Chlorinated solvent-contaminated groundwater is found at many Department of Defense sites. The objective of this demonstration was to determine if complete reductive dechlorination of tetrachloroethene (PCE) and trichloroethene (TCE) could be stimulated in groundwater through the introduction of a microbial culture containing halorespiring bacteria, specifically Dehalococcoides ethenogenes.
The reductive dechlorination of PCE and TCE is mediated by microorganisms, but generally the process stalls at cis-1,2-dichloroethene (c-DCE). Dehalococcoides ethenogenes, found in cultures such as Pinellas and KB-1, has been shown to reduce these compounds to ethene. At Kelly Air Force Base in Texas, researchers from GeoSyntec Consultants augmented groundwater contaminated with PCE by adding KB-1 culture to a hydraulically isolated plot where water was recirculated between one injection well and three extraction wells. Electron donors (i.e., methanol and acetate) were added to a total concentration of 7.2 millimolar. Residence time within the test cell was approximately 8 days.
Prior to the demonstration, limited reductive dechlorination to c-DCE was occurring. With the addition of electron donor, PCE decreased and c-DCE concentrations increased. At 72 days following addition of the KB-1 culture, ethane was detected and PCE, TCE, and c-DCE were at their lowest concentrations. To test the robustness of the KB-1 culture, the system was shut down and the addition of electron donor was discontinued. Complete dechlorination continued after one year of shutdown. Sulfate, added to the system 1.5 years after shutdown to determine if reductive dechlorination would decrease due to electron donor competition, did not affect reductive dechlorination. In a control plot with the addition of electron donor only (i.e., biostimulation), complete dechlorination was not achieved. Estimated full-scale costs for a 5-acre chlorinated solvent plume with a 15 to 25-foot below ground surface aquifer were $402,000 for bioaugmentation and $948,000 for pump-and-treat.
The advantages of bioaugmentation over conventional technologies, such as biostimulation and pump-and-treat, are reduced costs to achieve end goals, complete reduction of the contaminant, utilization of the aquifer as an in situ bioreactor, no aboveground treatment, minimization of wastewater, and preservation of the water balance. Technology limitations involve establishing the culture to compete with indigenous microorganisms, a need for sufficient permeability to allow manipulation of groundwater flow, and the dependency on adequate distribution of the culture within the subsurface. (Project Completed - 2004)