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ATC Associates Inc. <br /> 1117 Lone Palm Avenue,Suite 201 <br /> Modesto,California 95351 <br /> 209-579-2221 <br /> (Y/ATC. Fax:209-579-2225 <br /> AS SO C I A T E $ INC. <br /> 7.0 LOW RISK CLOSURE CRITERIA <br /> Criterion 1: The source of the UST release has been identified and removed. <br /> The UST system which was removed in 1987 appears to have been the source of the release.Data <br /> collected from the site do not suggest a release from other sources. <br /> 1a Criterion 2: The vertical and horizontal extent of subsurface impact has been adequately <br /> characterized. <br /> The vertical extent of adsorbed-phase hydrocarbons is localized in the vicinity of the former UST <br /> system. Historic soil analytical data indicate the area of greatest impact is in the vicinity of SB3 <br /> and VW3. The highest concentration of TPHg, 1,900 mg/Kg, was reported in the soil sample <br /> collected from SB3 at a depth of 20 feet bgs. Based on the laboratory analytical data, petroleum <br /> La impacted soil extended from 15 to 45.5 feet bgs in the vicinity of the former UST location and at <br /> VEW3. Low concentrations of petroleum hydrocarbons were detected in soil samples collected <br /> from MW2 at depths of 40.5 and 50.5 feet bgs. The petroleum hydrocarbon concentrations <br /> �., appeared to attenuate with depth and did not appear to extend to the water table. These levels are <br /> relatively low, less than the ESLs, and likely to have diminished as a result of the SVE effort and <br /> natural attenuation. <br /> Dissolved hydrocarbons in groundwater have been adequately assessed laterally and vertically. <br /> Data collected from historical subsurface investigations suggest that the contaminant plume <br /> appears to be localized in the vicinity of MW 1,MW2,and MW3. <br /> Criterion 3: The dissolved hydrocarbon plume is not migrating. <br /> Mann-Kendell statistical data(Section 4.6) indicate the dissolved-phase hydrocarbon plume is not <br /> migrating. Recent TPHg, TPHd, BTEX, and MTBE concentrations trends are either decreasing <br /> or stable, with the exception of MTBE data modeled from MW1 and MW2. MTBE <br /> concentrations are below the MCL. Refer to Appendix C for supporting data. <br /> La <br /> Criterion 4: No water wells, deeper drinking water aquifers, surface water, or other sensitive <br /> receptors are likely to be impacted. <br /> La <br /> Historical data collected at the site suggest that the localized contaminant plume is defined within <br /> the vicinity of the release source(i.e. former UST system). <br /> r" 8.0 CONCLUSIONS <br /> * + Based on laboratory analytical results, plume stability statistics, historical contaminant plume <br /> LM distribution and field observations, the lateral and vertical extent of petroleum hydrocarbon- <br /> impacted soil and groundwater are adequately defined. <br /> w.. <br /> S:Environmenta11625771Reports12010 closure summary.doc 10 <br /> V.. <br /> LM <br />