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0 Work Plan for Groundwater Monitoring Well Installation, Soil Gas and Groundwater Investigation, and Groundwater Monitoring Army Aviation Support Facility, Stockton, California 3.4 GROUNDWATER MONITORING WELL INSTALLATION After all site preparation activities and direct push borings and sampling are complete, groundwater monitoring wells will be installed at the approximate locations shown on Figure 4 (Appendix A). The rationale for the well locations proposed herein and indicated in Figure 4 is to provide better definition of groundwater conditions (gradient and chemistry) in the vicinity of, and upgradient and downgradient of the former USTs. Data collected during the groundwater grab and soil gas sampling will also be used to determine final well placement. Data from these new wells will assist in developing a remedial strategy for the site, as appropriate. Four 4-inch diameter polyvinyl chloride (PVC) wells (MW-4 through MW-7)will be installed at the location of the former 20,000-gallon USTs, and to the north and south, generally downgradient and upgradient of the former USTs, respectively. The wells will be installed using a HSA drill rig with 10'/4-inch-diameter augers and will be constructed of schedule 40 PVC consistent with California Well Standards and the San Joaquin County Well Standards (2005) for groundwater well installation. The first 5 feet of each boring will be advanced using a hand auger prior to implementation of drilling using the HSA rig. Once groundwater has been encountered, soils will be cored continuously for detailed analysis of lithology below the groundwater table for purposes of determining an appropriate well screen interval, and to ensure that a conduit for contaminants is not extended into a deeper water-bearing zone. Previous investigations indicate that groundwater is approximately 35 to 40 feet bgs at the site. In turn, the wells will generally be screened from approximately 10 feet above (30 feet bgs) to 20 feet below (60 feet bgs) the static groundwater level using 0.010-inch slotted screen and a filter pack of#2/12 sand extending a minimum of 2 feet above the top of the screen interval. The proposed screen interval may be modified based on the soils encountered during drilling. The filter pack will be sealed with 3 feet of hydrated bentonite chips or pellets. A bentonite/cement grout will be placed above the seal to approximately 3 feet bgs. Each well will be fitted with a lockable, water tight well cap, and completed with a 12-inch-diameter flush mount, traffic-rated well vault set with positive drainage to facilitate runoff of storm water away from the well vault. 3.5 SOIL SAMPLING DURING WELL INSTALLATION During installation of the groundwater wells, soils will be sampled at 5-foot intervals using a split-spoon sampler driven ahead of the auger flights for laboratory analysis and to allow visual evaluation of the well bore. This evaluation will also include preparation of a boring log for each well. Continuously cored samples below the groundwater table will not be analyzed for contaminants. Soil samples for laboratory inorganic laboratory analysis consistent with the methodologies described below will be collected in sterile 8-ounce glass containers (or stainless steel/brass sleeves, covered with Teflon tape and sealed with PVC caps). Pre-packed 25-gram En-Core@ samplers will be used to collect volatile samples. All samples will be submitted to a California-certified laboratory under chain-of-custody (COC) documentation to be analyzed for: • TPH-g and TPH-d using USEPA Method 8015-Modified; • VOCs including Naphthalene, BTEX, and Fuel Oxygenates using USEPA Method 826013; and • Total Lead using USEPA Method 601013. The full list of analytes is shown on Table 1. Soil samples will be labeled with a unique sample designation identifying the location and depth, in that order(e.g., MW4-5'). Sample designations, collection times, and analysis information will be recorded in the field notebook and on the COC form(s). OTI E 12