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be the most permeable deposits in the valley, however, they are generally not tapped by wells. Flood-basin deposits consist largely of fine-grained beds that restrict the vertical movement of water. Geology: A review of boring logs from this and previous studies show that with the exception of the silty, sandy gravel backfilled area, the predominant soil types at the site are clays and silts. Sandy silts and sandy clays are present across the site to approximately 10 to 15 feet below grade (fbg). Underlying these units are fine-to medium-grained sands, sandy silts, and sandy clays, ranging in thickness from less than 1 to approximately 3 feet thick. A medium-stiff to stiff, silty clay underlies the sandy zone, and persists to a depth of at least 32 fbg. Saturated soils are generally encountered at a depth of between 10 and 11 fbg. Ground Water Quality and Usage: Ground water in San Joaquin County is designated by the California Regional Water Quality Control Board in the Basin Plan to have beneficial or potentially beneficial use. Currently, however, the uppermost water-bearing zone does not yield water used for domestic, industrial, or municipal purposes; this may be attributed to the degraded nature of this shallow aquifer. Background ground water quality tests performed on ground water from upgradient and downgradient wells in December 1992, indicates that EPA Secondary MCLs were exceeded by the following constituents: chloride, sulfate, aluminum, iron, and manganese. i 3.0 BACKGROUND SITE CONDITIONS • 75 onsite and 12 offsite soil borings have been drilled at the site between 1988 and 1994, and 12 onsite test holes were excavated by backhoe in 1989. The maximum concentrations of total petroleum hydrocarbons as gasoline (TPH-G) and total petroleum hydrocarbons as diesel (TPH-D) detected at the site prior to remedial efforts were 1,900 and 8,200 parts per million (ppm), respectively. • Eight onsite and four offsite ground water monitoring wells are present at the site. • Onsite, adsorbed-phase hydrocarbons were excavated to below the standing water level in July 1990. Greater than 6,000 cubic yards of soil was hauled to appropriate disposal facilities. Approximately 7,300 cubic yards of clean, imported fill was then compacted and brought to grade. Figure 2 indicates the approximate limits of the area of excavation. • Dissolved-phase hydrocarbons have historically been detected in Monitoring Wells AW-3 and AW-9, along the northern margin of the site. 2