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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 5 Based on our preliminary site investigation BSK recommends the following: 1) That further investigation be performed to characterize the vertical and lateral extent of PFAS in subsurface soils and groundwater. 2) That the City be notified that a sample of municipally supplied water(obtained near the wash rack at PRA 1) contained Perfluorobutanoic Acid (PFBA; CAS # 375-22-4) at an estimated concentration (J flagged value)of 7.4 ng/L, which is above the MDL and below the RL of 10.0 ng/L. This recommendation has been implemented by the County since the issuance of the initial version of this report. 3) That a copy of this report be submitted to the Central Valley Regional Water Quality Control Board(CVRWQCB), the GeoTracker website, and the SJCEHD. Following the submission of BSKs Revised PSI, the CVRWQCB issued a letter titled, "Assess Full Extent of Per- and Poly-Fluorinated Substances (PFAS) Contamination in Soil and Groundwater at Stockton Metropolitan Airport, 5000 S. Airport Way, Suite 202, Stockton, San Joaquin County, California" dated April 8, 2022, requesting additional investigation of PRA 1 and PRA 2 at SCK to define the full extent of constituents in groundwater and soil with respect to vertical and lateral extent of constituents. The requests from the CVRWQCB letter dated April 8, 2022, are as follows: "The primary goal of the additional site assessment is to delineate the full extents of PFAS contamination in both lateral and vertical directions in soil and groundwater as a result of the application and release of Aqueous Film-Forming Foam from the Airport, in order to establish an accurate Conceptual Site Model with respect to the fate and transport of the PFAS plume in groundwater. Additionally, the Central Valley Water Board requires that the Work Plan includes discussion on the stormwater flow conditions at the Airport to evaluate the potential that PFAS compounds are transported offsite via stormwater.At a minimum, the Work Plan shall include the following elements: 1. Proposed numbers, locations, and depths of soil borings to fully delineate the PFAS contamination in soil at PCA-1 [PRA-1] and PCA-2 [PRA-2]; 2. Proposed numbers, locations, and depths of screen intervals of groundwater monitoring wells, including clustered wells with different screen depths, as needed, to fully delineate the PFAS groundwater plume at the source zones of PCA-1 [PRA-1] and PCA- 2 [PRA-2], and any downstream areas potentially impacted by the PFAS plume that may have originated from the Airport, 3. Methods to investigate or confirm the groundwater flow directions, gradients, and velocities at the Airport,