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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 2.4 MARCH 2008 SITE INVESTIGATION The site investigation conducted by URS in March 2008 included the collection of soil and grab groundwater samples from twelve hollow-stem auger (HSA) borings (B-3 through B-14). These samples were analyzed by both on-site and off-site laboratories. Sampling locations for the March 2008 investigation are indicated in Figure 3. Soil and grab groundwater samples were analyzed for TPH-g, TPH-d, VOCs, and BTEX compounds by USEPA Method 8260B, and lead by USEPA Method 6010B. Lead was detected in soil samples from all 12 borings, but below its PRG of 150 mg/kg. No VOC or BTEX compounds were reported in any soil samples. The only boring reporting petroleum hydrocarbon detection above action levels (Tri-Regional Board, 1990) was B-9, which is located at the southern portion of the 20,000-gallon USTs excavation (Figure 3). Reported diesel concentrations in B-9 were 1,840 mg/kg and 156 mg/kg at depths of 30 and 35 feet bgs, respectively. Diesel was not detected in any other borings (north, northeast, or southeast), and the URS report concluded that diesel in soil had been delineated. Grab groundwater samples were collected from each boring between approximately 38.5 feet and 40 feet bgs, corresponding to first groundwater encountered. Contaminants in groundwater exceeding respective MCLs were benzene in borings B-5 (6.0 pg/L) and B-9 (1.5 pg/L), carbon tetrachloride (CT) in boring B-9 (16 pg/L), TCE in boring B-13 (8.3 pg/L), and cis-1,2- dichloroethene (cis-1,2-DCE) in boring B-9 (18 pg/L). TPH-g was detected in borings B-4 and B-5, adjacent to the former 5,000-gallon USTs, at concentrations of 740 pg/L and 410 pg/L, respectively. TPH-d was only detected in boring B-9, at a concentration of 11,000 pg/L. Petroleum-related contaminants in groundwater were found generally only in areas in the immediate vicinity of the former USTs. Likewise, contaminants commonly found in solvents and degreasers (tetrachloroethylene [PCE], CT, TCE, and cis-1,2-DCE) were detected in groundwater samples in the southern portion of the site, generally upgradient from the former USTs. Groundwater contamination was defined laterally to the north (downgradient), east, and west (cross-gradient), but not to the south (upgradient). 3.0 PROPOSED ACTIVITIES This section describes the specific activities associated with the implementation of the scope of work described in Section 1.1, Project Objectives and Scope. All proposed field activities will be conducted in accordance with the APP/SSHP (Appendix B). 3.1 PERMITTING The SJCEHD is the lead agency that oversees the installation of groundwater wells within San Joaquin County. In accordance with SJCEHD requirements, one well permit application will be filed for the four proposed groundwater monitoring wells and the 30 proposed soil gas/grab groundwater sample borings. Once all appropriate SJCEHD permits have been received, field activities will be scheduled. 3.2 SITE PREPARATION All proposed field activities will be coordinated in advance with the CVRWQCB and appropriate CA ARNG personnel. Site preparation activities will be coordinated by OTIE's Field Manager, and will be initiated by a site visit to mark out proposed invasive drilling locations (i.e. well and boring locations). The Field Manager will further ensure that those locations are cleared for underground utilities or other obstructions. This clearance will include: OTIE 7