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0 3.2 HYDROGEOLOGY Most of the ground water used in eastern San Joaquin County is pumped from units below the Riverbank and Turlock Lake Formations. Signifi- cant yields from these formations are obtained only from scattered wells tapping discontinuous sand and gravel layers. EMCON conducted a survey of wells within a 1 -mile radius of the site in order to establish local ground -water conditions. Through area reconnaissance and review of Department of Water Resources records, 38 water -supply wells were located. Specific well construction data were available only for 10 wells. These data show that the cased depths for local wells range from 75 to 368 feet and average 164 feet. Most of the local wells are used for irrigation. Extensive irrigation and resulting ground -water withdrawal began around 1907 and have increased steadily since. As a result of pumping, ground -water levels have declined significantly over the years. The water level in a well within 1 mile of the site has declined from an elevation of 48 feet MSL in 1929 to -27 feet MSL in 1987. Water -level data acquired by EMCON during the current investigation indicate that current water levels in wells in the area range from elevations of -19 to -27 feet MSL (depths of 125 to 150 feet). The local gradient is very flat over most of the area, but appears to be generally to the west-southwest. Ground water was found in site wells (both water -supply and monitoring wells) at depths ranging from 124 to 156 feet. Results of both in situ and laboratory permeability testing are sum- marized on Table 3 of Appendix A. Test results confirm the low per- meability of generally fine-grained sediments in the over 100 -foot - thick unsaturated zone. Permeabilities from tests by EMCON range from 1 x 10-6 to 2 x 10-8 centimeters per second (cm/sec). Similar values were obtained during previous site investigations by another