U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the Technical Meeting, Colorado Springs, Colorado, September 20-24, 1993, Water-Resources Investigations Report 94-4015

Overview of Research at the Cape Cod Site: Field and Laboratory Studies of Physical, Chemical, and Microbiological Processes Affecting Transport in a Sewage-Contaminated Aquifer

CONTENTS

• ABSTRACT
• INTRODUCTION
• SITE DESCRIPTION
• SEWAGE PLUME
• RESEARCH ON TRANSPORT PROCESSES
  • Physical Transport
  • Chemical Processes
  • Microbial Processes
• SUMMARY
• REFERENCES

Abstract

The Cape Cod Toxic-Substances Hydrology research site in Falmouth, Mass., is the focus for multidisciplinary field studies of the physical, chemical, and microbiological processes affecting transport in a sewage-contaminated sand and gravel aquifer. Past and current research at the site is summarized in a synopsis of 62 papers. Past research has examined the distribution and fate of contaminants in a sewage plume that is more than 4.5 kilometers long and originates from the Otis Air Base wastewater-treatment facility. Current research includes efforts to characterize specific physical, chemical, and microbiological processes affecting transport and fate of solutes, bacteria, and protozoa at the site. These processes are characterized by means of laboratory experiments, such as batch and column tests; field experiments, such as natural-gradient tracer tests; and investigations of the sewage plume. The results of the research show the importance of physical and chemical heterogeneity, limited dispersion, and chemical processes at the sediment-water interfaces in determining the fate of contaminants in this aquifer.

INTRODUCTION

The U.S. Geological Survey (USGS) Cape Cod Toxic-Substances Hydrology research site is located in the northern part of Falmouth, Mass. (fig. 1). This site is the focus for multidisciplinary field studies of the physical, chemical, and microbiological processes affecting the movement of contaminants in a sewage plume originating from Otis Air Base. It was selected for study as part of a nationwide program of the USGS to describe the transport and fate of contaminants in the subsurface. The purpose of this paper is to review past research efforts at the Cape Cod site and to introduce the research topics discussed in other papers in this proceedings. Earlier summaries of research at the Cape Cod site can be found in reports by LeBlanc (1984b), Franks (1987), Ragone (1988), Mallard and Ragone (1989), and Mallard and Aronson (1991).

SITE DESCRIPTION

The study area is on a broad sand and gravel glacial-outwash plain that was formed during the last Pleistocene glacial retreat. The outwash plain slopes southward to Nantucket Sound and is pitted with many kettle holes, some of which contain ponds. The area contains several valleys that transect the plain from north to south. Most of these valleys do not contain streams but have wetlands at their southern ends.

The top 30 to 50 m (meters) of outwash are composed of stratified, medium to coarse sand with some gravel. The sand and gravel overlies fine sand and silt that locally contains lenses of clay, silt, sand, and gravel. These unconsolidated sediments overlie a crystalline (granodiorite) bedrock surface, which generally slopes to the southeast through the study area.

On the basis of measured values for similar sediments on Cape Cod, LeBlanc (1984c) estimated that the horizontal hydraulic conductivity of the sand and gravel in the study area ranges from 60 to 90 m/d (meters per day). Results of an aquifer test conducted in the study area in 1984 indicate that the horizontal hydraulic conductivity of the sand and gravel may locally be as high as 120 m/d (Garabedian and others, 1988). The horizontal hydraulic conductivity of the underlying fine sand and silt is estimated to be about one-tenth that of the sand and gravel (LeBlanc, 1984a). The crystalline bedrock is assumed to have a very low hydraulic conductivity; therefore, the bedrock is considered to be the bottom of the regional ground-water flow system.

Ground water in the unconsolidated sediments is under unconfined (water table) conditions. The water table slopes toward the south at about 1.5 m/km (meters per kilometer) (fig. 1). Seasonal variations in recharge from precipitation produce an annual water-table fluctuation of 0.3 to 0.9 m; the highest levels are in the spring and the lowest are in the fall.

Ground-water recharge to the study area occurs primarily from precipitation and underflow from upgradient areas. Little surface-water runoff occurs because the sandy soils are very permeable. Estimated recharge to the aquifer is 0.5 meters per year, or about 45 percent of the total precipitation (LeBlanc, 1984a). Estimated rates of horizontal ground-water velocity in the sand and gravel range from 0.2 to 0.6 m/d. These estimates are based on an average hydraulic gradient of 1.5 m/km, a horizontal hydraulic conductivity of 60 to 120 m/d, and a porosity of 30 to 40 percent (LeBlanc, 1984c).

