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AECOM Report on Soil Vapor Sampling and Human Health Risk Assessment 36 <br /> years). Environmental degradation has not been accounted for in the calculation of risks for <br /> the site. However, it is possible that compound concentrations evaluated in this report may <br /> decrease over the exposure duration due to biodegradation of petroleum hydrocarbons <br /> (USEPA, 1997).There is sufficient oxygen(11%)present for biodegradation to occur on site, <br /> as indicated in the laboratory analysis reports (Attachment B). The model does not account <br /> for this decrease in concentration over time and therefore, it is possible that it may <br /> overestimate potential risks for biodegradable petroleum hydrocarbons. <br /> 3.3 Vapor Intrusion Modeling and Risk Evaluation Summary <br /> A vapor intrusion evaluation was performed to assess the potential carcinogenic and non-carcinogenic <br /> inhalation risk to a resident and a commerclaVindustrial worker in a curentifuture on-site building from <br /> VOCs in soil vapor that may potentially volatilize into indoor air.The current and likely future land use <br /> conditions will be commercial1industrial; however, a potential future residential scenario was evaluated <br /> as a conservative measure for completeness. The evaluation was conducted based on the soil vapor <br /> results collected from the October 2010 sampling event, which were those data determined to be of <br /> sufficient quality for risk assessment. <br /> An evaluation of all available soil vapor data, collected from the May, September, and October 2010 <br /> sampling events was also conservatively performed for inclusion in the uncertainty assessment.The <br /> risk evaluation, conservatively based on the maximum detected soil vapor concentrations from all <br /> available sampling events, indicates that the potential ELCR and HI associated with the vapor <br /> intrusion pathway estimated for a resident and a commercial/industrial worker scenario are below <br /> CalEPA's and USEPA's target cumulative risk range and target HI level. <br /> December 15,2010 <br /> 60146576-M10 <br />