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Supplemental Calculationsfir r Closure Review September 8, 2011 Former BP Station No. 11193 3202 West Hammer Lane, Stockton, CA located at least 40 feet up-gradient of the dispenser islands, a suspected source area, and Site investigations have not identified a source in the vicinity of the wells. The February 2011 concentrations are within the historical range, and the relatively non-mobile constituent is not expected to significantly migrate. Although concentrations of DRO have historically fluctuated in the shallow zone, shallow DRO concentrations have not significantly migrated, the impacts are localized and immobile, and the concentrations are adequately delineated by the current monitoring well network. While a decreasing concentration trend cannot be demonstrated for DRQ in all wells, including deep well MW-13, decreasing trends are evident in wells MW-3 and MW-7, and it is clear that the reported concentrations are well below concentrations considered low-risk by the SWRCB Task Force. A declining trend in surrounding wells for relatively non- mobile constituents such as DRO is considered to be evidence that the overall extent of the contaminant plume is shrinking. Insufficient data above the laboratory reporting limit (for shallow monitoring wells currently exceeding the water quality objective of 100 ug/L) is available to perform a regression analysis and predict a timeframe to reach water quality objectives for DRO. ESTIMATED MASS REMAINING The estimated residual masses of DRO and MTBE in soil were calculated using soil sample information obtained from historical Site investigation summaries and the San Joaquin County Environmental Health Department (SJCEHD), since no table of historical soil sample results was available. Three detections of DRO in soil have been reported. During a November 1994 investigation, DRO was reported in sample TPHIS-10 at a concentration of 1 milligram per kilogram (mg/kg), in sample TD-2 at a concentration of 7,600 mg/kg, and in sample TD-3 at a concentration of 3,400 mg/kg. For the purposes of the calculation, a 10-foot thick soil column was used, which represents the depth of a typical surface release (approximately 2 feet below ground surface [bgs]) down to the reported groundwater depth of 12 feet bgs). Closure Solutions has conservatively estimated the volume of impact to be 10 feet by 10 feet by 10 feet deep. The estimated residual mass of DRO in soil is approximately 1,331.12 pounds. Closure Solutions notes that the laboratory analytical report indicates that sample TPH1S-10 contained a lower boiling point hydrocarbon mixture, quantified as diesel, but the chromatogram did not match the typical diesel fingerprint, and samples TD-2 and TD-3 contained a mixture of diesel and lower boiling point hydrocarbons, quantified as diesel, but the chromatograms did not match the typical diesel fingerprint. Analytical data used to estimate the soil mass was collected nearly 17 years ago, and the estimated residual mass is likely much lower following nearly 17 years of natural attenuation. Calculations and assumptions are presented in Attachment A. 2 of 4