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
Thomas E. Imbrigiotta (U.S. Geological Survey, 810 Bear Tavern
Rd., Suite 206, W. Trenton, NJ 08628) and Mary Martin (U.S. Geological Survey,
810 Bear Tavern Rd., Suite 206, W. Trenton, NJ 08628)
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.
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
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
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
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
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.
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
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