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• • Draft Supplemental Environmental Impact Report Page N.G-9 Forward Inc. Landfill 2018 Expansion Project • below ground surface (bgs). Underlying the younger alluvium are older alluvial deposits • (Victor Formation and the Laguna Formation) that consist of similar interbedded sandy silt, clayey silt, fine to medium-grained sand. Beginning at approximately 100 feet bgs, increasing coarse-grained sands and gravelly lenses have been noted in the boring logs for site monitoring • wells. As stream channel deposits, these sedimentary units interfinger and are laterally • discontinuous. The Victor Formation is underlain by the Laguna Formation of Plio-Pleistocene age (two million years ago). These older alluvial deposits thicken to about 650 feet in the Stockton area (DWR 2006). The main area of VOC groundwater plume impact from the combined landfill is associated with the northern section of the landfill in the area • downgradient of what used to be the Austin landfill Unit, based on the unlined nature of that • landfill unit and historical data of the late 1980s and early 1990's collected around the Austin Unit, which showed significantly higher concentrations of such contaminants of concern (COCs)such as the dissolved VOCs than found associated with the downgradient (southern) • Forward Unit. The deeper water bearing units north of the landfill are likely affected by inputs • from both units (GLA 2017a,b,c; GRA 2018a). • As the groundwater cleanup and monitoring are regulated for the Forward Landfill,the source . of the contamination does not affect the evaluation of whether the proposed landfill expansion addressed in this EIR would contribute additional impacts to groundwater or require additional mitigation measures. In general, any incremental additional potential impact to groundwater from this relatively small expansion—compared the whole landfill—would be small. Hydrogeologic conditions at the site are measured by a series of monitoring wells, as depicted on Figure IV.G-1. Historically high groundwater in the vicinity of the Landfill occurred in 1974 with groundwater being at an elevation of-2 to-5 feet amsl. As discussed in the regional hydrology, and confirmed in quarterly monitoring reports, groundwater flows in the project • area are generally to the north, sometimes with a northeast component in response to pumping • of agricultural and municipal wells east of Stockton. Figure IV.G-1 shows the most recent groundwater(elevation) contours, published in 2018, which illustrate the generally northward groundwater flow direction, with a local northeasterly flow from the northeastern comer of the landfill (GLA 2018a). • The Water Quality Control Plan (Basin Plan) for the Sacramento River and San Joaquin River Basins indicate the site is located in the Duck-Little John Hydrologic Area of the San Joaquin • River Basin. The San Joaquin Hydrologic Basin is a major regional aquifer system and • groundwater in the basin is used for industrial, domestic, and agricultural purposes. • The first encountered groundwater in the site area ranges from about 60 to 80 feet bgs and the . depths to water fluctuate by as much as 15 feet between wet and dry seasons. No regionally . continuous fine-grained layers have been identified in the older alluvial formations and groundwater is largely unconfined. However, due to the fluvial depositional environment, laterally discontinuous layers of fine-grained soil are present and local areas of groundwater confinement are known to occur(DWR 2003; GLA 2017a, 2018a, 2018b). The area surrounding . the landfill is primarily agricultural and there are numerous supply wells and irrigation • distribution tanks with piping networks to discharge pumped groundwater to the crops in the