This project examined the potential for biological degradation of TCE in karst aquifers. Biodegradation is often presumed to be an irrelevant attenuation process in karst aquifers because of short ground-water residence times, lack of bacteria, and unsuitable environmental conditions. To address these issues, hydrologic, biological, and geochemical information was gathered. The greatest challenge of this investigation was interpreting the results within the framework of the complex karst hydrology. Examining biodegradation in the shallow water-bearing zone near the top of the limestone bedrock was relatively straightforward. Identification was possible of a chlorinated-solvent plume, a general ground-water flow path, and a sequential pattern of oxidation-reduction zones in the shallow water-bearing zone. Two lines of evidence, geochemical and chlorinated-ethene data, sufficed to demonstrate the occurrence of TCE biodegradation in the shallow anaerobic zone (fig. 28). Since the aim of this study was to determine if biological degradation of chlorinated solvents was occurring in the karst aquifer, further examination of the shallow water-bearing zone was not done.
Byproducts of TCE reductive dechlorination were detected in water samples from the deeper karst aquifer; however, additional data was required to determine if the degradation byproducts were simply transported from the shallow water-bearing zone or if they were the result of biodegradation processes within the karst aquifer. Also, the complexity of the karst aquifer system made computerized fate-and-transport models useless for this site. In the karst aquifer, ground-water zones in close proximity to each other often varied in hydrology, biology, and geochemistry.
Geochemical parameters and chlorinated solvents measured quarterly in deep wells provided some understanding of spatial and temporal patterns in the karst aquifer. These data demonstrated that conditions were favorable for reductive dechlorination or cometabolic degradation pathways in various parts of the karst aquifer. After four sampling events, evidence showed that temporal patterns were not adequately characterized by the quarterly sampling schedule. Continuous monitoring devices were placed in four karst aquifer wells to gather information on temperature, pH, DO, ORP, specific conductance, and water levels to monitor aquifer stability. These continuous monitoring devices provided some of the most important information for identifying areas capable of sustaining biological degradation processes. The continuous monitoring devices confirmed water storage areas and active flow zones in the karst aquifer by linking specific conductance, ORP, and DO with hydrogeologic and weather information. This continuous monitoring information was used to interpret the bacteria and microcosms results (fig. 29).
Biological information similar to that described in other studies (Wilson and others, 1996) was gathered during the investigation. Additional biological data were needed because of the widespread perception that karst aquifers are not able to sustain large microbial populations. Determining if a sufficiently large and diverse microbial population capable of biodegrading the chlorinated solvents existed in the karst aquifer was critical for this investigation (fig. 30). This line of evidence was established by identifying bacteria known to degrade chlorinated solvents using molecular methods and traditional plate counts. Microcosms prepared from non-enriched well water demonstrated cometabolism or reductive dechlorination of TCE. Data resulting from these activities provided biological evidence that sufficient microbes were present in the karst aquifer and were capable of degrading TCE under expected site conditions.
Hydrogeology was the principal factor influencing the biological and geochemical conditions in the karst aquifers (fig. 30). Therefore, the chemical and biological evidence had to be considered in the context of the hydrology. For example, temporal and spatial continuity of the aquifer geochemistry and ORP had to be taken into account before applying the results from the microcosm studies to the karst aquifer. Results from the microcosm studies indicated that anaerobic reductive dechlorination was possible in each well given strong anaerobic conditions. However, geochemical conditions in some parts of the aquifer fluctuate with the weather, preventing reductive dechlorination from occurring to the same extent as in the microcosms.
Although hydrology had the principal influence on the karst system, the appropriate geochemical conditions and biological assemblages also had to be present for TCE biodegradation to occur. Individually, hydrologic, biological, or chemical observations were insufficient to prove that TCE biodegradation was occurring in the karst aquifer. Together they provided convincing evidence that anaerobic and aerobic biological degradation processes were active in distinct areas of the karst aquifer.
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