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v %W01 Site Background Information Former CURTISS PONTIAC-BUICK 2450 Toste Road, Tracy, California The site is situated within a retail business development in a business/agricultural area of western Tra cy, and co nsists of a r oughly tr iangular par cel of Ian d bordered on the northwest by Toste Road. A large showroom/office structure for auto sales is located on the site. REGIONAL GEOLOGICAL/HYDROGEOLOGICAL SETTING The site is situated within the Great Valley Geomorphic Province of California, a large, elongate northwest tending, asymmetric structural trough. The Great Valley Province has been filled with thick sequences of sediment ranging in age from J urassic to Recent, creating a nearly flat lying alluvial plain extending from the Tehachapi Mountains in the south to the Klamath 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 thebasement complex of the povince halve not beencompletely 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 area of Capay clay soil. Capay clay is located in interfan basin or alluvial fan areas, and generally consists of very deep, rmderately well-drained grayish brown and dark grayish brown clay. The soil formed in alluvium derived from mixed rock sources. The Modesto, Riverbank and Turlock Lake Formations and overlying Recent alluvium are the principal so urces of domestic ground water in the 13 ,500-square mile Sa n Joaquin Valley Ground W ater Bas in (Basin 5-22). Thi s b asin i s d rained pr imarily by the San Joaquin River. The near est su rface wa ter feature i n the v icinity of the p roperty i s t he OI d R iver approximately 1,300 feetnorth of the site.Ground waterat the site is currently encountered at a depth of approximately 6 to 8 feet below surface grade (bsg), and has a northerly flow direction. Ground water occurs in a geneally silty clay extending from surface grade to the total depths of the wells. Apparently discontinuous sand layers occur sporadically across the site, from approximately 8 feet bsg to 12 feet bsg and from approximately 14 feet bsg to 18 feet bsg. Advanced GeoEnvironmenfal,Inc.