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i . I Site Investigation Work Plan j Nella Oil Station #21 , 40052 Highway 41 , Oakhurst, California and the Pleistocene Modesto and Riverbank formations topping the stratigraphy , both Quaternary deposits . The alluvium is a collection of unconsolidated gravel and sand in the fan areas, while the interfan areas are primarily composed of silt, sand , and clay . The Modesto and Riverbank Formations are predominantly poorly consolidated gravel with sand and silt. The thickness of these undifferentiated formations ranges to 150 feet. These deposits are moderate to highly permeable. i Tertiary deposits incorporate the Pliocene-recent flood basin deposits, the Pliocene-Pleistocene Laguna Formation and the Miocene-Pliocene Mehrten Formation . The Pliocene-recent flood basin deposits are exposed in the delta area of the San Joaquin Valley. They are essentially fine-grained transported versions of the Laguna, Riverbank, and Modesto Formations and recent deposits. This unit's thickness ranges from 0 to 1 ,400 feet and has low permeability . The Pliocene-Pleistocene Laguna Formation consists of discontinuous stream-laid sand and silt with minor amounts of clay and gravel . The thickness ranges from 400- 1000 feet, and permeability ranges from moderate-high . The Miocene-Pliocene Mehrten Formation is composed of j moderately-well indurated andesitic sand and sandstone with conglomerate interbeds. The thickness ranges from 400-600 feet in the Stockton area. Site-specific geology is available through the four CPT profiles generated from the Dpcember 2007 site investigation . The site is predominantly silty clay and clayey silt mixtures with interspersed layers of fine-coarse sand . The sand layers show a variety of depths and thickness, but among the four borings there appears to be four relatively consistent zones of sand within the following intervals: 25-27 , 56-66, 92- 105„ and 120- 125 feet bgs. Rockworks© generated cross-sections between the CPT borings ar provided as Figures 4-6. 2 .2 .2 Hydrooeology '1 The eastern San Joaquin Valley is the southernmost section of the Great Valley Geomorphic Province , a broad basin bordered by the Sierra Nevada on the east and Coast Range Complex to the west. The San Joaquin Subbasin is composed of unconsolidated and semiconsolidated sedimentary deposits, bordered by the Modesto, Delta-Mendota , and Tracy Subbasins to the l south , southwest and west, respectively. The San Joaquin Subbasin is drained by the San Joaquin River and its tributaries northward and discharges into the San Francisco Bay . Annual precipitation ranges from 11 inches in the southwest to 25 inches in the northeast. The aquifer system consists of the late Tertiary to Quaternary-aged continental deposits detailed previously in the Geology section , with the upper Quaternary deposits being moderately to highly permeable and the lower Tertiary deposits ranging in permeability from low to high . Groundwater beneath the site ranges from a depth of 25 feet bgs to 35 feet bgs, depending upon the time of year and weather history . The gradient is to the south-southeast. 2 . 2 . 3 Plume Characteristics The plume is primarily composed of petroleum hydrocarbons , specifically TPH- _ yv�iuith a more restricted concentration of the fuel oxygenates benzene and DIPE . In the DeeemlSer 2007 CPT site investigation , petroleum compounds were found only in boring P-2 approximately 15 feet east of the main office into South EI Dorado Street. TPH-G was detected in groundwater at 35 feet bgs at 1 , 680 arri and benzene was detected in groundwater at 35 feet bgs at 30 . 7 ppm . �- DIPE was detected at a range o f depths , and corresponding concentrations The fuel oxygenate P 9 9 P ranged from 837 to 4 . 04 pp . The second quarter 2008 monitoring event revealed petroleum hydrocarbon compound de actions in all . four wells. Dissolved TPH-G was detected in � � � 3