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0 0 Work Plan for Groundwater Investigation and Groundwater Monitoring Well Installation Field Maintenance Shop #24, Stockton, California supply areas, a latrine, and offices, a vehicle inspection rack, hazardous materials storage sheds, a concrete wash pad with an associated holding tank, and the location of the former USTs and associated fuel dispensers. The two former USTs (one 5,000 -gallon diesel and one 10,000 -gallon gasoline) were removed in April 1996, and 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. Groundwater grab sampling performed in association with previous site investigation in 2004 (Versar) and 2007 (URS) confirmed that site groundwater has been impacted with petroleum hydrocarbons and related VOCs. 1.3 SITE SETTING AND GEOLOGY Current site features, along with locations associated with historical and current soil and/or groundwater sampling events, are indicated in Figure 3 (Appendix A). Approximately 90 percent of the surface at the site is paved, including the area around the former USTs and groundwater monitoring wells. 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). 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 interbedded silty clays, clayey silts, sands, and silty to clayey sands (OTIE, 2010a). A geophysical survey conducted at the CA ARNG facility on a neighboring divisional area to the northeast, indicated the site is underlain by approximately 15 feet of fine unconsolidated soils overlying coarser, consolidated sediments (Versar, 2007). Boring logs from previous subsurface investigations at the site are relatively consistent with the geophysical interpretation, with fine silts and clays to depths of approximately 20 to 30 feet below ground surface (bgs) overlying predominantly coarser, relatively continuous sands and silts (OTIE, 2010a). 1.4 REGIONAL AND SITE HYDROGEOLOGY Stockton is located in the San Joaquin Groundwater Basin as defined in Department of Water Resources Bulletin 118-80 (1980). The upper 700 to 800 feet of the unconsolidated sediments in the San Joaquin Basin contain a number of water -bearing zones. The distribution of water - bearing zones is directly related to the subsurface geology in any particular area. For the most part, first encountered groundwater occurs primarily in unconfined conditions; however, due to distribution of clay lenses in the sediments, groundwater can occur in semi -confined conditions as well. Regional groundwater flow in the vicinity of the site is generally southwesterly toward the valley trough, in the case of unconfined aquifers. OTIE 3