The biodegradation of chlorinated ethenes in aquifers consisting of unconsolidated material has been well documented; however, chlorinated-ethene biodegradation has not been adequately investigated in karst areas even though chlorinated-solvent biodegradation products have been detected in karst aquifers. Factors that affect the biodegradation of chlorinated solvents include hydrology, microbiology, and geochemistry.
Three categories of metabolic processes are involved in the biological degradation of chlorinated ethenes. Reductive dechlorination is an anaerobic process in which chlorinated ethenes are used as electron acceptors and results in a sequential dechlorination from PCE to TCE to DCE to VC to ethene. Cometabolism is an aerobic process in which chlorinated ethenes are degraded as a result of oxygenase enzymes, such as methane and ammonia monooxygenase, inserting oxygen into TCE, DCE, or VC molecules. Cometabolism needs additional substrates like methane or ammonia, as well as dissolved oxygen, to sustain the process. Direct oxidation is an aerobic or mildly anaerobic process in which lightly chlorinated ethenes (DCE and VC) are used as electron donors. At a given site, one or all of these processes could be occurring depending on environmental conditions.
Most studies of contaminant biodegradation in fractured rock or karst settings examined biodegradation in the overburden or saprolite above bedrock. The lack of studies examining biodegradation in karst aquifers may be due to the widespread perception that the hydrologic and microbiologic characteristics of karst aquifers prevent biodegradation of contaminants in karst aquifers. Previous research, however, has indicated that large volumes of water may be isolated from active ground-water flow paths in karst aquifers. Other studies have shown that water from bedrock aquifers may contain large and diverse microbial populations, which include bacteria responsible for the degradation of chlorinated ethenes.
The biodegradation of chlorinated ethenes was examined at a TCE contaminated karst site in Middle Tennessee. A shallow water-bearing zone is present at the site and chlorinated-ethenes are transported along a trough in the bedrock surface. Some deep wells intersect fractures that are part of the active ground-water flow system of the karst aquifer, whereas other deep wells intersect fractures isolated from the active ground-water flow system. Pump-and-treat wells completed in the upper part of the Ridley Limestone draw down water levels in many of the deep wells and affect local ground-water flow.
Multiple lines of evidence usually are needed to evaluate potential biodegradation processes. These lines of evidence normally include: (1) geochemical data that indicate depletion of electron donors and acceptors and increasing concentrations of metabolic byproducts, (2) chemical data that indicate decreasing concentrations of chlorinated solvents and increasing concentrations of degradation products, and (3) laboratory or field microbiological data that indicate the bacteria present at a site can degrade contaminants. Chlorinated-ethene, ethene, and ethane data for water samples from shallow and deep wells were obtained from TDEC-DSF files. Additional data-collection activities conducted by the USGS included periodic water-quality sampling of selected shallow and deep monitoring wells, collection of water samples from selected deep wells for bacteria identification and enumeration, continuous monitoring of water quality and water levels for selected deep wells, and microcosm studies.
Multiple lines of evidence indicate that reductive dechlorination was the dominant biodegradation process occurring in the anaerobic shallow water-bearing zone underneath the manufacturing building. Ground water in the shallow water-bearing zone was influenced by precipitation recharge as the ground water moved away from the manufacturing building. The recharge water supplied DO and diluted the contaminated water in this transition zone between the anaerobic zone upgradient and the aerobic zone downgradient. Geochemical data indicate aerobic bacteria capable of cometabolizing or directly oxidizing the chlorinated ethenes were active in the aerobic zone downgradient, further decreasing the concentration of chlorinated ethenes in the shallow water-bearing zone. Undoubtedly some of the water, carrying with it bacteria, contaminants, electron donors, and electron acceptors, in the shallow water-bearing zone also migrates down into the karst aquifer.
Water-quality conditions in the karst aquifer varied both spatially and temporally. Significant concentrations of chlorinated-ethene biodegradation products were detected in the karst aquifer; however, the biodegradation products could have been transported from the shallow water-bearing zone. Because of the complex hydrology at the site, it was not possible to identify a chlorinated-ethene plume or discrete oxidation-reduction zones. The deep wells were treated as individual areas instead of as a direct continuum along a flow path. For example, well 12D is thought to intersect an active flow conduit in the aquifer. Wells 1D and 3D intersect water-bearing zones in the aquifer with less active flow. The data indicated that samples from wells which intersect the active ground-water flow system of the karst aquifer (well 12D) contained bacteria that dechlorinated TCE; however, the geochemical conditions present would limit the occurrence of reductive dechlorination. Samples from wells which intersected fractures isolated from the active ground-water flow system (well 1D) contained bacteria which could dechlorinate TCE. Anaerobic conditions persisted in these zones and the geochemical conditions in these fractures were suitable for reductive dechlorination.
A vast consortia of bacteria capable of degrading chlorinated ethenes were detected in the water samples from the karst aquifer. Bacteria capable of cometabolism and direct oxidation of chlorinated ethenes were identified in water samples from wells that fluctuated between aerobic and anaerobic conditions (wells 2D and 3D). During periods of anaerobic conditions, constituents essential to cometabolism such as methane, ethane, and ammonia could be produced. Microcosm results indicated that the aerobic bacteria in samples from the karst aquifer could quickly (within less than 3 weeks) degrade TCE.
The greatest challenge to this investigation was interpreting the results within the framework of the complex karst hydrology. Data such as continuous water-quality monitoring and microbiological data were necessary to compose sufficient evidence that significant biodegradation occurred in the karst aquifer. Continuous monitoring provided some of the most useful information about the geochemical conditions and variability in the karst aquifer. Together, the multiple lines of evidence helped to identify the relation between hydrology, geochemistry, and biology and the biodegradation of chlorinated ethenes in the karst aquifer.
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