments behave brittlely.

Schlottman, J.L., Scholl, M.A., and Cozzarelli, I.M., 1999, Identifying ground-water and evaporated surface-water interactions near a landfill using deuterium, 18oxygen, and chloride, Norman, Oklahoma, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3 - Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 509-520.

Schmidt, J.C., Andrews, E.D., Wegner, D.L., Patten, D.T., Marzolf, G.R., and Moody, T.O., 1999, Origins of the 1996 controlled flood in Grand Canyon, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., the 1996 controlled flood in Grand Canyon, monograph no. 110: American Geophysical Union.

The March 1996 controlled flood in Grand Canyon resulted from a decade-long evolution in scientific thinking about the appropriate role of floods in management of the Colorado River in Grand Canyon. The flood was implemented after 5 consecutive years in which proposals to conduct a similar event were rejected; final implementation of the 1996 flood necessitated revision of the definition of the appropriate basin-wide runoff conditions that would trigger such a flood. The flood partly resulted from a multi-year effort to reform the Colorado River Storage Project Act that had culminated in passage of the Grand Canyon Protection Act in 1992. The flood itself consisted of a 4-day period of steady discharge of 227 m³/s, an 11-hr period of increasing discharge to a peak of 1274 m³/s that lasted for 7 days, a 45-hr period of recession, and a 4-day period of steady discharge at 227 m³/s. This event was partly a demonstration of the potential role of floods in regulated river management and also provided an opportunity for scientists to make measurements about physical and biological processes during flood conditions.

Scholl, M.A., Cozzarelli, I.M., Christenson, S.C., Breit, G.N., and Schlottman, J.L., 1999, Aquifer heterogeneity at Norman landfill and its effect on observations of biodegradation processes, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3 - Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 557-568.

Short, J.W., Kvenvolden, K.A., Carlson, P.R., Hostettler, F.D., Rosenbauer, R.J., and Wright, B.A., 1999, Natural hydrocarbon background in benthic sediments of Prince William Sound, Alaska--Oil vs. Coal: Environmental Science and Technology, v. 33, p. 34-42.

The source of the background hydrocarbons in benthic sediments of Prince William Sound (PWS), AK, where the 1989 Exxon Valdez oil spill (EVOS) occurred, has been ascribed to oil seeps in coastal areas of the Gulf of Alaska (GOA). We present evidence that coal is a more plausible source, including (i) high concentrations of total PAH (TPAH), between 1670 and 3070 ng/g, in continental shelf sediments adjacent to the coastal region containing extensive coal deposits; (ii) PAH composition patterns of sediments along with predictive models that are consistent with coal but not oil; (iii) low ratios (<0.2) of triaromatic steranes to methylchrysenes found in sediments and coals, contrasting with the high ratios (11 and 13) found in seep oil; and (iv) bioaccumulation of PAH in salmon collected within 100 m of the Katalla oil seeps but not in filter-feeding mussels collected near oilfield drainages 9 km from the seeps, indicating negligible transport of bioavailable PAH from Katalla seeps to the GOA. In contrast with oil, PAH in coal are not bioavailable, so the presence of coal in these benthic sediments confers no adaptive benefit to biota of the marine ecosystem with respect to PAH insults from anthropogenic sources.

Simon, N.S., Spencer, R.J., and Cox, T., 1999, Distribution of and relation among mercury and methylmercury, organic carbon, carbonate, nitrogen and phosphorus, in periphyton of the South Florida Ecosystem: Toxicological and Environmental Chemistry, v. 69, p. 417-433.

Periphyton samples from Water Conservation Areas, Big Cypress National Preserve, and Everglades National Park in south Florida were analyzed for concentrations of total mercury, methylmercury, nitrogen, phosphorus, organic carbon, and inorganic carbon. Concentrations of total mercury in periphyton decrease slightly along a gradient from north-to-south. Both total mercury and methylmercury are positively correlated with organic carbon, nitrogen, and phosphorus in periphyton. In horizontal sections of periphyton mats, total mercury concentrations tend to be the largest at the tops and bottoms of the mats. Methylmercury concentrations tend to be the largest near the bottom of mats. These localized elevated concentrations of methylmercury suggest that there are 'hot spots' of methylmercury in peeriphyton.

Point measures of flow speed and suspended-sand concentration were made from cableway 293-km downstream from Glen Canyon Dam during the 1996 controlled flood. The data demonstrate a systematic fining of the suspended load in the Colorado River, a reduction in near-bed sand concentration with time, and a strong secondary circulation that very effectively transported suspended sand toward the channel margins. In the center of the river, the primary flow was well represented by steady, horizontally uniform flow theory, with a shear velocity of 0.081 m/s and a sand grain related roughness parameter of 4.5 x 10⁶ m; at the channel margins the primary flow exhibited a distinct internal boundary layer with a shear velocity of approximately 0.081 m/s and an outer boundary layer with a shear velocity of approximately twice that value. The secondary circulation was caused by long wavelength irregularities in the rockfall-produced sloping banks of the approximately trapezoidal channel. The primary flow was forced upward and toward the river center by these topographic features causing a fully 3-dimensional circulation. The upward forced vertical velocities apparently, the upwelling zone degraded to an irregular, bank-parallel boil line. Downwelling occurred over a broad region in the center of the river, but also was concentrated along well-defined convergence zones over which woody debris concentrated. This secondary circulation was very effective in transporting suspended sand toward the channel margins at the bottom, then lifting it in the boils and depositing it inshore of the boil line on the rivermarks.

