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amec- <br /> 613 CTCE, <br /> and below the line where 613CDCE > 613CTCE. CSIA data for eight of the ten <br /> groundwater samples with paired CSIA data plot below the line (M-10A, M-6A, M-6C1, M-6C2, <br /> M-17C1, M-20C1, M-25C1 and M-25C2), indicating that substantial cDCE degradation has <br /> occurred by the time groundwater has reached these locations. The presence of vinyl chloride <br /> in groundwater samples from M-17C1 and M-20C1 is an indication that the cDCE destruction <br /> mechanism is in part by transformation to vinyl chloride. Data plotting farthest below the <br /> dashed line indicate the greatest amount of cDCE destruction. These samples were collected <br /> from monitoring wells M-6A, M-6C1, M-10A and M-25C1, and are characterized as having <br /> TCE concentrations greater than cDCE with no vinyl chloride. The results provide an indication <br /> that either cDCE and vinyl chloride have been degraded in the aquifer system represented by <br /> these samples, or that TCE is being directly mineralized to non-chlorinated breakdown <br /> products. <br /> CSIA data for samples collected from TH-10 and M-31 C1 plot above the line in Figure B.4-6 <br /> indicating that cDCE production from dehalorespi ration of TCE exceeds the rate of cDCE <br /> conversion to vinyl chloride. This scenario is consistent with the earlier stages of <br /> dehalorespi ration which appears to be a viable process closer to the WWTP as discussed in <br /> Section 3.2.1. Because TH-10 and M-31 C1 are closer to the WWTP, the residence time of <br /> COCs in groundwater that may have originated from discharges in that area would be smaller, <br /> and therefore an earlier stage of dehalorespi ration in this area would be anticipated, as a <br /> greater amount of net COC destruction is expected at greater distances and groundwater <br /> travel times. <br /> If net COC destruction removes carbon from the total moles of chlorinated COCs, then the <br /> average stable carbon isotope ratio for [TCE+cDCE+vinyl chloride] would be expected to <br /> increase. The average or bulk stable carbon isotope ratio (613Cbulk)for [TCE+cDCE+vinyl <br /> chloride] is calculated by mass balance as follows: <br /> 613Cbulk=[X TCE(613CTCE)+y cDCE(613CDCE)+ z vinyl chloride (613CVC)]/[X + y+ z] Equation 3 <br /> Where <br /> x=moles of TCE in groundwater sample, <br /> y=moles of cDCE, and <br /> z=moles of vinyl chloride. <br /> Figure B.4-7 shows a plot of 613Cbu1k versus distance from the WWTP, where 613Cbulk values <br /> increase from -31.7%o at TH-10 (approximately 350 feet south of the WWTP area) to -2.9%0 at <br /> M-10C1 approximately 4,200 feet north of the WWTP area. Total COC concentrations are also <br /> lower with distance from the WWTP, and vinyl chloride is absent beyond 2,000 feet (Figure <br /> B.4-7). This is a strong indication that net destruction of CDCs has been occurring in the <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 134-14 <br />