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amec�9 <br /> 2. Under oxic conditions, cDCE and vinyl chloride are susceptible to microbial <br /> mineralization to carbon dioxide with little or no accumulation of intermediate <br /> degradation products. <br /> 3. Under highly-reducing (anoxic) conditions such as sulfate-reducing and methanogenic <br /> environments, reductive dechlorination of TCE is the predominant biodegradation <br /> pathway, leading to the accumulation of ethene and less chlorinated intermediates <br /> (cDCE and vinyl chloride). <br /> 4. Destruction of cDCE and vinyl chloride without ethene production also occurs under <br /> anoxic conditions. <br /> Redox conditions based on groundwater samples from monitoring wells are summarized in <br /> Table B.4-4. Based on Table B.4-4, the reductive dechlorination of TCE is a viable process <br /> near the WWTP, where DO concentration is < 1 mg/L, oxidation-reduction potential (ORP) is <br /> negative, nitrate is absent and dissolved methane concentrations in groundwater samples <br /> indicate that groundwater in this area is methanogenic. Interpretation of these parameters <br /> indicates that groundwater near the WWTP is strongly reducing and TCE destruction by a <br /> microbiologically-mediated reductive dechlorination process called dehalorespi ration is viable <br /> if appropriate microbial communities are present. Dehalorespi ration is a viable process after <br /> dissolved oxygen and nitrate are depleted in the aquifer system due to an excess of dissolved <br /> organic carbon (Bennett et al., 2007). Evaluation of groundwater samples from beneath the <br /> site indicate that conditions are oxic to nitrate reducing, and production of cDCE and vinyl <br /> chloride from the biotransformation of TCE would not be expected under these conditions. <br /> This conclusion is consistent with the evaluation of monitoring data collected to date, which <br /> indicates that TCE is the primary COC in upper aquifer groundwater beneath the site. <br /> Groundwater beneath the site is currently addressed with three upper aquifer extraction wells. <br /> Redox conditions near the WWTP indicate that ongoing biodegradation of TCE by <br /> dehalorespi ration will continue to be a viable process. In areas farther from the WWTP, cDCE <br /> and vinyl chloride can be biodegraded by aerobic and anaerobic oxidative processes, <br /> suggesting that these compounds could be removed from the aquifer system without causing <br /> the accumulation of cDCE, vinyl chloride and ethene. <br /> 3.2.2 Aquifer Minerals and Abiotic Degradation <br /> Naturally-occurring ferrous iron minerals are solid-phase reductants that can reduce TCE to <br /> environmentally benign end products by a pathway called R-elimination, where TCE is <br /> transformed to a very short-lived intermediate called chloroacetylene, which rapidly self- <br /> decomposes to non-chlorinated ethene, ethane and ultimately carbon dioxide (Cwiertny and <br /> Scherer, 2010). The results of laboratory studies have shown that magnetite is associated with <br /> this process (Ferrey et al., 2004). Magnetite is known to exist in aquifer sediments of the Ripon <br /> AMEC Geomatrix, Inc. <br /> \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.4\Attach B-4.docx 134-8 <br />