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 Activities on the Transport and Fate of Chlorinated Solvents in Ground Water at Picatinny Arsenal, New Jersey, 1991-93

Contents

• ABSTRACT
• INTRODUCTION
• HYDROGEOLOGY AND GROUND-WATER CONTAMINATION
• ESTIMATED MASS DISTRIBUTION OF TRICHLOROETHYLENE AND PRELIMINARY SOLUTE MASS BALANCE
• CURRENT RESEARCH ACTIVITIES
  • Chemical Processes Affecting Transport of Chlorinated Solvents in the Saturated Zone
  • Transport of Chlorinated Solvents in the Unsaturated Zone
  • Biotransformation of Chlorinated Solvents in the Saturated Zone
  • Solute-Transport Modeling of Chlorinated Solvents in the Saturated Zone
  • Remediation Processes for Chlorinated Solvents in the Saturated and Unsaturated Zones
• SUMMARY
• REFERENCES

Abstract

The U.S. Geological Survey is conducting an interdisciplinary research study of ground-water contamination by chlorinated solvents, particularly trichloroethylene (TCE), at Picatinny Arsenal in north-central New Jersey. This paper summarizes the results of ongoing research studies investigating the processes of desorption, volatilization, and biotransformation and their effects on the fate and transport of TCE in the ground-water system.

Results of flow-through column experiments showed that contaminated aquifer sediments can act as a continuing source of TCE to the ground water. A one-dimensional model was developed to simulate the column desorption results. The model simulates an initial, fast-stage desorption by an equilibrium process and a second, slower-stage desorption by a kinetic mechanism. Concentrations of TCE sorbed to soil did not differ significantly whether the samples were air dried overnight prior to methanol extraction or whether the extraction was conducted on wet samples and the results were corrected for the TCE content of the soil moisture.

Results of a field experiment to study the dynamics of TCE volatilization in the unsaturated zone during infiltration indicated that an equilbrium distribution of TCE between soil gas and soil water was not achieved. A two-phase transport model of gas and aqueous phases was capable of simulating the field soil-gas and soil-water TCE concentrations after modification to include a constant-flux term for desorption of TCE from soil to water.

Anaerobic biotransformation rates calculated on the basis of measured field TCE concentrations and estimated ground-water travel time between sites generally were greater than those previously measured in laboratory soil microcosm experiments.

A reactive two-dimensional multispecies transport model is being used to simulate desorption, volatilization, and microbial degradation of TCE along the central axis of the plume by using rates estimated from results of other studies at the site. The formation and transport of TCE degradation products cis-1,2-dichloroethylene and vinyl chloride also are simulated.

Aerobic cometabolic biotransformation of TCE and cis-1,2-dichloroethylene can be stimulated in soil microcosms constructed with soils from the unsaturated zone near Building 24 at the arsenal if the indigenous methanotrophic bacteria are supplied with appropriate amounts of oxygen, methane, and nutrients.

Preliminary results of a study to determine whether surfactants can enhance the removal of TCE from aquifer sediments during pump-and-treat remediation indicate that addition of the nonionic surfactant Triton-X 100 is effective in artificially increasing the rate of mass transfer of TCE from soil to the aqueous phase.

INTRODUCTION

The U.S. Geological Survey is conducting an interdisciplinary research study of ground-water contamination by chlorinated solvents and other contaminants at Picatinny Arsenal in north-central New Jersey. The objectives of the study are to (1) identify and quantify the chemical, physical, and biological processes that control the movement and fate of these contaminants, particularly trichloroethylene (TCE), in the subsurface; (2) determine the relative importance of these processes in transporting TCE through the ground-water system; and (3) develop predictive models of contaminant transport.

