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La ATC Associates Inc. Y YV 1117 Lone Palm Avenue,Suite 201 Modesto,California 95351 209-579-2221 ATC ass O C I ATE s t N c. Fax:209-579-2225 La 4.5 Subsurface Vapor Petroleum hydrocarbon vapors were extracted from the subsurface over a two year period using MW2 as the extraction well. MW2 is screened from approximately 45 to 65 feet bgs. Data associated with the Lem SVE effort indicates the majority of petroleum hydrocarbon vapors have been removed from within the horizontal vacuum ROI of approximately 30 feet associated with MW2. Subsurface vapor samples with ` respect to potential vapor intrusion have never been collected from the site. Shallow subsurface vapor L samples are typically collected to evaluate the potential for soil vapor intrusion scenarios and to evaluate if the analytes of interest detected pose a risk to human health. Since shallow subsurface vapors have never been collected from the site, no evaluation was be made with respect to subsurface vapor intrusion. However, active remediation of vadose zone soil would be expected to reduce the risk of soil vapor intrusion. 4.6 Stability of Chemicals of Concern rv. A statistical evaluation of groundwater trends associated with the ten most recent groundwater sampling events were conducted for monitoring wells MW I and MW2 using the Mann-Kendall statistical model. The non-parametric Mann-Kendall test was used to evaluate time related trends in TPHg, TPHd, BTEX, and MTBE data. The Mann-Kendall test is considered well suited to the dataset because it can be used for data that are non-parametric (does not have a specific distribution, such as normal or log normal) and the dataset can contain data at irregularly spaced intervals. In addition, the dataset can contain elevated values compared to the average(outliers)or data reported as below the practical quantitative limit. Based on the Mann-Kendall test results included in Appendix C, TPHg, benzene, toluene, and ethyl benzene concentrations detected in samples collected from M W 1 during the last ten sampling events were determined to be stable. Xylenes concentrations were determined to be non-stable and TPHd and MTBE concentrations were determined to be decreasing and increasing, respectively. A decreasing trend in concentrations of TPHg, TPHd, and BTEX were noted in groundwater samples collected from MW2. An increasing trend with respect to MTBE was noted in groundwater samples collected from MW2. It is important to note that the MTBE concentrations have been reported at concentrations below the MCL since 2005. The results of constituent trend analyses associated with groundwater samples collected from ' MW I and MW2 during the last ten sampling events are summarized below. LW Mann-Kendall Statistical Test Summary 80%Confidence level Well TPHg TPHd Benzene Toluene Ethylbenzene Xylenes MTBE 1D MWI Stable Decreasing Stable Stable Stable Non-Stable Increasing MW2 Decreasing Decreasing Decreasing Decreasing Decreasing Decreasing Increasing * If no trend exists at the 80%confidence level,then the stability was test run. * 5.0 EFFECTIVENESS OF REMEDIAL ACTIONS As previously indicated in Sections 1.2 and 4.2,an SVE system was operated at the site for approximately 16,500 hours from January 17, 2007 to February 13, 2009. Operation of the SVE system was terminated La when extracted volatile organic vapor concentrations became asymptotic and too low to justify sustained F S:Environmental1625771Repons12010 closure summary.doe 8 V 1r