Intrinsic and enhanced in situ biodegradation of trichloroethene in a deep, fractured basalt aquifer

A process was developed for evaluating and implementing intrinsic and enhanced in situ biodegradation of trichloroethene (TCE) in ground water at the field scale. The field site was the Idaho National Engineering and Environmental Laboratory's Test Area North (TAN). The Snake River Plain Aquifer, Eastern Idaho's sole source aquifer, is located in deep, fractured basalt beneath TAN. Successful implementation of the process resulted in regulatory acceptance of intrinsic and enhanced in situ biodegradation as the main components of the plume remediation strategy at TAN.;The evaluation of intrinsic biodegradation processes indicated that intrinsic reductive dechlorination of TCE to dichloroethene (DCE) was occurring in the residual source area, but appeared to be electron donor limited. Natural attenuation of TCE was also apparent under aerobic conditions in the large, low concentration plume fringe. A tracer-corrected method developed to distinguish between dispersion and degradation estimated TCE degradation to occur with a half-life of about 10 to 20 years. Additional field and laboratory data provide the basis for the hypothesis that intrinsic aerobic cometabolism may be responsible for the natural attenuation. A numerical model was developed to test the aerobic degradation hypothesis at TAN, focusing on the occurrence of degradation, as opposed to the degradation mechanism. Several hypothesis-testing approaches were used, all of which supported the aerobic degradation hypothesis. The model also demonstrated the importance of distinguishing between dispersion and degradation in natural attenuation evaluations.;Enhanced in situ biodegradation was evaluated in the residual source area of the plume. High concentrations of lactate were added over an 8-month period, followed by 4 months of continued monitoring while residual lactate fermentation products served as the primary electron donors. Complete transformation of TCE to ethene was observed in a large zone across the 60-m contaminated thickness of the fractured basalt aquifer. Reductive dechlorination was strongly correlated with terminal electron accepting processes. The degradation efficiency of the system increased substantially while lactate fermentation products served as electron donors, suggesting that dechlorinating bacteria may have competed more effectively with methanogens or other populations without regular lactate additions.