By S. E. Church,
U. S. Geological Survey,
Mineral Resources Program
P.O. Box 25043, MS 973,
Denver, CO 80225,
schurch@usgs.gov
Public policy during the 1800's encouraged mining in the western United States. Mining on Federal lands played an important role in the growing economy creating national wealth from our abundant and diverse mineral resource base. The common industrial practice from the early days of mining through about 1970 in the U.S. was for mine operators to dispose of the mine wastes and mill tailings in the nearest stream reach or lake. As a result of this contamination, many stream reaches below old mines, mills, and mining districts and some major rivers and lakes no longer support aquatic life. Riparian habitats within these affected watersheds have also been impacted. Often, the water from these affected stream reaches is generally not suitable for drinking, creating a public health hazard. The recent Department of Interior Abandoned Mine Lands (AML) Initiative is an effort on the part of the Federal Government to address the adverse environmental impact of these past mining practices on Federal lands. The AML Initiative has adopted a watershed approach to determine those sites that contribute the majority of the contaminants in the watershed. By remediating the largest sources of contamination within the watershed, the impact of metal contamination in the environment within the watershed as a whole is reduced rather than focusing largely on those sites for which principal responsible parties can be found.
The scope of the problem of metal contamination in the environment from past mining practices in the coterminous U.S. is addressed in a recent report by Ferderer (1996). Using the USGS 1:2,000,000-scale hydrologic drainage basin boundaries and the USGS Minerals Availability System (MAS) data base, he plotted the distribution of 48,000 past-producing metal mines on maps showing the boundaries of lands administered by the various Federal Land Management Agencies (FLMA). Census analysis of these data provided an initial screening tool for prioritization of watersheds in the western U.S. A different approach to the scope of the abandoned mine problem (Church et al., 1996a) is shown by the water quality data collected by the States under the Clean Water Act, section 305(b). These data document the stream reaches affected by metals from naturally occurring sources as well as from mining, or mineral resource extraction. Permitted discharges from active industrial and mine sites are not covered in the 305(b) data base.
Local citizens and state and federal agencies are all part of the collaborative decision process used to select the drainage basins chosen for the AML Initiative pilot studies. Data gathered by these three entities were brought to bear on the watershed selection process. The USGS prepared data available from Federal data bases in the form of interpretative GIS products. Maps of the states of Colorado (Plumlee et al., 1995) and a similar study of the state of Montana (USGS, unpublished data) were used to select the Animas watershed in southwestern Colorado and the Boulder watershed southwest of Helena Montana as the pilot study areas for the AML Initiative. Thus, the watersheds selected for study were public decisions made on the basis of available scientific data. The role of the U.S. Geological Survey in the Abandoned Mine Land Initiative is outlined in Buxton et al. (1997).
The watershed approach to metals contamination in the environment has been studied in several drainage basins (Church et al., 1993, 1994, 1995, 1996b; Kimball et al., 1995). The underlying principles used to successfully discriminate between sources and to quantify the impact of these sources on the environment are the subject of this report.
The primary methods used in the study of metal contamination in the environment are:
The leach is a 2M HCl-H2O2 digestion of a 2-g aliquot of a minus-80 to minus-100 mesh composite stream-sediment sample. Samples are collected above and below the confluence of main tributaries, as well as in the tributaries themselves, within the watershed. From this initial collection, trace-element geochemical and Pb-isotopic signatures provide an overview of the geochemical background, the presence of major sources of metals in the environment, the trace-metal and Pb-isotopic signatures of specific sources, and their relative metal loadings within the watershed. Early studies indicated that the Pb-isotopic composition in the hydrous iron- and manganese-oxide phases changed in a stream reach at the tens to hundreds of meters scale (Gulson et al., 1992). Thus, the Pb-isotopic signature of mining districts could be used to quantify their contribution to the metal loads in the stream sediments and in colloids dispersed downstream from a point source. In addition to the Pb-isotopic signature of contaminant sources, the trace-metal signature of contaminant sources is also given by the metals extracted from these hydrous iron- and manganese-oxide phases.