Swetnam, T.W., Allen, C.D., and Betancourt, J.L., 1999, Applied Historical Ecology: using the past to manage for the future: Ecological Applications 64, p 1189-1206.

Applied historical ecology is the use of historical knowledge in the management of ecosystems. Historical perspectives increase our understanding of the dynamic nature of landscapes and provide a frame of reference for assessing modern patterns and processes. Historical records, however, are often too brief or fragmentary to be useful, or they are not obtainable for the process or structure of interest. Even where long historical time series can be assembled, selection of appropriate reference conditions may be complicated by the past influence of humans and the many potential reference conditions encompassed by nonequilibrium dynamics. These complications, however, do not lessen the value of history; rather they underscore the need for multiple, comparative histories from many locations for evaluating both cultural and natural causes of variability, as well as for characterizing the overall dynamical properties of ecosystems. Historical knowledge may not simplify the task of setting management goals and making decisions, but 20th century trends, such as increasingly severe wildfires, suggest that disregarding history can be perilous.

We describe examples from our research in the southwestern United States to illustrate some of the values and limitations of applied historical ecology. Paleoecological data from packrat middens and other natural archives have been useful for defining baseline conditions of vegetation communities, determining histories and rates of species range expansions and contractions, and discriminating between natural and cultural causes of environmental change. We describe a montane grassland restoration project in northern New Mexico that was justified and guided by an historical sequence of aerial photographs showing progressive tree invasion during the 20th century. Likewise, fire scar chronologies have been widely used to justify and guide fuel reduction and natural fire reintroduction in forests. A south-western network of fire histories illustrates the power of aggregating historical time series across spatial scales. Regional fire patterns evident in these aggregations point to the key role of interannual lags in responses of fuels and fire regimes to the El Nino-Southern Oscillation (wet/dry cycles), with important implications for long-range fire hazard fore-casting. These examples of applied historical ecology emphasize that detection and explanation of historical trends and variability are essential to informed management.

Sueker, J.K., Turk, J.T., and Michel, R.L., 1999, Use of cosmogenic 35S for comparing ages of water from three alpine-subalpine basins in the Colorado Front Range: Geomorphology, v. 27, p. 61-74.

High-elevation basins in Colorado are a major source of water for the central and western United States; however, acidic deposition may affect the quality of this water. Water that is retained in a basin for a longer period of time may be less impacted by acidic deposition. Sulfur-35 (³⁵S), a short-lived isotope of sulfur (t_1/2 = 87 days), is useful for studying short-time scale hydrologic processes in basins where biological influences and water/rock interactions are minimal. When sulfate response in a basin is conservative, the age of water may be assumed to be that of the dissolved sulfate in it. Three alpine-subalpine basins on granitic terrain in Colorado were investigated to determine the influence of basin morphology on the residence time of water in the basins. Fern and Spruce Creek basins are glaciated and accumulate deep snowpacks during the winter. These basins have hydrologic and chemical characteristics typical of systems with rapid hydrologic response times. The age of sulfate leaving these basins, determined from the activity of ³⁵S, averages around 200 days. In contrast, Boulder Brook basin has broad, gentle slopes and an extensive cover of surficial debris. Its area above treeline, about one-half of the basin, is blown free of snow during the winter. Variations in flow and solute concentrations in Boulder Brook are quite small compared to Fern and Spruce Creeks. After peak snowmelt, sulfate in Boulder Brook is about 200 days older than sulfate in Fern and Spruce Creeks. This indicates a substantial source of older sulfate (lacking ³⁵S) that is probably provided from water stored in pore spaces of surficial debris in Boulder Brook basin.

Surian, N., Andrews, E.D., 1999, Estimation of geomorphically significant flows in alpine streams of the Rocky Mountains, Colorado (USA): Regulated Rivers: Research & Management, v. 15, no. 4, p. 273-288.

Streamflows recorded at 24 gauging stations in the Rocky Mountains of Colorado were analyzed to derive regional regression equations for estimating the natural flow duration and flood frequency in reaches where the natural flows are unknown or have been altered by diversion or regulation. The principal objective of this analysis is to determine whether the relatively high, infrequent, but geomorphically and ecologically important flows in the Rocky Mountains can be accurately estimated by regional flow duration equations. The region considered in this study is an area of relatively abundant runoff, and, consequently, intense water resources development. The specific streams analyzed here, however, are unaltered and remain nearly pristine. Regional flow duration equations are derived for two situations. When the mean annual discharge is known, flows greater than or equal to 10% of the time can be estimated with an uncertainty of plus or minus 9% for the 10% exceedance flow, to plus or minus 11% for the 1.0% exceedance flow. When the mean annual discharge is unknown, the relatively high, infrequent flow can be estimated using the mean basin precipitation rate (in m³/s), and basin relief with an uncertainty of plus or minus 23% for the 10% exceedance flow to plus or minus 21% for the 1.0% exeedance flow. The uncertainty in estimated discharges using the equations derived in this analysis is substantially smaller than has been previously reported, especially for the geomorphically significant flows which are relatively large and infrequent. The improvement is due primarily to the quality of streamflow records analyzed and a well-defined hydrologic region.