Figure 1. Location of study area, showing sewage plume, water-table contours, and tracer-test site. (50k)

SEWAGE PLUME

Disposal of treated sewage onto infiltration sand beds at Otis Air Base (fig. 1) since 1936 has created a plume of contaminated ground water (LeBlanc, 1984 b, c) that is 0.8 to 1.1 km (kilometers) wide, 23 m thick, and more than 4.5 km long (fig. 1). The plume moves to the south in the direction of ground-water flow and is overlain by up to 15 m of uncontaminated ground water derived from precipitation that recharges the aquifer. Part of the plume discharges to Ashumet Pond (fig. 1), which is located about 500 m from the disposal beds.

The plume of sewage-contaminated ground water is characterized by elevated concentrations of dissolved solids, boron, chloride, sodium, phosphorus, ammonium, nitrate, detergents (LeBlanc, 1984c), and volatile organic compounds (VOC), including dichloroethene, trichloroethene, and tetrachloroethene (Thurman and others, 1984; Barber and others, 1988). Boron, chloride, and sodium appear to be conservative and nonreactive constituents that are attenuated primarily by hydrodynamic dispersion. Phosphorus movement is greatly retarded by adsorption onto the sediments (LeBlanc, 1984c); colloidal precipitation of iron-phosphate compounds also may affect phosphorus transport near the disposal beds (Gschwend and Reynolds, 1987; Backhus and Gschwend, 1990; Backhus and others, 1993).

Ammonium is the predominant nitrogen species in the center of the plume within 1.5 km of the disposal beds (LeBlanc, 1984c). The distribution of ammonium is caused, in part, by adsorption onto the aquifer sediments, which retards the movement of ammonium (Ceazan and others, 1989). Beyond 1.5 km, the predominant nitrogen species changes to nitrate. Nitrate cannot be detected in the center of the plume immediately downgradient of the disposal beds, even though nitrate concentrations in the sewage effluent are as high as 16 mg/L (milligrams per liter) (as nitrogen (N)), because microbially mediated denitrification has converted the nitrate to nitrogen gas (Smith and others, 1991b).

Detergent (methylene-blue-active subtances) concentrations exceed 0.5 mg/L from 0.9 to 3.0 km downgradient from the disposal beds. This distribution of detergents reflects the use of nonbiodegradable detergents during 1946-64 (LeBlanc, 1984c; Thurman and others, 1986; Thurman and others, 1987). Although more than 90 percent of the biodegradable detergents are presently removed by the wastewater-treatment facility, the remaining detergents which enter the aquifer degrade slowly and are detected as far as 1.5 km from the disposal site (Field and others, 1992a, 1992b).

Elevated VOC concentrations are present in two zones in the study area. The source of the VOC zone immediately downgradient from the disposal beds may be unrelated to the sewage disposal because the VOC's are found beneath the sewage plume. However, the VOC zone 500 to 2,600 m downgradient from the disposal beds is thought to originate from the sewage-treatment facility because the VOC's are found within the sewage plume (Thurman and others, 1984; Barber and others, 1988; Barber and others, 1992b). VOC concentrations in the downgradient zone exceed 50 micrograms per liter, which suggests that these compounds are mobile and not readily degraded in the sandy aquifer (Barber, 1988; Barber and others, 1988).

Bacterial population counts as large as 2 x 10⁶/mL (per milliliter) are found in ground water near the disposal site; these counts decrease by an order of magnitude more than 1 km from the beds (Harvey and others, 1984). These numbers appear to correlate with the availability of degradable organic compounds; concentrations of dissolved organic carbon (DOC) decreasefrom 12 mg/L to less than 2 mg/L over the same distance (Thurman and others, 1986; Harvey and Barber, 1992; Metgeand others, 1993). Assays of microbial activity have been madefor ground water and aquifer sediments because more than 90 percent of the bacteria are attached to silt- and clay-sized particles (Harvey and George, 1987; Smith and Duff, 1988). These assays show that rates of microbially mediated denitrification are greatest in water and sediment collected from a 1- to 2-m-thick zone near the top of the plume. The presence of the thin zone of elevated microbial activity demonstrates the need for sampling ground water and aquifer sediments at closely spaced vertical intervals (Smith and others, 1991a.)

RESEARCH ON TRANSPORT PROCESSES

Past research efforts at the Cape Cod Toxic-Substances Hydrology research site have focused on defining and describing the extent of ground-water contamination in the sewage plume. Current research at the site includes efforts to characterize specific physical, chemical, and microbiological processes affecting the transport and fate of solutes and microorganisms in the aquifer. These processes are characterized using small-scale laboratory experiments, such as batch and column tests; intermediate-scale field experiments, such as natural-gradient tracer tests with transport distances of 1 to 280 m; and large-scale investigations of the sewage plume.

