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• Work Plan for Groundwater Monitoring Well Installation at the California Army National Guard Stockton Field Maintenance Shop, Stockton, California 3.3 GROUNDWATER WELL INSTALLATION After all site preparation activities are complete, groundwater wells will be installed at the approximate locations shown on Figure 2. These locations vary somewhat relative to the proposed locations indicated in the URS SI Report, as the URS locations were apparently designed to be what can be described as non -detect, point of compliance locations. The objective for the well locations proposed herein and indicated in Figure 2 is to provide a better assessment of the margins of the groundwater contaminant plume, which will assist in developing a remedial strategy, if appropriate. One 6 -inch diameter polyvinyl chloride (PVC) well will be installed in the center of the contamination plume, and four 4 -inch diameter PVC wells will be installed at strategic locations in roughly north, south, east, and west directions from the center of the plume (Figure 2). The wells will be installed using a HSA drill rig and will be constructed of schedule 40 PVC. The first five feet of each boring will be advanced using a hand auger prior to implementation of drilling using the HSA rig. Based on previous data indicating that groundwater is approximately 40 to 45 feet bgs at the site, the wells will be screened from approximately 10 feet above (35 feet bgs) to 20 feet below (65 feet bgs) the static groundwater level using 0.020 -inch slotted screen and a filter pack of #3 sand. To facilitate the use of one or more of the wells as part of a potential remedial system in the future, each well will be completed with a 2 -foot by 3 -foot flush mount, traffic -rated concrete well vault. Consistent with California standards for groundwater well installation, a 12-1/4 inch diameter • boring will be completed for the 6 -inch diameter well. For the 4 -inch diameter wells, a 10-1/4 inch diameter boring will be completed. A minimum of 48 hours after installation, to allow sufficient time for well materials to cure, the wells will be developed. The purpose of developing new monitoring wells is to remove the residual materials remaining in the well after installation has been completed. Development procedures will include pumping compressed air into the water column (at a depth of 2-3 feet below static water level), and flushing water up and out of the well and into waste containers. This procedure will be continued until water removed from the well is clear and free of any visible sediment, and the physical parameters of temperature, pH, and conductivity have stabilized. Development water will be containerized and stored onsite until characterized and transported offsite for disposal. 3.4 SOIL SAMPLING DURING WELL INSTALLATION During installation of the groundwater wells, soils will be cored continuously using a split -spoon sampler driven ahead of the auger flights to allow visual evaluation of the entire lithologic section within the well bore. This evaluation will include preparation of a boring log for each well, and detailed analysis of lithology as the groundwater table is approached for purposes of determining an appropriate well screen interval. At intervals within the core where potential contamination is noted, based on visual and/or olfactory observation, and where significant changes in lithology are noted, discrete soil samples will be collected for chemical analysis using the split -spoon sampler. For purposes of this scope of work, we assume that at least one sample will be collected for chemical analysis at 5 -foot intervals between the surface and the top of the groundwater table in each well bore. Soil samples will be collected in sterile glass • containers, submitted to a California -certified laboratory under chain of custody (COC) documentation and analyzed for: OTIE