Implementing accelerated anaerobic bioremediation at large, multisource solvent plumes, Dover AFB, DE

Many Department of Defense facilities that maintain vehicles, aircraft, and equipment historically have used large quantities of degreasing agent, usually chlorinated solvents. Disposal and use of solvents were not controlled as they are today and sometimes resulted in significant environmental contamination. Dover Air Force Base has been maintaining aircraft for more than 50 years. The various maintenance activities and past spent solvent disposal practices have contaminated the shallow water table aquifer in several locations. Large chlorinated solvents plumes below the Base originate from multiple source areas and extend to over a mile in length. Remediation of these plumes, in an acceptable time frame, presents unique challenges. Feasibility studies and subsequent Records of Decision determined that source areas and the core of the groundwater plumes (where concentrations were highest) would be actively treated to reduce contaminant mass while the more distal portions of the groundwater plumes vuld be addressed with natural attenuation and monitoring. The source areas and core of the plumes were actively treated using accelerated anaerobic biodegradation (AAB). A mixture of sodium lactate and emulsified vegetable oil were used as the substrate to promote the growth of solvent-degrading indigenous microbial communities and create and maintain the needed anaerobic environment. This mixture was delivered and distributed in the subsurface using two methods: a mobile direct- push rig and permanent wells. Direct injection was used in the source areas to more densely focus substrate placement. Prior to injection, a direct-push rig equipped with a membrane interface probe (MIP) was used to confirm and delineate the source areas. This ensured that the substrate was injected into the most contaminated area and at the correct depth interval. The MIP work was very successful in significantly reducing the originally defined source areas and resulted in significant cost savings. Extraction and injection wells were used to create reactive zones (or transects) across the core of the plumes. The wells were spaced about 50 feet apart along lines perpendicular to groundwater flow. Mobile treatment trailers injected substrate mixed with extracted groundwater from alternating wells creating a push-pull effect to distribute the substrate across the plume. Analytical data confirmed that the injected substrate quickly creates a reducing environment. Soon after injection, field indicators revealed the aquifer becoming more reducing: sulfate and dissolved oxygen were falling, and iron, total organic carbon, and conductivity were increasing. It is anticipated that the degradation of parent contaminants, primarily PCE and TCE, will accelerate and that increases in the degradation products cis-1,2-DCE, vinyl chloride, and ultimately methane, ethene, and ethane will be observed.