Remediation of a chlorinated solvent source area using in situ accelerated anaerobic biodegradation at dover air force base, delaware

A solvent source area located under a mission-critical maintenance facility, Building 719, at Dover Air Force Base (DAFB), Delaware, has been the target of a 5-year in situ accelerated anaerobic biodegradation (AAB) remediation effort. The primary solvents used for parts cleaning at the site were trichloroethene (TCE) and 1,1,1- trichloroethane (1,1,1-TCA). TCE concentrations as high as 280,000 micrograms per kilogram (μg/kg) in soil and 21,000 micrograms per liter (μg/L) in groundwater have been detected in samples collected at the site. 1,1,1-TCA concentrations have been several orders of magnitude lower than TCE concentrations. The AAB system was installed around the outside of the building and consists of four extraction wells, 12 injections wells, and 20 monitoring wells. The system controls groundwater flow in order to maintain flow under the building and through the source area. Sodium lactate and dibasic ammonium phosphate are pulsed into the flow stream to provide sufficient carbon substrate and metabolic nutrients for microbial dechlorination. The AAB system has been operating since February 2002 and has created an anaerobic reactive zone around the injection wells where reductive dechlorination occurs. As site contaminants are captured by the extraction wells and recycled through the reactive zone, typically over 97 percent of the chlorinated ethenes are degraded. All contaminants, except TCE, are below their respective maximum contaminant levels (MCLs) in the key monitoring wells closest to the source area. TCE concentrations in a few of these monitoring wells remain slightly above its MCL. Due to the success of the AAB system, the current active pumping system is being converted to a passive system. This conversion involves distributing emulsified vegetable oil (a slow release substrate) under the building as a long-term supply of organic carbon. This will maintain the anaerobic environment needed to degrade any residual contaminants remaining in the source area. Monitoring of the passive AAB will continue to ensure contaminant rebound does not occur.