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C 3-12 estimates for hydraulic conductivities of sandy silts and very fine to fine sands. The calculated hydraulic conductivities are summarized on Table 7. Aquifer pumping tests completed by Camp Dresser McKee Inc. (1999) and groundwater modeling that was completed for the existing Austin Road Landfill by Herst & Associates/Andrews Environmental Engineering ([HA/AEE], 2001) indicate that the hydraulic conductivity of upper aquifer materials in the area is significantly higher (250 ft/d) than was measured in the slug and bail test results. This higher value is consistent with values listed in the literature (Driscoll, 1986) for medium- to coarse-grained sands and appears to most closely approximate the types of materials encountered within water -bearing zones in monitoring wells at the site. Using the hydraulic conductivity value selected by HA/AEE (250 ft/d), an average groundwater gradient of 0.003 percent, and an estimated effective porosity of 0.25, the calculated average linear groundwater velocity at the site is: v = [(K)(h/I)]/n = [(250 ft/d) (0.003)] / 0.25 = 3 ft/d Where: v = velocity (cm/sec) K = hydraulic conductivity (cm/sec) n = effective porosity h/I = gradient 3.7.6 Regional Water Quality The San Joaquin hydrologic basin has experienced overdraft for many years. In the Central San Joaquin Irrigation District, which encompasses Forward Landfill, water levels have dropped approximately 70 feet in the past 30 years. Based on groundwater elevation contours published by the San Joaquin County Flood Control and Water Conservation District, the regional groundwater is now typically 10 to 25 feet below msl and flows to the north- northeast. Declining water levels throughout the Stockton area have induced the eastward movement of poor -quality water from the delta sediments. Migration of these saline waters had already impacted the utility of groundwater as far east as Stockton at the time of a 1981 study by the U.S. Geological Survey (USGS) (San Joaquin County Flood Control and Water Conservation District, 1990). The San Joaquin County Flood Control and Water Conservation District has documented a decline in water quality for the Stockton area due to a general increase in chlorides. The regional water quality in the area around Forward Landfill meets the State's Drinking Water Standard. The water is a calcium-magnesium/bicarbonate water type, with total dissolved solids (TDS) concentrations under 500 mg/I. Forward Landfill SWT Engineering Joint Technical Document - April 2014 z:\projects\allied waste\forward\five year permit rvw 2013\jtd-5 yr pr 2013\text\sec 3_final.doc