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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 1.5 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 area is generally southwesterly toward the valley trough, in the case of unconfined aquifers. Based on previous investigations performed by Versar(2004) and URS (2008), as well as recent investigations at the nearby Stockton FMS performed by OTIE (2010), depth to groundwater in the vicinity of the site is approximately 35 to 40 feet bgs. Limited groundwater gradient data specific to this site were obtained as a result of those investigations, but the gradient is assumed to be generally flat, with an average of 0.004 feet/foot estimated for the site (Versar, 2004; URS, 2008). Groundwater flow is estimated to be generally toward the north, based on historical groundwater measurements collected from the three existing AASF monitoring wells, as well as measurements from wells at the nearby Stockton FMS facility. 1.6 CONCEPTUAL SITE MODEL It is necessary to consider both native and fill materials in order to evaluate the potential movement of contaminants in the subsurface at the site. For example, coarser units will facilitate movement of contaminants in groundwater and in soil gas within the vadose zone. Since finer sediments (clays and silts) overlie coarser sediments (sands and silty sands interbedded with some silts) at the site (based on site-specific and regional data), it is expected that vapor transport within the upper sediments (i.e. finer materials)will be limited. Still, underground utilities, sewers, and storm water runoff conveyance piping may act as conduits for contaminant transport because coarse fill material is typically used as backfill in these areas. The chemicals of potential concern (COPCs) for the site, listed on Table 1 (Appendix B), are TPH-d, TPH-g, fuel oxygenates, and VOCs (including TCE). The primary release mechanism for petroleum impacts to groundwater at the site are leaks and other forms of discharge from the former USTs. The potential mechanism of this impact is the leaching of contaminants downward into the coarser-grained units and surrounding groundwater. There is limited potential for surface water infiltration because, as previously mentioned, approximately 90 percent of the surface area at the site is paved. However, because limited petroleum constituents are still present in subsurface soils, they could possibly continue to migrate downward toward groundwater. The potential exists for chlorinated solvent impacts to groundwater as well, through infiltration of run-off water from the current and former aircraft wash racks and/or a potential release from the associated surficial drainage system. 2.0 PREVIOUS ENVIRONMENTAL INVESTIGATIONS The areas of environmental concern for this proposed work are the former locations of the diesel and gasoline USTs and the existing and former aircraft wash racks and associated storm drainage conveyance (Figure 2). The three former 5,000-gallon and two former 20,000-gallon USTs, which were both single-walled steel tanks, were installed sometime in the early 1950s when the CA ARNG facility first began site operations. The subsections below describe the environmental investigations and field activities that have occurred at the site. OTI E 5