SITE INFORMATION AND CORRESPONDENCE

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Soil and Groundwater Investigation Report Mountain House Development Site#6 July 2008 Depth-discrete vertical profile samples were not collected in Area 2 and Area 3. 5.2.2 Suitability of the Groundwater for Drinking TDS concentrations ranged from 2,260 to 2,560 milligrams per liter(mg/L)in samples collected from SB-20 and SB-25,respectively(Table 6),which exceed the following current California Department of Health Services (Ca1DHS)Maximum Contaminant Levels(MCLs)for drinking water(Ca1DHS,March 2008): • 500 mg/L,recommended. • 1,000 mg/L,upper. • 1,500 mg/L, short term, as well as: • RWQCB Water Quality Goals(WQG), Ca1DHS and US EPA drinking water standards: 500 mg/L(RWQCB,August 2007). • RWQCB WQG,Agricultural Water Quality Goal: 450 mg/L(RWQCB,August 2007). • RWQCB WQG,US EPA Ambient Water Quality Criteria for Taste and Odor or Welfare: 250 mg/L(RWQCB,August 2007). The groundwater at this site exceeds numerous State of California and federal TDS standards for drinking water as well as agricultural-use criteria. Therefore, groundwater at this site will not be used as a drinking water source. 6. CONCLUSIONS AND RECOMMENDATIONS SAIC reviewed the findings of this investigation and the previous soil and groundwater investigation and concludes the following: • Field observations and laboratory analytical results indicate that there are at least three areas of affected soil and groundwater: o Area 1 ■ The vertical extent of affected soil has been defined by borings SB-9, SB-10, SB-11, and SB-15, at depths ranging from 35 to 39.5 feet bgs. Soil samples were not collected from borings SB-26 and SB-8 at deeper depths corresponding to where affected soil was detected in adjacent borings (SB-9, SB-10, SB-11, and SB-15). Therefore,the lateral extent of affected soil in this area is undefined. A source area has not been identified during field activities as evidenced by the absence of shallow affected soil. The down-gradient and up-gradient extents of affected groundwater in this area have been defined at borings SB-26 and SB-8,respectively. The vertical extent of affected groundwater has been defined by borings SB-10 and SB-15 at 35 and 30 feet bgs, respectively. o Area 2 ■ The affected soil and groundwater in this area has not been entirely delineated. Affected soil and groundwater was not encountered in borings SB-17, SB-19, SB-20, and SB-24, which delineates the southeastern extent of Area 2. Soil samples were not collected from borings SB-4, SB-6, and SB-7 at deeper depths corresponding to where affected soil was detected in adjacent borings (SB-13, SB-14, SB-16, and SB-18). Therefore,the lateral extent of affected soil in the northeastern portion of the area is undefined. The vertical extent of affected soil has been defined by borings SB-5, SB-13, SB-14, SB-16, and 8 AAe From Science to Solutions