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Work Plan for Groundwater Monitoring Well Installation, Soil Gas and Groundwater Investigation, and Groundwater Monitoring Army Aviation Support Facility, Stockton, California AST shares fuel dispensers, located approximately 50 feet to the northwest, with an existing 15,000-gallon UST also containing JP-8 fuel. A release of approximately 5 gallons of fuel occurred south of the former USTs in April 2002, impacting approximately 20 square feet of soil. The spill was reported and immediately excavated following the release. According to CA ARNG and CVRWQCB representatives, a wash rack located along the southwestern margin of the property was periodically used for degreasing operations. A drain in the central portion of the wash rack is designed to discharge into a clarifier, which ultimately discharges to the City of Stockton sanitary sewer system to the north. That system is still in operation, although degreasing operations are no longer performed there according to site representatives. A former wash rack located along the southern site margin is believed to have also been used for degreasing operations, but was abandoned concurrent with construction of the active wash rack. The only indication of the former wash rack is a vertical PVC drain pipe, cut off at ground level, and believed to have discharged into the storm drain system flowing to the east (Figure 2). Periodic surface runoff from the former wash rack was reportedly believed to flow to the east along a surficial drainage swale located on the southern site margin, approximately 50 feet north of the fence, where it discharged into a storm drain inlet also tied to the City of Stockton storm sewer system. These surface and subsurface drainage features are indicated in Figure 2. 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 groundwater monitoring wells (Figure 2). Some grasses are present, particularly along the southern perimeter fence line of the site, where the surficial drainage area noted above is located. 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. This system drains approximately 1.5 miles north of the site into Little John Creek, then into French Camp Slough, and finally into the San Joaquin River(Versar, 2004; URS, 2008). 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). Geologic materials beneath the site consist primarily of artificial fill and interbedded silty clays, clayey silts, sands, and silty to clayey sands (URS, 2008). A geophysical survey conducted at the CA ARNG on a neighboring divisional area to the northwest, indicated the site is underlain by approximately 15 feet of fine unconsolidated fill overlying coarser, consolidated sediments (Versar, 2007). Boring logs for subsurface investigations in the vicinity of the AASF former UST locations are relatively consistent with the geophysical interpretation, with fine silts and clays to depths of approximately 20 feet bgs overlying predominantly coarser, relatively continuous sands and silts (URS, 2008). OTIE 4