15.Enzymatic and histopathologic alterations of the digestive gland, grill, gonad, and kidney were studied in Asian clam (Potamocurbula amurensis) in April, 1997 from each of four United States Geological Survey (USGS) stations in the San Francisco Estuary. Station were selected based on differing body burdens of metallic contaminants in clams (Stn 4.1>6.1>8.1>12.5) observed over 7 years. Because no pristine sites are known within the estuary and because no laboratory-reared stocks of P. amurensis were available, claims from station 12.5 served as reference animals. Histopathologic analysis revealed no lesions in clams collected from station 12.5. Mild digestive gland atrophy and moderate distal kidney tubular vacuolation were seen in clams collected from station 8.1. Mild digestive gland atrophy, moderated kidney tubular atrophy, and moderate gill inflammation were seen in clams collected from station 6.1. Lesions found only in clams from station 4.1 were: (1) severe inflammation and moderate atrophy of primary ducts and diverticula, and decreased numbers of heterophagosomes and heterolysosomes in diverticula of the digestive gland; (2) severe gill inflammation; (3) severe kidney tubular atrophy; (4) severe ovarian and testicular inflammation and necrosis; (5) decreased numbers of mature ova; and (6) decreased numbers of glycogen storage cells in the ovary and testis. Localization of specific enzymes including adenosine triphosphatase (ATP), acid phosphatase (ACP), alkaline phosphatase (ALKP), gamma-glutamyl transpeptidase (GGT), and glucose-6-phosphate dyhydrogenase (G6PDH) was performed and correlated, in serial sections with glycogen (PAS) and haematoxylin and eosin stains. Enzymatic analysis revealed: (1) increased digestive diverticula ATP in stations 6.1 and 4.1; (2) decreased digestive diverticula ACP in stations 6.1 and 4.1 and proximal kidney tubular ACP deficiency in station 4.1; (3) no ALKP differences among stations; (4) increased distal kidney tubular GGT at station 12.5 and decreased distal kidney tubular GGT at station 4.1; (5) decreased digestive diverticula G6PDH in all station except 12.5 and decreased proximal kidney tubular G6PDH in stations 8.1 and 6.1. It is possible that other anthropogenic and natural stressors may have affected the results in this study. However, the prevalence and increased severity of lesions in clams with highest metal body burden suggests a contaminant-associated etiology. Enzymatic and histopathologic biomarker alterations identified in this study were positively correlated with the metal body burden. Clams with the higher prevalence of diseases and enzyme alterations also showed a lower condition index and glycogen content in the month when histopathological assessment was performed. Further study will seek to develop enzymatic and histopathologic biomarkers for use in controlled laboratory conditions to help validate the field study.

Keller hypothesized that at high flow, near-bed velocities in pools exceed velocities in riffles and create pool scour. Pools, however, typically have larger cross-sectional areas of flow at bankfull discharge. This condition raises an inconsistency with Keller's velocity reversal hypothesis and the one-dimensional continuity of mass equation. To address this problem, a model of pool maintenance and sediment sorting is proposed that relies on constriction of flow by recirculating eddies and flow divergence over the exit-slopes of pools. According to the model, a narrow zone of high velocity occurs in the center of pools, creating scour. Along the downstream end of pools, an uphill climb of particles up the pool exit-slope promotes sediment deposition. The model is tested with field and flume measurements of velocity, water-surface elevation, and size of bed sediments in recirculating-eddy influenced pools. Local reversals of the water-surface gradient were measured in the field and a velocity reversal was created in the flume. The reversals that were measured indicate higher gradients of the water surface over the upstream portions of pools and higher velocities in pools at high flow. The distribution of bed sediments collected in the field also support the proposed model of pool maintenance.

Thorn, K.A., and Aiken, G.R., 1999, Polar metabolites of crude oil, in Morganwalp, D.W., and Buxton, H.T., eds., U.S. Geological Survey Toxic Substances Hydrology Program - Proceedings of the Technical Meeting, Charleston, South Carolina, March 8-12, 1999--Volume 3 - Subsurface Contamination from Point Sources: U.S. Geological Survey Water-Resources Investigations Report 99-4018C, p. 123-132.

To, T. B., Nordstrom, D. K., Cunningham, K. M., Ball, J. W., and McCleskey, R. B., 1999, A new method for the direct determination of dissolved Fe(III) concentration in acid mine waters: Environmental Science and Technology, v. 33, no. 5, p. 807-813.

A new method for direct determination of dissolved Fe(III) in acid mine water has been developed. In most present methods, Fe(III) is determined by computing the difference between total dissolved Fe and dissolved Fe(II). For acid mine waters, frequently Fe(II) >> Fe(III); thus, accuracy and precision are considerably improved by determining Fe(III) concentration directly. The new method utilizes two selective ligands to stabilize Fe(III) and Fe(II), thereby preventing changes in Fe reduction-oxidation distribution. Complexed Fe(II) is cleanly removed using a silica-based, reversed-phase adsorbent, yielding excellent isolation of the Fe(III) complex. Iron(III) concentration is measured colorimetrically or by graphite furnace atomic absorption spectrometry (GFAAS). The method requires inexpensive commercial reagents and simple procedures that can be used in the field. Calcium(II), Ni(II), Pb(II), Al(III), Zn(II), and Cd(II) cause insignificant colorimetric interferences for most acid mine waters. Waters containing >20 mg of Cu/L could cause a colorimetric interference and should be measured by GFAAS. Cobalt(II) and Cr(III) interfere if their molar ratios to Fe(III) exceed 24 and 5, respectively. Iron(II) interferes when its concentration exceeds the capacity of the complexing ligand (14 mg/L). Because of the GFAAS elemental specificity, only Fe(II) is a potential interferent in the GFAAS technique. The method detection limit is 2 mg/L (40 nM) using GFAAS and 20 mg/L (0.4 mM) by colorimetry.

