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amec— <br /> How <br /> are the CSIA results used to assess the extent of COC biodegradation? <br /> The extent of degradation of CDCs is related to the extent of fractionation in a predictable <br /> manner described by the simplified form of the Rayleigh equation (Wilson, 2010; EPA, 2008; <br /> Clark and Fritz, 1997): <br /> S13Ct = 613Co + Elnf (2) <br /> Where 613 C represents the 13C/12C of the sample as reported by the laboratory in %o; 813Co <br /> represents the carbon isotopic composition of the original non-degraded COC source (i.e. at <br /> time zero, where time t at 613 C represents the residence time of the COC in the aquifer system <br /> since the release occurred); s represents the enrichment factor and f represents the fraction of <br /> COC remaining at time=t since degradation started (f is the concentration of COC in a <br /> groundwater sample divided by the initial groundwater concentration at the time of release, <br /> often decades earlier; f is the fraction of non-degraded material remaining relative to the initial <br /> amount after degradation has occurred over a certain period of time). <br /> Equation 2 can be rearranged to solve for f if the other values are known to a reasonable <br /> degree of certainty. <br /> The value of s is also usually not known accurately and is typically based on published values <br /> from laboratory studies (Wilson, (2010) reports a range of values from -13.8 to -2.5%o). If the <br /> published range of s is large, the extent of biodegradation calculated with equation 2 can vary <br /> greatly. In this assessment, it was possible to calculate both 613Co and s using site data, such <br /> that a relatively narrow range of values for f could be estimated. If f can be estimated and the <br /> timing of the release is known, a bulk degradation rate can also be estimated. <br /> 3.3.2 Stable Carbon, Chlorine and Hydrogen Isotope Ratios of COCs in Groundwater <br /> Selected groundwater samples were analyzed by ZymaX for to determine the stable carbon <br /> isotope ratios of TCE (613CTCE), cDCE (613CDCE) and vinyl chloride(613CVC); a subset of these <br /> samples were analyzed for chlorine and hydrogen isotope ratios of TCE (837CITCE and BDTCE, <br /> respectively). The results are summarized in Table B.4-5. Where 613CTCE and 637CITCE values <br /> were measured in groundwater samples from the same well in Q2 and Q3, the difference in <br /> the value was 1%o or less, except for M-25C1 (the difference in Q2 and Q3 613CTCE values was <br /> 1.4%o).This result provides an indication that the stable carbon and chloride isotope ratios <br /> generally are consistent between events and reproducible (typically, results are reported to <br /> ±0.5%o for carbon). Duplicate pairs of hydrogen isotope ratios were not analyzed. <br /> A plot of 613CTCE versus 837CITCE values for groundwater samples is shown in Figure B.4-4 and <br /> compared with the isotopic composition reported for known manufacturers of TCE reported in <br /> EPA (2008). The 637CITCE values for groundwater samples from EU-3, M-1 OA, EW-1, M-6C1, <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 134-12 <br />