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Remedial Investigation QAPP Stockton Army Aviation Support Facility,California WORKSHEET #17 & 18: SAMPLING DESIGN AND RATIONALE Worksheets #17 & #18 provide the sampling design and rationale, in addition to the sampling locations and methods, for the Stockton AASF. The sampling program was designed to meet the DQOs established in Worksheet #11, while considering the CSM summarized in Worksheet #10. As discussed in Worksheet #10, Three A01s were identified at Stockton AASF during the SI (AECOM, 2023). SL exceedances were reported in samples from two of the A01s, A01 1 and A01 3. A01 2 will also be included in further investigations in the RI, because of detected concentrations of PFAS compounds close to SLs and the limited numbers of samples collected during the SI in A01 2.The areas to be investigated are: • A01 1:Airfield • A01 2:Wash Rack • A01 3: Butler Building Parking FTA The RI Mobilization 1 sampling program was designed to refine the nature and extent of PFAS in the environment at each release area and downgradient within the investigation area,conduct preliminary screening and evaluate the results as part of the comprehensive HHRA and ERA, and define more precise boundaries for potential response actions during the FS, if warranted.Analytical data from the SI (AECOM, 2023)were used to refine the approach for the RI. Data collected during the RI Mobilization 1 will be used to refine the approach for additional monitoring well installation and groundwater, soil, surface water, and sediment sampling during the RI Mobilization 2, should all data quality objectives not be met during Mobilization 1. The sampling program for the RI Mobilization 1 will include: • Collection of surface soil samples for analysis of PFAS around the identified PFAS release areas at the A01 1 Airfield and A01 3 Butler Building FTA. • Collection of VAP grab groundwater and soil samples for analysis of PFAS for lateral and vertical characterization of PFAS impacts at select areas upgradient and downgradient of the identified PFAS releases at A01 1 Airfield and A01 3 Butler Building FTA. • Installation of new permanent shallow groundwater monitoring wells on-Facility, including one in A01 2 Wash Rack area. • Collection of groundwater samples for analysis of PFAS from new permanent monitoring wells. • Collection of groundwater samples for analyses of THPg,TPHd,and VOCs for evaluation of potential co- contamination with the FMS#24 petroleum investigation • Hydraulic conductivity testing(slugtesting)and geotechnical soil sample collection to determine aquifer permeability at select monitoring well locations. • Collection of samples from Rydberg Creek for analysis of PFAS. • Collection of synoptic water levels in the new permanent monitoring wells and in 3 existing monitoring wells in the FMS#24 area. The following subsections provide the specific sampling design and rationale for the selection of sampling locations for the RI.The proposed locations of samples to be collected as part of the RI are presented on Figure 17-1 through Figure 17-3.The specific design rationale for each task, in addition to the sampling locations and PARSONS Worksheet#17&18:Sampling Design and Rationale Page 112