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Remedial Investigation QAPP Stockton Army Aviation Support Facility,California will be performed using sonic drilling and push-ahead sampling methods. The initial sample will be collected near the top of the water table,and then samples are anticipated to be collected at a frequency of approximately one every 15 feet or more along the vertical profile of the aquifer being characterized. Frequency and number of samples may vary based on field conditions, soil logging observations, and/or interpreted hydrostratigraphy. Groundwater samples will be analyzed under standard TAT for the target list of 40 PFAS by USEPA Method 1633, as specified in Worksheet#15.VAP sampling will be performed in accordance with the procedures presented in Worksheet#14 and further detailed in Parsons SOP PFAS ENV-14 Vertical Aquifer Profiling(Appendix D). The three most western VAP groundwater samples (GW-01, GW-02, and GW-03) will also be analyzed for potential COPCs from the FMS#24, including TPH-GRO,TPH-DRO,and VOC, as specified in Worksheet#15. 17.4 Groundwater Monitoring Well Sampling Design and Rationale During RI Mobilization 1, groundwater samples will be collected from newly installed monitoring wells onsite at Stockton AASF. In order to confirm shallow groundwater flow directions and establish a monitoring network for shallow groundwater, twelve shallow groundwater monitoring wells will be installed onsite during the RI Mobilization 1.The rationale for the new monitoring wells and the target screen intervals is provided in Table 17- 4. The proposed new permanent monitoring well locations are shown on Figure 17-2. The shallow wells will be installed at the top of groundwater in the surficial aquifer, which is comprised of unconsolidated Quaternary sediments made up of alluvial and lake deposits, using DPT or sonic drilling methods. Permanent monitoring wells will be installed in accordance with the procedures presented in Worksheet#14 and further detailed in Parsons SOP PFAS ENV-05 Soil Borings and Monitoring Well Installation (Appendix D). During RI Mobilization 1,groundwater samples will be collected from all newly installed wells on a quarterly basis as established in Table 17-4. Based on the results of RI Mobilization 1, additional monitoring wells may be installed in RI Mobilization 2 to further delineate the potential PFAS releases. Groundwater samples will be analyzed under standard TAT for the target list of 40 PFAS by USEPA Method 1633,with a subset of samples(all newly installed wells during the first quarterly sampling only)analyzed for TSS (USEPA Method SM2540D),TDS (USEPA Method SM2540C), TOC (USEPA Method SW9060A), pH (USEPA Method 9040C), as specified in Worksheet #15. Groundwater samples will be collected in accordance with the procedures presented in Worksheet#14 and further detailed in Parsons SOP PFAS ENV-03 Groundwater Sampling(Appendix D). During RI Mobilization 2, additional permanent monitoring wells will be proposed at locations to be determined based on the results of RI Mobilization 1. Locations may be selected at shallow, intermediate or deep intervals to delineate the nature and extent of PFAS impacts in groundwater. Additional Mobilization 2 permanent monitoring wells will be presented in a separate QAPP Addendum with the relevant QAPP worksheets. All permanent monitoring wells installed during the RI will be sampled for PFAS for four quarters. 17.5 Hydraulic Conductivity Testing Design and Rationale Slug testing (aquifer testing) will be completed to evaluate aquifer permeability at the newly installed shallow groundwater monitoring wells during RI Mobilization 1. The well locations and screen intervals for slug testing are included in Table 17-4.The well locations are shown on Figure 17-2. Up to three slug tests (rising or falling head) will be performed at each well by using different initial displacements. Both rising and falling head tests will be conducted, if possible. Falling head tests will not be performed in wells screened at the top of the water table(i.e., not fully saturated screen).A pressure transducer and data-logger will be employed to record the head changes. Data will be evaluated using the approach recommended by Butler (Butler, 2020). Plots of head PARSONS Worksheet#17&18:Sampling Design and Rationale Page 115