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i The lithology encountered during the drilling of monitoring wells MW-1 through MW-3 consisted predominantly of yellowish-brown, brown, and reddish-brown sandy and clayey silts interbedded with reddish-brown sands, and overlying bluish-gray, clayey silts The uppermost aquifer is located within the bluish-gray silts Ground water was encountered during drilling at about 45 feet below surface grade (bsg) in the borings ' The nearest surface water channel is the Mormon Slough, located approximately 3,500 feet to the north The nearest mayor navigable waterway is the San Joaquin River, including its ' associated tributaries, located approximately two miles to the west During inclement weather, on-site runoff is directed into a series of surface drains which empty into a subsurface drainage system along the western perimeter This drainage system is believed to flow in a southerly Idirection ASSESSMENT HISTORY IInitial UST Removal and Sampling Activities Originally, the site housed three diesel underground storage tanks (two 10,000-gallon capacity I USTs, one 5400-gallon capacity UST, and one 515-gallon capacity waste-oil UST) which were excavated and removed on June 12, 1989 by Wathall Construction of Stockton The attached piping and dispensers were also removed Analytical results of the soil samples collected in native soil at a depth of 121/2 feet at the bottom of the waste-oil UST excavation revealed that elevated levels of Total Recoverable Petroleum Hydrocarbons (TRPH) at 4,500 parts per million (ppm) were present Soil contamination was detected in the sample collected from the center of the west wall of the diesel UST excavation at a depth of 13 feet Laboratory analytical results revealed a diesel (TPH-d) concentration of 650 ppm No detectable concentrations of suspected contaminants were revealed in samples collected from the northwest, southwest, and eastern portions of the diesel UST excavation, all three piping samples, and one sample collected from the soil underlying the easternmost dispenser Water Works of Escalon, California prepared a Problem Assessment Report dated April 4, 1990 following a completed investigation of soil and ground-water contamination On November 27, 1989, soil was excavated from the waste-oil UST excavation to a depth of 18 feet Soil contamination was detected in the excavation along the south and east walls Apparently, no confirmational samples were collected, the excavation was evidently backfilled with clean soil Water Works then completed one ground-water monitoring well, MW-1, east of the UST excavation area and one angled boring, ASB-1, to the south It was advanced beneath the Iexisting shop building Laboratory analyses of soil samples collected during the drilling of MW- 1 revealed minor concentrations of TRPH in soil samples collected from depths of 15 feet (1 5 ppm), 20 feet (2 7 ppm), and 30 feet (4 1 ppm) No detectable concentrations of BTEX were W W Iwo Inc Project No 33236 00 August 1995 2