From Ground Water Currents, September 1996, Issue No. 16

Natural Attenuation Research at Dover

An extensive four-year research program is in progress at Dover Air Force Base in Delaware. The Dover study is being conducted by the Remediation Technologies Development Forum's (RTDF) Bioremediation of Chlorinated Solvent Consortium. This study will contribute greatly to a fundamental scientific understanding of natural attenuation as a remedy for chlorinated solvent contamination. To date, acceptance of natural attenuation of chlorinated solvents has been largely site specific. The protocol being used at Dover and the information to be obtained will be applicable for the assessment and understanding of other sites. The Dover site offers the opportunity not only to assess the contributions of reductive dehalogenation but other multiple processes as well, including both aerobic and anaerobic reactions.

Dover Air Force Base was chosen over the many other sites evaluated for the study because: (1) the plume is well characterized; (2) analyses of ground water chemistry provided clear evidence that chlorinated solvent contaminants are being biodegraded; (3) the deep zone of the aquifer has relatively simple geology and is underlain by a thick confining layer; (4) access for sampling and testing is good and the site is easily reached by offsite personnel and visitors; (5) the base has a proactive environmental program.

The plume contains primarily TCE and dichloroethene (DCE), with smaller amounts of vinyl chloride. It occupies an area north and south of U.S. Highway 113 approximately 9,000 feet long and 3,000 feet wide. There are multiple sources of solvent contamination in the area north of the highway, as well as several minor sources of petroleum hydrocarbons. There appear to be at least three sources of TCE.

Based on currently available information, natural biological attenuation of PCE and TCE is occurring in the ground water as evident from the presence of daughter products such as cis-DCE, vinyl chloride and ethylene. Methanogenesis appears to be the dominant microbial process in the anaerobic portion of the aquifer. The rate and extent of this process is likely to be controlled by the geochemistry, particularly the availability of natural or anthropogenic co-substrates which are needed to drive the reductive dehalogenation reaction. However, the results appear to suggest that there are multiple biodegradation pathways involved (i.e., anaerobic and aerobic). Based on the distribution of the contaminants and the geochemical parameters, aerobic biodegradation pathways may also determine the fate of the chlorinated ethenes. The extent of ground water contamination by TCE is greater than the size of the cis-DCE and vinyl chloride plumes. The cis-DCE and vinyl chloride can be degraded in aquifer samples under aerobic conditions in the absence of any additional co-substrate. Vinyl chloride has been shown to be used as a carbon source for growth of aerobic bacteria. Based on these observations, cis-DCE and vinyl chloride may be degraded via aerobic biodegradation mechanisms. This conclusion is supported by preliminary results of laboratory microcosm studies conducted using soil from the Dover site. Under aerobic conditions, 14-C labeled DCE and vinyl chloride appear to be degraded to carbon dioxide.

TCE concentrations in the ground water range up to 20 milligrams per liter (mg/L). The TCE concentration declines rapidly near Highway 113. TCE is degraded before reaching the St. Jones River to the south of the plume.

DCE concentrations are over 10 mg/L in two areas. The DCE is primarily cis-1,2-DCE, the isomer produced by biodegradation of TCE. Chemically manufactured DCE can be distinguished from biogenic DCE because chemically manufactured DCE contains a mixture of isomers, of which cis-DCE is a minor component. The DCE plume overlaps the TCE plume. DCE concentrations also decline rapidly south of Highway 113.

There is a smaller vinyl chloride plume with concentrations up to 1 mg/L. Since vinyl chloride was never used on the base, the Consortium believes that is present as a biodegradation product of DCE. If DCE were being lost primarily by reduction to vinyl chloride, we should be able to detect low, transient concentrations of vinyl chloride throughout the area containing DCE, regardless of the relative degradation rates of the two compounds. The area containing vinyl chloride, however, is considerably smaller than the DCE plume.

Ethylene is also present, showing that complete reductive dehalogenation of TCE does occur in the deep zone. The amount of ethylene is small, however, i.e., 50 micrograms per liter or less. This is much too low to account for the observed losses of TCE and DCE.

The project at Dover is in the second year of the four-year study. The evidence clearly demonstrates that active intrinsic remediation of chlorinated solvents is occurring. The key evidence supporting this conclusion is as follows. First, the contaminant plumes are "stacked," indicating that the more mobile contaminants are being destroyed before they can move away from the less mobile contaminants. Second, the chloride ion concentration increases as the solvent concentration declines. The increase is large enough to account for the entire observed loss of solvents. Third, there is clear field evidence of reductive dehalogenation and oxidation, and possible evidence for co-oxidation.

In addition to the Dover study mentioned in this issue and the St. Joseph Superfund Site (see Natural Bioremediation of TCE in the September 1993 issue of Ground Water Currents), natural attenuation of chlorinated solvents is being examined at a number of sites around the world. We will be reporting to you on all of this research as it comes to fruition.

Information for this article was, for the most part, excerpted from Symposium on Natural Attenuation of Chlorinated Organics in Ground Water, Dallas, Texas, September 11-13, 1996, pp. 93-97 and Intrinsic Remediation of Chlorinated Solvents in Groundwater, from Conference on Intrinsic Bioremediation, London, England, March 18-19, 1996.