Physical Transport

A major objective at the Cape Cod site has been to relate the dispersion of solutes to the heterogeneity of the aquifer's hydraulic properties. A direct measure of the dispersion of solutes in the aquifer was obtained by a spatial-moments analysis of a large-scale natural-gradient tracer test (fig. 1) conducted during 1985-88 (LeBlanc and others, 1991; Garabedian and others, 1991). Bromide, a nonreactive tracer, was monitored with a three-dimensional array of about 10,000 sampling points in an abandoned gravel pit as the tracer moved 280 m through the aquifer. The spatial-moments analysis indicates that the dispersivity is about 1.0 m in the direction of flow (longitudinal), about 0.02 m in the transverse horizontal direction, and about 0.002 m in the transverse vertical direction (Garabedian and others, 1991). The extremely detailed bromide distributions were also used by Knopman and others (1991) to test various sampling-design strategies.

Application of a numerical model of density-dependent flow (LeBlanc and Celia, 1996) to the large-scale natural-gradient tracer test shows that the density difference between the ambient ground water and the tracer solution was sufficient to cause part of the downward movement of the tracer cloud observed during the test. The density-induced downward movement was most important during the first 37 days of transport when the density difference was greatest. Intermittent recharge from precipitation also caused part of the downward movement.

Hess and others (1992) used the theoretical stochastic transport equations of Gelhar and Axness (1983) to estimate aquifer macrodispersivity at the Cape Cod site from the statistical properties of the hydraulic-conductivity distribution. The statistics were obtained from the analysis of nearly 1,500 measurements of hydraulic conductivity that were made using borehole-flowmeter tests and permeameter analyses of cores (Wolf and others, 1991) near the location of the large-scale tracer test. The range of estimated longitudinal macrodispersivity is 0.35 to 0.78 m; this range is similar to the longitudinal dispersivity observed in the large-scale tracer test.

Reilly and LeBlanc (1996) conducted a well-purging experiment to test the hypothesis that the spatial heterogeneity of hydraulic conductivity and water chemistry in the aquifer near a well can cause the chemical composition of the water sampled from the well to vary temporally as the well is pumped. They observed temporal trends in specific conductance and concentrations of ferrous iron and calcium during well purging which agree with the hypothesis that the trends are due to flow and solute transport in the heterogeneous aquifer in the immediate vicinity of the well and not to the purging of standing water in the well. Morin and others (1988) used borehole geophysical logs to demonstrate that disturbance of the formation during drilling can affect the spatial heterogeneity of hydraulic conductivity adjacent to the well.

In addition to the above efforts to characterize aquifer properties and their effects on flow and solute transport at spatial scales of 1 to 280 m, recent efforts also have focused on large-scale characterization of the aquifer. Moench and others (1996) conducted an aquifer test at a partially penetrating well in the sand and gravel aquifer near the site of the large-scale natural-gradient tracer test and analyzed the water-level-drawdown data by type-curve matching. The analysis indicates that the horizontal hydraulic conductivity is 105 m/d and that the ratio of vertical to horizontal hydraulic conductivity is about 1:2, values that are consistent with estimates obtained from a stochastic analysis of detailed measurements of hydraulic conductivity made at the same site (Hess and others, 1992).

Masterson and Walter (1996) developed a preliminary regional-scale ground-water-flow model of western Cape Cod and used the model to examine the pathlines of contaminants emanating from the sewage-disposal beds. Modifications to aquifer properties used in the model resulted in similar simulated hydraulic-head distributions and rates of ground-water discharge to streams, but markedly different contaminant pathlines. An accurate representation of the regional hydrogeologic framework, especially the thickness and hydraulic conductivity of the fine sand and silt beneath the permeable sand and gravel, was needed to simulate the observed path of the sewage plume accurately, even though the simulated heads and discharge rates were insensitive to these aquifer properties in the model.