Topping, D.J., Rubin, D.M., Nelson, J.M., Kinzel, P.J., III, and Bennett, J.P., 1999, Linkage between grain-size evolution and sediment depletion during Colorado River floods, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A., eds., The 1996 controlled flood in Grand Canyon: Washington, D.C., American Geophysical Union, Geophysical Monograph 110, p. 71-98.

Suspended-sediment concentrations decrease and suspended sediment and bed sediment coarsen significantly during floods or seasonal flood periods in the Colorado River in Grand Canyon. We present evidence of these processes for the 1996 controlled flood, examine their role in producing the sedimentologic signatures of both modern and historical flood deposits, and suggest how these processes may be exploited to optimize flood releases to achieve management objectives. In general, the processes that cause grain-size evolution in the Colorado River arise because of sediment supply limitation and a mismatch between the timing of tributary sediment supply to the Colorado River and high sediment-transporting events in the mainstem. The system fines immediately following large tributary sediment inputs, and the system coarsens as the bed sediment is winnowed during subsequent mainstem flows.

Rock-fragment cover occurs on hillslopes in many parts of the world and may be especially prevalent in arid and semiarid regions. These rock veneers form by a variety of process sets, each resulting in a condition of approximate hillslope equilibrium and stability. It is proposed that the long-term influx of rock fragments to a point on a hillslope must equal the long-term outflux from that point to approach stability, regardless of the dominant process set. Thus, the route toward equilibrium and stability is represented by a mass-balance equation integrating rock-fragment influx, accumulated rock mass, and rock fragment outflux. This model of hillslope development may be applicable to a variety of process-controlled hillslope types such as talus slopes, debris fans, and those resulting from the gully gravure process set. Where changes in rock-fragment fluxes occur, due to natural or human-induced disturbances to the hillslope system, the model depicts the return toward equilibrium and stability.

Uhle, M.E., Chin, Y., Aiken, G.R., and McKnight, D.M., 1999, Binding of polychlorinated biphenyls to aquatic humic substances: The role of substrate and sorbate properties on partitioning: Environmental Science and Technology, v. 33, p. 2715-2718.

Two ortho- (2,2',5 and 2,2',5,6') and a non-ortho- (3,3',4,4') substituted polychlorinated biphenyl (PCB) congeners were used to study the effects of sorbate structure in binding processes to two lacustrine fulvic acids. Binding constants were determined by solubility enhancement of the solutes by the fulvic acids. The binding of the orthotrichlorobiphenyl was significantly less than the non-orthosubstituted tetrachlorobiphenyl to both fulvic acids. Surprisingly, the measured ortho-trichlorobiphenyl binding constant to both fulvic acids was approximately the same as the ortho-substituted tetrachlorobiphenyl. The effect of the chlorines in the ortho position inhibits free rotation around the 1,1' carbon bond, thereby making the molecule less able to interact effectively with the fulvic acid substrate relative to its non-ortho-substituted congeners. Finally, binding of all three PCBs to the Great Dismal Swamp fulvic acid was significantly higher than for the Pony Lake sample. This observation is attributable to the former substrate's higher degree of aromaticity and polarizability, which can potentially interact more favorably with the PCBs through an increase in van der Waals type interactions.

This note introduces a set of eight papers devoted to a detailed study of two sediment cores from San Francisco Bay with an overview of the region and a chronology of human activities. Data used in this study to constrain the range of sediment ages at different depths include ²³⁴Th , ²¹⁰Pb , ¹³⁷Cs , ^239,240Pu , and ¹⁰Be concentrations in the sediment and the ¹⁰C age of shell fragments. In order of first detectable appearance in the record, the indicators of contamination that were analyzed include PAHs>Hg>Ag, Cu, Pb, Zn>DDT, PCB>foraminiferal Cd/Ca. This study also documents a large memory effect for estuarine contamination caused by sediment mixing and resuspension. Once an estuary such as San Francisco Bay has been contaminated, decades must pass before contaminant levels in surface sediment will return to background levels, even if external contaminant inputs have been entirely eliminated.

A five-year dissolved Cd time series from San Francisco Bay and adjacent coastal water shows that the composition of surface water towards the mouth of the estuary is determined largely by the effect of coastal upwelling. Cd concentrations inside and outside the estuary (0.2-1.0 nmol/kg) increase as Cd-rich deep water is advected to the surface near the coast during spring and summer. On average, the mean Cd concentrations inside San Francisco Bay (0.54 nmol/kg) during 1991-1995 was significantly higher than outside (0.35 nmol/kg), however. Surface samples collected throughout San Francisco Bay confirm an internal Cd source unrelated to river discharge. The Cd content of the test of a benthic foraminifer (Elphidiella hannai) in a dated sediment core from San Francisco Bay was measured to determine if the water column Cd enrichments in San Francisco Bay could be related to the rapid development of the watershed. The method is based on the observation that the Cd/Ca ratio of carefully cleaned tests of foraminifera is determined by the dissolved Cd content of overlying water at the time of test formation. Pre-industrial foraminiferal Cd/Ca ratios in the sediment core average 274 ± 15 nmol/mol (n=19) nmol/mol. Foraminiferal Cd/Ca ratios increased to 386 ± 33 nmol/mol (n=19) over the past several decades indicating a 40% increase in the mean Cd content of surface water in Central San Francisco Bay. We suggest that, in addition to Cd discharges into the estuary, indirect consequences of agricultural development in the Central Valley of California could have contributed to this increase. This new method to reconstruct estuarine contamination is not affected by some of the processes that complicate the interpretation of changes in bulk sediment metal concentrations.

