SITE INFORMATION AND CORRESPONDENCE

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' Lee Higgins -2 19 May 2011 Mountain House development Site#6 ' Assessment (PEA) Guidance Manual. In conjunction, the cancer and non-cancer toxicity values promulgated by the California Office of Environmental Health and Hazard Assessment (OEHHA) were also used to quantitatively estimate potential risk and ' hazard. SAIC indicated that it is unlikely that risks and hazards estimated for the HHSE would be underestimated. ' • In accordance with the PEA guidance, analytical results for benzene, toluene, ethyl- benzene, and xylenes (BTEX) and polynuclear aromatic hydrocarbons (PAHs) were considered. Lab results of 49 soil samples collected from 38 borings at depths between ' 0 and 10 feet below ground surface (bgs) were used. The HHSE was performed considering the BTEX and PAH constituents detected in shallow soil. ' • Groundwater analytical results for general minerals indicated that the shallow groundwater exceeds several California and Federal total dissolved solids for drinking water as well as some agricultural purposes. Shallow groundwater is not likely to be ' used as a drinking water source and the direct exposure route was considered incomplete and not evaluated quantitatively for the HHSE. ' • SAIC reviewed Department of Water Resources files to identify water supply wells within 2,000 feet. Five wells identified northwest downgradient; two wells lateral-cross gradient were previously sampled during the Mountain House Development Site #3 ' investigation; analytical results were less than laboratory method detection limits. Considering the distance from the Site, it is unlikely these wells would be affected by the TPH compounds in groundwater. Site characterization information describes exposure point concentrations (EPCs) that were used to calculate the upper confidence limit (UCLs) with a statistical software program. Since soil and groundwater were delineated, and the characterization dataset adequately describes Site conditions, the EPCs were calculated as the UCL using the mean soil concentration for each constituent of potential concern (COPC). ' . In accordance with the National Contingency Plan, an incremental lifetime cancer risk of 1E-06 (i. e., 1 in 1 million), was used as a point of departure for the HHSE. For non- cancer hazard indices, a hazard index of 1 or less is considered an acceptable health hazard level. The HHSE was prepared to estimate future onsite residential risk and hazards under current site conditions. In addition, the evaluation was developed using ' health protective, conservative risk and hazard algorithms developed by DTSC and provided within the PEA Guidance Manual and by California OEHHA cancer and non- cancer toxicity values. ' . Future oral and direct contact exposures to onsite soils and cancer risks were estimated and the risk for individual COPCs and summation of risks of all COPCs for these ' exposure routes were determined. Potential future indirect inhalation exposures to onsite soils and cancer risk were also estimated. The combined total potential cancer risk to a hypothetical future resident from for oral, dermal, and indirect inhalation 1 exposures was estimated to be 5E-06 which is above the lifetime cancer risk of 1 E-06, but within the acceptable risk range of 1 E-06 to 1 E-04 for exposure to onsite soils.