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Supplemental Calculations four Closure Review September 8, 2011 Former BP Station No. 11193 3202 West Hammer Lane,Stockton, CA Closure Solutions identified only one soil sample containing MTBE that was collected above the current water table for use in the residual mass calculation. To provide a more conservative estimate of residual MTBE mass in soil, the SJCEHD requested that six additional samples containing MTBE, which were collected beneath the former underground storage tanks (USTs) at 15 feet bgs be used in the calculation. Closure Solutions notes that these sample locations are currently below the water table by up to 4 feet. The residual mass calculation includes two discreet areas: one for soil within the vadose zone and one for soil beneath the water table. During facility upgrades conducted in December 1998, MTBE was reported in sample D-3, collected beneath a dispenser, at a concentration of 1.8 mg/kg. MTBE was also reported in six samples collected beneath the former USTs at concentrations of 74, 11, 40, 64, 80, and 11 mg/kg. t For the purposes of the calculation, it is assumed that sample D-3 was collected at 2 feet bgs, which represents the depth of a typical surface release and extended to groundwater (approximately 12 feet bgs). Closure Solutions has conservatively estimated the volume of impact to be 10 feet by 10 feet by 10 feet deep. For samples collected beneath the former USTs, Closure Solutions averaged the concentrations and conservatively estimated the volume of impact to be the lateral limits of the former UST excavation (estimated to be 25 by 38 feet) with a vertical thickness of 10 feet. The total estimated residual mass of MTBE in soil is approximately 53.92 pounds. Closure Solutions notes that the analytical data used to estimate the soil mass was collected nearly 13 years ago, and the estimated residual mass is likely much lower following nearly 13 years of natural attenuation. Calculations and assumptions are presented in Attachment A. To determine residual mass in groundwater, Closure Solutions assumed that the concentrations of DRO and MTBE detected in the groundwater samples collected during the February 2011 — monitoring event were representative of Site conditions. The accuracy of these calculations is based on the data available and the actual mass may be over represented or under represented. DRO was detected using silica gel cleanup in three wells (MW-9, MW-13, and VW-1) during the February 2011 monitoring event at an average concentration of 477 micrograms per liter (ug/L). The depth to water at the Site during the event was approximately 12 feet bgs and DRO was reported in deep well MW-13 which is screened down to 113 feet bgs. Therefore to be conservative, a water column of 101 feet was used in the calculation. Based on the assumptions of a 3,175 square foot area of impact, a vertical impact zone of 101 feet, and an assumed porosity of 30 percent; the estimated mass of DRO remaining in the dissolved phase is approximately 2.86 pounds (see attached calculations). As shown on Figure 4 in Stantee Consulting Corporations first quarter 2011 monitoring report, MTBE was detected in three wells during the February 2011 monitoring event at concentrations 3of4