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Remedial Investigation QAPP Stockton Army Aviation Support Facility,California methods, are included in Table 17-1 through Table 17-5. Analytical requirements to achieve project objectives and support the quantification of potential risks are detailed in Worksheet#15. Field activities will be completed in accordance with the procedures outlined in Worksheet#14 and the Parsons SOPs in Appendix D. In instances where deviations from this sampling design and rationale are made due to unforeseen conditions, a Field Change Request Form will be generated to document the change and request feedback from the Parsons Task Manager, USACE,and ARNG. 17.1 Potential Release Area Surface Soil Sampling Design and Rationale Based on the evaluation of available data in soil and groundwater, as presented in Section 10.5,the presence of PFAS has been confirmed in soil and groundwater within and downgradient of AOI 1 and A01 3, with exceedances of the SLs in soil (at AOI 3) and groundwater(at AOI 1 and A01 3). The exact locations and extent of the PFAS release areas are not fully understood. During the SI,groundwater elevation contours indicated that the primary groundwater flow direction at the Facility is primarily to the northeast,with local groundwater flow to the southwest in the vicinity of the Wash Rack and Butler Building Parking FTA(Figure 10-5). To address these data gaps, surface soil sampling will be utilized during RI Mobilization 1. The surface soil sampling program will refine the understanding of the suspected release areas at the Stockton AASF. In accordance with the goals of the project outlined in Worksheet#11, the surface soil sampling program will aid in the identification of PFAS release areas, new locations for permanent monitoring wells, and placement of subsurface soil borings to collect data for use in the risk assessments. The rationale for the proposed surface soil sampling program is based on identifying the presence of a potential PFAS release and defining the extent of that release. Since PFAS include a wide variety of compounds with different structures and reactive moieties, some PFAS compounds tend to sorb well to soils when released at the surface,and others are highly mobile and may move to the subsurface. Elevated surface soil concentrations of the less-mobile PFAS compounds are good indicators of a release area. Surface soil sampling will be conducted around the perimeter of the A01 1 Airfield site to identify the PFAS source area. At the A01 3 Butler Building FTA, surface soil samples will be collected around the surface soil SL exceedance at A0I03-02 in four cardinal directions and at 3 additional locations within the A01 3 area to identify the extent of the PFAS source area. A total of 16 surface soil samples(not including QC samples)will be collected at the Facility during Mobilization 1, including 9 samples within A01 1 Airfield and 7 within A01 3 Butler Building Parking FTA. The Mobilization 1 surface soil sample locations are shown on Figure 17-1. The rationale for the surface soil sample locations is provided in Table 17-1. Surface soil samples will be analyzed for the target list of 40 PFAS by USEPA Method 1633, as specified in Worksheet#15.Surface soil samples will be collected in accordance with the procedures presented in Worksheet#14 and further detailed in Parsons SOP PFAS ENV-02 Soil Sampling(Appendix D). Based on the results of the designated Mobilization 1 surface soil samples for each potential release area, additional surface soil sampling may continue during Mobilization 2 with "step-out" locations until the data are sufficient to allow for source area identification.Surface soil data are considered adequate when a local release area has been identified or decreasing concentrations less than the human health soil SLs in surface soil are observed in the four cardinal directions away from an area of elevated surface soil concentrations. If PFAS release areas are identified with concentrations greater than the soil SLs,subsurface soil characterization of the release areas will occur as defined in Section 17.2. Additionally, permanent monitoring wells may be moved or PARSONS Worksheet#17&18:Sampling Design and Rationale Page 113