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0 • <br />Work Plan for Groundwater Investigation and <br />Groundwater Monitoring Well Installation <br />Field Maintenance Shop #24, Stockton, California <br />highest response peaks will be sampled in addition to samples being collected from different soil <br />matrices (clay/silt vs. sand). The soil and groundwater confirmation sampling is described <br />further in Sections 3.4 and 3.7, respectively. <br />3.4 GROUNDWATER MONITORING WELL INSTALLATION <br />Following analysis of the MIP data, locations will be selected for the installation of two new <br />groundwater monitoring wells. The new wells will be located within the primary source area for <br />petroleum hydrocarbons and related compounds, as refined using the results of the MIP data as <br />described above. In order to provide confirmation of the MIP results (e.g., lithology, qualitative <br />analytical results, etc.), these two wells will be placed adjacent to select MIP boring locations. <br />Additionally, to address the spatial gap between FMS-MW2, an upgradient well that has <br />historically indicated the second highest concentrations of petroleum hydrocarbons and related <br />compounds in groundwater, and perimeter well FMS-MW3, one additional groundwater <br />monitoring well will be installed between these two wells, as shown in Figure 4. This well is <br />anticipated to provide further understanding of the distribution of petroleum hydrocarbon and <br />related compound impacts upgradient and/or crossgradient of the primary source area. <br />Boreholes for installation of groundwater monitoring wells will be advanced using 8-% inch HSA <br />drilling equipment operated by a C-57 licensed well drilling contractor, under the direct oversight <br />of a qualified geologist/hydrogeologist working under the supervision of a California PG. The <br />wells will be constructed of 2 -inch schedule 40 polyvinyl chloride (PVC) consistent with <br />California Well Standards and the San Joaquin County Well Standards (2005) for groundwater <br />well installation. <br />The first 5 feet of each boring will be advanced using a hand auger prior to implementation of <br />drilling using the HSA rig. Split spoon soil samples will be collected at predetermined depths <br />based on MIP data (if applicable) or at a frequency of one every five feet until groundwater is <br />penetrated. Once groundwater has been encountered, soils will be cored continuously for <br />detailed analysis of lithology below the groundwater table for purposes of determining an <br />appropriate well screen interval, and to ensure that a conduit for contaminants is not extended <br />into a deeper water -bearing zone. Historical monitoring data from FMS-MW5 indicates that <br />groundwater in this area is approximately 31 to 34 feet bgs. In turn, the wells will generally be <br />screened for 20 feet, with approximately 15 feet of screen below the water table (to <br />approximately 50 feet bgs) using 2 -inch diameter 0.010 -inch slotted screen and a filter pack of <br />#2/12 sand extending a minimum of 2 feet above the top of the screen interval. The proposed <br />screen interval may be modified based on the soils encountered during drilling. The filter pack <br />will be sealed with 3 feet of hydrated bentonite chips or pellets. A bentonite/cement grout will be <br />placed above the seal to approximately 3 feet bgs. Each well will be fitted with a lockable, water <br />tight well cap, and completed with a 12 -inch diameter flush mount, traffic -rated well vault set <br />with positive drainage to facilitate runoff of storm water away from the well vault. <br />3.4.1 Soil Sampling During Well Installation <br />During installation of the groundwater wells, soils will be sampled for laboratory analysis and to <br />allow visual evaluation of the well bore as described above. This evaluation will also include <br />preparation of a boring log for each well. Spilt spoon samples collected for laboratory analysis <br />will serve as confirmation sampling to corroborate qualitative MIP data. Continuously cored <br />samples collected below the groundwater table will not be analyzed for COPCs. Samples will <br />be periodically screened for petroleum hydrocarbons using an organic vapor analyzer (OVA) or <br />PID. The site geologist will log soils using the Unified Soil Classification Scheme (USCS) for <br />OTIE 12 <br />