Stable carbon isotope biogeochemistry of a shallow sand aquifer contaminated with fuel hydrocarbons
Introduction
In the subsurface environment, contamination by organic compounds often results in the complete consumption of available O2 by indigenous microorganisms and the development of anaerobic conditions (Goerlitz et al., 1985, Cozzarelli et al., 1990, Cozzarelli et al., 1994, Chapelle et al., 1995). The microbially mediated degradation processes change both spatially (Fang et al., 1997) and temporally (Chapelle et al., 1995) in the subsurface. Under oxic conditions, microorganisms use dissolved O2 as the terminal electron acceptor in the oxidation of organic contaminants. Under anaerobic conditions, microorganisms capable of utilizing alternative electron acceptors (NO3, Fe(III), SO4 and CO2) become important (Evans, 1977). The determination of predominant terminal electron accepting processes (TEAPs) in contaminated aquifers is important in design and implementation of intrinsic or engineered bioremediation schemes. Stable C isotope ratios (δ13C) of soil gas and ground-water dissolved inorganic C (DIC) have been used as monitoring tools to determine terminal electron accepting processes and biodegradation of petroleum hydrocarbons in aquifers (Aggarwal and Hinchee, 1991, National Research Council, 1993, Aggarwal et al., 1997, Landmeyer et al., 1996, Conrad et al., 1997 and references cited therein). It has also been noticed that stable isotope measurement alone can yield ambiguous results because of the overlapped δ13C values of petroleum hydrocarbons and indigenous plant material Suchomel et al., 1990, Aggarwal et al., 1997. As such, the combined use of δ13C and 14C measurement of soil gas and ground-water DIC were proved to be more effective in hydrocarbon source apportionment and demonstration of biodegradation Suchomel et al., 1990, Conrad et al., 1997. The present study was conducted in an aquifer with high carbonate content, active recharge, and historic contamination of JP–4 fuel hydrocarbons. The purpose of this paper is to show that measurement of δ13C of ground-water DIC combined with biogeochemical measurements can be a useful tool to monitor TEAPs in the subsurface and to identify hydrologic and microbiological processes that affect the δ13C of ground water DIC.
The study area was the KC–135 Crash Site at the former Wurtsmith Air Force Base (WAFB), located near the town of Oscoda in northeastern Lower Michigan (Fig. 1). WAFB is bounded by a line of 24-m-high bluffs in the west, by the Van Etten Lake to the northeast, and by the Au Sable River to the south, which flows eastward and discharges into Lake Huron. The Base lies on an 8 km wide plain with the altitude of the land surface ranging from 177 to 229 m above mean sea level. The KC–135 Crash Site is located in the western portion of the Base (Fig. 1a). The aquifer was contaminated by JP–4 fuel resultant from the tragic crash of a KC–135 fuel tanker in October, 1988. At least 3000 gallons of JP–4 were spilled at the site, an unknown quantity of which burned as a result of the accident, and the remainder percolated into the ground.
The aquifer sediments consist of 30 to 76 m of unconsolidated glacial, deltaic and lacustrine deposits overlying the Mississippian-age Marshall Formation sandstone and Coldwater Shale bedrock (Rama Rao, 1992). The shallow deposits, extending from the surface to depths of approximately 9 to 24 m below land surface (bls), consist of fine to very coarse sands with occasional gravels. These deposits form the regional water table aquifer which overlays an impermeable lacustrine clay layer. The deeper aquifer, consisting of glacial till deposits (clay-rich silt, sand, and gravel) is separated from the shallow aquifer by the lacustrine clay and silt layer (Fig. 1b). This layer of fine sediment forms an aquitard that restricts the downward migration of contaminant plumes (ICF, 1994). However, the shallow aquifer is unconfined. Because of the flat topography and well-drained nature of the land, precipitation easily infiltrates the sandy soil (USAF, 1993). The depth of the ground-water table ranges from less than 3 m bls at the western part of the base to 7.6 m bls near Van Etten Lake (Fig. 1a).
The KC–135 Crash Site is located near the main runway of WAFB (Fig. 1), with a surface elevation of about 190.5 m above mean sea level (msl). Recent ground-water level measurements and historic data (USGS, 1990) indicate that ground-water flow at the KC–135 Crash Site is consistent with the regional ground-water flow direction (Fig. 1), southeast toward the Au Sable River, with a hydraulic gradient of 0.0024 m/m. Two-dimensional finite-difference flow models have been used to describe ground-water movement and to simulate water levels (Stark et al., 1983). Results indicate that ground-water flow was as low as 0.03 m/day and as high as 0.3 m/day, depending on the location. Slug tests conducted by WW Engineering and Science (1993) suggest an average hydraulic conductivity of 21 m/day. The ground-water flow velocity in the shallow aquifer is calculated to be 0.17 m/day, assuming an effective porosity of 0.3.
Section snippets
Methodology
This study was a part of the multidisciplinary study on the fuel hydrocarbons-contaminated aquifer at the KC–135 site. Samples for chemical analyses of fuel hydrocarbons and aromatic acids, mineralogical analyses and stable C isotope determinations were taken in June 1996, and samples for lipid analysis and CH4 determinations were collected in December 1995 and March 1997, respectively.
Aquifer mineralogy
Clay minerals constitute <4% of the aquifer matrix, as indicated by the clay index (Table 1). The sand fraction of the aquifer solids consist of 87% quartz, and average 4.5% of calcite and dolomite (Table 1). Generally, carbonate content varied more in a vertical than in a horizontal direction, with higher carbonate abundance in the deeper portions of the aquifer. For example, the surface layers of the aquifer (<3 m, ML–13 and ML–14) contain less than 1% calcite and dolomite, which increased in
Stable carbon isotopic geochemistry of dissolved inorganic carbon
The absence of contaminants in the upgradient zone, low DIC content in ground water, the oxygenated environment, and δ13C values of DIC suggest that the upgradient zone was unimpacted by the spilled JP–4 fuel and represents `background' conditions in the aquifer. It is likely that the aerobic oxidation of soil organic matter and dissolution of carbonates are the controlling factors of DIC δ13C values in the unimpacted upgradient zone. Historical and current water levels at the site indicate
Summary and conclusions
The C isotopic composition of DIC in a jet fuel-contaminated aquifer was studied to evaluate interrelationships between microbial oxidation of fuel hydrocarbons and aquifer biogeochemistry. In the upgradient zone, ground-water chemistry reflected the `background' characteristics of the aquifer, and δ13C of ground-water DIC was controlled primarily by rainwater infiltration and dissolution of soil gas CO2 and aquifer carbonates. Microbial oxidation of fuel hydrocarbon in the source zone under SO
Acknowledgements
The authors thank Mark Blankenship of USEPA–RSKERL for valuable assistance in the design and execution of the project, Mark Henry, Charles L. Major and Tony Brown for assistance in field sampling, and Nitin Barad, Tim Baker, and Hongming Chen for laboratory assistance, and Michael Gerdenich for collecting vegetation samples. We are grateful to Anton Reznicek of University of Michigan for plant species identification, Richard Cahill and Randall E. Hughes of the Illinois Geological Survey for
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