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2.2 Hydrogeologic Conditions The Water Quality Control Plan for the Sacramento River Basin and the San Joaquin River Basin (Basin Plan) places the site in the Duck-Littlejohns Hydrologic Area (31.40) of the San Joaquin River Basin. As a major regional aquifer system, the San Joaquin hydrologic basin is used for industrial, domestic, and agricultural purposes. Most of the production wells in the area are constructed to depths on the order of 500 feet and produce water from the Laguna Formation. Although there are no regionally significant fine-grained intervals, by the heterogeneous nature of the Laguna Formation, some localized confinement of groundwater may be present, though typically, groundwater is unconfined or locally semi -confined. It is recognized that the area's heavy reliance on groundwater associated with significant population growth and agricultural needs, has resulted in the basin being in overdrafted for at least the past 40 years, and water levels have continued to decrease throughout the region. The uppermost groundwater beneath the Forward Landfill occurs in the unconfined Victor Formation. Historical groundwater measurements in monitoring wells at the Forward Landfill indicate that groundwater occurs at a depth of approximately 50 to 70 feet below the ground surface with seasonal variations in the groundwater depth of as much as 10 feet; the highest groundwater elevations are observed during the winter/spring and lowest levels in the summer/fall. Groundwater elevations typically range from about -10 feet relative to MSL on the southwest corner of the Forward Unit to about -25 feet relative to MSL on the northern perimeter of the Austin Unit. Data acquired by GLA between 2003 and 2013 indicate that, groundwater generally flows to the north and northeasterly at a gradient between 0.001 and 0.003ft/ft. Locally, a southwest gradient has been noted in the northeast corner of the Forward Unit beginning in May 2011. This southwest gradient is believed to be a result of agricultural activities east of the site creating groundwater mounding. Although a broad range of hydraulic conductivity values have been assigned to the formational materials (e.g., clay, silt and sand) within the Victor Formation, slug tests performed in four wells at the Forward Landfill, resulted in an estimated hydraulic conductivity value of 0.04 cm/sec for the uppermost aquifer beneath the landfill. Based on a maximum hydraulic gradient of 0.003 ft/ft, and an estimated effective porosity of 0.35, the maximum groundwater velocity beneath the Forward Landfill is estimated to be approximately 355 feet per year. 2.2.1 Local Groundwater Impacts Forward Unit — Historically, low-level VOCs have been detected intermittently in wells at the Forward Unit. On behalf of Forward Inc., GLA prepared a letter report dated February 14, 2001 (RWQCB Case No. 2209), to assess the source and potential impacts of low-level VOCs in groundwater adjacent to the Forward Landfill. The letter report concluded that the low-level volatile organic compounds (VOCs) in groundwater (and lysimeter samples) at the Forward Unit are most likely due to landfill gas from unlined units WMU A/B. D:\2014-0012\FA finassur.doa 3 3/10/2014, Rev. 0