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3.2 Soil Borings and Monitoring Well Installation Three groundwater monitoring wells will be placed around the stockpile area at each facility as shown in Figures 2 and 3. As noted previously,the actual locations will depend on access restrictions posed at the site and the location of underground utilities..thouuh locations will be locate as close to the waste piles as possible. Based on preliminary information, the depth to groundwater is approximately 28 feet bgs at the Lathrop facility,and 32 feet bgs at the French Camp facility. During drilling,soil samples will be collected at selected intervals using a California modified split-spoon sampler lined with sample sleeves.The samples will be examined for lithologic characterization and described using the Unified Soils Classification System(USCS).Select samples may be analyzed for physical and chemical characteristics. Groundwater monitoring wells will be installed using hollow-stem auger drilling methods and will extend to a maximum depth of 45 feet bgs. The actual well depth and screen length will be determined based on the geologic conditions to monitor the uppermost aquifer. The wells will be constructed of 2-diameter PVC casing with a maximum of 120 feet of 0.01-slot screen. The slot size of the well screen may be modified based on observations of subsurface soil encountered during installation. The filter pack will extend from the base of the borehole to at least two feet above the top of the screen. A minimum two-foot thick bentonite transition seal will be placed on top of the filter pack.The remainder of the borehole to within one foot of the surface will be backfilled with cement grout. Groundwater well depth,sand pack height,and bentonite thickness may vary based on the depth of groundwater. Each well will be completed with a locking well cap. A protective steel casing or flush mounted well box will be set in concrete around the wellhead. The type of surface completion will be determined in the field for each well location. 3.3 Monitoring Well Development Initial well development will be conducted immediately after placing the filter pack by surging to settle the filter pack prior to placing the bentonite transition seal. Additional well development will completed after well installation by surging and pumping or bailing until physical parameters such as pH,temperature,and conductivity stabilize and the water appears clear or the well goes dry. If the subsurface soil is comprised primarily of fines, it may not be possible to achieve low levels of turbidity. 3.5.2 Groundwater Purging and Sampling Prior to sampling each groundwater monitoring well,the water level will be measured and the volume of water in the well casing will be calculated. A minimum of three well casing volumes will be purged using a bailer or low flow submersible pump(or equivalent methods). During the purge cycle,field measurements such as pH,conductivity,temperature,and visual observations will be recorded to verify well stabilization. The groundwater in the well will be considered stabilized when three successive field parameter measurements are within 10 percent of the previous measurement.Groundwater will continue to be purged if the field measurements have not stabilized or until a maximum of 5 casing volumes have been removed. Alternatively if conditions are suitable.micro-purging techniques may be utilized. If a well has poor recovery and goes dry during purging,the groundwater will be allowed to recover to 80 percent of it's original well volume or for a maximum of 15 minutes. The well will then be purged again and allowed to recover to 80 percent or a maximum of 1 hour prior to sampling. Instrument calibration will be conducted at the beginning of each field day,and thereafter if suspect or erroneous readings occur. Samples will be collected by submersible pump or bailer(or equivalent methods)and transferred to laboratory-provided containers appropriate for the analyses to be conducted.Samples collected for dissolved metals will be filtered at the laboratory prior to analysis. Groundwater sample will be analyzed for the following parameters: ■ Total Petroleum Hydrocarbons for diesel (TPH-d)using EPA Test Method 8015 modified for diesel. ■ Total and dissolved chromium using EPA Test Method 6010B. _