Industrialization and urbanization around San Francisco Bay as well as mining and agriculture in the watersheds of the Sacramento and San Joaquin rivers have profoundly modified sedimentation patterns throughout the estuary. We provide some constraints on the onset of these erosional disturbances with ¹⁰Be data for three sediment cores: two from Richardson Bay, a small embayment near the mouth of San Francisco Bay, and one from San Pablo Bay, mid-way between the river delta and the mouth. Comparison of pre-disturbance sediment accumulation determined from three ¹⁴C-dated mollusk shells in one Richardson Bay core with more recent conditions determined from the distribution of ²¹⁰Pb and ²³⁴Th [see Fuller, C.C., van Geen, A., Baskaran, M., Anima, R.J., 1999, above] shows that the accumulation rate increased by an order of magnitude at this particular site. All three cores from San Francisco Bay show subsurface maxima in ¹⁰Be concentrations ranging in magnitude from 170 to 520 x 10⁶ atoms/g. The transient nature of the increased ¹⁰Be input suggests that deforestation and agricultural development caused basin-wide erosion of surface soils enriched in ¹⁰Be , probably before the turn of the century.

Venkatesan, M.I., de Leon, R.P., van Geen, A., and Luoma, S.N., 1999, Chlorinated hydrocarbon pesticides and polychorinated biphenyls in sediment cores from San Francisco Bay: Marine Chemistry, v. 64, p. 85-98.

Sediment cores of known chronology from Richardson and San Pablo Bays in San Francisco Bay, CA, were analyzed for a suite of chlorinated hydrocarbon pesticides and polychlorinated biphenyls to reconstruct a historic record of inputs. Total DDTs (DDT=2,4'- and 4,4'-dichlorodiphenyltrichloroethane and the metabolites, 2,4'- and 4,4'-DDE, -DDD) range in concentration from 4-21 ng/g and constitute a major fraction (>84%) of the total pesticides in the top 70 cm of Richardson Bay sediment. A subsurface maximum corresponds to a peak deposition date of 1969-1974. The first measurable DDT levels are found in sediment deposited in the late 1930's. The higher DDT inventory in the San Pablo relative to the Richardson Bay core probably reflects the greater proximity of San Pablo Bay to agricultural activities in the watershed of the Sacramento and San Joaquin rivers. Total polychlorinated biphenyls (PCBs) occur at comparable levels in the two Bays (<1-34 ng/g). PCBs are first detected in sediment deposited during the 1930's in Richardson Bay, about a decade earlier than the onset of detectable levels of DDTs. PCB inventories in San Pablo Bay are about a factor of four higher in the last four decades than in Richardson Bay, suggesting a distribution of inputs not as strongly weighed towards the upper reaches of the estuary as DDTs. The shallower subsurface maximum in PCBs compared to DDT in the San Pablo Bay core is consistent with the imposition of drastic source control measures for these constituents in 1970 and 1977 respectively. The observed decline in DDT and PCB levels towards the surface of both cores is consistent with a dramatic drop in the input of these pollutants once the effect of sediment resuspension and mixing is taken into account.

Voytek, M.A., Priscu, J.C., and Ward, B.B., 1999, The distribution and relative abundance of ammonium-oxidizing bacteria in six antarctic lakes determined by PCR amplification: Hydrobiologia, v. 401, p. 113-130.

Marked differences in the concentrations of major ions and cations, macronutrient chemistry and general trophic status exist among the lakes of the McMurdo dry valleys in Antarctica. These differences have been attributed to both variations in stream inputs and in situ lake processes. This study examines the role of nitrifying bacteria in nitrogen transformations in these lakes. Applying two polymerase chain reaction (PCR) assays targeting the 16S rRNA genes of ammonia-oxidizing bacteria and the active site of the ammonia monooxygenase gene (amoA), the distribution of ammonia-oxidizers was examined in six Antarctic lakes: Lake Bonney, Lake Hoare, Lake Fryxell and Lake Joyce in the Taylor Valley, Lake Miers in the the Miers Valley and Lake Vanda in the Wright Valley. Using a two stage amplification procedure, ammonia-oxidizers from both the beta and gamma- subclasses of the Proteobacteria were detected and their relative abundances were determined in samples collected from all sites. Ammonia-oxidizers were detected in all lakes sampled. Members of the gamma subclass were only present in the saline lakes. In general, nitrifiers were most abundant at depths above the pycnocline and were usually associated with lower concentrations of NH₄ and elevated concentrations of NO₃ or NO₂. The distribution of nitrifiers suggests that the primary N₂O peak observed in most of the lakes was produced via nitrification. Preliminary data on the rate of nitrification support the occurrence of nitrification and the presence of nitrifiers at the depth intervals where nitrifiers were detected. In all lakes, except Lake Miers, the data indicate that nitrifying bacteria have an important role in the vertical distribution of nitrogen compounds in these systems.

Wagner, B.J., 1999, Evaluating data worth for ground-water management under uncertainty: Journal Water Resources Planning and Management, ASCE, v. 125, no. 5., p. 281-288.