Metals extracted by this leach from sediments derived from unmineralized drainages are those metals released by chemical weathering (Church et al., 1987). Recovery of the total amount of the metal from weathered silicate detritus contained in the sediments from unmineralized areas is very low, nominally only a few ppm. In contrast, in mineralized areas or areas affected by oxidation of mine wastes, the rapid weathering caused by the oxidation of pyrite upon exposure to air and water results in the release of acid and metals from the exposed sulfides. Precipitation of these metals, either from solution in the form of colloids or on the surface of sand grains in the stream sediment, overwhelms the contribution from chemical weathering of silicate rock and results in a substantial metal loading downstream from source areas. Thus, the contrast between metals released by weathering of detritus from unmineralized tributary drainage basins relative to that released from the weathering of sulfides in undisturbed mineralized areas or from finely ground sulfides exposed by the mining of mineral deposits accentuates the trace-metal signature from those point sources of metal contamination. Using these tools, we can effectively discriminate between metals released by natural weathering from unmineralized areas and those metals derived from anthropogenic activities such as mining (Church, 1996).
The fundamentals of the methods used are illustrated by the study of the Camanche Reservoir in the northern California foothills belt of the Sierra Nevada Mountains east of Stockton, California (Church et al., 1997). Camanche Reservoir was formed by the damming of the Mokelumne River in 1964. Immediately upstream near the inlet for Camanche Reservoir is the Penn Mine which produced more than 1,000,000 tons of ore from a massive sulfide ore body between 1860 and 1955 when it closed. Historically, acidic waters carried metals from the mine and the waste rock directly into the Mokelumne River. The waters of the reservoir flooded the mine workings. Metal-laden acidic mine waters drained into the reservoir. Pardee Reservoir, located on the Mokelumne River immediately above the Penn Mine, has served as a trap for metals since it was built in 1928. Regional geochemistry studies have shown that there are no other mineral deposits contributing metals to Camanche Reservoir. The Pb-isotopic data are used to quantify the percentage of the metal from the Penn Mine in the sediments of the reservoir. These percentages are calculated using the following equation:
where:
RC is the 206Pb / 204Pb value determined at the source of the contaminant.
Similar equations can be used for 208Pb / 204Pb values as well. Generally, there is not sufficient analytical resolution to make accurate calculations using 207Pb / 204Pb values unless the source of the contaminant is more than two billion years old because of the short half-life of the parent isotope for 207Pb, 235U.
The source of the trace metals in the sediments is readily shown to be from the Penn Mine by the close tracking of the primary ore metals, copper and zinc, with the distribution of lead as a function of distance downstream from the Penn Mine. Note that the maximum concentration of extractable or labile lead in the sediments is only 40 ppm directly below the Penn Mine, an enrichment of a factor of eight over the background value in sediments from the Mokelumne River immediately above the Penn Mine. Concentrations of labile copper and zinc reach about 1,000 ppm and 3,000 ppm respectively in the sediments below the Penn Mine, that is, an enrichment of more than 20-fold for copper and more than 30-fold for zinc above the geochemical background. Metals not associated with the ore deposit (for example, Co, Ti, Sr) show no enrichment as a function of distance from the Penn Mine (Church et al., 1997) and clearly indicate that the source of the copper and zinc was from the Penn Mine. This conclusion is supported by the fact that the Pb-isotopic compositions measured in the metal-contaminated sediments lie on a mixing line between the Pb-isotopic composition of the unmineralized sediments and the ores from the Penn Mine (Church et al., 1997).
The upper Arkansas River drainage basin covers a large area, approximately 6,000 km2 and drains the east side of the central Rocky Mountains in central Colorado. Geochemical and Pb-isotopic studies of the stream sediments in this basin are ongoing (Church et al., 1993, 1994). Mineral deposits of several types and different ages were exploited in several mining districts within the basin (Church, 1996). Geochemical mapping (Smith, 1994) and Pb-isotopic studies have delineated the effect of the Leadville mining district on the metal load in the sediments (Church et al., 1993, 1994) and the colloids in the Arkansas River (Kimball et al., 1995). The distribution profile for labile lead clearly shows that the major source of the metal load in the sediments and the colloids in upper Arkansas River drainage basin is the Leadville mining district. Lead concentrations do not approach crustal abundance values until the contaminants are diluted by sediments derived from that portion of the watershed underlain by Mesozoic and Tertiary sedimentary rocks just above Pueblo Reservoir 220 km downstream from the headwaters of the Arkansas River. Approximately 95 percent of the labile lead in the sediments of the Arkansas River 50 km downstream from Leadville, Colorado was derived from the Leadville mining district (Church et al., 1994). In the upper Arkansas River below Chalk Creek, additional tributary sources of lead having different Pb-isotopic compositions contribute to the metal load in the sediments. Thus, the Pb-isotopic signature can be used to discriminate between various metal sources within the drainage basin. Studies of the Alamosa (Church et al., 1995) and the Animas River drainage basins (Church et al., 1996b) also demonstrate the power of Pb-isotopic data, coupled with leachate geochemistry, to quantify the metal signatures of the contaminant, to discriminate between different metal sources, and to assign specific metal loads to those sources.
