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S ECOR <br /> 6.2 GROUNDWATER <br /> As presented also in Table 3, onsite groundwater EPCs (95% UCL of four most recent 2006-2007 <br /> monitoring events) were compared to California MCLs and Tier 1 groundwater ESLs based on drinking <br /> water toxicity, indoor air impact, chronic freshwater aquatic habitat goals (even though there are- <br /> ,no <br /> surface water bodies i6 the vicinity of the Site), and ceiling values (odor, taste, etc.)for current and future <br /> land use evaluation. Although the receptors of concern at the Site are commercial/industrial workers and <br /> utility/construction workers, residential ESLs werealso presented in Table 3 for reference';purposes <br /> (RWQCB, 2005). California MCL, Tier 1 ESL based on residential drinking water toxicity, and USEPA ' <br /> Region 9 residential drinking water PRG were exceeded by benzene (slight exceedance, 954 UCL of <br /> 2.12 pgIL versus MCL of 1 pg/L). Tier 1 groundwater ESLs, based on indoor air impact levels, were not <br /> exceeded by any COPCs and there are no corresponding ESLs for DIPE. . , <br /> This Tier 1 'RBCA comparison for groundwater is extremely conservative for the following reasons: <br /> • Groundwater is not a potential direct contact exposure route at the Site: <br /> o There�are no current or anticipated uses of the impacted water within the timeframe <br /> projected to meet water quality objectives. <br /> o There are no existing water supply wells identified within a radius of 2,000 feet from the <br /> Site. <br /> o Chemical constituents are not migrating offsite (as such, estimation of the distance the <br /> residual plume will travel before water quality objectives are achieved is not needed). <br /> o On-site contamination represents a decreasing source strength due to biodegradation.. <br /> k - <br /> . w <br /> i <br /> F <br /> 14 <br />