Debris flows from 600 tributaries in Grand Canyon periodically deposit poorly sorted sediment on debris fans along the Colorado River. Before regulation, mainstem floods maintained fans and rapids as highly-reworked deposits dominated by large boulders. After regulation, the reduced peak discharges have entrained only particles up to 1 m in diameter, and debris fans have aggraded. We measured the effects of the 1996 controlled flood on 18 recently aggraded debris fans. At most sites, fan area decreased by 2-42%, volume decreased by 3-34%, distal margins became armored with a lag of cobbles and boulders, the width of the reworked zone increased by 4-30 m, and river constrictions decreased slightly. Stream power decreased in most rapids because water-surface fall decreased and rapids widened. The amount of reworking is a function of stream power and the elapsed time between debris flow and flood. The effectiveness of future floods of similar magnitude in reworking debris fans will depend in part on the release history and extent of armoring in the period between debris flow and flood.

Webb, R.H., Melis, T.S., Griffiths, P.G., Elliott, J.G., Cerling, T.E., Poreda, R.J., Wise, T.W., and Pizzuto, J.E., 1999, Lava falls rapid in Grand Canyon, effects of late Holocene debris flows on the Colorado River: U.S. Geological Survey Professional Paper 1591, 90 p.

Lava Falls Rapid is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Debris flows in 1939, 1954, 1955, 1966, and 1995, as well as prehistoric events, completely changed flow through the rapid. Floods cleared out much of the increased constrictions, but releases from Glen Canyon Dam, including the 1996 controlled flood, are now required to remove the boulders deposited by the debris flows.

Webb, R.H., Wegner, D.L., Andrews, E.D., Valdez, R.A., and Patten, D.T., 1999, Downstream effects of Glen Canyon Dam on the Colorado river in Grand Canyon: A review, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R.A, The controlled flood in Grand Canyon: Scientific experiment and management demonstration: Washington, D.C. American Geophysical Union, Geophysical Monograph 110, p. 1-21.

Glen Canyon Dam, completed in 1963, has altered geomorphic and ecological processes and resources of the Colorado River in Grand Canyon. Before the dam was completed, the river transported large quantities of sediment during spring floods as large as 8500 m³/s. After 1963, dam releases typically were less than 900 m³/s with large diurnal fluctuations and little sediment. The 2-yr peak discharge decreased by a factor of 2.5, resulting in aggraded rapids and a large increase in riparian vegetation. The clearwater releases from the dam eroded sand deposited on the bed and banks. Although pre-dam water temperatures varied seasonally, dam releases typically are about 8 degrees C year round. Because of the clear, cold water and reduced flooding, post-dam aquatic productivity is considerably higher in the tailwater. Rainbow trout and other non-native fishes are now common, 3 native species have been extirpated, and the remaining species, including the endangered humpback chub, cannot successfully reproduce in the river.

Weedman, S.D., Paillet, F.L., Edwards, L.E., Simmons, K.R., Scott, T.M., Wardlaw, B.R., Reese, R.S., and Blair, J.L., 1999, Lithostratigraphy, geophysics, biostratigraphy, and strontium-isotope stratigraphy of the surficial aquifer system of eastern Collier County and northern Monroe County, Florida: U.S. Geological Survey Open-File Report 99-0432, 125 p.

In 1997, ten cores were drilled in eastern Collier County and northern Monroe County, within the limits of the Big Cypress National Preserve. These cores represent a continuation of the study of seven cores in western Collier County begun in 1996 and reported in Weedman and others (1997) and Edwards and others (1998). This joint U.S. Geological Survey and Florida Geological Survey project is designed to acquire subsurface geologic and hydrologic data in southwest Florida to extend current ground-water models, thereby expanding the utility of these models for land and water management. In this report we describe the lithostratigraphy, geophysical logging, sedimentological analysis, dinocyst biostratigraphy, and strontium-isotope stratigraphy of these ten cores.

The three geophysical logs (natural gamma-ray, induction conductivity, and neutron porosity) assumed to be related to formation lithology and water quality show that a number of clay-rich zones are present in all of the boreholes, and that pore-water conductivity increases with depth. The clay-rich zones are confirmed by visual examination of core material and sedimentological analysis.

The relative transmissivity calculated at 10-foot-thick intervals shows that in six of the boreholes, high values are associated with the shallow aquifer in the 0-40 ft interval. Two of the boreholes (the most northerly and the most easterly) showed relatively higher values of transmissivity in permeable zones at or somewhat below 100 ft in depth. Core geology and logs indicate that the deeper aquifers are not more permeable than similar deeper zones in the other boreholes, but rather that the shallow aquifer appears to be less permeable in these two boreholes.

The Arcadia (?) Formation was only penetrated in the deepest core where it is late Miocene in age. The Peace River Formation was penetrated in all but the two westernmost cores. It yields a late Miocene age, based on both dinocysts and strontium-isotope stratigraphy. The top is an irregular surface. Age and stratigraphic relations suggest that the upper part of the Peace River and lower part of the unnamed formation are at least partially equivalent laterally.

The unnamed formation was recovered in every core. It is thinnest in the northernmost core and thickest to the west. Ages calculated from strontium isotopes range from 6.9 to 4.6 million years ago (late Miocene to early Pliocene). The top of the unnamed formation is deepest to the north and it becomes shallower to the southwest.

The Tamiami Formation also was recovered in every core and consistently yields early Pliocene ages; it yields late Pliocene ages near the top in two cores. The age and lateral relations strongly suggest that the lower part of the Tamiami Formation and the upper part of the unnamed formation are lateral facies of each other.

The Fort Thompson (?) Formation, Miami Limestone, and undifferentiated siliciclastic sediments and limestone at the very top of the cores were not dated.

Wershaw, R.L., 1999, Molecular aggregation of humic substances: Soil Sci., v. 164, p. 803-813.

