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Workplan for PFAS Sampling and Analysis Project Number:60657245 1 .3 Hydrogeology The Site is within the Central Valley which contains the Central Valley aquifer system. The system is generally characterized as an unconfined aquifer interspersed with clay lenses. The Corcoran Clay divides the upper and lower aquifers. Saline water is found in the bottom extent of the lower aquifer(Galloway and Riley, 1999). The stratigraphic framework that has evolved from data collected from Site investigations shows that the sedimentary sequence beneath the Site can be divided into five units based on lateral and vertical distribution of fine- and coarse-grained sediments ("A"zone,"B"zone,"B/C"aquitard,"C"zone,and"D"zone).The five zones identified beneath the Site extend eastward and downgradient of the Site and characteristics of each zone are similar. The work described in this work plan will target the shallowest water-bearing zone,the A-Zone. The A-Zone typically extends from ground surface to approximately 20 to 25 feet below ground surface (bgs). The upper 10 to 15 feet of the A-Zone consists of silt and clay with lesser amounts of interbedded sand. Sand units in the upper part of the A-zone are typically one to two feet thick and are laterally discontinuous.The lower part of the A-Zone consists of interbedded clay, silt, and sand. The Stockton Terminals Technical Committee (STTC), which consists of Buckeye Terminals, LLC (Buckeye) and Tesoro, monitors groundwater at both the Buckeye and Tesoro facilities for petroleum constituents of concern in accordance with Monitoring and Reporting Program No. R5-2017-0826. Groundwater is monitored by the STTC via a network of monitoring wells shown in Figure 1-4.Three monitoring wells located on the west end of the West Terminal on Tesoro property(SP-3, SP-4, SP-5) and shown on Figure 1-4 are part of the STTC well network. Based on 2020 monitoring well data, the depth to A-Zone groundwater beneath the Site ranges from approximately 1 foot to 9 feet bgs and the direction of groundwater flow in the A-Zone is generally to the east-southeast (Figure 1-5) at a hydraulic gradient of 0.002 feet/foot(Orion, 2021). Based on groundwater monitoring, shallow groundwater present beneath the Site and vicinity show a consistent southeasterly direction of groundwater flow away from the nearby San Joaquin River. The direction of shallow groundwater flow beneath the Site reflects the recharge of shallow groundwater from the nearby San Joaquin River. 1 .4 Pathways and Receptors Regional groundwater flow in the Stockton area is greatly affected by extensive pumping from groundwater production wells located several miles to the northeast and east of the Site. Pumping and the resulting cones of depression have created regional flow patterns toward these pumping depressions. Regional flow patterns beneath the Port of Stockton area are to the northeast and water quality is generally poor due to saltwater intrusion induced by over pumping in the previously mentioned area. As a result, water use from production wells in the Port of Stockton area is generally restricted to industrial and irrigation uses. A search with California Water Board's Groundwater Information System (LAMA)within a one-mile radius of the Site identified two water supply wells marked as"other"and two municipal wells.The"other"water supply wells are located between Navy Drive and Washington Street. The available data show the last sampling event occurred in 1979. The two municipal wells are located across a branch of the San Joaquin River, north of Navy Drive. AECOM contacted California Water Service Company (CWSC), the local water purveyor providing drinking water to Central Stockton area within which the Site is located. CWSC indicated that they have no municipal wells within a 1- mile radius of the Site(S. Romero, May 25, 2021, personal communication). As described in Section 1.1, stormwater from the facility is contained on site for inspection and released to Port of Stockton ditches along Navy Drive. Thus, water does not freely flow off site from the Site.As indicated in Section 1.1, any stormwater that is released to the Port of Stockton drainage ditch is directed to a Port of Stockton holding pond, located across Navy Drive to the south of the Site(Figure 1-2). The main surface water body near the Site is the San Joaquin River where branches of the river are located approximately 1,100 feet to the southwest and 1,600 feet to the northwest of the West Terminal(Figure 1-2). Prepared for:Tesoro Logistics Operations LLC AECOM 5