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amec— <br /> M-31 <br /> C1 and M-6C2 ranged from -2.5%o to -1.9%o which is within the range of values reported <br /> for TCE manufactured by PPG and StanChem, but outside of the range reported for DOW and <br /> ICI. Wilson (2010) reported that the published range in 613CTCE values for non-degraded <br /> manufactured TCE is from -33.5%o to -27.8%o. The 613CTCE values shown in Figure B.4-4 are all <br /> greater than this range thus providing direct evidence that biodegradation of TCE has <br /> occurred. A linear correlation between 637CITCE and 613CTCE values is apparent in Figure B.44, <br /> indicating that 637CITCE values also increased with biodegradation, however, the magnitude of <br /> fractionation was much smaller for chlorine isotopes than was observed for carbon and <br /> therefore 613CTCE values appear to be a better indication of COC degradation. <br /> A plot of 613CTCE versus 6DTCE values for groundwater samples is shown in Figure B.4-5 and <br /> compared with the isotopic composition reported for known manufacturers of TCE reported in <br /> EPA (2008). The 6DTCE values for groundwater samples from M-1 OA, EW-1, M-6C1, M-31 C1 <br /> and M-6C2 ranged from +121%o to +143%o which is substantially lower than the published <br /> range of values reported for TCE manufactured by PPG, StanChem, DOW and ICI (where <br /> 6DTCE > +400%o). No correlation between 6DTCE and 613CTCE values is apparent in Figure B.4- <br /> 5. The 6DTCE values also are lower than the range reported manufactured TCE but much <br /> higher than the range reported for TCE that would have formed as a breakdown product of <br /> tetrachloroethene, which would have much lighter (negative) 6DTCE values (EPA, 2008). The <br /> published data on manufactured 6DTCE values and groundwater 8DTCE values from chlorinated <br /> solvent release sites are limited; this complicates the interpretation of the 6DTCE values and <br /> therefore the values are not discussed further in this report. <br /> 3.3.3 Direct Evidence of Net COC Destruction <br /> For the purposes of this analysis, the term "net COC destruction" refers to a decrease in the <br /> total moles of chlorinated ethenes in the groundwater system. Specifically, TCE--).cDCE would <br /> not result in net COC destruction because the total number of moles of chlorinated COCs has <br /> not changed. However the biotransformation of vinyl chloride to non-chlorinated end products <br /> (ethene and chloride or CO2 and chloride)would result in net COC destruction. Direct <br /> evidence of COC mineralization can be obtained by comparing paired 613CTCE and 613CDCE <br /> values for each groundwater sample where both values could be measured. According to <br /> Equation 2, cDCE produced from the reduction of TCE will always be "lighter" or depleted in <br /> 13C compared to the parent TCE based on the magnitude of the enrichment factor s. <br /> Therefore, if 613CDCE > 613CTCE , then cDCE is being mineralized or transformed to vinyl choride <br /> (Wilson, 2010). <br /> A plot of all pairs of 613CTCE versus 613CDCE values is shown in Figure B.4-6, along with the <br /> relative molar concentrations of TCE, cDCE and vinyl chloride for each groundwater sample. <br /> The dashed line shown in Figure B.4-6 divides the plot into two areas: above the line 613CDCE < <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 134-13 <br />