Chemical Processes

More than 30 tracer tests have been conducted to determine geochemical controls on reactive transport in the heterogeneous aquifer. Two reactive tracers, lithium and molybdate, were monitored as part of the 1985-88 large-scale natural-gradient tracer test and were found to be significantly retarded relative to bromide (LeBlanc and others, 1991). Adsorption of lithium, a cation, occurs on the mineral surfaces and, more significantly, inside the weathered grains (Wood and others, 1990), where adsorption is controlled by diffusion into pores inside the grains. The diffusion-controlled adsorption resulted in a skewed distribution of lithium, with higher concentrations near the leading edge of the solute cloud and lower concentrations in the trailing edge. Adsorption of molybdate (an oxyanion of molybdenum) was affected during the tracer test by the presence of the sewage plume, which caused variations with depth in pH and concentrations of phosphate, an oxyanion that competes with molybdate for adsorption sites (Stollenwerk and Kipp, 1990).

The effect of hydrologic and geochemical processes on metal-ion transport was evaluated in a series of laboratory experiments and small-scale tracer tests conducted at the Cape Cod site (Davis and others, 1996). The results of 12 natural-gradient tracer tests performed during 1988-92 using zinc, nickel, chromium, selenium, and ethylenediaminetetraacetic acid (EDTA) show that adsorption-desorption, aqueous-complexation, and oxidation-reduction reactions affect the transport of the reactive metals during transport through the uncontaminated, oxic zone and the sewage-contaminated, mildly reducing zone of the aquifer. The transport of chromium showed a marked dependence on its speciation and the chemical composition of the ambient ground water (Kent and others, 1994; Anderson and others, 1994). The results of the laboratory experiments and small-scale tracer tests were used to design a natural-gradient tracer test, which began in April 1993, to examine transport of a complex mixture of reactive metal ions (Davis and others, 1996). The large-scale test, in which eight tracers (bromide, chromium, zinc, copper, lead, nickel, potassium, and EDTA) were dissolved in 10,000 liters of water and injected as a pulse into the aquifer, will involve detailed observation of the tracer cloud as it moves as much as 200 m through the sand and gravel.

The geochemical properties of the aquifer sediments that control metal-ion transport are being investigated in conjunction with the tracer experiments to determine their potential use as indicators of the spatial variability of metal adsorption in the aquifer. Fuller and others (1996) show that lead and zinc are adsorbed primarily by iron- and aluminum-oxide coatings on the surfaces of the quartz grains. The amount of adsorption, which varies by a factor of two to four, is related to the amount of iron and aluminum that can be dissolved from the aquifer material by partial chemical extraction when the amounts that can be adsorbed and dissolved are normalized to surface area.

Hess and others (1996) compared the spatial variability of metal-ion adsorption to estimates of hydraulic conductivity based on the results of laboratory experiments on 375 sediment samples from 14 boreholes at the tracer-test site. Zinc and lead adsorption were measured in batch experiments (Fuller and others, 1996), whereas hydraulic conductivity was estimated from grain-size distributions. There is a statistically significant, but weak, negative correlation between lead adsorption and hydraulic conductivity. Garabedian and others (1988) used a stochastic analysis to demonstrate that a negative correlation similar to that observed for lead adsorption increases the apparent dispersion of a sorbing solute.

Geochemical heterogeneity of the sand and gravel also affects the sorption of organic compounds. Barber and others (1992a) and Barber (1994) showed that sorption of chlorobenzenes to the Cape Cod sediments increases as the particle size decreases because the fine-grained sediments have a larger total surface area, a greater abundance of magnetic minerals, and higher concentrations of sediment organic carbon than coarse-grained sediments in the aquifer.

Adsorption of metals ions in the sewage plume (Rea and others, 1996) has prevented significant transport of the metals zinc, copper, and lead away from the sewage-disposal beds. The extent of adsorption is affected by vertical gradients of pH characteristic of the transition between the sewage-contaminated ground water and the uncontaminated ground water that overlies the plume (Smith and others, 1991a).

Adsorption to the aquifer sediments also has retarded the transport of phosphate in the sewage plume (Rea and others, 1996). Phosphate concentrations in the ground water near the disposal beds are similar to those measured in the treated sewage. Because phosphate sorbs strongly to the sediments, the high concentrations in ground water indicate the presence of a large reservoir of phosphate sorbed to the aquifer sediments. The reservoir of sorbed phosphate may be remobilized after sewage disposal is stopped, as planned in late 1995, and some phosphorus may be transported in ground water that discharges to Ashumet Pond (fig. 1). A series of laboratory column experiments (Stollenwerk, 1996) indicates that the phosphate may desorb slowly as the sewage plume is flushed naturally from the aquifer. About 160 pore volumes were eluted from the column before phosphate concentrations decreased to levels generally considered by limnologists not to cause eutrophication of lakes (about 0.05 mg/L).

