Combining ZVI and Organic Substrates for Full-Scale Treatment of Dilute Trichloroethene Plume in an Aerobic Aquifer

Background/Objectives.A chlorinated ethene plume, consisting primarily of trichloroethene, at the Concord Naval Weapons Station (CNWS) extended approximately 700 feet down gradient from the source area and up to 100 feet below ground surface. The effected aquifer consists of unconsolidated silt, sands and clays and is unconfined and semiconfined. Groundwater in the treatment area is highly aerobic and had sulfate concentrations up to 250 mg/L. The dechlorinating bacteria, Dehalococcoides sp., was not detected in the aquifer. An Enhanced Anaerobic Bioremediation (EAB) pilot test demonstrated complete degradation the chlorinated ethene (CE) concentration from approximately 5,000 microgram per liter (μg/L) to less than 5 μg/L in approximately 500 days. Although EAB alone was effective, the Navy wanted to evaluate a more aggressive approach to achieve site cleanup. A Design Optimization Test (DOT) was conducted to evaluate enhancement of the biological approach by In Situ Chemical Reduction (ISCR). The ISCR approach was demonstrated to treat the CEs substantially quicker than the EAB alone by enhancing abiotic degradation of dichloroethene following biological reduction of trichloroethene.

Results/Lessons Learned. The injection process distributed substrates a minimum of 15 feet from the injection point. The ISCR process rapidly degraded TCE, dichloroethene (DCE) and vinyl chloride (VC) to below MCLs in the majority of the plume during the first injection event. Small areas of the plume in which substrate was not effectively distributed was treated with a second injection event. The reduced treatment time in the ISCR approach is attributed to β-elimination of DCE compared to the hydrogenolysis pathway in the EAB approach. This aggressive approach was demonstrated to effectively treat a laterally and vertically extensive CE plume in an aerobic aquifer.