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APPENDIX D GROUNDWATER VOLUME ASSUMPTIONS AND CALCULATIONS : TPH-G Stallworth Auto Sales 602 North California Street, 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 two depth zones (20 to 50 feet bsg and 50 to 90 feet bsg) and four contoured zones (i . e . 1001000 pg/l , 10,000 pg/l , 1 ,000 pg/I and 100 pg/I ) 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 March 2013 groundwater sampling event sampling event and selected grab groundwater data collected between in 2000 and 2008 are considered to be representative of current dissolved site conditions . • The thicknesses of the dissolved plumes were derived using chosen zones between 20 and 50 feet bsg and 50 to 90 feet bsg . Thicknesses were further evaluated in cross- sectional view. • The two depth zones depicting the lateral dissolved extent ( Figures 12 and 13) were 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 20 to 50 foot zone , the program determined the area of the 100 , 000 pg/I contour interval to be 44 ft2 , the 10 , 000 pg/I contour interval to be 337 ft2F the 1 , 000 pg/I interval to be 736 ft2 and the 100 pg/I contour interval to be 1 ,346 W. For the 50 to 90 foot zone the program determined the area of the 100 , 000 pg/I contour line to be equal to 129 ft2• ,the 10 , 000 pg/I contour to be equal to 1 ,909 W , the 1 , 000 contour line to be equal to 3,434 ft' and the 100 pg/I contour line to be equal to 6,310 W. • The area and volume of each contoured zone can be estimated utilizing the program to determine the area between the above referenced 1001000 pg/I , 1 ,000 pg/I and 100 pg/I contour intervals (Figures 10 through 13) 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 10 through 13) and "h" is the inferred height based on the selected interval and cross-section . 20 to 50 feet bsg groundwater plume mass calculation : 1 ) For the estimated 100 , 000 pg/I inner contour interval ; A100,000 = 44 ft' and h = 10 ft (average thickness)