The above studies indicate that the geochemical environment greatly affects the transport and fate of reactive contaminants. An earlier study (Lee, 1991) had shown that the principal process affecting the ambient chemistry of the shallow, uncontaminated ground water in the study area is the carbon-dioxide-controlled hydrolysis of sodium feldspar. Lee (1996) measured carbon dioxide concentrations in soil gases from the unsaturated zone at 20 sites. The measurements show that carbon dioxide concentrations vary significantly from site to site, depending on land use, and may cause related variations in ambient water chemistry over the study area.

Microbial Processes

The microbial populations and their activity and transport in the aquifer have been characterized by several techniques. The use of small-scale tracer tests to measure microbial activity directly in the aquifer at a site where denitrification is occuring was tested using methane and hexafluoroethane as tracers (Smith and others, 1991c). These dissolved gases were transported without retardation, but concentrations of methane apparently decreased because of biodegradation. Acetylene, a dissolved gas, was subsequently used as a tracer because it inhibits the final step in the denitrification process and allows in situ measurement of the denitrification rate (Brooks and others, 1996). The measured rates of denitrification were generally lower than those reported for aquatic sediments or measured in the laboratory with sediments from the Cape Cod site, suggesting that laboratory methods may overestimate the rate of denitrification in the aquifer. Brooks and others (1992) reported that measurements of denitrification in the laboratory may also be biased when chloramphenicol, a common amendment in microbial-activity assays, is used.

The denitrification studies suggest that biodegradation can remove contaminants from aquifers. Smith and others (1994) identified several strains of bacteria that consume nitrate when hydrogen or formate are added to sediment samples collected from the zone of active denitrification in the sewage plume. The degradation of trichloroethylene was observed in sediment from the Cape Cod site that had been innoculated with genetically engineered bacteria (Krumme and others, 1993). Survival of the parental strain of the genetically engineered bacteria for more than 100 days after it had been injected into the aquifer indicates that laboratory strains of bacteria can survive for extended periods of time in the subsurface and may be effective agents for bioremediation (Krumme and others, 1994; Thiem and others, 1994).

Tracer tests conducted at the Cape Cod site show that bacteria can be transported significant distances through the aquifer (Harvey and others, 1989). In one test, the bacteria moved about 7 m at the same rate as a nonreactive tracer, bromide, under a natural hydraulic gradient, although the concentrations of bacteria declined relative to the concentration of bromide. Adsorption and differential size exclusion (filtering) of bacteria from small pores can vary significantly in the heterogeneous aquifer sediments and greatly affect bacterial transport and attenuation (Harvey and Garabedian, 1991, 1992; Harvey and others, 1993). Geochemical conditions, such as the pH of the ground water and the presence of oxyhydroxide coatings on the sediment grains, also affect the transport of bacteria (Scholl and Havey, 1992). In column experiments using sediment from the Cape Cod site, an increase in pH from 5.8 to 7.9 resulted in a 70 percent decrease in bacterial attachment (Metge and others, 1996). Bacterial attachment was also affected by changes in the amount and type of DOC and the ionic strength and divalent-anion concentration in the ground water. The pH also affects the attachment of bacteriophage (bacteria-specific viruses) to the Cape Cod sediments (Kinoshita and others, 1993).

A survey of the abundance of protozoa in the sewage plume showed that their abundance typically varies from 10,000 to 100,000 per gram of sediment and is directly related to the concentration of DOC in the ground water (Kinner and Harvey, 1996). The protozoa consist largely of small (2-3 micrometer-long) flagellates. It had been assumed that the role of protozoa in the subsurface ecosystem would be similar to their role in other environments--that is, as predators of bacteria. But their small size and correlation with the concentration of DOC suggests that they feed directly on the organic compounds.

The transport of protozoa was examined by means of several small-scale, natural-gradient tracer tests (Harvey and others, 1996). The retardation and immobilization of the protozoa were several orders of magnitude greater than the retardation and immobilization of bacteria in the same experiments. The high degree of immobilization and retardation may be related to the surface chemistry of the protozoa and not to their size.

SUMMARY

Past research efforts at the Cape Cod Toxic-Substances Hydrology research site have focused on defining and describing the extent of ground-water contamination in a sewage plume emanating from the Otis Air Base sewage-treatment facility on Cape Cod, Mass. Current research at the site includes efforts to characterize specific physical, chemical, and microbiological processes affecting the transport and fate of solutes and bacteria in the aquifer. These processes are characterized by means of small-scale laboratory experiments, such as batch and column tests; intermediate-scale field experiments, such as natural-gradient tracer tests with transport distances of 1 to 280 m; and large-scale investigations of the sewage plume.

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