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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment <br /> Executive Summary <br /> At the request of the Central Valley Regional Water Quality Control Board (RWQCB), soil vapor has <br /> been investigated at the former Montgomery Wards, LLC (Wards)auto service center site located at <br /> 5400 pacific Avenue, and the former Unocal service station site (No. 5098), located at 5606 Pacific <br /> Avenue, Stockton, California-The results of this investigation were used to assess the potential risk to <br /> building occupants posed by soil vapor. The site is currently, and is expected to remain, a commercial <br /> facility for the foreseeable future. <br /> Soil vapor was sampled in May, September, and October of 2010. Sampling was conducted as <br /> proscribed by the California Department of Toxic Substances Control (DTSC), New York State <br /> Department of Health(NYSDH), and Chevron guidance. Irregularities in the May sampling included an <br /> inability to achieve target vacuum levels in the sample canister, and the detection of leak tracer gas <br /> (i.e., helium). These irregularities led to a subsequent sampling of SV-1 in September,during which <br /> target vacuum was also not achieved and tracer gas was not analyzed.A"seasonal" sampling event <br /> was conducted in October, during which sampling and quality control criteria were met. <br /> Based on the October sample, the excess lifetime cancer risk(ELCR)and hazard index(HI)results <br /> are below the California EPA and USEPA's target cancer risk range (1x10 to 1 x10')and total HI of <br /> 1,for the residential scenario.Therefore, based on these results, concentrations of detected volatile <br /> compounds in soil vapor do not pose a human health concern at the site for the property's current <br /> industrial/commercial use. <br /> An uncertainty in the risk analysis for the October event was the elevated soil vapor concentrations <br /> previously observed during the May and September events.To address this uncertainty, additional <br /> evaluation of the May and September data was conducted. Although the May and September <br /> samples were not analyzed for air-phase hydrocarbons (APH), the data collected during these <br /> sampling events was manually calculated for incorporation in a risk assessment. Using this procedure, <br /> the ELCR and HI results for the May and September events were also determined to be below the <br /> CaIEPA and USEPA's target cancer risk range of 1 x106 to 1x10 4 and total HI of 1,for the residential <br /> scenario.Therefore,the highest detections observed from the site (i.e., the May and September <br /> events),which were reflective of lower quality data, also do not indicate a human health concern in a <br /> residential scenario at the site. <br /> Based on the results of the additional evaluation of the May and September data, the uncertainty <br /> associated with the October evaluation was eliminated. Risk evaluation results for each of the three <br /> events shows no elevated risk to human health. Additionally,the commercial use of the property <br /> represents an additional safety factor, as no risk was determined for a residential scenario.Therefore, <br /> no additional investigation is recommended. In the absence of other risk concerns at the site, a <br /> request for closure is recommended. <br /> December 15,2010 <br /> 6014657&M10 <br />