Humic substances (HS) form molecular aggregates in solution and on mineral surfaces. Elucidation of the mechanism of formation of these aggregates is important for an understanding of the interactions of HS in soils and natural waters. The HS are formed mainly by enzymatic depolymerization and oxidation of plant bipolymers. These reactions transform the aromatic and lipid plant components into amphiphilic molecules, that is, molecules that consist of separate hydrophobic (nonpolar) and hydrophilic (polar) parts. The nonpolar parts of the molecules are composed of relatively unaltered segments of plant polymers and the polar parts of carboxylic acid groups. These amphiphiles form membrane-like aggregates on mineral surfaces and micelle-like aggregates in solution. The exterior surfaces of these aggregates are hydrophilic, and the interiors constitute separate hydrophobic liquid-phases.

White, A.F., Blum, A.E., Bullen, T.D., Vivit, D.V., Davison, V., Schultz, M., and Fitzpatrick, J., 1999, The effect of temperature on experimental and natural chemical weathering rates of granitoid rocks: Geochimica et Cosmocheimca Acta, v. 63, p. 3277-3291.

The effects of climatic temperature variations (5-35 ^oC) on chemical weathering are investigated both experimentally using flow-through columns containing fresh and weathered granitoid rocks and for natural granitoid weathering in watersheds based on annual solute discharge. Although experimental Na and Si effluent concentrations are significantly higher in the fresh relative to the weathered granitoids, the proportional increases in concentration with increasing temperature are similar. Si and Na exhibit comparable average apparent activation energies (E_a) of 56 and 61kJ/mol, respectively, which are similar to those reported for experimental feldspar dissolution measured over larger temperature ranges. A coupled temperature-precipitation model, using an expanded database for solute discharge fluxes from a global distribution of 86 granitoid watersheds, produces an apparent activation energy for Si (51 kJ/mol), which is also comparable to those derived from the experimental study. This correlation reinforces evidence that temperature does significantly impact natural silicate weathering rates.

Effluent K concentrations in the column study are elevated with respect to other cations compared to watershed discharge due to the raped oxidation/dissolution of biotite. K concentrations are less sensitive to temperature, resulting in a lower average Ea value (27 kJ/mol) indicative of K loss from lower energy interlayer sites in biotite. At lower temperatures, initial cation release from biotite is significantly faster than cation release from plagioclase. This agrees with reported higher K/Na ratios in cold glacial watersheds relative to warmer temperate environments. Increased release of less radiogenic Sr from plagioclase relative to biotite at increasing temperature produces corresponding decreases in ⁸⁷Sr/⁸⁶Sr ratios in the column effluents. A simple mixing calculation using effluent K/Na ratios, Sr concentrations and ⁸⁷Sr/⁸⁶Sr ratios for biotite and plagioclase approximates stoichiometric cation ratios from biotite/plagioclase dissolution at warmer temperatures (35 ^oC), but progressively overestimates the relative proportion of biotite with decreasing temperature. Ca, Mg, and Sr concentrations closely correlate, exhibit no consistent trends with temperature, and are controlled by trace amounts of calcite or exchange within weathered biotite. The inability of the watershed model to differentiate a climate signal for such species correlates with the lower temperature dependence observed in the experimental studies.

Accessory calcite, present at concentrations between 300 and 3000 mg kg^-1, occurs in fresh granitoid rocks sampled from the Merced watershed in Yosemite National Park, CA, USA; Loch Vale in Rocky Mountain National Park CO USA; the Panola watershed, GA USA; and the Rio Icacos, Puerto Rico. Calcite occurs as fillings in microfractures, as disseminated grains within the silicate matrix, and as replacement of calcic cores in plagioclase. Flow-through column experiments, using de-ionized water saturated with 0.05 atm. CO₂, produced effluents from the fresh granitoid rocks that were dominated by Ca and bicarbonate and thermodynamically saturated with calcite. During reactions up to 1.7 yr, calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates, principally biotite. Mass balance calculations indicate that most calcite had been removed during this time and accounted for 57-98% of the total Ca released from these rocks. Experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution. The lack of excess Ca and alkalinity indicated that calcite had been previously removed by natural weathering.

The extent of Ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks. High Ca/Na ratios relative to plagioclase stoichiometries indicate excess Ca in the Yosemite, Loch Vale, and other alpine watersheds in the Sierra Nevada and Rocky Mountains of the western United States. This Ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains. In contrast, Ca/Na flux ratios are comparable to or less than the Ca/Na ratios for plagioclase in the subtropical Panola and tropical Rio Icacos watersheds, in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport-limited weathering. These results indicate that the weathering of accessory calcite may strongly influence Ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces.

Wickland, K.P., Striegl, R.G., Schmidt, S.K., and Mast, M.A., 1999, Methane flux in subalpine wetland and unsaturated soils in the southern Rocky Mountains: Global Biogeochemical Cycles, v. 13, p. 101-113.

