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Lacustrine deposits are composed of fine-grained material, clay and silt interbedded <br /> with sands and conglomerates, and formed during a time when lakes and marshes <br /> existed within the Valley. <br /> The geologic formations found within San Joaquin/Stanislaus County are typically <br /> composed of the Basement Complex, the Ione Formation, the Valley Springs Formation, <br /> the Mehrten Formation, the Tulare Formation and recent alluvium. The basement <br /> complex is composed of crystalline igneous and metamorphic rocks and lies beneath <br /> the sedimentary units. The Ione Formation, deltaic in nature, is composed of claystone <br /> r and sandstones with a small percentage of conglomerates. The Valley Springs <br /> Formation, alluvial in nature, is composed of rhyolitic sandstones, siltstones, and <br /> claystones. The Mehrten Formation, alluvial in nature, is composed of andesitic <br /> conglomerates, sandstones, siltstones, and claystones. The Tulare Formation, lacustrine <br /> in nature, is composed of claystone, sandstone and gravels that have been alternately <br /> deposited in oxidizing and reducing environments. Within the Tulare Formation (or <br /> Turlock Lakes Equivalent) exists the Corcoran Clay Member, a prominent aquitard <br /> composed of interbedded clay units, is found throughout the area. The Riverbank and <br /> Modesto Alluvial Formations lie above this unit. Lastly, Quaternary river and flood <br /> plain deposits, consisting of clays, silts, sands and gravels overly the formations as soil <br /> deposits. <br /> i <br /> SITE SPECIFIC SOILS AND GEOLOGY <br /> The Riverbank WWTP site lies within alluvial deposits with parent materials <br /> originating from sandy granitic alluvium. Soils found at the City of Riverbank WWTP <br /> are composed of loamy sand to sandy loam. The site specific geology near surface <br /> geology is dominated by the Riverbank and Modesto Formation age units, including <br /> gravel and sediment deposits from the Stanislaus River. Additional detail is provided <br /> below. <br /> 1.2 HYDROGEOLOGY <br /> REGIONAL HYDROGEOLOGY <br /> As per Bulletin 118, the Riverbank WWTP lies in the southwesterly portion of the <br /> Eastern San Joaquin Subbasin of the San Joaquin Basin. The Eastern San Joaquin <br /> Subbasin in bounded by the Stanislaus River to the south, the San Joaquin River to the <br /> west, Dry Creek to the north, and the Sierra Nevada crystalline basement to the east. <br /> City of Riverbank Wastewater Treatment Plant Monitoring Well Installation May 2006 <br /> DE Project No.:109-06 3 Rev.0 <br />