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' 12J • June 1996 ' AGE-NC Project No 95-0130 Page 2 of 5 ' 23 INITIAL INVESTIGATION ' On 28 and 29 December 1994, five soil borings (B 1 through B5) were advanced at the site Borings B 1, B3, and B5 were advanced to depths of 35 feet below surface grade (bsg) near the area of the ' former USTs Borings B2 and B4 were advanced to depths of 50 and 65 feet bsg, respectively Petroleum hydrocarbons were detected in soil samples throughout each boring ' On 08 and 09 January 1995, four additional soil borings (MW-1, MW-2, MW-3 and 136) were advanced at the site Boring B6 was advanced to a depth of 65 feet bsg Borings MW-1 through MW-3 were advanced to depths of 80 feet bsg and completed as 2-inch diameter ground water ' monitoring wells Petroleum hydrocarbon-impacted soil and ground water were detected in samples collected from each monitoring well Table 1 summarizes the analytical results of all boring and monitoring well soil samples 24 REGIONAL GEOLOGIC SETTING a 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 sediments 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 Klamath Mountains in the north The western and eastern boundaries of this province are comprised of the California Coast Range and the Sierra Nevada Mountains, respectively Rocks composing the basement floor of the province have not been completely defined but are believed to be of metamorphic and igneous origin The Great Valley Province is split into two mayor divisions which are identified as the Sacramento and San Joaquin Valleys ' 25 GROUND WATER DEPTH AND GRADIENT The water-table surface beneath the site currently occurs at a depth of approximately 51 feet bsg The uppermost ground water flows in an unconfined aquifer With depth, the aquifer becomes semi- confined due to the presence of heterogeneous layers of sediment Review of Lines of Equal Depth Of Water Wells Fall 1993 published by the San Joaquin County i Flood Control and Water Conservation District suggests that regional ground water flow is towards the northeast Conversely, previous quarterly ground water monitoring events have indicated that local ground water flow beneath the site is towards the southeast i