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
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Overview of Research on the Transport, Microbial Degradation,
and Remediation of Hydrocarbons at a Subsurface Gasoline-Spill Site in
Galloway Township, New Jersey
by
Arthur L. Baehr (U.S. Geological Survey, West Trenton, N.J) and
Jeffrey M. Fischer (U.S. Geological Survey, West Trenton, N.J)
CONTENTS
Abstract
Since 1988, the U.S. Geological Survey has conducted research on the
fate and transport of hydrocarbons at a site of subsurface gasoline contamination
in Galloway Township, New Jersey, Work has focused on three interrelated
areas of research: ground-water contaminant geochemistry, estimation of
microbial degradation rates of hydrocarbons on the basis of rates of gas
transport in the unsaturated zone, and vapor-extraction remediation design.
Contaminant-geochemistry work has produced a water-quality data base that
indicates zones of aerobic degradation adjacent to zones of anaerobic degradation
with sharp chemical-concentration gradients in narrow interfacial zones.
The analysis of gas transport in the unsaturated zone has provided a method
for quantifying rates of aerobic hydrocarbon biodegradation in the capillary
zone and shallow ground water. Mathematical models have been developed to
simulate vapor-extraction remediation and to optimize system design. Research
activity in each area is detailed and the project bibliography is updated.
INTRODUCTION
In 1988, the U.S. Geological Survey (USGS) began a field-oriented research
project at a site of subsurface gasoline contamination from a leaky underground
storage tank on a farm in Galloway Township, New Jersey (fig. 1). The objective
of the original project, which was conducted in cooperation with the New
Jersey Department of Environmental Protection and Energy, was to determine
factors controlling the performance of vapor-extraction remediation. During
the installation and sampling of the initial monitoring network in the fall
of 1988, it became clear that the site afforded a unique opportunity to
conduct field and laboratory research on the fate and transport of hydrocarbons
in the subsurface.
The scope of the project was expanded to allow for more extensive characterization
of contamination in the unsaturated zone and shallow ground water before
site remediation. The project data base provides for evaluation of the natural
attenuation and microbial degradation of hydrocarbons. In 1989, the site
was selected as a USGS Toxic Substances Hydrology program research project.
The project workplan in its formative stage was presented at the technical
meeting held in Monterey, Calif., in 1991. The purpose of this paper is
to summarize research activity and update the project bibliography to include
results reported since the Monterey meeting.
The Galloway Township study site is located in the New Jersey Coastal
Plain about 30 mi northwest of Atlantic City (fig. 1). Unconsolidated sediments
beneath the site are mostly medium-grained sands, with some clay layers.
The unsaturated zone is typically about 9 ft thick. Its lower boundary is
a perched water table which, depending on precipitation history, may be
coincident with the regional water table. Ground water in the regional aquifer
usually flows to the southeast. Fischer, Smith, and Baehr (1996) describe
the hydrogeology of the site. The monitoring network (fig. 1) consists of
27 nests of vapor probes, 13 ground-water monitoring wells, 2 multilevel
ground-water samplers, 5 vapor-extraction wells, 2 neutron-probe access
pipes, and a nest of thermistors (Fischer and others, 1991). The composition
of unsaturated-zone gases is determined by analysis of gas samples collected
from the vapor-probe network (Baker and others, 1991). Ground-water contamination
and inorganic chemistry is determined by analyzing samples collected from
wells and submerged vapor probes (Baedecker and others, 1991; Cozzarelli
and others, 1991). Gibs and others (1993) report on the construction and
sampling of the multilevel samplers.
![baehr.final.fig1b](baehr.final.fig1b.gif)
Figure 1. Location of subsurface
gasoline-spill study site in Galloway Township, N.J. (33 k)
HYDROCARBON CONTAMINATION GEOCHEMISTRY
Results of initial studies at the site revealed that gasoline
was present as a separate-phase liquid on the regional
water table in the immediate vicinity of well DEP-1 and
on the perched water table in the vicinity of the exhumed
tank (fig. 1). Dissolved hydrocarbons identified include
benzene, toluene, C2-C3-, C4-alkylbenzenes,
and napthalene (Baedecker and others, 1991; Phinney and
Cozzarelli, 1996). The areawide distribution of contaminants
was described by Baedecker and others (1991) and Fischer, Smith,
and Baehr (1996). Contamination in perched water has
persisted at high concentrations in the vicinity of the exhumed
tank. Hydrocarbon contamination on the regional water
table is downgradient from the source and concentrations vary
with time, depending on the direction of ground-water flow. The
concentration of total aromatic hydrocarbons decreases
downgradient, as a result of dilution, volatilization, and biodegradation.
