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Revised Workplan-Additional Site Investigation for PFAS BSK Project E20-016-01F Stockton Metropolitan Airport(SCK) March 1, 2023 Stockton, California Page 13 • location map including, borings, and monitoring wells installed in the first phase of investigation, and proposed locations of monitoring wells for the second phase of the investigation, • field boring logs, and field sample logs (if applicable), and • analytical reports. The following project approach will be utilized for the Second Phase of investigation at SCK: 1. Advance four (4) borings based on results from the First Phase of investigation and sample groundwater using BESST INC's, SimulProbe sampler, or similar sampling device. Soil samples would be collected at 1 ft, 5 ft, and 10 ft bgs for chemical analysis. Groundwater samples would be collected in 10-ft intervals once groundwater is encountered and will continue to attempt to be sampled to a depth of approximately 40 ft below static groundwater level (i.e., groundwater encountered at 50 ft bgs would be sampled down to 90 ft bgs). 2. Review analytical results from vertical groundwater sampling. This will be completed on a rush basis to avoid an additional drill rig mobilization fee. 3. Following the review of analytical results from vertical groundwater sampling, install 4 additional monitoring wells.Two(2)of the monitoring wells would be placed in the vicinity of PRA 1 and two (2) of the monitoring wells would be placed in the vicinity of PRA 2. One of the two monitoring wells installed in the vicinity of PRA 2 would be installed approximately within 50 ft of the 2020 Preliminary Investigation's Temporary Well TW-2. These monitoring wells will be used to define the lateral extent of groundwater contamination, as well as further assess groundwater gradient, and flow direction. Soil samples would be collected at 1 ft, 5 ft, and 10 ft bgs for chemical analysis in each of the four(4) monitoring well borings.The placement of the four(4) monitoring wells will be dependent on review of analytical results from the first phase of the investigation, discussion of the "Progress Report" submitted to the CVRWQCB following the first phase of investigation, and results from vertical groundwater sampling during the second phase of the investigation. 4. Prepare a comprehensive report of findings documenting both phases of the investigation.