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Evidence of Chlorinated-Ethene Biodegradation at a Karst Site in Middle Tennessee

Byl, T.D., and Williams, S.D., 2000, Evidence of chlorinated-ethene biodegradation at a karst site in Middle Tennessee [abs.], in Tennessee Water Resources Symposium, 10th, Burns, Tenn., 2000, Proceedings: Tennessee Section of the American Water Resources Association, p. 1B-39.

Abstract

Field and laboratory investigations examined chlorinated-ethene biodegradation in a karst aquifer contaminated with trichloroethylene (TCE). The study site, located in Middle Tennessee, was selected because of the presence of TCE degradation byproducts in the karst aquifer, available site hydrologic information and chlorinated-ethene analytical data. The site is underlain by Ordovician limestone characterized by thin soil, vertical fractures, and horizontal bedding plane openings and dissolution openings. Additional chemical, biological and hydrological data were collected to evaluate if the occurrence of TCE degradation byproducts in the karst aquifer was the result of biodegradation in the aquifer or simply transport into the karst aquifer from the soil zone. Geochemical analysis established that sulfate-reducing conditions, essential for reductive dechlorination of chlorinated solvents, existed in parts of the contaminated karst aquifer. Other areas of the aquifer fluctuated between anaerobic and aerobic conditions and contained compounds associated with cometabolism, such as ethane, methane, ammonia and dissolved oxygen. A large, diverse bacteria population inhabited the contaminated aquifer. Bacteria known to biodegrade TCE and other chlorinated ethenes, such as sulfate-reducers, methanotrophs, and ammonia-oxidizers, were identified from karst-aquifer water using the RNA-hybridization technique. Results from microcosms using raw karst-aquifer water showed that aerobic cometabolism and anaerobic-reductive dechlorination degradation processes were possible when appropriate conditions were established in the microcosms. These chemical and biological results provide circumstantial evidence that several biodegradation processes were active in the aquifer. Additional site hydrologic information was developed to determine if appropriate conditions persisted long enough in the karst aquifer for these biodegradation processes to be significant. Continuous monitoring devices placed in four wells during the spring of 1998 documented a dual-phase system within the karst aquifer characterized by dynamic areas where active flow occurs and stable areas isolated from active flow. The areas of active flow in the karst aquifer fluctuated between anaerobic and aerobic conditions in response to rain events. Bacterial communities and geochemical conditions conducive to cometabolism were associated with this dynamic environment. In contrast, the pH, specific conductance, dissolved oxygen concentrations (low) and oxidation-reduction potentials (low) changed very little in the stable areas isolated from active flow. These stable areas in the karst aquifer had geochemical conditions and bacterial communities conducive to reductive dechlorination of chlorinated ethenes. In summary, multiple lines of evidence developed from biological, chemical and hydrological data demonstrate that a variety of chlorinated-ethene biodegradation processes were active in this karst aquifer.

 


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