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Site Background Information BOB'S MUFFLER SHOP 466 Moffat Boulevard, Manteca, California ' The site is situated in a commercial area of Manteca, California and consists of a roughly triangular parcel of land bordered on the northeast by Moffat Boulevard. Structures on the site include one small building which formerly housed the muffler shop,a tall canopy,and two former pump islands. ' REGIONAL GEOLOGICAL/HYDROGEOLOGIC SETTING The site is situated within the Great Valley Geomorphic Province of California, a large, elongate, ' northwest-trending, asymmetric structural trough. The Great Valley Province has been filled with thick sequences of sediment ranging in age from Jurassic to Recent, creating a nearly flat lying alluvial plain that extends from the Tehachapi Mountains in the south to the Klammath Mountains ' in the north. The western and eastern boundaries of this province are comprised of the California Coast Range and the Sierra Nevada, respectively. Rocks composing the basement complex of the province have not been completely defined but are believed to be of metamorphic and igneous ' origin. The Great Valley Province has been subdivided into two major divisions, identified as the Sacramento and San Joaquin Valleys. Based on the General Soil Map from the San Joaquin County Soil Survey,published by the United States Department of Agriculture Soil Conservation Service in 1992, the site is situated within an areaofDelhi-Veritas-Tinnin soil.Delhi-Veritas-Tinnin soil is found ondunes,alluvial fans,and low ' fan terraces, and generally consists of moderately well-drained to somewhat excessively-drained, coarse-textured and moderately coarse-textured soils that are deep to a cemented hardpan or are very deep. The Modesto, Riverbank, and Turlock Lake Formations and overlying Recent alluvium are ' the principal sources of domestic ground water in the 13,500-square mile San Joaquin Valley Ground Water Basin (Basin 5-22). This basin is drained primarily by the San Joaquin River. ' The nearest surface water features in the vicinity of the property are the San Joaquin River and Little John Creek, located approximately 5 miles southwest and north, respectively, of the site. Ground water at the site is currently encountered at a depth between 20 and 25 feet below surface grade (bsg), and has a northwesterly flow direction. Presently, ground water occurs in an interval of predominantly clayey silt. Informally designated layer 2, this layer is approximately 15 feet thick and occurs between 20 feet and 35 feet bsg.Apparently discontinuous,3-to 5-foot thick sand lenses occur sporadically within layer 2. WELL SURVEY A well survey was performed in October 1999 to identify municipal,domestic,industrial,irrigation, or monitoring wells and septic tanks located within a 2,000-foot radius of the site. The nearest ground water wells were two City of Manteca wells (No. 7 and No. 19), located approximately Advanced GenEavironmental,Inc.