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i Project Description —Report of Composting Site Information Tracy Materia/Recovery Facility and Transfer Station 30703 S. MacArthur Drive, Tracy, Califomia 95377 Regional Groundwater H drolo The site is underlain by the Tulare Formation, the principle source of groundwater in the area. The Tulare Formation serves as the major reservoir for subsurface pumping of water for the west sides of San Joaquin and Stanislaus Counties. The Tulare Formation is composed of semi-consolidated, poorlysorted, discontinuous deposits of clay, silt, and gravel. The Tulare Formation is exposed as a thin band in the lower foothills to the west of the sit and dips steeply northeastward into the San Joaquin valley. The formation dips on the order of 15 to 20 degrees at its outcrop area, but flattens and thickens in the vicinity of Tracy. A regionally extensive clay layer, known as the Corcoran Clay Member of the Tulare Formation, occurs near the top of the Tulare Formation. The Corcoran Clay is the most extensive hydrologic confining layer in the San Joaquin valley, and underlies about 3,500 square miles of bottom land and western slopes. In the Tracy area, the Corcoran Clay can vary from near zero to about 150 feet thick. in the Tracy area, the Corcoran Clay occurs at a depth of approximately 300 feet below the ground surface. The clay ranges from 20 to 100 feet in thickness. Groundwater occurs under unconfined(water table), semi-confined, and confined conditions within the western margin of the San Joaquin Valley. There are roughly two separate producing zones in the Tracy area; unconfined, and confined. The Corcoran Clay divides the groundwater system into a lower confined zone and an upper unconfined (and semi-confined) zone. Water quality in the upper, unconfined aquifer above the Corcoran Clay is relatively poor, and variable in quality. Beneath the Corcoran Clay, the confined aquifer contains water of higher quality. High wafer yields are reported from wells developed within the sediments below the Corcoran Clay, Most of the larger irrigation wells, the industrial wells and the municipal wells obtain their water supply from below the Corcoran Clay. Small domestic wells often obtain their supply from above the confining Corcoran Clay. The site is in the Tracy-Patterson Ground Water Storage Unit The estimated groundwater storage capacity of the Tracy-Patterson storage unit is 4,040,000 acre feet. In the Tracy- Patterson area, yields to wells from the Tulare Formation range from about 40 gallons per minute to 3,300 gallons per minute, and most wells average more than 1,000 gallons per minute (Hotchkiss and Balding, 1971). Groundwater Flow The direction of groundwater flow in the upper unconfined aquifer pprs toward the northeast, indicating a source of recharge near the western edge of the basin (Department of Water Resources, 1967). Recharge of the upper aquifer is from deep percolation of irrigation water, infiltration of rainfall, seepage of stream flow out of Corral Hollow Creek and other creeks, and subsurface inflow from the consolidated rocks adjacent to the basin. The direction of groundwater flow in the lower confined zone is north to northwest, and appears to be water originating from the eastern side of the San Joaquin valley(Department of Water Resources, 1967). 10 Edgar&Associates,Inc. Tracy Compost Fac ProjDesc042310 G