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' GEOMATRIX <br /> potential for additivity due to multiple constituents, do not pose an unacceptable risk to future <br /> on-site residents. These data indicate that future use of the site for residential purposes should <br /> ' not be restricted due to past impacts by petroleum hydrocarbons. <br /> Groundwater data suggest that the residual petroleum present in the soil is not providing a <br /> ' significant source of dissolved petroleum constituents to groundwater. Groundwater samples <br /> collected from the two locations in the assumed downgradient direction from affected soil <br /> ' (GB4 and GB5) did not contain dissolved petroleum at concentrations above water quality <br /> ' objectives (WQOs)that have been established by the RWQCB at similar sites such as the <br /> Dividend property. Based on our review of the chromatograms, "detections" of petroleum <br /> ' constituents in the grab groundwater sample collected from within the affected soil are likely <br /> the result of interference from petroleum-affected soil particulates (<0.7 microns in size for <br /> ' TPHd analysis) entrained within the sample. <br /> While it is likely that residual petroleum present in the soil is not providing a significant source <br /> ' of dissolved petroleum constituents to groundwater,based on discussions held on 18 August <br /> 1998 with Mr. Michael J. Infurna of SJCPHS, Geomatrix recommends installing two temporary <br /> ' groundwater monitoring wells near borings GB-1 and GB-4 to further assess this conclusion <br /> ' (see Figure 3): These wells will be sampled once after they are installed, and again <br /> approximately one month later to confirm initial sampling results. Geomatrix will recommend <br /> ' that the site receive regulatory closure (if groundwater is not affected above WQOs) or be <br /> transferred to the RWQCB for future regulatory oversight(if groundwater is affected above <br /> ' WQOs). <br /> ' 1:Ow SaW780\surIdrpt.dm 6 <br />