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' 05 May 1997 AGE-NC Project No 95-0173 Page 11 of 29 yields approximatelSl, 11 3 gallons of gasoline in the remaining impacted soil at the site (Appendix 1=) Assuming that wells MW-1 through MW-3, VW-1, VW-2, AW-1, AW-2, soil borings B1 through B3 and B6 are located within the petroleum hydrocarbon impacted soil area, a short diameter of 120 feet, a long diameter of 130 feet and a height of 65 feet were used to estimate a coned elliptical volume of impacted soil The distribution of dissolved petroleum hydrocarbons were approximated by utilizing four co-centric elliptical sequences, with the inner sequence containing ' the higher petroleum hydrocarbon concentrations An average concentration of 5,000 ppm was used to calculate the total volume of remaining hydrocarbons in the inner sequence, 1,000 ppm and 100 ppm were utilized to calculate the volume within the middle sequences, and 50 ppm was utilized to calculate the total volume of the outer sequence Petroleum hydrocarbon-impacted soil volume assumptions and calculations are also included in Appendix G Ground water has been sampled at the site on sax occasions During the ground water sampling events, substantial petroleum hydrocarbon concentrations have been detected in ground water samples collected from MWA through MW-4, MW-6, MW-9, VW-1, VW-2 Significant concentrations of dissolved petroleum hydrocarbons have been detected from AW-1 Atth a screened Interval of 50 to 60 feet bsg, and from AW-2 with a screened interval of 60 to 70 feet bsg Additional assessment is warranted northwest of MW-6, southeast of MW-3, and southeast of MW- 9 for delineation of the dissolved petroleum hydrocarbon plume Further vertical assessment is warranted in the area between AW-1 and AW-2 for vertical delineation 6.0 POTENTIAL IMPACT ON NEARBY SURFACE AND GROUND WATER Typically, ground water would represent the migration medium for contaminants over significant 1 horizontal distances Sites located In up-gradient or cross-gradient directions from the subject property are not Iikely to be negatively impacted by the subject property The nearest surface water feature to the site is the Calaveras River, located approximately 4,500 feet northwest(cross-gradient) ' from the site Based upon the distance and crossgradient direction from the site, it is very unlikely that impacted ground water at the site represents a significant pollution threat to water in the Calaveras River Drinking water near the site is supplied by the City of Stockton (California Water Service Cal r Water)from municipal wells First encountered ground water is not used for drinking or irrigation, although all ground water in the State of California is considered to be of potential beneficial use One Cal Water municipal well is located 650 feet northeast of the site, while another Cal Water municipal water well is located 1,350 feet southeast of the site (See Section 2 5 ) Based on the distance and the upgradient direction to the site, the municipal water well located southeast of the site poses minimal concern However,the municipal water well (Station 4)located 650 feet northeast 1 Advanced GeoEnvironmental,Inc