Stable Carbon Isotope Evidence for Intrinsic Bioremediation of Tetrachloroethene and Trichloroethene at Area 6, Dover Air Force Base

Abstract

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 delta13C values of the primary contaminants spilled at the site, PCE and TCE. For TCE, downgradient wells show delta13C values as enriched as -18.0/1000 as compared to delta13C values for TCE in the source zone of -25.0 to -26.0/1000. The most enriched delta13C 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.

Assessment of Degradation Pathways in an Aquifer with Mixed Chlorinated Hydrocarbon Contamination Using Stable Isotope Analysis

“…On the basis of published isotopic enrichment factors for the PCE-TCE dechlorination step (Table 1), it is unlikely that TCE with a δ 13 C <-45‰ can be produced from PCE with a δ 13 C of -28.7‰. Furthermore, the δ 13 C of TCE at the site (-36.2 to -45.0‰) is clearly distinct from TCE observed at other sites (δ 13 C >-30.7) where PCE with an isotopic signature comparable to this site acted as its source (1,10,11). Thus, TCE in the groundwater was most probably derived from 1,1,2,2-PCA, which is consistent with the depleted δ 13 C values of -39.1‰ of 1,1,2,2-PCA in the source area.…”

Stable carbon isotope analysis to distinguish biotic and abiotic degradation of 1,1,1-trichloroethane in groundwater sediments

“…Carbon isotope analysis has been used to demonstrate reductive dechlorination of chlorinated ethenes in groundwater (Hunkeler et al, 1999(Hunkeler et al, , 2011Sherwood Lollar et al, 2001;Imfeld et al, 2008) and streambed sediments (Abe et al, 2009). Sherwood Lollar et al (2010 studied the isotope fractionation during biodegradation of 1,1,1-TCA and 1,1-DCA by a dehalobacter-containing mixed culture in whole-cell and cell-free extract microcosms.…”

Evaluating the fate of chlorinated ethenes in streambed sediments by combining stable isotope, geochemical and microbial methods

“…Furthermore, assessment of concentration patterns can be ambiguous since they are also affected by physical processes such as dilution, especially at the surface-groundwater interface, and because non-unique end products may be formed such as CH 4 and CO 2 (Bradley and Chapelle, 2000). Recently compound-specific isotope analysis was proposed as a complementary tool to assess the degree of reductive dechlorination (Hunkeler et al, 1999;Sherwood Lollar et al, 2001;Hunkeler et al, 2002;Song et al, 2002). The method relies on differences in reaction rates between molecules with light and heavy isotopes, which lead to the enrichment of heavy isotopes in the remaining substrate.…”