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.
Soil columns were constructed in support of the Remediation
Technologies Development Forum accelerated biodegradation study at Dover Air Force Base to evaluate the
impact of amendments on the anaerobic reductive
dechlorination of trichloroethene (TCE) in Dover soil.
Dechlorination of TCE to cis-dichloroethene (c-DCE) was
observed in the columns using lactate, lactate and methanol,
butyrate, glutamate and 1,2-propanediol, or toluene as
electron donors, in combination with vitamins and other
supplemental nutrients. However, the c-DCE formed was not
further dechlorinated using any of these amendments.
Subsequent inoculation of two columns with a competent, non-native TCE-dechlorinating culture resulted in the dechlorination of TCE to ethene after 30 days. Once the culture was
established, dechlorination of TCE to ethene was complete
in the first several centimeters of the columns at TCE
influent concentrations of 4 mg/L. The culture was also
able to dechlorinate TCE to ethene when TCE influent
concentrations were increased to 170 mg/L. These results
suggest that a critical bacterial population was missing
in these soils and that bioaugmentation is an appropriate
remedial strategy under such circumstances.
The biodegradation potential of [14C]dimethylsilanediol, the monomer unit of polydimethylsiloxane, in soils was investigated. Dimethylsilanediol was found to be biodegraded in all of the tested soils, as monitored by the production of 14CO2. When 2-propanol was added to the soil as a carbon source in addition to [14C]dimethylsilanediol, the production of 14CO2 increased. A method for the selection of primary substrates that support cometabolic degradation of a target compound was developed. By this method, the activity observed in the soils was successfully transferred to liquid culture. A fungus, Fusarium oxysporum Schlechtendahl, and a bacterium, an Arthrobacter species, were isolated from two different soils, and both microorganisms were able to cometabolize [14C]dimethylsilanediol to 14CO2 in liquid culture. In addition, the Arthrobacter sp. that was isolated grew on dimethylsulfone, and we believe that this is the first reported instance of a microorganism using dimethylsulfone as its primary carbon source. Previous evidence has shown that polydimethylsiloxane is hydrolyzed in soil to the monomer, dimethylsilanediol. Now, biodegradation of dimethylsilanediol in soil has been demonstrated.
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