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0 2. HYDROGEOLOGIC CONCEPTUAL SITE MODEL The hydrogeologic Conceptual Site Model(CSM)was evaluated in relation to identifying data gaps in the extent of and potential transport pathways of sulfolane between the on-Site source area and the Simplot backup water supply well (BSW), Simplot primary water supply well (PSW), and the LWD municipal water supply wells. A discussion of the geology, groundwater transport, sulfolane impacts, and data gaps related to the CSM are discussed below. 2.1 Geology The Site is located in the San Joaquin Valley Groundwater Basin and is underlain by Corcoran Clay, which is a former lake deposit that forms a regionally extensive aquitard in the San Joaquin Valley(Department of Water Resources [DWR], 2016; DWR, 1981). Below the Site,the Corcoran Clay is encountered at a depth of approximately 210 feet below ground surface (ft bgs). The Corcoran Clay dips to the east and is encountered at a depth of approximately 270 ft bgs at LWD-10 (Figure 3). Overlying the Corcoran Clay are alluvial deposits made up of sands and gravels with smaller amounts of silts and clays. Generalized stratigraphic layering based on existing geologic and geophysical logs is shown in Figures 3. Below the Site, the stratigraphy appears to consist of four general layers above the top of the Corcoran Clay from shallowest to deepest: a silty sand referred to as the Water Table zone in previous reports, a sand referred to as the Shallow zone, a sandy silt to sandy clay referred to as the Intermediate zone and a coarse sand and gravel referred to as the Deep zone. To the east of the Site, as the Corcoran Clay dips down, the Deep zone appears to separate into three distinct stratigraphic layers: a sand, underlain by a sandy silt to sandy clay, underlain by a coarse sand and gravel (Figure 3). This separation creates six general stratigraphic layers above the top of the Corcoran Clay to the east of the Site in the vicinity of the water supply wells and may influence the fate and transport of sulfolane from the on-Site source area towards the water supply wells. 2.2 Groundwater Flow The regional groundwater flow direction at the Site is to the northwest; however, the groundwater flow direction is locally influenced by groundwater pumping from the remedy extraction wells and water supply wells. Within the deep coarse sand and gravel layer immediately above the Corcoran Clay, where the Simplot and LWD water supply wells are screened, the groundwater flow direction from the Site appears to be eastward towards EW-1213, EW-1513,the PSW and LWD-10 when the PSW and/or LWD-10 are pumping(Figure 4). The groundwater flow regime has varied over time in the area east of the Site due to fluctuation in ground pumping from supply wells. Currently, LWD-10 pumps at an average flowrate when operating of around 1,200 gallons per minute(gpm). Since 2012,the BSW has rarely been operated due to the presence of sulfolane and PSW has been pumping at an average flowrate of approximately 300 gpm. The horizontal and vertical gradients are highly influenced by groundwater pumping in the area east of the Site. When LWD-10 is pumping, the horizontal gradient in the deep sand and gravel layer between BSW and LWD-10 is generally around 0.002 feet per foot(ft/ft). When LWD-10 is turned off,the horizontal gradient in the same area is an order of magnitude lower and is Site Characterization Work Plan 2 June 22,2018