Ground water was sampled and analyzed to identify biogeochemical processes
occurring at the site and to identify degradation pathways. Baedecker and
others (1991) and Cozzarelli and others (1991), provide descriptions of
the sampling methods. Results of analyses for inorganic and organic chemical
compounds indicate active microbial degradation of hydrocarbons in ground
water. These data are reported by Baedecker and others (1991), Cozzarelli
and others (1991), Cozzarelli (1993), and Cozzarelli and Baedecker (1996).
Aerobic respiration and reduction of nitrate, sulfate, and iron oxides
by microbes have been identified as important processes for degrading hydrocarbons
at the site (Baedecker and others, 1991; Cozzarelli and others, 1991). The
perched water has been depleted of dissolved oxygen and nitrate, leaving
sulfate and iron reduction as the main biodegradation processes. In the
regional aquifer, aerobic respiration and nitrate reduction appear to be
the main hydrocarbon-degradation pathways as evidenced by results of multilevel
sampling beneath the water table. Results of microbial counting showed higher
numbers of hydrocarbon-degrading microorganisms in contaminated ground water
and sediment than in uncontaminated ground water and sediment (Mills and
Randall, 1991). The fate of organic acids, which are intermediate hydrocarbon-degradation
products, depends on the availability of electron acceptors for anaerobic
degradation processes (Cozzarelli, 1993).
ESTIMATION OF MICROBIAL DEGRADATION RATES
OF HYDROCARBONS ON THE BASIS OF GAS TRANSPORT RATES IN THE UNSATURATED ZONE
Environmental regulators concerned with the effect of gasoline spills
on ground-water quality have recently acknowledged that microbial degradation
of hydrocarbons is a significant mechanism for attenuating subsurface gasoline
contamination. Methods for quantifying degradation rates at a given site,
however, have not been established. The approach developed for the Galloway
Township site is to identify stoichiometric relations among oxygen consumption,
carbon dioxide production, and hydrocarbon degradation. Once these relations
are established, fluxes of oxygen and carbon dioxide through the unsaturated
zone are used to estimate in situ biodegradation rates.
Baehr and others (1991) provide an overview of this method. Oxygen diffuses
toward the water table where it is used by microbes to aerobically degrade
hydrocarbons. Carbon dioxide, which is an end product of this degradation,
diffuses toward the surface. At the Galloway Township site, hydrocarbons
are not detected in the gaseous phase in most parts of the unsaturated zone,
whereas the underlying perched water is virtually anoxic and contains high
concentrations of dissolved hydrocarbons (Cozzarelli and others, 1991; Cozzarelli,
1993). These data, combined with the presence of an abundance of hydrocarbon-degrading
bacteria at the water table (Mills and Randall, 1991), indicate that aerobic
hydrocarbon degradation occurs near the top of the perched water. Thus,
the rates of oxygen and carbon dioxide transport are related to the rate
of aerobic degradation of hydrocarbons.
The relation between the rates of oxygen consumption and carbon
dioxide production by hydrocarbon-degrading bacteria is
complicated by the oxidation of ferrous iron. Also, the presence
of intermediate metabolites, such as organic acids, detected
in the ground water (Cozzarelli, 1993) indicates that aerobic
degradation is not always complete. If these pathways
can be neglected for the purpose of determining an approximate
stoichiometry, then complete hydrocarbon mineralization
(for example, for toluene: C7H8 + 9O2
7CO2 + 6H2O) may provide a working approximation.
This working approximation was substantiated in the laboratory by conducting
respiration experiments with hydrocarbon-contaminated sediment (Noland,
1993; Noland and others, 1996). Furthermore, Lahvis (1993) determined that
the ratio of oxygen and carbon dioxide fluxes in the unsaturated zone above
contaminated perched water at the Galloway Township site was close to that
predicted by using the mineralization working approximation.
A value for the effective diffusion coefficient for a constituent in
unsaturated porous media is required for calculating diffusion rates through
the unsaturated zone. Cores of sediment were extracted from the unsaturated
zone at the Galloway Township site and a technique was developed for estimating
effective diffusion coefficients (Fischer, Baker, and Baehr, 1996). The
method involves measuring changes in tracer concentration along the length
of the core. A mathematical model developed by Baehr and Bruell (1990) is
used to calculate the effective diffusion coefficient.