From 1960 to 1981, the wastewater-treatment system in Building 24, which housed a metal-plating facility, discharged wastewater daily into two 8-ft-deep, sand-bottomed settling lagoons behind the building (fig. 1) (Benioff and others, 1990). The wastewater contained trace metals, other inorganic ions used in plating solutions, and degreasing solvents (Imbrigiotta and Martin, 1991). From 1973 to 1985, solvent vapors from a degreasing unit were allowed to condense in an improperly installed overflow pipe and discharged into a 4-ft-deep dry well in front of Building 24. TCE was the degreasing solvent used from 1960 to 1983. The infiltration of wastewater from the lagoons and chlorinated solvents from the dry well has created a plume of contaminated ground water downgradient from Building 24.

This paper (1) briefly describes the hydrogeology and current ground-water contamination at the Building 24 research site at Picatinny Arsenal, (2) presents and discusses a preliminary solute-transport mass balance for dissolved TCE in the shallow aquifer at the arsenal, and (3) summarizes the significant findings of the ongoing research studies at the Building 24 research site at the arsenal.

Figure 1. (A) Location of Building 24 study area at Picatinny Arsenal, New Jersey, and areal extent of trichloroethylene plume and (B) vertical distribution of trichloroethylene concentrations, October-November 1991. (Location of section A-A' is shown in figure 1 (A). (50k)

HYDROGEOLOGY AND GROUND-WATER CONTAMINATION

Picatinny Arsenal is located in a glaciated valley. The contamination plume at the Building 24 site is in a 50- to 70-ft-thick unconfined aquifer consisting primarily of coarse to fine sand with some gravel and some discontinuous silt and clay layers.

On the basis of results of aquifer-test analysis and the calibration of a multilayered ground-water-flow model, the horizontal hydraulic conductivity of the unconfined aquifer is estimated to be 50 to 360 ft/d and the estimated ratio of horizontal to vertical hydraulic conductivity is 100 to 1 (L.M. Voronin, U.S. Geological Survey, written commun., 1993).

Long-term water-table altitudes average about 696 ft above sea level at Building 24, the contaminant source area, and about 686 ft above sea level at Green Pond Brook, the natural ground-water discharge point for the site. The general flow pattern in the unconfined aquifer is south-southeast from the edge of the glacial sediments to Green Pond Brook, with a slight downvalley component. Within the unconfined aquifer, flow generally is horizontal, with some downward flow near Building 24 and upward flow near Green Pond Brook. Estimated ground-water-flow velocities, based on calibrated flow-model hydraulic conductivities and measured head gradients, are about 1 to 3 ft/d in the plume area.

Ground-water contamination measured in the unconfined aquifer in 1987 and 1989 has been described previously by Sargent and others (1990) and Imbrigiotta and others (1991). Results of these studies showed that TCE was the most widespread organic contaminant in the system. Results of analyses of water samples collected from 53 wells in October and November 1991 confirmed that the areal extent of the TCE contaminant plume had changed little since the 1987 synoptic sampling (fig. 1a). The plume extends 1,640 ft. from Building 24 to Green Pond Brook, where it is about 1,000 ft wide. The plume area in which TCE concentrations are greater than 10 mg/L is estimated to cover 1.4 x 10⁶ ft². The vertical distribution of TCE along the central axis of the plume (fig. 1b) indicates that the highest concentrations (> 10,000 mg/L) still are found near the base of the unconfined aquifer midway between Building 24 and Green Pond Brook. TCE concentrations greater than 1,000 mg/L are present immediately downgradient from the source area. In 1991, TCE concentrations in water samples from most wells at the site are similar or slightly lower than those found previously.

The total estimated mass of dissolved TCE was calculated from results of six sets of synoptic water-quality analyses of samples collected during 1987-91. The estimated mass of dissolved TCE within the plume below the water table is about 1,000 kg. This mass is equal to about 660 L of pure TCE. The estimate of the mass of dissolved TCE within the plume appears to depend on the number of samples in which TCE concentrations exceeded 10,000 mg/L and the volume of ground water each of these samples is assumed to represent. As much as 60 to 70 percent of the total mass of dissolved TCE in the plume was estimated to be associated with wells in which TCE concentrations exceeded 10,000 mg/L.

