Tetrachloroethylene (PCE) and trichloroethylene (TCE) are among the most frequently detected ground water contaminants at industrial sites, including many DoD facilities. Due to the high cost and uneven performance of traditional remediation technologies, monitored natural attenuation is emerging as a new technology for ground water remediation of pollutants such as these. In addition, there is growing interest in active remediation technologies that employ abiotic minerals. PCE and TCE are susceptible to reductive dechlorination by microorganisms as well as reduced minerals such as iron sulfide (FeS). Unlike biological reductive dechlorination, which often results in accumulation of harmful intermediates such as cis 1,2-dichloroethylene (cis-DCE) and vinyl chloride (VC), abiotic mineral-mediated dechlorination of PCE and TCE tends to result in complete transformation to non-toxic products such as acetylene. To more accurately apply natural attenuation and other remediation technologies, a greater understanding of the geochemical factors affecting the rates of purely abiotic reductive dechlorination of PCE and TCE is needed. Additional tools are also needed to determine whether or not abiotic reductive dechlorination is occurring at a particular site, and its relative importance compared to microbial dechlorination under a variety of geochemical conditions.

Chlorinated aliphatic hydrocarbons (CAHs), such as tetrachloroethene (PCE) and trichloroethene (TCE), are ubiquitously pollutants in aquifer sediments and groundwater due to their heavy usage in industry and inappropriate disposal in the last century. Among about 1300 NPL (National Priorities List) sites, PCE and TCE are the two most frequently detected hazardous contaminants. Engineered bioremediation, including biostimulation and bioaugmentation, is a promising technology to clean those PCE and/or TCE contaminated sites. However, in many contaminated groundwater systems and hazardous waste sites, pH can be lower than 5 to 6. And release of HCl (strong acid) from anaerobic reductive dechlorination may lower the pH of groundwater. Besides, another main source of acidity comes from the fermentation of additive electron donors such as alcohols, organic acids and etc. Decreasing pH has been proved to be detrimental to the microbes that dechlorinated PCE or TCE. We intended to enrich and isolate microorganisms, which can perform anaerobic reductive dechlorination at low pH environments, by establishing microcosms, which will be beneficial to in situ bioremediation. We also screened some existing cultures for dechlorinating activity at low pH and determined the pH tolerance of consortium BDI, which had been successfully, applied for in situ bioremediation. Besides, this study investigated and explored the effects of solids on BDI consortium under low pH conditions. Generally, various dechlorinating pure cultures and consortium BDI show highest dechlorination rates and extent at circumneutral pH. Only Sulfurospirillum multivorans among tested cultures dechlorinated PCE to cDCE at pH 5.5. The screening efforts suggest that microbes capable of dechlorination below pH 5.5 are not common. It was observed that solids play an important role for enhancing microbial activities under low pH conditions. And BDI consortium can recover from up to 8 weeks exposure to low pH conditions, although the VC-to-ethene dechlorination step was affected.