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GEOMATRIX <br /> Approach (there currently are no WQOs for TPHmo); it is included herein as supplemental <br /> information. Additionally,because there is no potentiai source for MtBE at the site, this <br /> information is also supplemental. <br /> Groundwater samples were collected by Geomatrix on 5 November 1998 and submitted to <br /> Friedman and Bruya Laboratories of Seattle, Washington, for the following analyses: <br /> • PAHs using EPA Method 8270 SIM <br /> • TPHd using EPA Method 8015M with silica gel cleanup based on EPA Method <br /> 3630M. <br /> Samples were collected and analyzed for PAHs using EPA Method 8270 SIM in order to <br /> achieve lower detection limits than those achievable with Method 8270. These lower detection <br /> limits are required to compare PAH concentrations to WQOs. Silica gel cleanup was used as <br /> part of the TPHd analysis to remove any potential positive interferences from soluble biogenic <br /> material in groundwater samples. This soluble biogenic material can either be naturally <br /> occurring or can result from the intrinsic biodegradation of the residual petroleum. The use of <br /> silica gel cleanup has been approved by the RWQCB as part of the Consistent Technical <br /> Approach. <br /> 4.0 RESULTS <br /> This section presents the results from the soil and groundwater investigations conducted by <br /> Radian in July and August 1998 and by Geomatrix in October and November 1998. Soil <br /> sampling locations are presented on Figure 3; groundwater elevation and analytical data are <br /> shown on Figure 4. Soil and groundwater analytical results are summarized in Tables 3 <br /> through 5. Laboratory reports are presented in Appendix D. <br /> 4.1 HYDROGEOLOGY <br /> The soil underlying the site is composed primarily of silts and clays from ground surface to <br /> approximately 25 feet bgs,with sandy gravel lenses predominantly at depths of 25 feet bgs and <br /> greater. The water table is present at approximately 73 to 75 feet above mean sea level (MSL), <br /> or 30 to 32 feet bgs(Table 2). Groundwater elevation data collected from wells MW-2, MW-3, <br /> and MW-4 indicate that the groundwater gradient is approximately 4.01 to the east-northeast <br /> (Figure 4). MW-1 was not used to estimate the groundwater gradient because of separate- <br /> phase petroleum in this well(estimated to be approximately 6 inches thick). <br /> 1Lsf31dep[dasa\Doe_Safe15000sr501 11SI HRA Rpt.doc 7 <br />