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amec�9 <br /> groundwater system downgradient of the WWTP area. The samples from monitoring wells M- <br /> 6C1, M-25C1 and M-25C2 plot below the overall 13C-enrichment with distance trend shown in <br /> Figure B.4-7 and generally contain a higher proportion of TCE. This may be indicative of a <br /> different COC source. <br /> 3.3.4 Quantitative Assessment and Timeframe of COC Destruction <br /> This section illustrates the use of CSIA to estimate the extent of COC loss to date and the <br /> range of net COC destruction rates in the subject aquifer system. Critical parameters for this <br /> calculation are: <br /> 1. The initial stable carbon isotope ratio of TCE (613Co) prior to degradation <br /> 2. the stable carbon isotope enrichment factor (a) <br /> 3. the residence time (t) for COCs in the aquifer system <br /> Initial 613CTCE of TCE <br /> As shown in Figure B.4-7, the value of 613Cbu,k is lowest (least enriched in 13C) in the sample <br /> from TH-10, which is closest to the WWTP, where the early stages of dehalorespi ration are <br /> active and destruction of vinyl chloride has not yet become a major process in comparison to <br /> cDCE production. If the amount of of vinyl chloride destruction near TH-10 is not great, then <br /> the calculated 613Cbu,k value of-31.8%o would theoretically be equal to 613Co for TCE. This <br /> value is near the middle of the range reported for freshly manufactured TCE (-33.5 to -27.8%o; <br /> Wilson, 2010) and therefore appears to a reasonable approximate of the 613Co. <br /> Estimation of the Stable Carbon Isotope Enrichment Factor (E) <br /> The enrichment factor s can be calculated using Equation 2 if f, 613Co and 613Ct are known, <br /> where 613Ct is the carbon isotope ratio for TCE in the groundwater sample. Assuming vinyl <br /> chloride has not been significantly degraded at TH-10, f can be calculated from the <br /> concentrations of TCE, cDCE and vinyl chloride reported in the sample for TH-10 as follows: <br /> f--[x]/[x + y + z] Equation 4 <br /> where x, y, and z are the moles of TCE, cDCE and vinyl chloride in the groundwater sample <br /> from TH-10. Based on equation 4 and the COC concentrations reported for TH-10 (Table B.4- <br /> 5), f is calculated to range from 0.08 to 0.09, meaning that 91 — 92% of TCE has been <br /> converted to cDCE and vinyl chloride by dehalorespi ration at TH-10, causing the 613CTCE value <br /> to increase from 6 <br /> 13Co=-31.8%o to 613Ct =-20.3%o. Equation 2 was used to calculate an <br /> enrichment factor (s) of-4.5%o, assuming X0.08 and the initial and final 613CTCE values were <br /> 6 <br /> 13Co=-31.8%o and 613Ct =-20.3%o, respectively. This value is within the range of published <br /> values for s (-2.5 to -13.8%o) reported in Wilson (2010). The enrichment factor for cDCE and <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 134-15 <br />