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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 3 2.4 PRA 1—Wash Rack Figure 3 shows the location of PRA 1 and approximate AFFF usage area within the PRA. The wash rack area dates to at least May 1993, based on evaluation of historical Google Earth imagery. The wash rack apron area is paved with thick reinforced concrete for aircraft traffic and is presented in Figure 5. The wash rack has been used for servicing Aircraft Rescue and Firefighting (ARFF) vehicles, ARFF testing and training using mostly water, and nozzle-testing on the cement pad. ARFF training occurred on the wash rack apron on a quarterly basis for approximately five years. A stormwater retention basin is located adjacent to and immediately east of PRA 1. The drainage basin is fenced off and has approximate dimensions of 230 ft by 150 ft. PRA 1 is located approximately 4,000 ft northeast of French Camp Slough. 2.5 PRA 2—Former Runway near Present Cargo Area The former runway area was used for fire-fighting training exercises that were conducted between 1994 and 2009. The training exercises incorporated the use of AFFF and is detailed in Figure 3. Figure 6 shows the location of PRA 2 with the footprint of the former runway superimposed over the recent aerial photograph base map. The former runway had a triangular configuration with an approximately 2,000- foot long northern leg oriented in a southwest to northeast direction and an approximately 2,500-foot long eastern leg that was oriented in a generally north to south direction. Evaluation of historical aerial imagery available on Google Earth indicates that the former runway was eliminated between March and August of 2002, and it appears that the runway pavement was removed. Between 2002 and 2004, the original triangle configuration of the former runway remained. By 2004, an air cargo ramp was installed to the north of the former runway area. In 2005, the air cargo ramp was connected to Runway 11L-29R via a new taxiway. Portions of the airport property have been used for planting row crops since the 1990s. For more information regarding farming in the vicinity of PRA2, please see Section 2.4 of BSKs Revised PSI report. 2.6 Project History On March 20, 2019, the State Water Resources Control Board (SWRCB) issued a California Water Code (CWC) Section 13267 Order WQ 2019-0005-DWQ(Order) for the Determination of the Presence of PFAS Impacts which was addressed to 27 California airports, including the Stockton Metropolitan Airport(SCK), based upon its use of Aqueous Film-Forming Foam (AFFF) fire-fighting compounds. AFFF is a liquid concentrate that consists of perfluorinated surfactants with a foam stabilizer and viscosity control agents. AFFF serves as both as a barrier to exclude air or oxygen and, in addition, produces an aqueous film on the fuel surface that can suppress the generation of fuel vapors. AFFF contains high concentrations of PFAS compounds. The Order specifies that if AFFF or other PFAS compounds were disposed of,discharged