Buxton, H.T., Nimick, D.A., von Guerard, Paul, Church, S.E., Frazier, Ann, Gray, J.R., Lipin, B.R., Marsh, S.P., Woodward, Daniel, Kimball, Briant, Finger, Susan, Ischinger, Lee, Fordham, J.C., Power, M.S., Bunck, Christine and Jones, J.W., 1997, A science-based, watershed strategy to support effective remediation of abandoned mine lands: Fourth Annual International Conference on Acid Rock Drainage, Vancouver B.C. Canada (in press).
Church, S.E., 1996, Indicators and discriminators used to separate metal sources from contrasting upstream mining districts: Proceedings of the Symposium on the Application of Geophysics to Engineering and Environmental Problems, Environmental and Engineering Geophysical Society, Wheat Ridge, Colorado, p. 261-268.
Church, S.E., Alpers, C.N., Vaughn, R.B., and Briggs, P.H., (1997) Use of lead-isotopes as natural tracers of metal contamination: A case study of the Penn Mine and Camanche Reservoir, California (submitted to Applied Geochemistry).
Church, S.E., Ferderer, D.A., Robinson, Rob, and Russell, Carol, 1996a, Watershed affected by mining in the western United States: U.S. Geological Survey Administrative Report, 8 p.
Church, S.E., Fey, D.L., Vaughn, R.B., and Ferderer, Dave, 1996b, Geochemical studies of sediments in the Animas River watershed, Colorado: Geological Society of America, Abstracts with Programs, v. 28, no. 7, p. 156.
Church, S.E., Holmes, C.W., Briggs, P.H., Vaughn, R.B., Cathcart, James, and Marot, Margaret, 1993, Geochemical and lead-isotope data from stream and lake sediments, and cores from the upper Arkansas River drainage: Effects of mining at Leadville Colorado on heavy-metal concentrations in the Arkansas River: U.S. Geological Survey Open-File Report 93-534, 61 p.
Church, S.E., Mosier, E.L., and Motooka, J.M., 1987, Mineralogical basis for the interpretation of multi-element (ICP-AES), oxalic acid, and aqua regia partial digestions of stream sediments for reconnaissance exploration geochemistry: Journal of Geochemical Exploration, v. 29, p. 207-233.
Church, S.E., Wilson, S.A., and Briggs, P.H., 1995, Geochemical and lead-isotopic studies of stream and river sediments, Alamosa River basin, Colorado: U.S. Geological Survey Open-File Report 95-250, 71 p.
Church, S.E., Wilson, S.A., Vaughn, R.B., and Fey, D.L., 1994, Geochemical and lead-isotopic studies of river and lake sediments, upper Arkansas River basin, Twin Lakes to Pueblo, Colorado: U.S. Geological Survey Open-File Report 94-412, 44 p.
Ferderer, D.A., 1996, National overview of abandoned mine land utilizing the Minerals Availability System (MAS) and geographic information systems technology: U.S. Geological Survey Open-File Report 96-549, 42 p.
Gulson, B.L., Church, S.E., Mizon, K.J., and Meier, A.L., 1992, Lead isotopes in iron and manganese oxide coatings and their use as an exploration guide for concealed mineralization: Applied Geochemistry, v. 7, p. 495-511.
Kimball, B.A., Callender, Edward, and Axtmann, E.V., 1995, Effects of colloids on metal transport in a river receiving acid mine drainage, upper Arkansas River, Colorado, U.S.A.: Applied Geochemistry, v. 10, p. 285-306.
Plumlee, G.S., Streufert, R.K., Smith, K.S., Smith, S.M., Wallace, A.R., Toth, M.I., Nash, J.T., Robinson, Rob, Ficklin, W.H., and Lee, G.K., 1995, Map showing potential metal-mine drainage hazards in Colorado, based on mineral-deposit geology: U.S. Geological Survey Open-File Report 95-26, scale 1:750,000.
Smith, S.M., 1994, Geochemical maps of copper, lead, and zinc, upper
Arkansas River drainage basin, Colorado: U.S. Geological Survey Open-File
Report 94-408, 15 p.
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