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' and west of the site The former location of UST 81713-2 and adjacent USTs and features is <br /> presented on Figure 2 <br /> ' 2.2 SITE HYDROGEOLOGY <br /> Rough and Ready Island is located within the tidal delta of the San Joaquin River which <br /> ' drains the San Joaquin Valley of California Previous investigations conducted by Kleinfelder <br /> Associates (Kleinfelder, 1988) describe shallow and intermediate aquifers on the island During <br /> ' drilling, a shallow aquifer was penetrated at a depth of 5 to 20 feet below ground surface (bgs) and an <br /> intermediate aquifer was located at a depth of approximately 35 to 50 feet bgs The stratigraphy of <br /> ' the area is highly variable in both lateral and horizontal extent because of the location of the site at <br /> the margin of a delta near the floodplain of the San Joaquin River <br /> The water table beneath the island is shallow and was observed at depths from 2 to 7-112 feet <br /> ' bgs across the island (Kleinfelder, 1988) During April 1992, water was not present in the UST <br /> excavation pit, which had a depth of 8 feet bgs (PRC, 1992b) The general ground-water flow <br /> direction is expected to be to the south by southwest near the UST location, based upon water levels <br /> reported by Kleinfelder (1988) <br /> ' 2.3 SITE BACKGROUND <br /> Under the Navy's abandoned tank program, abandoned fuel tank 81713-2 at NAVCOMMSTA <br /> Stockton was excavated and removed from the site in accordance with the Tank Closure Plan, <br /> ' NAVCOMMSTA Stockton (PRC, 1992a) UST 81713-2 had dimensions of 5 feet in diameter by 25 <br /> feet in length, which allows for a capacity of approximately 3,500 gallons The tank was used to <br /> ' store leaded gasoline <br /> ' High photoionization detector (PID) readings ranging up to 6,000 parts per million (ppm) <br /> were observed during excavation of the UST This required the use of vapor suppressing foam <br /> ' during excavation to prevent an explosion hazard Upon removal of the UST, extensive surface <br /> corrosion of the tank was observed along with approximately 25 1 5-centimeter diameter holes along <br /> ' the bottom of the UST <br /> SAMPLAN 162 <br /> 044AmurATW-05A M 4 <br />