ARCHIVED REPORTS_2011 REVISED FEASABILITY STUDY

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amec�9 As shown in Table B.4-6, percent net COC removal increases with distance from the WWTP, meaning that cDCE and vinyl chloride are produced from biotransformation of TCE near the WWTP, and then these compounds are destroyed as groundwater conditions change to more oxidizing farther downgradient. At TH-10, no net removal is evident from the CSIA data, consistent with cDCE and vinyl chloride production due to the highly reducing conditions in this area (Table B.4-4) and relatively close proximity to the reported release area. At greater distances from the WWTP, percent net COC removal increases, where at M-17C1, approximately 1,750 feet hydraulically downgradient of the WWTP, 87 to 88% net COC removal is calculated. Assuming a groundwater velocity of 3 feet per day (AMEC, 2009), the travel time for groundwater at M-1 7C1 can be calculated as follows: Travel time (t) = (1,750 feet)/(3 feet/day) = 583 days Equation 6 An average half-life for bulk COCs can be calculated based on the calculated 87% COC removal for M-1 7C1 and the 583 day (1.6 year) timeframe (t) from Equation 6. First order degradation kinetics are assumed, where the rate constant for degradation (k) is calculated with Equation 7: k=-In(f)/t Equation 7 A value of k=-1.3/year is calculated using t = 1.6 years and f= 0.13 (Table B.4-6; M-17C1). A half-life (t1,2) of approximately 0.5 years was calculated from k using Equation 8: t112=0.693/k Equation 8 This is within the range of values typically reported for TCE, cDCE and vinyl chloride (reported half lives are generally less than one year; Rifai et al.,2010). Degradation rates are expected to be highly variable farther from the WWTP based on the different redox conditions and different relative efficiencies of intrinsic remediation processes as summarized in Table B.4-4. Redox conditions, potential degradation pathways and associated rates likely also are variable and change with distance from the WWTP (Table B.4-4). Nonetheless, the CSIA data indicate the all target COCs at the subject site, including vinyl chloride, are subject to degradation in the groundwater environment near the WWTP, and that the rate of degradation can be relatively rapid. Additional data will be collected during the implementation of the intrinsic remediation strategy as part of the groundwater remedy for the WWTP area. This implementation will provide additional site specific information that will be used to further characterize the spatial distribution of redox conditions and degradation pathways and rates. Prior to the remedy implementation, however, the geochemical evaluation performed to date, including the CSIA analysis and evaluation, provides strong evidence that intrinsic remediation is an effective remedial alternative that is worth including as part of the overall remediation plan for the WWTP. AMEC Geomatrix, Inc. \\oad-fs1\doc_safe\9000s\9837.006\4000 REGULATORYTS Assessment_Apx B_012711\Attachment B.4\Attach B-4.docx 134-17