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Human Health Screening Evaluation <br /> Former Vernalis Pump Station September 2,2010 <br /> 7. UNCERTAINTIES <br /> Section 7 discusses the uncertainties inherent to the human health cancer risk and non-cancer <br /> health hazard evaluation performed during this HHSE. Site-specific conditions that contribute <br /> most significantly to the uncertainty of the risk and hazard estimates are identified and discussed. <br /> Site characterization studies were conducted between 1995 through 2008 with the oversight of <br /> the Central Valley RWQCB. Soil samples were collected between 0 and 10 feet bgs from 33 soil <br /> borings. Detection limits for the COPCs in the boring samples ranged from 0.0005 to 7.3 mg/kg. <br /> Data of adequate analytical quality(e.g., low detection limits)were collected to support an <br /> HHSE. <br /> Radian analyzed two PAHs (benzo[b]fluoranthene and benzo[k]fluoranthene) as one constituent <br /> (benzo[b,k]fluoranthene). SAIC-Benham did not include the results for benzo(b,k)fluoranthene <br /> in the quantitative evaluation of risk because individual results were not known. The results of <br /> all samples analyzed for benzo(b,k)fluoranthene reported non-detect levels; as a result, excluding <br /> these data is not expected to have a significant impact on the conclusions of this HHSE. <br /> SAIC-Benham performed this HHSE at the request of the Central Valley RWQCB after it was <br /> satisfied that adequate groundwater monitoring data had been collected from the site. This site <br /> was well characterized during the previous site investigations; consequently, uncertainties in this <br /> HHSE were reduced to an acceptable level. The on-site soil impacts are believed to be well- <br /> characterized; therefore, SAIC-Benham evaluated EPCs as the UCL of the mean soil <br /> concentration for all COPCs evaluated in this HHSE, with the exception of naphthalene. The <br /> UCL is a conservative estimate of the mean concentration, and is unlikely to lead to <br /> underestimation of exposure concentrations. SAIC-Benham used a calculated UCL to represent <br /> the EPC to avoid underestimating the exposure concentration for naphthalene, which was <br /> detected in only two of 67 samples. Typically, when the detection frequency is very low (less <br /> than 4 to 5 percent), as was the case for naphthalene, the median concentration is used as the <br /> UCL. <br /> SAIC-Benham did not quantitatively evaluate the potential for vapor intrusion of volatile <br /> chemicals to indoor air from groundwater. However,benzene was detected in only one of <br /> 42 groundwater samples and the concentration was below the MCL; and toluene was detected at <br /> very low concentrations in only two of 42 groundwater samples. PAHs were only detected at <br /> low concentrations in groundwater samples, indicating that the potential risks associated with <br /> this pathway are low. <br /> SAIC-Benham estimated on-site exposures and potential resulting risks and hazards using <br /> algorithms developed by Cal/EPA DTSC, which were based on EPA and Cal/EPA DTSC <br /> guidance and methodology. The equations evaluated exposures to soils by dermal, oral, and <br /> inhalation exposure routes using health-protective, conservative reasonable maximum exposure <br /> parameters. Additionally, SAIC-Benham used cancer and non-cancer toxicity values <br /> promulgated by Cal/EPA OEHHA and EPA IRIS to quantitatively estimate potential risk and <br /> hazards. Therefore, it is unlikely that the risks and hazards estimated during the development of <br /> this HHSE are significantly underestimated. In fact, considering the conservative and health- <br /> protective assumptions used to develop this HHSE, it is likely that the actual on-site potential <br /> future risks and hazards will be lower than those presented here. <br /> 12 �a The Benham Companies,LLC <br /> ,..frt j`® A Wholly Owned Subsidiary <br /> From&mxe to Solanoss <br />