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 on the Transport, Microbial Degradation, and Remediation of Hydrocarbons at a Subsurface Gasoline-Spill Site in Galloway Township, New Jersey

CONTENTS

• ABSTRACT
• INTRODUCTION
• HYDROCARBON CONTAMINATION GEOCHEMISTRY
• ESTIMATION OF MICROBIAL DEGRADATION RATES OF HYDROCARBONS ON THE BASIS OF GAS TRANSPORT RATES IN THE UNSATURATED ZONE
• VAPOR-EXTRACTION-REMEDIATION DESIGN
• REFERENCES

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.

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., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings 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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Method for estimating rates of microbial degradation of hydrocarbons based on gas transport in the unsaturated zone at a gasoline-spill site in Galloway Township, New Jersey, in Mallard, G.E., and Aronson, D.A., eds., U.S. Geological Survey Toxic Substances Hydrology Program--Proceedings of the technical meeting, Monterey, California, March 11-15, 1991: U.S. Geological Survey Water-Resources Investigations Report 91-4034, p. 250-255.

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