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Revised Soil Gas Sampling 3r VVorkplan September 8, 2017 CA ARNG, FMS #24, Stocktoii, CA Page 2 facility, including the Army Aviation support Facility (AASF), the Armory, the Field Maintenance Shop (FMS), and the Combined Support Maintenance Shop (CSMS). The general site layout and related features are shown in Figure 2. SITE DESCRIPTION The Stockton FMS and three other divisional areas are State-owned facilities operated by the CA ARNG. The FMS consists of a one-story maintenance building composed of work bays, supply areas, a latrine, and offices; a vehicle inspection rack; hazardous materials storage sheds; and a concrete wash pad with an associated holding tank. Two former USTs and associated pump island were located approximately 100 feet southeast of the concrete wash pad (Figure 2). The 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 their associated piping indicated that releases of diesel fuel and gasoline had impacted site soil. Groundwater grab sampling performed in association with site investigations in 2004 (conducted by Versar) and 2007 (conducted by URS Corporation) confirmed that site groundwater was impacted with petroleum hydrocarbons and related volatile organic compounds (VOCs) (Versar, 2004 and URS, 2007). These initial sample locations are shown on Figure 2. SITE SETTING Approximately 90 percent (°/0) of the ground surface at the site is paved, including the area around the former USTs (Figure 2). Grass and shrubs are present along the fenced 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 toward the north, through underground pipes, into the adjacent airport storm water system (URS, 2007). ENVIRONMENTAL HISTORY SINCE 2010 Five soil borings were advanced and sampled and then converted to groundwater monitoring wells (FMS-MW-1 through FMS-MW-5) as part of a site investigation conducted by Oneida Total Integrated Enterprises (OTIE) in March to April 2010. The monitoring well locations are depicted on Figure 2. Soil and groundwater were analyzed for total petroleum hydrocarbons (TPH) as gasoline (TPHg); TPH as diesel (TPHd); VOCs including benzene, toluene, ethylbenzene, and xylenes (collectively referred to as BTEX); and fuel oxygenates. Soil samples were also analyzed for total lead. The key findings associated with the 2010 soil and groundwater investigation were presented in Final Groundwater Monitoring Well Installation and First Quarter 2010 Groundwater Monitoring Report, California Army National Guard Stockton Field Maintenance Shop, Stockton, California (OTIE, 2010). Physical soil conditions reported through the 2010 investigation indicate that the shallow first water bearing zone (35 to 40 feet below ground surface [bgs]) is dominated by interbedded sand units within fat clays and silty/sandy clays. Although trace concentrations of petroleum hydrocarbons constituents were found in soil samples collected from all five soil borings, only soil collected from FMS-MW-5 indicated marked soil impacts from petroleum hydrocarbons. "'"4"tar