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0 • <br /> September 21,2005 <br /> Mr.Nuel Henderson,P.G.,SJCEHD Project No.U6100-15-1 <br /> ARCO Station 6100,Tracy,California <br /> Additional Site Assessment Work Plan <br /> Page 6 <br /> illustrated on Figure 6 depict groundwater flow in <br /> The groundwater elevation contours <br /> to the N17°E strike calculated for the subsurface <br /> sand zone 2 that is nearly perpendicular <br /> stratum, with groundwater flow towards the down-dip direction. Based on this <br /> observation, groundwater movement in sand zone 2 appears to be significantly influenced <br /> by the dip of the subsurface stratum beneath the site. <br /> PROJECT APPROACH <br /> Stratus is proposing to advance five exploratory soil borings (B-44 through B-48) <br /> downdip/downgradient of the service station property to further characterize soil <br /> conditions and the extent of impact in these areas. Boring locations were selected based <br /> on requests by SJCEHD personnel in the August 11, 2005 letter, findings of previous <br /> investigation activities, and our understanding of subsurface petroleum hydrocarbon <br /> distribution and transport. Due to relatively difficult drilling conditions (hard soils), <br /> sampling of soil with a California-modified split spoon sampler is not practical. Stratus <br /> will therefore advance the borings using mud-rotary coring methods, similar to <br /> previously advanced borings B-33 to B-42. The proposed soil boring method will enable <br /> continuous recovery of subsurface strata and collection of depth-discrete soil samples at <br /> the desired intervals. <br /> A review of Figure 7, which illustrates gasoline range organics (GRO), benzene, and <br /> methyl tertiary butyl ether (MTBE) concentrations for well samples collected from wells <br /> screened within sand zone 2 (deeper screened wells) during the second quarter 2005, <br /> indicates that the lateral extent of petroleum hydrocarbon impact to groundwater remains <br /> undefined south and southeast of the site. Based on our understanding of the site <br /> geology, hydrogeology, and the findings of previous investigations, residual petroleum <br /> hydrocarbon impact in soil off-site is indicative of areas within the subsurface where <br /> petroleum hydrocarbon impacted groundwater will be encountered. <br /> Off-site, the highest concentrations of dissolved petroleum hydrocarbons in groundwater, <br /> and free product, occur in wells screened within sand zone 2 (MW-1, MW-10, MW-12, <br /> and MW-15). The northward extent of groundwater presence within sand zone 2 appears <br /> constrained by dry wells MW-3, MW-5, and MW-7. The southwestern extent of <br /> petroleum hydrocarbon impact appears constrained by the non-detection of GRO <br /> constituents, and the minimal presence of benzene (0.089 milligrams per kilogram <br /> [mg/Kg] maximum concentration) in soil samples collected from boring B-41. Proposed <br /> soil borings B-44 through B-48 will thus be advanced in locations south-southeast of well <br /> MW-12, and southeast, east, and northeast of well MW-15, to further assess the extent of <br /> petroleum hydrocarbon impact off-site. <br /> Once the analytical results of the soil samples collected from borings B-44 through B-48 <br /> have been received, and compared with geologic relationships observed, a network of <br /> K:\Bp-Arco for URS\6100\Work Plans\arco610o-workplan 09.2005.doc <br />