Environ. Sci. Technol. 2001, 35, 261-269
Stable Carbon Isotope Evidence for Intrinsic Bioremediation of Tetrachloroethene and Trichloroethene at Area 6, Dover Air Force Base B. SHERWOOD LOLLAR* AND G. F. SLATER Department of Geology, University of Toronto, Toronto, Ontario, Canada M5S 3B1 B. SLEEP Department of Civil Engineering, University of Toronto, Toronto, Ontario, Canada M5S 1A4 M. WITT AND G. M. KLECKA Toxicology & Environmental Research and Consulting, The Dow Chemical Company, 1803 Building, Midland, Michigan 48674 M. HARKNESS AND J. SPIVACK General Electric Research and Development Center, Schenectady, New York 12301
Area 6 at Dover Air Force Base (Dover, DE) has been the location of an in-depth study by the RTDF (Remediation Technologies Development Forum Bioremediation of Chlorinated Solvents Action Team) to evaluate the effectiveness of natural attenuation of chlorinated ethene contamination in groundwater. Compound-specific stable carbon isotope measurements for dissolved PCE and TCE in wells distributed throughout the anaerobic portion of the plume confirm that stable carbon isotope values are isotopically enriched in 13C consistent with the effects of intrinsic biodegradation. During anaerobic microbial reductive dechlorination of chlorinated hydrocarbons, the light (12C) versus heavy isotope (13C) bonds are preferentially degraded, resulting in isotopic enrichment of the residual contaminant in 13C. To our knowledge, this study is the first to provide definitive evidence for reductive dechlorination of chlorinated hydrocarbons at a field site based on the δ13C values of the primary contaminants spilled at the site, PCE and TCE. For TCE, downgradient wells show δ13C values as enriched as -18.0‰ as compared to δ13C values for TCE in the source zone of -25.0 to -26.0‰. The most enriched δ13C value on the site was observed at well 236, which also contains the highest concentrations of cis-DCE, VC, and ethene, the daughter products of reductive dechlorination. Stable carbon isotope signatures are used to quantify the relative extent of biodegradation between zones of the contaminant plume. On the basis of this approach, it is estimated that TCE in downgradient well 236 is more than 40% biodegraded relative to TCE in the proposed source area. 10.1021/es001227x CCC: $20.00 Published on Web 12/01/2000
2001 American Chemical Society
Introduction Previously stable carbon isotope analysis focused on the isotopic characterization of CO2 or dissolved inorganic carbon to assess bioremediation. This approach monitored changes in the δ13C value of the bulk carbon pool due to the addition of CO2-derived from biodegradation of hydrocarbon contaminants (1, 2). The results were often ambiguous, however, due to the difficulty in constraining multiple subsurface sources and sinks of CO2 and due to the effects of indigenous respiration, which can mask changes in δ13C values of the bulk carbon pool related to contaminant degradation unless such degradation rates are very high. High sensitivity GC/C/IRMS (gas chromatograph/combustion/isotope ratio mass spectrometry), through its ability to characterize the stable carbon isotope composition of individual compounds at low dissolved concentrations in groundwater, provides a new tool for investigation of hydrocarbon contamination. Early studies focused on characterizing differences in the isotopic signatures of contaminants from different sources in order to establish the potential range in δ13C values (3-7). Provided that this isotopic signature is conserved in the subsurface, stable carbon isotope analysis has the potential to identify different sources of contamination. Laboratory experiments have assessed the kinetic carbon isotope effects associated with key subsurface processes controlling contaminant fate and transport in groundwater. These studies showed that stable carbon isotope values are indeed largely conserved during nondegradative processes such as sorption (7, 8), dissolution (4, 9) and volatilization (7, 9, 10). Carbon isotope fractionation associated with these processes is small and, in many cases, less than analytical uncertainty. In contrast, laboratory studies have shown that for chlorinated hydrocarbons, degradation can involve large and reproducible kinetic isotope effects, producing systematic changes in the δ13C values of the residual contaminant (1116). During anaerobic microbial reductive dechlorination of chlorinated hydrocarbons, the light (12C) versus heavy isotope (13C) bonds are preferentially degraded, resulting in isotopic enrichment of the residual contaminant in 13C. Laboratory studies have shown that the magnitude of fractionation decreases with increasing degree of chlorination of the chlorinated ethene. Maximum isotopic enrichment in the residual contaminant relative to its initial isotopic composition of up to 5-6‰ has been observed for experiments involving degradation of tetrachloroethene (PCE) (15, 17). Enrichments of up to 14‰ have been observed for experiments