SITE INFORMATION AND CORRESPONDENCE

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Mr. Jeff Baker - 2 - 25 April 2008 Stockton Terminals Technicalomittee • ! up groundwater pollution migrating downgradient from the Site. The system is comprised of 57 A zone and 16 B zone wells connected to two 5-pounds-per-day (lbs/day) ozone generators. During the second, third and fourth quarters, depth to groundwater measurements ranged from about 1.4 feet below ground surface (bgs) in AR/MW-5A to 11.2 bgs in OW-1A. Monitoring well resurveying is proposed to resolve the A and B zone localized water table mounding typically present at wells PS/WC-1S and M and the radial inward flow at wells SP/WC-1S and M. Therefore, water elevations from these wells were not included in gradient calculations. During 2007, the direction of groundwater flow ranged from east-southeast in the A zone to the southeast in the B, C, and D zones. Horizontal gradients ranged from 0.002 foot per foot (ft/ft) in the C and D zones to 0.07 ft/ft in the A zone. The direction of the vertical gradient at the Site is predominantly downwards between the four hydrostratigraphic units. During 2007, the highest magnitudes of the downward gradients were 0.025 ft/ft between the A and B zones, 0.074 between the B and C zones, and 0.072 ft/ft between the C and D zones. Higher vertical gradients were calculated at the PS/WC=1 S and M well pair, but Regional Water Board staff understands the groundwater elevations from these wells should not be used until the proposed resurveying is completed. The highest COC concentrations typically underlie the BP and Time Oil facilities. As reported in the 1 February 2008 Fourth Quarter and Annual 2007 Site Status, Groundwater Monitoring and Remedial Summary Report (Annual Report), SPH or SPH sheen was observed in eleven A zone wells; 7 on the Arco Site, 4 on Time Oil, and one on Tesoro. Consistent with historical observations, no B, C, and D zone wells contained SPH. The highest COC concentrations observed in each hydrostratigraphic zone during the second, third, and fourth quarters of 2007, are tabulated below. Groundwater Zones A B C D cocs Maximum concentration (Ng/L)/Well ID TPHg 110,000/AR/MW-8A 16,000/AR/MW-11B 3,600/OW-9C 12,000/TS/MW-2D TPHd 160,000/TO/MW-1 18,100/AR/MW-1113 1,900/OW4C 2,200/TS/MW-2D benzene 9,600/TO/MW-1 9,400/AR/MW-11B 2,000/0W-9C 5,800/TS/MW-2D toluene 13,000/AR/MW-8A 110/AR/MW-11B 18/OW-9C 38/TS/MW-2D eth (benzene 4,300/AR/MW-10A 18/AR/MW-18B 5.1/OW-9C 4.3AR/MW-14D Total xylenes 16,000/AR/MW-10A 140/AR/MW-11B 23/OW-9C 59/TS/MW-2D MTBE 28,000/AR/MW-50 3,600/AR/MW-8B 200fTS/MW-1C 4.4/TS/MW-2D TBA 38,000/ MW-50 860/AR/MW-8B 400/TS/MW-1C 1,600 TS/MW-2D TAME 340/AR/MW-50 13/AR/MW-8B ND ND Explanation Ng/L micrograms per liter The OS system installed in 2004 is divided into two portions. The west OS system is downgradient of Tesoro and the east system is downgradient of BP. The STTC estimates that, collectively, the OS system has destroyed about 1,650 lbs of hydrocarbons since system operation commenced. This total represents an overestimation, however, since the lifetime