J. A. Kean, D. Graves, M Lodato, 2003. "Enhanced Reductive Dechlorination and the Relationship between Cis-1,2-DCE Accumulation and Methanogenesis." In: V.S. Magar and M.E. Kelley (Eds.), In Situ and On-Site Bioremediation"2003. Proceedings of the 7th International In Situ and On-Site Bioremediation Symposium (Orlando, FL; June 2003). ISBN 1-57477-139-6, Battelle Press, Columbus, OH.
Enhanced reductive dechlorination is a viable technology for treating chlorinated ethenes whether implemented through the addition of Hydrogen Releasing Compound (HRCTM), molasses, sodium lactate, vegetable oil, or other organic carbon sources. However, no technology works equally well at every site, and it is important to understand the complexities of the local microbial community. This paper describes a problem that appears relatively common at sites undergoing enhanced reductive dechlorination, the accumulation of cis-1,2-dichloroethene (cis-1,2-DCE). In addition, this paper will provide a cost analysis of this treatment technology.
Enhanced reductive dechlorination was stimulated by two HRC applications made at a dry cleaning site in Florida. The first application principally targeted the shallow surficial aquifer with 147 direct injection points and approximately 6,000 lbs. of HRC. The second application occurred 18 months later and targeted the lower surficial and intermediate aquifers with 128 delivery points and an additional 3,200 lbs. of HRC. As a consequence of the first application, PCE and TCE were quickly dechlorinated to cis-1,2-DCE and to some vinyl chloride (VC) in the shallow surficial aquifer indicating the occurrence of HRC mediated dechlorination. The limited amount of HRC applied to the lower surficial aquifer during the first application presumably prevented appreciable dechlorination in the deeper groundwater. The latter HRC delivery was focused on the deeper groundwater; however, the anticipated complete dechlorination did not occur, resulting in a dramatic increase in the cis-1,2-DCE concentration in the lower surficial aquifer.
The accumulation of cis-1,2-DCE in a biologically active aquifer provided an opportunity to investigate the reasons for the incomplete reductive dechlorination reaction. Groundwater samples were routinely analyzed for chlorinated ethenes, ethene, ethane, methane, pH, oxidation reduction potential, and dissolved oxygen. In addition to these analyses, soil and groundwater samples were also analyzed for volatile fatty acids (VFA), Dehalococcoides ethenogenes, and bioavailable iron. Vertical and horizontal groundwater gradients were examined to evaluate the movement of cis-1,2-DCE between the surficial and intermediate aquifers.
Using the analytical results, a conceptual model of the site emerged to explain the behavior of chlorinated ethenes in the aquifer. D. ethenogenes was present, only a small amount of bioavailable iron was present, and HRC released lactic acid that was further degraded to other VFA. The accumulation of cis-1,2-DCE was directly correlated with methanogenesis such that when the electron donor (HRC, lactate, and VFA) concentration was high, methanogenesis and partial dechlorination reactions dominanted but as the electron donor (and hydrogen concentration) decreased, methanogenesis subsided and cis-1,2-DCE dechlorination began to occur. These observations suggested that complete dechlorination of chlorinated ethenes depended on the abundance of electron donating compounds. Although this behavior may be specific to this site, the methods and approach used to investigate the competing processes provide a framework to assess sites for the potential for successful reductive dechlorination and to diagnose sites that behave in undesirable or unexpected ways.