Methane exchange between the atmosphere and subalpine wetland and unsaturated soils was evaluated over a 15-month period during 1995-1996. Four vegetation community types along a moisture (wetland, moist-grassy, moist-mossy, and dry) were included in a 100 m sampling transect situated at 3200 m elevation in Rocky Mountain National Park, Colorado. Methane fluxes and soil temperature were measured during snow-free and snow-covered periods. The range of mean measured fluxes through all seasons (a positive value represents CH₄ efflux to the atmosphere) were: 0.3 to 29.2 mmol CH₄ m^-2d^-1, wetland area; 0.1 to 1.8 mmol CH₄ m^-2d^-1, moist-grassy area; -0.04 to 0.7 mmol CH₄ m^-2d^-1, moist-mossy area; and -0.6 to 0 mmol CH₄ m^-2d^-1, dry area. Methane efflux was significantly correlated with soil temperature (5 cm) at the continuously saturated wetland area during snow-free periods. Consumption of atmospheric methane was significantly correlated with moisture content in the upper 5 cm of soil at the dry area. A model based on the wetland flux-temperature relationship estimated an annual methane emission of 2.53 mol CH₄ m^-2 from the wetland. Estimates of annual methane flux based on field measurements at the other sites were 0.12 mol CH₄ m^-2, moist-grassy area; 0.03 mole CH₄ m^-2, moist-mossy area; and -0.04 mol CH₄ m^-2, dry area. Methane fluxes during snow-covered periods were responsible for 25, 73, 23, and 43% of the annual fluxes at the wetland, moist-grassy, moist-mossy, and dry sites, respectively.

Wiele, S.M., Andrews, E.D., and Griffin, E.R., 1999, The effect of sand concentration on depositional rate, magnitude, and location in the Colorado River below the Little Colorado River, in Webb, R.H., Schmidt, J.C., Marzolf, G.R., and Valdez, R., eds., The controlled flood in Grand Canyon: American Geophysical Union, Monograph 110, p. 131-145.

A model of flow, sand transport, and bed evolution is used to examine processes during the 1996 controlled flood to examine the effect of low and high sand concentrations on depositional rate, magnitude, and location. During a flood on the Little Colorado River in January 1993, water discharge in the main stem was increased to about 950 m³/s and a sand volume sufficient to increase sand concentrations to about twice the sand concentration measured on the first day of the controlled flood was delivered to the main stem and redistributed downstream. This influx of sand led to large deposits in depressions in main channel pools and deposition in recirculation zones that the model predicts originated as separation deposits and spread downstream and into the recirculation zone. In contrast, during the controlled flood at a discharge of 1270 m³/s sand was eroded from the main channel and the delivery of suspended sand from the main channel to the recirculation zones was focused at the reattachment point. Similar depositional patterns were predicted by the model using sand inputs at the upper and lower bounds of measured sand transport at the controlled flood water discharge. Rates of deposition and deposit volume were proportional to the sand input to the reach.

The fundamental rationale for statistical downscaling is that the raw outputs of climate change experiments from General Circulation Models (GCMs) are an inadequate basis for assessing the effects of climate change on land-surface processes at regional scales. This is because the spatial resolution of GCMs is too coarse to resolve important sub-grid scale processes (most notably those pertaining to the hydrological cycle) and because GCM output is often unreliable at individual and sub-grid box scales. By establishing empirical relationships between grid-box scale circulation indices (such as atmospheric vorticity and divergence) and sub-grid scale surface predictands (such as precipitation), statistical downscaling has been proposed as a practical means of bridging this spatial difference. This study compared three sets of current and future rainfall-runoff scenarios. The scenarios were constructed using: (1) statistically downscaled GCM output; (2) raw GCM output; and (3) raw GCM output corrected for elevational biases. Atmospheric circulation indices and humidity variables were extracted from the output of the UK Meteorological Office coupled ocean-atmosphere GCM (HadCM2) in order to downscale daily precipitation and temperature series for the Animas River in the San Juan River basin, Colorado. Significant differences arose between the modelled snowpack and flow regimes of the three future climate scenarios. Overall, the raw GCM output suggests larger reductions in winter/spring snowpack and summer runoff than the downscaling, relative to current conditions. Further research is required to determine the generality of the water resource implications for other regions, GCM outputs and downscaled scenarios.

Winston, R.B., 1999, Upgrade to MODFLOW-GUI: Addition of MODPATH ZONEBUDGET, and additional MODFLOW packages to the U.S. Geological Survey MODFLOW-96 graphical-user interface: U.S. Geological Survey Open-File Report 99-184, 64 p.

Surface-water bodies are integral parts of groundwater flow systems. Groundwater interacts with surface water in nearly all landscapes, ranging from small streams, lakes, and wetlands in headwater areas to major river valleys and seacoasts. Although it generally is assumed that topographically high areas are groundwater recharge areas and topographically low areas are groundwater discharge areas, this is true primarily for regional flow systems. The superposition of local flow systems associated with surface-water bodies on this regional framework results in complex interactions between groundwater and surface water in all landscapes, regardless of regional topographic position. Hydrologic processes associated with the surface-water bodies themselves, such as seasonally high surface-water levels and evaporation and transpiration of groundwater from around the perimeter of surface-water bodies, are a major cause of the complex and seasonally dynamic groundwater flow fields associated with surface water. These processes have been documented at research sites in glacial, dune, coastal, mantled karst, and riverine terrains.

The effects of potential climate change on mean annual runoff in the conterminous United States (U.S.) are examined using a simple water-balance model and output from two atmospheric general circulation models (GCMs). The two GCMs are from the Canadian Centre for Climate Prediction and Analysis (CCC) and the Hadley Centre for Climate Prediction and Research (HAD). In general, the CCC GCM climate results in decreases in runoff for the conterminous U.S., and the HAD GCM climate produces increases in runoff. These estimated changes in runoff primarily are the result of estimated changes in precipitation. The changes in mean annual runoff, however, mostly are smaller than the decade-to-decade variability in GCM-based mean annual runoff and errors in GCM-based runoff. The differences in simulated runoff between the two GCMs, together with decade-to-decade variability and errors in GCM-based runoff, cause the estimates of changes in runoff to be uncertain and unreliable.