SITE INFORMATION AND CORRESPONDENCE

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Human Health Screening Evaluation Mountain House Development Site#6 January 18,2011 investigation activities and results are included as Appendix B. All available analytical data from previous investigations at the Site are included as Appendix C. 2. EXPOSURE PATHWAYS AND MEDIA OF CONCERN SAIC constructed a CSM that identifies the on-site contaminant sources, release mechanisms, transport media, exposure pathways, exposure routes, and potential human receptors associated with a site. SAIC evaluated the on-site residential land-use scenario for this Site as required in the PEA Guidance Manual. The CSM for this Site is shown on Figure 3. SAIC evaluated direct soil exposure pathways (incidental ingestion of soil and dermal contact with soil), and indirect inhalation exposure routes (inhalation of wind-blown dust for non- volatile constituents and inhalation of chemical vapors from soils for volatile constituents). The evaluation of risk associated with the soil exposure pathways is limited to the upper 10 feet of on-site soils, consistent with the on-site residential land-use scenario as described in the PEA Guidance Manual. Only analytical results from soil samples collected between 0 and 10 feet bgs (below ground surface)have been included in this HHSE (Tables 1 and 2). The analytical results for COPCs from all groundwater samples collected by Condor and SAIC are provided in Appendix C. Total dissolved solids (TDS) concentrations in groundwater samples collected at the Site ranged from 2,190 milligrams per liter(mg/L)to 2,560 mg/L (Table 3), which exceed the following current California Department of Public Health(CDPH) maximum contaminant levels (MCLs; CDPH, 2004) and Central Valley RWQCB Water Quality Goals (WQG; RWQCB, 2008) for drinking water: • CDPH MCL: 500 mg/L, recommended; • CDPH MCL: 1,000 mg/L,upper; • CDPH MCL: 1,500 mg/L, short term; • Central Valley RWQCB WQG, Criterion for beneficial use designation of municipal and domestic water supply: 3,000 mg/L; • Central Valley RWQCB WQG, CDPH, and EPA drinking water standards: 500 mg/L; • Central Valley RWQCB WQG, Agricultural Water Quality Goal: 450 mg/L; and • Central Valley RWQCB WQG, EPA Ambient Water Quality Criteria for Taste and Odor or Welfare: 250 mg/L. The TDS results for groundwater exceed California and federal TDS standards for drinking water as well as agricultural-use criteria. Therefore, shallow groundwater at this Site is not likely to be used as a drinking-water source. SAIC considered the direct groundwater exposure route incomplete for these reasons, and did not quantitatively evaluate that route in this HHSE. There is no surface water body within the area of the affected soil and groundwater; therefore, as shown on Figure 3, this pathway is also considered incomplete. Inhalation of volatiles from groundwater is a potential pathway; however, benzene, toluene, ethylbenzene, and total xylenes (BTEX)were not detected in groundwater, and other volatile organic compounds (VOCs; acenaphthene, chrysene, fluorene, and naphthalene) were detected at 3 SAIE from sclence to Solvtloas