ESTIMATED MASS DISTRIBUTION OF TRICHLOROETHYLENE AND PRELIMINARY SOLUTE MASS BALANCE

TCE may be present at the Building 24 research site in several phases: dissolved in water, as a vapor in the soil gas, and sorbed onto solid surfaces or associated with biota. TCE also may be present as a dense nonaqueous-phase liquid (DNAPL). No estimate of the amount of DNAPL TCE at the site has been made, however. The amounts of TCE in the dissolved, sorbed, and vapor phases for a block of aquifer and unsaturated zone immediately downgradient from Building 24 were estimated on the basis of actual measurements of TCE concentrations in samples of all three phases (fig. 2). Most of the mass of TCE in the system near Building 24 is associated with the sediments in both the unsaturated and saturated zones.

Figure 2. Mass distribution of trichloroethylene in the saturated and unsaturated zones immediately downgradient from Building 24 at Picatinny Arsenal, New Jersey. (50k)

A conceptual model of the physical, chemical, and biological processes that affect the transport and mass balance of TCE within the plume has been developed and presented previously by Imbrigiotta and Martin (1991). The physical processes of advection and dispersion in the saturated zone affect the movement of dissolved TCE and cause TCE to be removed from the system in the discharge to Green Pond Brook. TCE also is removed from the system by the biological process of anaerobic biotransformation (reductive dehalogenation) and the chemical processes of volatilization at the water table and sorption to saturated-zone sediments. Desorption of contaminated sediments can act as a source of TCE to the ground-water system. If TCE is present as a DNAPL, dissolution will result in an increase in dissolved TCE in the ground water.

Preliminary estimates of the fluxes of processes that affect the mass balance of TCE within the ground-water system at the Building 24 site are shown in figure 3. The estimated flux of TCE discharged to Green Pond Brook from the plume area, on the basis of measurements of TCE concentrations in ground water and base-flow discharge to the brook, is 1 to 2 mg/s. The flux of TCE volatilized from the water table is estimated to be about 0.1 mg/s on the basis of measured soil-gas TCE gradients and estimates of the physical charateristics of the unsaturated zone. Although volatilization appears to be a minor factor affecting the mass balance of TCE in the ground-water system, Martin (this volume) states that results of a modeling sensitivity analysis show volatilization to be an important mechanism for removing solutes and thereby affecting concentrations near the water table. Biotransformation probably is the mechanism by which most dissolved TCE leaves the ground-water system. First-order rate constants for TCE transformation ranging from less than 0.001 to 0.02/wk were estimated by Wilson and others (1991) on the basis of laboratory microcosm studies of soil from five sites within the plume area. By using the estimated mass of dissolved TCE, the flux of TCE lost from the plume through biotransformation is calculated to be in the range of 1 to 30 mg/s. Analogous first-order rate constants for TCE biotransformation calculated by Ehlke and others (1996) from field-measured TCE concentrations and time-of-travel data generally were higher than those measured in the laboratory experiments. Thus, the flux of TCE lost through biotransformation may actually be greater than that shown in figure 3.

Figure 3. Preliminary mass-balance estimates of fluxes of processes that effect the fate and transport of trichloroethylene in the ground-water system at Picatinny Arsenal, New Jersey. (50k)

Desorption of TCE from soils that have undergone long-term adsorption (years) and dissolution of DNAPL TCE probably are the processes by which most TCE enters the ground-water system. Because no estimate of the amount of DNAPL has been made, no estimate of TCE dissolution can be made. Three first-order rate constants of TCE desorption from shallow aquifer sediments at the arsenal made by Koller and others (1996) ranged from 0.003 to 0.015/wk. By assuming the estimated mass of TCE sorbed to the aquifer sediments to be three to four times the mass of TCE in the dissolved state, the estimated flux of TCE into the ground-water system through desorption is in the range of 15 to 85 mg/s. The rate constants were measured in laboratory flow-through columns by using uncontaminated water as the influent fluid. Desorption rates in the field probably would be lower where ground water containing TCE is flowing past the desorbing sediments. This flux estimate is made by assuming that, over long periods (years), the short-term desorption rate (weeks and months) is equal to the short-term adsorption rate.

