Technical and Regulatory Requirements for Enhanced In Situ Bioremediation of Chlorinated Solvents in Groundwater

Enhanced in situ bioremediation (EISB) of chlorinated solvents in groundwater involves the input of an organic carbon source, nutrients, electron acceptors, and/or microbial cultures to stimulate degradation. EISB systems may be used to remediate high concentration areas within plumes or source areas, to help provide containment of a chlorinated solvent plume, or as part of a treatment train downgradient from a primary cleanup or containment system. The major biological processes by which chlorinated solvent compounds degrade include anaerobic reductive dechlorination, aerobic cometabolism, and oxidation. Anaerobic reductive dechlorination involves the replacement of chlorine atoms in the chlorinated compound by hydrogen. An electron donor, either hydrogen gas or a precursor carbon compound, is necessary for the reduction to occur. Aerobic cometabolism involves the fortuitous degradation of chlorinated solvents by enzymes intended to metabolize compounds such as toluene, phenol, or methane. The organisms gain no benefit from the degradation, and may be harmed. Direct degradation of certain lesser chlorinated solvents can occur in either anaerobic or aerobic environments. A key factor in the design of EISB systems is the mechanism of delivery of the various amendments to the targeted portion of the groundwater plume. Various types of delivery mechanisms have been used, including dual vertical well recirculation, horizontal well recirculation, combinations of well-infiltration trench recirculation, direct liquid amendment injection, gas amendment injection, and pass-through or reactive cell designs. Each of these may have advantages or disadvantages depending upon the major objective of the project and site conditions. For sites in which treatment of high concentration portions of a plume is the goal, systems with either dual wells or other arrangements may provide semi-closed loops which reduce downgradient flow of contaminants while providing biotreatment.