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APPENDIX C POST-REMEDIATION GROUNDWATER MASS-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 using the average screening interval at the site (equal to 20 feet bsg)and by separating the shallow impact into three contoured zones (i.e. 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 from the most recent groundwater sampling events and selected grab groundwater data collected at the site are considered to be representative of current dissolved site conditions. • The thicknesses of the dissolved plumes were derived using the average screening interval of wells installed within the dissolved plume (20 feet). • A figure depicting the lateral extent of dissolved TPH-g at the site (Figure 7)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 shallow zone impact at the site the program determined the area of the 10,000 pg/I contour interval to be 549 ft2, the 1,000 pg/I contour interval to be 3,430 ft2 and the 100 pg/I contour interval to be 5,480 ft2. • The area and volume of each contoured zone can be estimated utilizing the program to determine the area between the above referenced 10,000 pg/I, 1,000 pg/I and 100 pg/I contour intervals (Figure 6) 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 6) and "h" is the inferred height based on the selected interval and cross-section. Shallow water bearing zone plume mass calculation: 1) For the estimated 10,000 pg/I inner contour interval; A,o,000= 549 ft2 and h = 20 ft (average screen interval) Utilizing TPH-g data from wells MW-1 (18,000 pg/1)and MW-9A (10,000 pg/1)and the 10,000 pg/I contour line (Figure 6), the average dissolved TPH-g concentration is estimated to be 12,666 pg/I (equivalent to 0.000012666 kilograms per liter [kg/1]). The volume is given by: V,o,000 = A,o.000 x h = 549 ft2 x 20 ft = 10,980 ft3