CURRENT RESEARCH ACTIVITIES

The ongoing studies at the Picatinny Arsenal site fall into five major areas of research: (1) chemical processes affecting transport of chlorinated solvents in the saturated zone, (2) transport of chlorinated solvents in the unsaturated zone, (3) biotransformation of chlorinated solvents in the saturated zone, (4) solute-transport modeling of chlorinated solvents in the saturated zone, and (5) remediation processes for chlorinated solvents in the saturated and unsaturated zones. A brief summary of the current research findings on each of these subjects is given in the following sections.

Chemical Processes Affecting Transport of Chlorinated Solvents in the Saturated Zone

Flow-through columns were constructed with sediments from four sites within the TCE plume to determine the rate of TCE desorption in the saturated zone. Desorption appeared to occur at all sites in two stages--an initial, rapid stage (days to weeks) during which 1 to 10 percent of the total sorbed mass of TCE was released, followed by a slow stage (months to years) during which the remaining 90 to 99 percent was desorbed. A column experiment with sediment artificially contaminated in the laboratory for only 5 days showed the same two-stage desorption, but 65 to 70 percent of the TCE mass was desorbed in the initial, rapid stage, and the remaining 30 to 35 percent was desorbed in the slow stage. A one-dimensional model was developed to determine the desorption rates by simulating the concentrations measured in the desorption column experiments. Results of the model simulations compared well with the column data by simulating initial, fast-stage desorption as an equilibrium process and a second, slower-stage desorption as a kinetic process. Model-calculated long-term (slow-stage) desorption rates of TCE from Picatinny Arsenal soils ranged from 0.5 x 10^-8 to 2.5 x 10^-8/s.

The effect of air drying TCE-contaminated soils prior to extraction with methanol to determine the concentrations of sorbed TCE was investigated. Aquifer-sediment samples collected at five locations within the TCE plume were split into two fractions; one fraction underwent extraction while wet and the other underwent extraction after being air-dried overnight. Concentrations of TCE in the wet soils were corrected for TCE in the soil moisture by subtracting the results of analyses of the ground-water samples. Comparison of the extraction results for air-dried and wet soils showed that, for four of the five samples, the concentration of sorbed TCE in the air-dried soils was not significantly different from that in the wet soils.

Transport of Chlorinated Solvents in the Unsaturated Zone

A study was conducted to quantify gas-water mass-transfer rates at the low flow rates of infiltrating water encountered in the unsaturated zone at the Building 24 site. This was done by conducting a field experiment under steady-state infiltration conditions and by using a mathematical model to simulate the results. A gas- and aqueous-phase transport model with desorption simulated as a constant-flux source of TCE at all depths was capable of simulating the field data. The gas-water mass-transfer rate constant used in the model was 4.5 x 10^-6/hr. Equilibrium between the gas- and water-phase concentrations of TCE was not observed during infiltration at the field site. Mass-transfer limitations between the soil-water and soil- solid phases also were observed during infiltration.

Biotransformation of Chlorinated Solvents in the Saturated Zone

Rates of anaerobic biotransformation of TCE and cis-1,2-dichloroethylene (cisDCE) were estimated by using field measurements from selected sites along a flow path in the plume and time of travel for ground water between the sites. The first-order biotransformation rates for TCE calculated in this manner ranged from less than 0.001 to 0.08/wk, whereas the first-order biotransformation rates for cisDCE ranged from less than 0.001 to 0.03/wk. The field-calculated biotransformation rates for TCE generally were more rapid than the biotransformation rates measured previously in the laboratory soil microcosms (<0.01 to 0.02/wk) (Wilson and others, 1991). Field-calculated biotransformation rates for cisDCE generally were lower than the biotransformation rates measured in laboratory soil microcosms (<0.01 to 0.18/wk) (Ehlke and others, 1991). These results show that field TCE-concentration data and time-of-travel data can yield biotransformation-rate estimates for TCE and cisDCE that are usually the same order of magnitude as those measured in laboratory soil-microcosm studies.