Two types of field experiments were conducted to evaluate hydrocarbon-biodegradation
rates. The first experiment involves measuring oxygen and carbon dioxide
profiles at the site under natural conditions prior to remediation. The
second experiment involves monitoring the redistribution of gases after
a period of vapor extraction has caused oxygen concentrations to increase
and carbon dioxide concentrations to decrease in the unsaturated zone to
near-atmospheric levels. A vapor-transport model developed by Lahvis (1993)
is used to compute rates of gas movement from gas-phase concentration data.
Lahvis and others (1996) conclude that the rates of gas movement at the
Galloway Township site indicate that significant rates of hydrocarbon degradation
may be limiting the spread of the contaminant plume in ground water.
Water-quality data collected by sampling near the perched water table
indicate that the aerobic degradation rate is highest in a small vertical
interval between the top of the capillary zone and 1 ft below the water
table. This information leads to the formulation of the hypothesis that
coexisting conditions of high moisture content and high concentrations of
oxygen and hydrocarbons, which prevail only within this small vertical interval,
are more favorable for the aerobic degradation of hydrocarbons than those
in the relatively dry overlying unsaturated zone and underlying anoxic ground
water. Construction of a field experiment to study these conditions over
the indicated spatial scale was not possible as a result of perched water-table
altitude variations. Therefore, a laboratory method to emulate biochemical
conditions near the water table was developed (Baker, 1993). Sediment collected
just above the water table was packed in glass columns. The upper part of
the column was unsaturated and the column water level was maintained at
a constant altitude. Hydrocarbon concentration was maintained as a constant
level in the saturated lower part of the column. Concentrations of hydrocarbons,
oxygen, and other constituents can be controlled by manipulating the end
reservoirs. Analysis of these experiments with a one-dimensional vapor-transport
model is reported by Baker (1993) and Baker and Baehr (1996). Hydrocarbon-degradation
rates within sediments maintained near capillary-zone moisture conditions
were an order of magnitude higher than those in the drier overlying unsaturated
sediments. Neither hydrocarbon concentrations nor oxygen concentrations
were limiting factors in these experiments.
VAPOR-EXTRACTION-REMEDIATION DESIGN
Remedial action for spills of petroleum products generally include an
effort to physically recover product accumulated on the water table by pumping
or bailing. Substantial portions of the spill typically remain trapped in
residual saturations in the unsaturated and capillary zones as well as in
a smear zone in shallow ground water caused by fluctuating water-table altitudes.
This part of the spill can pose a long-term threat to ground-water quality,
because it is a source of hydrocarbons that can be transported in aqueous
and gaseous phases. Vapor-extraction technology has been applied successfully
to remove additional hydrocarbons from the residual saturation source. The
success of this technology is based on the significant volatility of gasoline
hydrocarbons and the ability to induce an air-flow field in the unsaturated
zone with combinations of dry wells and trenches.
Mathematical models of the air-flow field can be used to help design
a vapor-extraction system. The ground-water-flow simulator MODFLOW (McDonald
and Harbaugh, 1988) has been adapted for this purpose. This code, called
AIR3D, and a users' manual currently are in preparation for distribution
in the public domain. Application of the air-flow code results in the definition
of the air-flow field corresponding to a chosen configuration of extraction
and injection locations. Welty and others (1992; 1996) extended the modeling
tool by coupling the three-dimensional air-flow model with mathematical
programming to define an optimal configuration of extraction and injection
locations. Publication of the computer code that implements this coupling,
called OPTAIR, and a users' guide in the public domain is planned.
The permeability of unsaturated porous media to air is required information
for air-flow simulations. Baehr and Hult (1991) developed analytical solutions
to the equation defining air flow to or from a single well screened in an
unsaturated zone to determine air-phase permeability from results of pneumatic
tests conducted at a research site near Bemidji, Minn. A computer code that
implements the analytical solutions has been developed. The code, called
AIR2D, and a users' manual is currently in preparation for publication in
the public domain. Joss and others (1992) report on applications of this
parameter-estimation technique at the Galloway Township site.
A mathematical model for simulating chemical transport associated with
vapor extraction and bioventing has been developed by Joss (1993). The model
can be used to simulate three-dimensional, coupled transport of multiple
chemical species and is expected to be a useful tool in analyzing and designing
vapor-extraction, bioventing, and air-sparging remediation systems. This
model is summarized by Joss and Baehr (1996).
REFERENCES
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- Distribution of organic and inorganic constituents in ground water
at Galloway Township, New Jersey, in Mallard, G.E., and Aronson, D.A.,
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of the technical meeting, Monterey, California, March 11-15, 1991: U.S.
Geological Survey Water-Resources Investigations Report 91-4034, p. 287-293.
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Report 95-4015.
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