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• � 5 <br /> 1 <br /> 4.2 DRILLING AND SOIL SAMPLING <br /> Approximately nine onsite and two offsite borings will be drilled to a depth of approximately 20 <br /> fbg or 10 feet below the deepest detected hydrocarbon concentrations. See Figure 2 for the <br /> approximate boring locations. Borings have been located in the most feasible positions allowed <br /> by site conditions to further characterize hydrocarbon concentrations as requested by the San <br /> Joaquin County Public Health Services Department letter received by Mobil Oil Corporation on <br /> February 1, 1994. The borings have been located according to the following constraints: <br /> i <br /> • To characterize hydrocarbon concentrations vertically below-the depth of the current <br /> water table (approximately 10 fbg), two borings will be drilled in locations that prior to' <br /> the 1990 remedial excavation contained elevated hydrocarbon concentrations: One boring <br /> will be drilled in the vicinity of the former underground gasoline storage tanks, and a <br /> second boring will be drilled in the vicinity of the former above ground diesel storage <br /> tank location. The borings will be advanced 10 feet beyond the deepest detected <br /> hydrocarbon concentrations. ! <br /> i <br /> I <br /> • To characterize possible hydrocarbon concentrations in•tthe vicinity along the southern <br /> edge of the 1990 remedial'excavation, five borings will be drilled The borings will be <br /> advanced 10 feet beyond the deepest detected hydrocarbon concentrations. <br /> • To further characterize soil hydrocarbon concentrations to the west of the site, one boring <br /> will be drilled. The boring will be located in the PGE right of way immediately west of <br /> the limits of the 1990 remedial excavation. <br /> • To further characterize soil hydrocarbon concentrations to the east of the site, one boring <br /> will be drilled. The boring will be located on the adjacent property immediately east of <br /> the limits of the 1990 remedial excavation. <br /> • To further characterize soil hydrocarbon concentrations on the northern edge of the site, <br /> two borings will be drilled.' The borings will be located onsite at the most northerly point <br /> allowed by site specific coditions. Several factors will limit locating additional borings <br /> further north of the site in Grantline Road. Overhead power lines run above the sidewalk <br /> on the south side of Grantline Road. Numerous utility lines prevent drilling in the east <br /> bound lanes of Grantline Road and under the sidewalk on the south side of the road. <br /> Previously proposed borings in the center of Grantline Road were not drilled as a <br /> Chevron pipeline is located under the center median of Grantline Road. In addition, the <br /> soil on the north side of Grantline Road has been characterized in borings completed in <br /> previous years and apparently has not been affected by hydrocarbons from the Mobil site. <br /> Soil samples, from borings inside,the`1990 remedial excavation, will.be collected at 2.5 foot <br /> intervals starting at a depth of 10 fbg (below the depth of backfill), through the capillary fringe <br /> down to the total depth of the boring. .Soil samples, from borings outside of the 1990 remedial <br /> excavation, will be collected at 5 foot intervals above ground water and at 2.5 foot intervals from <br /> 5 <br />