SITE INFORMATION AND CORRESPONDENCE

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Ms. Kathy Amaru Estimates of Contaminants in Soil4k Groundwater . PG&E McMullin Natural Gas Dehydration Station April 25, 2008 • To determine the average concentration of hydrocarbons in the volume of soil defined by the polygon, the impacted soil sample concentration and '/ of the corresponding ND detection limit were averaged; • In-place soil density was estimated at 130 ib/ft3 (59 kglft3). After determining the above data, the following equations were used to estimate the mass of each contaminant remaining in soil. Inputs and results to these equations are depicted in Table A. C, f� Si = 2 Xy, Xp s = Is; Where: C; = The detected contaminant concentration in soil in the ith polygon; R = The reporting limit of the detected contaminant; V;= The volume of impacted soil in the ith polygon; Si= The mass of contaminant remaining in soil in the ith polygon; p = Soil density; and S = The total mass of contaminant remaining in soil. Estimate of Contaminants Remaining in Groundwater To estimate the mass of contaminants in groundwater the following data were required: the hydrocarbon concentrations in groundwater, the volume of water to be considered, and soil porosity. Once these data were compiled, a calculation of remaining hydrocarbon mass was possible. To determine these data, the following assumptions were made: • All contaminant concentrations are assumed to be '/ of the detection limits for the estimation because no contaminants were detected in the most recent groundwater monitoring event of May 2004; • Mass of remaining contaminants is estimated within Site boundaries of 85ft x 100ft, and vertically from the top of the groundwater table (as measured in the groundwater monitoring event of May 2004) to 16ft (depth of the 2002 excavation at the Site); and • Soil porosity is assumed to be 50%. The above data were used as inputs to calculate the mass of hydrocarbons in groundwater. The inputs and results of calculations are depicted in Table B. Page 2 of 3