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Investigation of In-situ Biogeochemical Reduction of Chlorinated Solvents in Groundwater by Reduced Iron Minerals
Whiting, K., P. Evans, B. Henry, J. T. WILSON, C. Lebron, AND E. Becvar. Investigation of In-situ Biogeochemical Reduction of Chlorinated Solvents in Groundwater by Reduced Iron Minerals. Presented at Battelle Conference, Monterey, CA, May 05 - 09, 2009.
To make bservations regarding biogeochemical transformation
Biogeochemical transformation is a process in which chlorinated solvents are degraded abiotically by reactive minerals formed by, at least in part or indirectly from, anaerobic biological processes. Five mulch biowall and/or vegetable oil-based bioremediation applications for treating chlorinated organic compounds in groundwater were investigated to 1) identify the key parameters which are indicative of effective treatment by biogeochemical transformation processes, 2) develop a streamlined methodology for predicting the conditions under which biogeochemical transformation will occur, and 3) identify strategies for optimizing system performance by the engineered stimulation of biogeochemical transformation. The systems chosen for the evaluation included two mulch biowall applications at Altus Air Force Base, Oklahoma; a biowall application at Dover Air Force Base, Delaware; a biowall/vegetable oil application at Seneca Army Depot, New York; and an emulsified vegetable oil application at Dugway Proving Ground, Utah. The solid phase mulch or vegetable oil impacted sediment was analyzed for mineral content using iron and sulfur extractions, electron microprobe analysis, and organic carbon analyses. Groundwater (upgradient, downgradient, and within the treatment zones) was analyzed for metals, major anions and cations, field geochemical parameters, volatile organic compounds, and stable carbon isotopes. The degradation signature was evaluated by the reduction in parent compounds (TCE) relative to a lack of intermediate dechlorination products (abiotic dechlorination), or to the accumulation of intermediate dechlorination products (sequential biotic dechlorination). Geochemical modeling was performed to correlate the occurrence of abiotic dechlorination to the potential for formation of reduced metal sulfides. The most notable observation was that biogeochemical transformation was most evident within the mulch biowalls where the amount of organic material, iron (from backfill sand), and sulfate (in groundwater or from amendments) were high. Biotic processes were more evident downgradient of the biowalls. The two most important parameters that were indicative of the occurrence of abiotic dechlorination were pH and the total surface area of the iron sulfide minerals present. Degradation rate constants increased with increasing pH and iron sulfide surface area. The amounts of other reduced-iron phases, such as magnetite, ferrous silicates, and green rusts were limited in the samples analyzed, and thus were of minor importance within the applications investigated.