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Laboratory, field, and digital solute-transport-modeling studies demonstrate that microorganisms indigenous to the shallow ground-water system at Laurel Bay Exchange, Marine Corps Air Station Beaufort, South Carolina, can degrade petroleum hydrocarbons in gasoline released at the site. Microorganisms in aquifer sediments incubated in the laboratory under aerobic and anaerobic conditions mineralized radiolabeled carbon 14-toluene to 14C-carbon dioxide with first-order rate constants of Kbio -0.640 per day and Kbio = -0.003 per day, respectively. Digital solute-transport modeling using the numerical code SUTRA revealed that anaerobic biodegradation of benzene occurs with a first-order rate constant near Kbio = -0.00025 per day. Sandy aquifer material beneath Laurel Bay Exchange is characterized by relatively high hydraulic conductivities (K^ = 8.9 to 17.3 feet per day), average ground-water flow rate of about 60 feet per year, and a relatively uniform hydraulic gradient of 0.004 feet per foot. The sandy aquifer material also has low adsorptive potentials for toluene and benzene (both about K^ = 2.0 x 10"9 cubic feet per milligram), because of the lack of natural organic matter in the aquifer. The combination of this ground-water-flow rate and absence of significant adsorptive capacity in the aquifer permits toluene and benzene concentrations to be detected downgradient from the source area in monitoring wells, even though biodegradation of these compounds has been demonstrated. Solute-transport simulations, however, indicate that toluene and benzene will not reach the Broad River, the nearest point of contact with wildlife or human populations, about 3,600 feet west of the site boundary. These simulations also show that contamination will not be transported to the nearest Marine Corps property line about 2,400 feet south of the site. This is primarily because the source of contaminants has essentially been removed, and the low adsorptive capacity of the aquifer sediments has prevented the occurrence of an adsorbed, continuous source of petroleum hydrocarbons. Therefore, digital simulations of toluene and benzene transport at Laurel Bay Exchange indicate that intrinsic bioremediation could be a successful remediation alternative for prohibiting transport of dissolved toluene and benzene to the Broad River. INTRODUCTION An unknown volume of gasoline from the underground storage tank system at the Laurel Bay Exchange, Marine Corps Air Station (MCAS) Beaufort, S.C., was detected in the subsurface in September 1991. A contamination assessment made during March and April 1993 indicated that petroleum hydrocarbon concentrations in soils and ground water exceeded maximum contaminant levels (MCL's) for ground water established by the South Carolina Department of Health and Environmental Control (SCDHEC) (U.S. Navy Department, 1993). Recommendations were made to reduce these concentrations to compliable levels and included (1) tank excavation (source removal) and thermal treatment (incineration) of the removed soil, (2) ground-water extraction and reinjection, and (3) enhancement of naturally occurring biodegradation through engineering processes at the site (U.S. Navy Department, 1993). The tanks and surrounding sediments were excavated and removed in 1992 pursuant to SCDHEC regulations (F. Araico, Marine Corps Air Station Beaufort, written commun., 1994). The U.S. Geological Survey (USGS), in cooperation with the Southern Division, Naval Facilities Engineering Command, U.S. Navy, and MCAS Beaufort quantified hydrologic, geochemical, and microbial data in sediments from Laurel Bay Exchange to determine if contaminant transport will occur to pre-defined points-of-contact (POC) such as adjacent surfacewater bodies. The objective of this study is to establish a framework for assessing intrinsic bioremediation or a remedial alternative consistent with recent guidance documents prepared by the SCDHEC. Purpose and Scope The results presented in this report summarize a laboratory-, field-, and numericalmodeling-based study focused on the assessment of intrinsic bioremediation of gasoline in the shallow aquifer at Laurel Bay Exchange, MCAS Beaufort, S.C. (hereafter referred to as either Laurel Bay or the site). This study provides the necessary criteria to support intrinsic bioremediation as a feasible remedial strategy compared to the more costly, engineered alternatives offered in the Navy's original assessment report. This study consisted of (1) delineating the 1994 location of two different hydrocarbon plumes at the site (benzene, toluene, ethylbenzene, and total xylenes BTEX, and the gasoline additive methyl-tert-butyl-ether ~ MTBE); (2) assessing the hydrologic conditions in the shallow aquifer system, and identifying potential points of ground-water discharge that coincide with points-of-contact (POC) or MCAS property boundaries; (3) determining the rate of toluene degradation under aerobic and anaerobic aquifer conditions using radiolabeled toluene, and the degree of toluene adsorption onto aquifer sediments; (4) using a digital solute-transport model to estimate the anaerobic degradation rate of benzene, and a laboratory study to assess the degree of benzene adsorption onto aquifer sediments; and (5) conducting a digital solute-transport analysis designed to estimate rates of toluene and benzene transport in the aquifer in order to estimate the extent of toluene and benzene transport to delineated POC or property lines by incorporating data gathered in steps 1-4. Description of Study Area The Laurel Bay Exchange, MCAS, in Beaufort, S.C., is located on Port Royal Island, in the Sea Islands of the Atlantic Coastal Plain physiographic province (fig. 1). The service station at the site handled gasoline that leaked from underground storage tanks into the silty sand sediments that comprise the Pleistocene to Holocene surficial terrace deposits on Port Royal Island. The surface sediments at the site, to a depth of around 40 ft, consist of Holocene (<7,000 years Before Present) fine-grained sand and represent a back-barrier island depositional fades. The lithology of these sediments is shown in a representative lithologic log of a boring near LB-EX-14 (fig. 2). The uncontaminated sediments at the site are comprised of fine-to-coarse grained sands and some silt coated with ferric oxyhydroxides. This indicates the relatively shallow depth of the ground water and its saturation with dissolved oxygen. The amount of ferric oxyhydroxide coating diminishes, however, as depth and petroleum hydrocarbon concentrations increase. Framework for Assessing Intrinsic Bioremediation The efficiency of intrinsic bioremediation to limit contaminant migration in ground-water systems can be assessed by quantitatively comparing rates of contaminant transport to rates of biodegradation. If rates of transport by ground water are fast relative to rates of biodegradation, then contaminants may migrate with ground water and potentially reach POC with human or wildlife populations. Conversely, if transport rates are slow relative to biodegradation rates, contaminant migration will be more confined and be less likely to reach POC. In either case, the efficiency of intrinsic bioremediation can be assessed at a particular site by evaluating the presence or absence of contaminant transport to pre-determined POC. Thus, this assessment includes determining the hydrologic (rates of ground-water flow), geochemical (adsorption properties of the aquifer material), microbial (rates of degradation), and demographic (POC) constraints in the system. The U.S. Geological Survey (USGS), in cooperation with the Southern Division, Naval Facilities Engineering Command, U.S. Navy, and MCAS Beaufort, has developed a framework for assessing the efficiency of intrinsic bioremediation that is based on these components. In this framework, hydrologic, geochemical, and microbial data are integrated using a digital solutetransport model to determine rates of contaminant transport from a source area to pre-defined POC such as adjacent surface-water bodies. This framework is consistent with recent guidance documents prepared by the SCDHEC and is designed for application at sites where ground-water contamination is the principle issue of concern.

water-resources investigations report

Assessment of intrinsic bioremediation of gasoline contamination in the shallow aquifer, Laurel Bay Exchange, Marine Corps Air Station Beaufort, South Carolina