Solute-Transport Modeling of Chlorinated Solvents in the Saturated Zone

A modified version of the USGS SUTRA transport code (Voss, 1984) is being used to simulate areally variable desorption, volatilization at the water table, and microbial degradation of TCE. The transport of degradation products, cisDCE and vinyl chloride, also is simulated. The reactive multispecies solute-transport code was previously described by Martin (1991). The modified code simulates the transport and reaction of any number of species at the same time.

Sensitivity simulations with higher simulated ground-water velocities than those in the calibrated model generally resulted in increased simulated concentrations of TCE and decreased simulated concentrations of the degradation products. Sensitivity analysis on dispersivity showed that the simulated concentrations were too high or too low compared to measured concentrations when dispersivity rates were other than those used in the calibrated model. Results of sensitivity simulations run with various desorption and degradation rates generally showed that use of the laboratory estimates provided reasonable simulated concentrations.

Remediation Processes for Chlorinated Solvents in the Saturated and Unsaturated Zones

The feasibility of using aerobic cometabolic biotransformation of gas-phase TCE in the unsaturated zone as a remediation process was tested. In this process TCE is degraded as a consequence of stimulating methane degradation. Soil cores were collected near the contaminant source, where the soil-gas concentration of TCE was greatest (43 mg/L), and were used to construct soil microcosms. Results of the soil-microcosm study showed that biotransformation of TCE was rapid (16 (mg/L)/d) in acclimated soil having a 1.2-percent methane headspace. Thus, this remediation process is feasible at the Picatinny Arsenal site and occurs at a much faster rate than anaerobic TCE biotransformation processes. Further studies to optimize the TCE, methane, oxygen, and nutrient concentrations are planned.

A study was begun to determine whether the addition of the nonionic surfactant Triton X-100 to ground water can artificially increase the rate of TCE mass transfer from aquifer sediments was begun. Soil samples from the field were used in laboratory experiments conducted with continuous-flow stirred tank reactors, with and without Triton X-100. Preliminary results indicate that the rate of desorption is increased by 15 to 20 percent by the presence of Triton X-100 in the aqueous phase. Two possible mechanisms that could be responsible for the increased desorption rate are proposed: (1) the addition of Triton X-100 above its critical micelle concentration increased the apparent water solubility of TCE and thus increased the concentration gradient between the sorbed and aqueous phases, or (2) the presence of Triton X-100 increased the mass-transfer coefficient. Future experiments are planned to determine precisely the mechanism affecting the rate of desorption.

SUMMARY

The results of the ongoing interdisciplinary studies at the Picatinny Arsenal research site in north-central New Jersey have been synthesized to yield preliminary estimates of the TCE mass distribution and the TCE mass balance (transport fluxes) in the ground-water system. Contaminated sediments are the primary repository for TCE in both the saturated and unsaturated zones. Desorption of TCE from contaminated sediments is a significant long-term source of TCE to the ground-water system. Anaerobic biotransformation of TCE apparently is the most important process for removal of TCE from the ground-water system. These results can be used to guide future investigations of other sites contaminated with chlorinated solvents. In addition, some of the results of the remediation studies may help in the eventual cleanup of such sites.

Plans for future work at Picatinny Arsenal include (1) determination of the presence or absence of DNAPL TCE at the site and its relative importance as a source of TCE compared to desorption; (2) application of the modified solute-transport model to test hypotheses of plume formation, plume aging, and the expected effectiveness of different remediation processes in cleaning up the plume; (3) direct measurement of the TCE flux volatilizing through the unsaturated zone; (4) field testing of surfactants as a method to enhance desorption of TCE from contaminated sediments; (5) application of aerobic cometabolic biotransformation of TCE in the field to test its effectiveness as a remediation process at pilot scale; and (6) application and evaluation of other remediation technologies for chlorinated solvents at the field scale.

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