focusing on degradation of trichloroethene (TCE) (13, 15-17). Experiments that characterized the isotopic composition of both the degrading parent compound and daughter products indicate that, when initially produced, daughter products are more isotopically depleted in 13C than the parent compound, reflecting the preferential biodegradation of light 12C-containing molecules. The daughter products begin to show the characteristic isotopic enrichment trends very quickly, however, as they themselves undergo subsequent biodegradation to the next less chlorinated compound (13, 15, 17). In contrast to the source differentiation application described above, the objective of this study was to determine if isotopic fractionation and the enrichment trends produced by biodegradation in the laboratory during reductive dechlorination of PCE and TCE could be used as a means of * Corresponding author e-mail:
[email protected]; telephone: (416)978-0770; fax: (416)978-3938. VOL. 35, NO. 2, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Site map of Area 6, Dover Air Force Base, showing names of wells in the anaerobic part of the plume sampled for the isotopic study. Source areas identified in previous work are shown as ovals (20). The lateral extent of contamination of the deep aquifer is also shown. Transect I refers to a sampling transect approximately downgradient and upgradient of source area 1 (wells 211/212), encompassing downgradient wells 236 and 353, and upgradient wells 325, 323, and 322. Transect II refers to a sampling transect approximately downgradient and upgradient from source area 2 (well 326), encompassing downgradient wells 316, 315, 235, ir7, 354, 355, and upgradient well 324. identifying and monitoring intrinsic biodegradation in the field. Sturchio et al. (18) used variations in stable Cl isotope ratios of TCE to evaluate natural attenuation in an aerobic aquifer. Hunkeler et al. (15) focused on using the large carbon isotope fractionation associated with cis-DCE, VC, and ethene to verify that extensive reductive dechlorination was occurring in a PCE-contaminated aquifer. To our knowledge, the present study is the first to provide definitive evidence for reductive dechlorination of chlorinated hydrocarbons at a field site based on the δ13C values of the primary contaminants spilled at the site, PCE and TCE.
Site Background Area 6 at Dover Air Force Base (Dover, DE) is part of the industrial portion of the base where solvents were commonly used for aircraft and vehicle maintenance, painting, stripping, and welding. Spills and past waste disposal practices from multiple sources of varying and unknown ages resulted in introduction of a variety of contaminants into the groundwater. While some petroleum hydrocarbon sources exist, the majority of the contamination at the identified source areas consisted of chlorinated solvents, both PCE and TCE (19, 20). While Area 6 contains a number of different source areas and resultant plumes, this study focused on two major source areas (1 and 2) and the contamination upgradient and downgradient from these source areas (Figure 1). Since 1995, Area 6 has been the location of an in-depth study by the RTDF to evaluate the effectiveness of intrinsic bioremediation (natural attenuation) of PCE and TCE contamination in groundwater and sediments. This location was chosen because of its relatively homogeneous geology (12-17 m of unconsolidated Pleistocene coarse sands and gravelssthe Columbia Formation), the presence of a shallow aquifer within these deposits underlain by a confining layer, and indications that sufficient biological activity exist in the aquifer (19, 20). Groundwater flows from a recharge area in the northern portion of the study area toward the Saint Jones River to the south and southwest at a rate of approximately 14.4 m/year (Figure 1) (21). 262
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Summary of Conceptual Model for the Site. Results of the geological, hydrogeological, and biogeochemical study and computer modeling of the site have been previously published (20, 21). Introduction of PCE and TCE into the permeable soil overlying the aquifer resulted in vertical migration of DNAPL (dense nonaqueous phase liquid) to the deep zone of the aquifer (21, 22). As PCE and TCE were the only chlorinated solvents used in Area 6, the presence of daughter products (cis-DCE, VC, and ethene) in the deep aquifer is indicative of biological reductive dechlorination. Additional lines of biogeochemical evidence for intrinsic bioremediation via reductive dechlorination include the delineation of the three plumes of daughter products entirely within the PCE and TCE plumes and largely within the anaerobic zone of the aquifer. Within the anaerobic zone, increasing soluble chloride ion concentrations correlate with decreases in chlorinated ethene concentrations, and high dissolved H2 levels (>10nM) correlate with high dissolved CH4 concentrations (100-500 µg/L) and depleted levels of dissolved oxygen (
