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APPENDIX C PRE-REMEDIATION GROUNDWATER VOLUME ASSUMPTIONS AND CALCULATIONS: TPH-G Super Center Mart 701 East Charter Way, Stockton, California Assumptions: • The distribution of maximum total petroleum hydrocarbons quantified as gasoline (TPH-g) concentrations on the site can be approximated by separating the impacted area into one depth discrete zone (equal to well screening interval lengths; 20 feet) and four contoured zones (i.e. 100,000, 10,000 pg/1, 1,000 pg/I and 100 pg/1) within the zones. • The effective porosity of soil at the site is estimated to be 30% (i.e. silt/sandy silt mixtures). • Data collected prior to corrective action (ISCO injections) during the 27 October 2010 monitoring event and grab groundwater samples historically collected during investigations was used to estimate the pre-remediation mass of dissolved TPH-g for the site. • The thickness of the dissolved plume was derived using the well screening length of 20 feet. • The zone depicting the lateral dissolved extent (Figure 5) was generated utilizing a computer assisted drafting program. The program has a specific function to calculate the area of a selected object, such as each area between the above referenced TPH-g isoconcentration intervals. For the impacted area , the program determined the area of the 100,000 pg/I contour interval to be 86 ft2, the 10,000 pg/I to be 1,985 ft2, the 1,000 pg/I to be 4,147 ft2 and the 100 pg/I contour interval to be 9,589 ft2. • The area and volume of each contoured zone can be estimated utilizing the program to determine the area between the above referenced 100,000 pg/I, 10,000 pg/l, 1,000 pg/I and 100 pg/I contour intervals (Figure 5) and then multiplying the area by the thickness as interpreted above. The volume of the TPH-g plume can be calculated by the following formula: V= AREA-h, where "area" is determined through the computer assisted drafting program (Figure 9) and "h" is the inferred height. 1) For the estimated 100,000 pg/I inner contour interval; A100,000 = 86 ft2 and h = 20 ft Utilizing TPH-g data from MW-3 (210,000 pg/1) and the 100,000 pg/I contour line (Figure 9), the average dissolved TPH-g concentration is estimated to be 155,000 pg/I (equivalent to 0.000155 kilograms per liter [kg/1]). The volume is given by: V100,000 = A100,000 x h = 86 ft2 x 20 ft = 1,720 ft3 Water occupies the porosity in the soil, which is estimated to be 30% of the soil volume, so the total volume of water in the saturated portion is approximated by: V100,000= (0.30)(1,720 ft3)= 516 ft3