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Work Plan for Groundwater Monitoring Well Installation at the California Army National Guard Stockton Field Maintenance Shop, Stockton, California The two former USTs (one 5,000 -gallon diesel and one 10,000 -gallon gasoline) were removed in April 1996 by Seward Schreder Construction. Soil samples collected during removal and excavation of the USTs and associated piping indicated that releases of diesel fuel and gasoline have impacted site soil. Analytical results for soil samples collected during UST and piping removal indicated concentrations of TPH-g up to 12,000 milligrams per kilogram (mg/kg) and TPH-d up to 7,100 mg/kg (URS, 2007). In addition, BTEX constituents were detected in soil at concentrations up to 6.2 mg/kg (ethylbenzene) and 580 mg/kg (total xylenes) (Versar, 2004). 1.4 SITE SETTING AND GEOLOGY Approximately 90 percent of the surface at the site is paved, including the area around the former USTs and the proposed locations of the groundwater monitoring wells (Figure 2). Some grasses and shrubs are present along the fence perimeter of the site. The site has been graded to promote surface drainage into storm drain inlets. Catch basins collect rainwater and divert the runoff to the north through underground pipes into the adjacent airport storm water system (URS, 2007). The site lies within the Sacramento Valley, which is composed of unconsolidated Quaternary sediments from alluvial and lake deposits. The Sierra Nevada Mountains to the east are a source of the alluvial deposits resulting from erosion. Also contributing to the deposits are the Sacramento Valley Coast Ranges to the west. Both ranges consist of a wide variety of metamorphic and sedimentary rocks (ranging widely in age and composition). In addition, the . Stockton Fault, which trends northeast -southwest, is located within the city limits, and the Midland Fault Zone (trending north) is located approximately 25 miles northwest of the site (Versar, 2004). • Based on previous investigations performed by Versar and URS, depth to groundwater in the vicinity of the site is approximately 40 to 45 feet bgs. Limited groundwater gradient data were obtained as a result of those investigations, but the gradient is assumed to be generally flat, with an average of 0.004 foot/foot estimated for the site (Versar, 2004). OTIE