Obstacles to Complete PCE Degradation during Reductive Dechlorination
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J. A. Kean, D. Graves, K, Bishop, E. Mott-Smith, M. Lodato, 2002, "Obstacles to Complete PCE Degradation during Reductive Dechlorination, "in Proceedings of the 3rd International Conference on the Remediation of Chlorinated and Recalcitrant Compounds, May.

Reductive dechlorination is a viable technology for treating chlorinated ethene contaminated groundwater especially when facilitated by the addition of hydrogen releasing compound (HRCTM), molasses, sodium lactate, vegetable oil, or some other organic carbon source. However, no technology works equally well at every site. This paper describes a problem that appears relatively common at sites undergoing enhanced reductive dechlorination, the accumulation of cis-1,2-DCE.

Enhanced reductive dechlorination was accomplished by two HRC applications made at a dry cleaning site in Florida. The first application principally targeted the shallow surficial aquifer with 144 direct injection points and approximately 3300 lbs. of HRC. The second application occurred 18 months later and targeted the lower surficial and intermediate aquifers with 146 delivery points and an additional 3300 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 subsequent HRC delivery was focused on the deeper groundwater; however, the anticipated dechlorination did not occur. In fact, the cis-1,2-DCE concentration increased in the lower surficial aquifer.

Site data have been evaluated in an attempt to understand the behavior of the chlorinated ethenes in the presence of HRC. Results from the first application of HRC in the shallow aquifer clearly indicated the reductive dechlorination of PCE and TCE and to a much lesser extent cis- 1,2-DCE and VC dechlorination. Only low concentrations of ethene were observed. This indicated that complete dechlorination was occurring but that the rate was much less than the dechlorination rates for PCE and TCE. Site samples were examined for the presence of Dehalococcoides ethenogenes, groundwater geochemistry was critically examined, the concentration of lactic acid and other metabolic acids was determined, and the influence of climatic conditions on groundwater gradients was evaluated. The experience gained at this site should not preclude the application of accelerated reductive dechlorination at other sites; rather, the lessons learned thus far should be used to develop more rigorous site qualification procedures and design criteria.

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