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Draft Environmental Impact Report Page IV.F-10 Forward Inc. Landfill Expansion The depths to water measured in the borings installed by CDM in 1998 ranged from 61 to 84.6 feet bgs or-24 to--42 feet msl. Water levels have subsequently dropped with most wells showing depths to groundwater at about 60 feet bgs in 2008 for the Forward and Austin Road landfill units. As discussed in the regional hydrology, groundwater flows in the project area are F,44 generally to the north, sometimes with a northeast component.Figure IV.F-2 shows the May 2008 groundwater (elevation) contours,which illustrate the groundwater flow direction,which is generally northward. Groundwater gradient (the steepness of the slope of the groundwater flow) varies seasonally, y being reflective of the weather cycles and pumping, with a steeper, north-trending gradient in Ffidry years and a more gentle, northeast gradient in wet years (Kleinfelder, 1988; CDM, 1999). The dry year gradient is apparently affected by greater drawdown in the area northeast of Stockton. Perched water may occur intermittently during the wet season, above relatively impermeable silts or clays in the sediment sequence. Hydrogeological data from the Forward Landfill site indicate that vertical and horizontal transmissivities are highly varied, and there is substantial interconnection between upper and lower aquifers. The development of a regional cone of depression east of Stockton, in response to pumping of agricultural and municipal wells, indicates the connectedness of the regional aquifers. The flow directions and gradients F are consistent with the groundwater quality results collected during CDM(1999a) and AEE (2001a and 2001b) investigations. The groundwater gradient, even with its variation,has been relatively flat and consistent, ranging between 0.001 and 0.002 ftlft between 1991 and 2007 } (GLA, 2007) In general, the available information suggests that the primary aquifer under the site is an unconfined aquifer within the Victor Formation. No widely correlatable shallow aquifer sands, " y or silt and clay aquicludes or local aquitards (barriers)were reported, which reduces the ability to predict lateral groundwater flow or vertical interconnections. Groundwater tests associated with the Forward Landfill site determined that the rate of groundwater flow in the uppermost regional aquifer was in the range of 0.08 to 0.8 feet per day, which is a relatively high flow rate (Kleinfelder, 1991). Due to the heterogeneous nature of the Victor Formation sediment units, aquifer properties,including conductivity and rate of flow, are anticipated to vary by a factor of F, 10 or greater, depending on location. The location of the main expansion area upgradient of the Forward Landfill and upgradient (west) of the Austin Road landfill unit means that, over time, should subsurface contamination occur the groundwater will flow towards the extraction well points downgradient. F.1 Groundwater Quality Site Groundwater Studies New groundwater and hydrochemical data has been collected and modeled since the Consolidated Forward Inc. Landfill EIR(2003) was completed. Numerous studies and groundwater monitoring events have been completed per the RWQCB Order(s)in an effort to F` track trends and monitoring and mitigate the offsite groundwater plume. F.