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Weston Solutions, Inc. Remedial Action Work Plan for the San Joaquin County Human Services Agency Site—Stockton, California GEOLOGY AND HYDROGEOLOGY 3.2 REGIONAL GEOLOGIC INFORMATION The following information is from the Department of Water Resources (DWR) California's Groundwater Bulletin 118, San Joaquin Valley Groundwater Basin, Eastern San Joaquin Subbasin Groundwater Basin Number: 5-22.01 updated in 2006. The Site is located in the Eastern San Joaquin Subbasin and is drained by the San Joaquin River and several of its major tributaries,namely,the Stanislaus, Calaveras, and Mokelumne Rivers. The San Joaquin River flows northward into the Sacramento and San Joaquin Delta and discharges into the San Francisco Bay. Water-bearing formations of significance in the Eastern San Joaquin Subbasin consist of the Alluvium and Modesto/Riverbank Formations, Flood Basin Deposits, Laguna Formation, and Mehrten Formation. The Mehrten Formation is considered to be the oldest freshwater-bearing formation on the east side of the basin, even though the underlying Valley Springs Formation produces minor quantities. Alluvium and Modesto/Riverbank Formations(Undifferentiated).These units 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 while clay, silt, and sand are dominant in the interfan areas. These units range in thickness from a thin veneer on the east side of the basin to over 150 feet near the center of the basin. Groundwater occurs unconfined within these units. Because these units are limited in thickness, most wells penetrate them to tap deeper aquifers in the area. Flood Basin Deposits. This unit is exposed in the Delta area of the San Joaquin Valley. These deposits are basinward, fine-grained forms of the Laguna, Riverbank, Modesto, and Recent formations and,therefore,range in age from Pliocene to Recent. They are generally much finer grained with a higher percentage of fine sand and clays than their depositional equivalents to the east and west. Occasional gravel beds occur along the present waterways and are probably representative of the type of underlying lithology distribution. This unit ranges in thickness from 0 to 1,400 feet. Groundwater in this unit occurs under unconfined to confined conditions. The unit, in general, has low permeabilities and may create semi-confined to confined conditions when interfingered with the Alluvium and Modesto/Riverbank Formations. 3-3 February 2021