Enhanced reductive dechlorination-a broader perspective

Abstract

Enhanced reductive dechlorination of chlorinated compounds has been applied via in situ engineered anaerobic systems in increasing numbers over the past decade. Knowledge gained from these field applications, combined with academic literature, allows us to have a broader understanding of the technology. Microbial communities can be engineered through manipulation of aquifer inputs, creating in-situ reactive zones. Over a short segment of the groundwater flow path, the microbial biomass is shifted to a succession of facultative degraders and fermenters, sulfate reducers, acetogens and methanogens. At low-to-moderate electron donor loading rates, the aquifer microbial continuum is dominated by species that produce C-1 compounds and acetate as the fermentation products. When electron donor loading increases to high levels a significant portion of the electron donor flows into pathways that generate fatty acids. The network of anaerobic metabolic processes is introduced as an electron donor cascade. Bacteria occupying several points in the electron donor metabolic cascade have been shown to reductively dechlorinate halogenated compounds. Biostimulation using late-cascade compounds such as lactate and met hanol bypass much of the productive dechlorinating communities. These observations were developed from the authors' research and published reports of numerous other field applications and lab research. Enhanced reductive dechlorination of chlorinated compounds has been applied via in-situ engineered anaerobic systems in increasing numbers over the past decade. Knowledge gained from these field applications, combined with academic literature, allows us to have a broader understanding of the technology. Plumes of contaminated groundwater historically were managed by conventional pump-andtreat programs. These programs often require long-term system operation, are expensive to operate and maintain, and rarely are effective in meeting stringent groundwater cleanup objectives. Other technologies, such as conventional air sparging, lack the ability to address sorbed-phase contaminants, often resulting in a rebound effect of contaminant concentrations upon system shutdown. The propagation of in-situ reactive zones through reagent injection is an emerging remedial strategy that has been evaluated, developed, and implemented only within the past decade. The strategy is gaining attention due to the increasing recognition of the limitations of pump-and-treat and air sparging systems, and the ability to implement, in-situ, most of the treatment processes otherwise used in above-ground systems. ARCADIS pioneered the in-situ reactive zone strategy and has implemented more than 100 in-situ reactive zones for enhanced reductive dechlorination, at a broad range of sites, since 1993. These sites have included a variety of constituents to be treated, including tetrachloroethene (PCE), trichloroethene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), carbon tetrachloride, pentachlorophenol, and chlorinated pesticides in various groundwater concentration ranges with numerous hydrogeologic settings (including shale and karst limestone bedrock, low permeability glacial tills and saprolite, and high permeability alluvium and glacial outwash environments). To successfully apply this technology, it is crucial to understand the processes that occur in the reactive zones and under what conditions reductive dechlorination will occur. The following sections detail these processes, and provide case studies to illustrate this technology. © 2005 Springer Science + Business Media, Inc.