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' 11 November 2004 AGE-NC Protect No 99-0624 ' Page 4 of 13 ' Between November 1999 and October 2004,eighteen ground water monitoring events were performed at the site to monitor dissolved petroleum hydrocarbon concentrations in ground water and to characterize ground water flow direction and gradient on the site Generally, ' TPH-g was detected in water samples collected from wells MW-1, MW-2 and MW-3 at concentrations as high as 23,000 }tg/l, BTEX compounds from wells MW-1 through MW-3 at concentrations as high as 1,700 µg/1, and MTBE from wells MW-1, MW-2, MW-3 and ' MW-7 at concentrations as high as 19,000 µg/1 ' Based on the current area bounded by wells MW-1 through MW-7,the lateral extent of the dissolved petroleum hydrocarbon plume is not known towards the south and west The vertical extent is marginally defined beneath the UST tank pit area (MW-7) and approximately 85 feet south (downgradient)of the former UST tank pit area(CPT-1) Analytical results of groundwater and soil samples are included in Tables 1 and 2 24 REGIONAL GEOLOGIC SETTING According to a Geologic Map of California, published in 1966 by the California Department of Conservation Division of Mines and Geology, the site area is within the Great Valley Geomorphic Province(GVGP)of California, a large, elongate, northwest trending structural trough The GVGP is subdivided into two mayor divisions designated the Sacramento and San Joaquin Valleys, which have been filled to the present elevation with thick sequences of sediment ranging in age from Jurassic to present day, 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 Mountains, respectively Based on a 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 area is described as being within the Tokay-Acampo (TA)association The TA soils are located within low fan terraces along the Mokelumne River, and generally consist of moderate well drained, moderate coarse textured soils that are deep to a cemented hardpan The TA soils generally formed in alluvium derived from granitic rock sources 3.0 SITE CONCEPTUAL MODEL I Based on the subsurface investigations at the subject property, AGE developed a subsurface model of the site soil stratigraphy comprised of five informal zones- a vadose(unsaturated)zone and four • hydrogeologic units Hydrogeologic units may be defined as significant vertical intervals of soil IAdvanced CeoEnvironmental,Inc