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tT&T Trucking Subsurface Investigation Results and Site Conceptual Model Page 4-5 4 2 Hydrogeology Geologic logs from previous investigations (Leeward, 1997 and BC 1998) and the Department of Water Resources Well Drillers Report of the new water supply well to the north were reviewed to assess the local stratigraphy of the area The collection of water levels and groundwater sampling of the four groundwater monitoring wells have been conducted on a quarterly basis beginning in April 1999 to the present This section briefly describes the regional geology, site groundwater quality, hydrostratigraphic units, hydraulic parameters and water levels 4 2 1 Re oval Hydrogeology ' The Site is located within the Eastern San Joaquin Subbasin that is part of the San Joaquin Valley Groundwater Basin (Department of Water Resources [DWR], 2003) It is bounded to the south, southwest, and west by the Modesto, Delta-Mendota, and Tracy Subbasins, respectively and on the northwest and north by the Solano, South American, and Cosumnes Subbasins The Solano and South American are subbasins of the Sacramento Valley Groundwater Basin The Eastern San Joaquin Subbasin is drained by the San Joaquin River and several of its major tributaries namely the ' Stanislaus, Calaveras, and Mokelumme Rivers The San Joaquin River flows northward into the Sacramento and San Joaquin Delta and discharges into the San Francisco Bay Annual precipitation within the subbasin ranges from about 15 inches on the west to about 22 inches on the east (DWR 2003) The surficial soil of the Site has been identified as the Jacktone clay, a soil in the ' Jackstone-Hollenbeck-Stockton series Soils in this series are range from somewhat poorly drained to moderately well drained and nearly level soil These soils are considered to be moderately deep alluvium soil overlying a hardpan, and have moderate infiltration rates with fine textures (United States Department of Agriculture, 1992) ' Water bearing formations of significance consist of recent alluvium and the Modesto/Riverbank Formations, Flood Basin Deposits,Laguna Formation, and Mehrten Formation The Mehrten Formation is considered to be the oldest fresh water bearing formation on the east side of the basin, even though the underlying Valley Springs Formation produces minor quantities (DWR, 2003) First water beneath the Site hes within the recent alluvium and Modesto/Riverbank Formations that ' were deposited as alluvial fans deposited by the Mokelumne River and are exposed within the subbasin along a band approximately 15 miles wide that extends from about Stockton eastward These units are recent to Late Pleistocene in age and consist primarily of sand and gravel in the fan ' areas and clay, silt, and sand in the interfan areas and are over 150-ft thick near the center of the subbasin Groundwater occurs unconfined within these units with well yields as high as 650 gallons per minute (gpm) reported (DWR, 2003) Because these units are limited in thickness, most wells penetrate them in order to tap deeper aquifers in the area Average specific yields in the 10- to 200-ft depth range vary from about 7 to 15 percent ' P\23000\23954 T1 trucking\C111 Summary R(.port\I'INAi 1&i I rucking DOC