Biodegradation of Chlorinated Ethenes at a Karst Site in Middle Tennessee


The biodegradation of chlorinated solvents such as trichloroethylene (TCE) in aquifers consisting of unconsolidated materials has been well documented, and several guidelines have been developed from those studies (Remediation Technologies Development Forum, 1997; U.S. Environmental Protection Agency, Region 4, 1997; and Wiedemeier and others, 1998). The guidelines for evaluating sites contaminated with chlorinated solvents incorporate hydrogeology, microbiology, and geochemistry, and ground-water modeling. A large component of those guidelines is for evaluating if significant biodegradation is taking place. One environment where chlorinated-solvent biodegradation has not been adequately investigated is karst aquifers because of the complex hydrology associated with these aquifers. Although chlorinated-solvent biodegradation products such as cis-1,2-dichloroethylene (cDCE) have been detected in karst aquifers, little research has been done in examining if biodegradation occurred in the karst aquifers or if the biodegradation products originated elsewhere.

Approximately 40 percent of the United States east of the Mississippi River is underlain by various types of karst aquifers (Quinlan, 1989), and more than two-thirds of the State of Tennessee is underlain by carbonate rocks and can be classified as karst (Wolfe and others, 1997). Potential industrial sources of ground-water contamination are common in karst regions; however, the fate and transport of contaminants such as chlorinated solvents in karst areas are poorly understood because of the distinctive hydraulic characteristics of karst aquifers (Field, 1993). Ground-water models that predict the fate and transport of contaminants in sandy aquifers have limited application to karst aquifers. Most natural attenuation and bioremediation guidelines specify that ground-water models are not applicable in fractured rock or karst aquifers (U.S. Environmental Protection Agency, 1997).

The U.S. Geological Survey (USGS), in cooperation with the Tennessee Department of Environment and Conservation, Division of Superfund (TDEC-DSF), is conducting a study of the occurrence, fate, and transport of chlorinated solvents in karst regions of Tennessee. One objective of this study was to examine the role of biodegradation in the fate and transport of chlorinated solvents in karst aquifers. This report presents results from field and laboratory data collected to examine chlorinated-ethene biodegradation at a TCE-contaminated karst site in Middle Tennessee. The study site was selected because of the presence of chlorinated-solvent degradation products in ground water and because of the availability of hydrologic and chlorinated-ethene data in TDEC-DSF files. The objectives of this report are to:

  1. Summarize the current understanding of:
  1. chlorinated-ethene biodegradation processes and
  2. biodegradation of organic compounds in bedrock aquifers.
  1. Characterize the microorganisms and geochemical conditions present in ground water at the study site.
  2. Evaluate which chlorinated-ethene degradation processes may be occurring in ground water at the study site.
  3. Document lessons learned from this study that may assist future investigations of chlorinated-solvent biodegradation in karst areas.

The body of the report is divided into six main sections. The first section is a general overview of chlorinated-ethene degradation processes. The second section reviews previous studies relevant to the effects of hydrology, bacteria, and geochemical conditions on the biodegradation of organic compounds in bedrock aquifers. The third section describes the study site with emphasis on hydrogeology and monitoring and remedial activities. The fourth section describes methods and procedures used to collect and interpret geochemical, chlorinated ethene and degradation product, and microbiological data. The fifth section presents results and interpretation of the multiple lines of evidence used to examine chlorinated-ethene degradation at the study site. Finally, the sixth section describes lessons learned during this study.

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