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deposits of clays, silts, and very silty or clayey fine-grained sands to a depth of about 100 feet. Layers of gravel and medium- to coarse- grained arkosic sand are common between 100 and 200 feet below the ground surface. B. GROUND -WATER OCCURRENCE AND MOVEMENT Most of the ground water used in eastern San Joaquin County is pumped from the Tertiary Arroyo Seco and underlying Laguna Formations. Although overlying Quaternary units, such as the Riverbank and Turlock Lake Formations, generally contain less ground water, some wells in the region draw large yields from discontinuous sand and gravel layers within these units. Ground water is primarily recharged by percola- tion from surface waters originating in the foothills of the adjacent Sierra Nevada Mountains. The San Joaquin County Flood Control and Water Conservation District has monitored ground -water levels and quality in the County since 1971. Ground -water elevation contour maps from the District (1985 to 1987) indicate ground -water elevations in wells range from approxi- mately -10 to -20 feet mean sea level (MSL) in the site vicinity. The regional flow gradient is shown to be to the southwest. EMCON conducted a well survey and identified 38 water supply wells on and within 1 mile of the site (see Figure 3). Additional wells may exist. Water -level depth measurements, taken in August 1987, in 16 of these wells ranged from 121 to 164 feet (approximately -19 to -27 feet MSL; see Table 1). The local gradient is very flat over most of the area, but appears to be generally to the west-southwest (see Fig- ure 4). Actual local flow directions are anticipated to be influenced seasonally by pumpage of irrigation wells. Ground water is found in site wells (G-1, G-2, and W-1) at depths of 124 to 143 feet (-19 to -21 feet MSL). Water -level measurements from water supply Well W-2 are not considered reliable due to the large PJ9 9390207.000 3