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13 October 1998 AGE-NC Project No. 98-0504 Page 2 of 5 collected from beneath the USTs and dispenser island at concentrations up to 39 parts per million; total petroleum hydrocarbons quantified as gasoline (TPH-g) were detected at concentrations up to 1,400 ppm; and volatile aromatics (benzene, toluene, ethyl benzene and xylenes: BTEX) were detected at concentrations up to 190 ppm. Approximately 200 tons of soil were excavated during removal of the USTs. After removal procedures were completed, the soil was placed back into the excavation because of strong hydrocarbon odors and the proximity to a school. Two laboratory-compositedsamples were collected from the stockpiled soil prior to placement into the excavation. TPH-g, BTEX and MTBE and TBA were all detected in the composite samples. 2.4. SITE HYDROGEOLOGIC CONDITIONS The closest surface water feature to the site is Smith Canal and Yosemite Lake, located approximately 550 feet north of the site. Water from Smith Canal drains into the San Joaquin River Deep Water Channel and is primarily used for commercial and recreational boating. Based upon well information obtained from nearby sites, ground water flow direction is northerly and is currently estimated to occur at a depth of approximately 25 feet below surface grade (bsg). Ground water occurs in a generally sandy layer extending from approximately 20 feet bsg to 36 feet bsg. The sedimentary units are relatively horizontal and continuous over the area. 2.5. REGIONAL GEOLOGIC CONDITIONS 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 Jurassic to Recent, creating a nearly flat lying alluvial plain, which extends from the Tehachapi Mountains in the south to the KIamath 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. 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. Adi,anced GcoEnvironmental,Inc.