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' AECOM Report on soil Vapor Sampling and Human Health Risk Assessment <br /> i <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 center site located <br /> 5400 Paces Avenue, nd the fornmer Unocal service stationontgomery Wards, uste (No. 5098),ards)auto1 loce cated at 5606 Pacfict <br /> 1 Avenue, Stockton, posedCalifornia.soie results l vapor. The sites currently, and s expected to remain,,potentialhis investigation were used to assess the comme cial <br /> building occupants posed by P <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 /> targt vacuum was also not wase <br /> econducted in October, duriingvwhich sampling and analyzed.A <br /> criteriaewserreampling event <br /> mela <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 1x104)and total HI of <br /> 1,for the residential scenario.Therefore, based on these results, concentrations of�tre�cted volatile <br /> property's <br /> ' compounds in soil vapor do not pose a human health concern at the site for the r s current <br /> industrial1commercial 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 mese <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 1 x10' 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 /> r <br /> December 15,2010 <br /> 60146576M10 <br />