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0 4.2 PotentialVadoseZone Impacts' The vadose zone is the unsaturated soil zone above the groundwater table. The Forward Landfill site is located in an area where the vadose zone is typically about 50 to 60 feet thick. In semi -arid areas similar to Stockton, subsurface movement of LFG is considered the most significant means by which the vadose zone may be affected by the WMU. 4.3 Potential Groundwater Impacts Recognizing that the expansion areas will be lined with a Subtitle D liner system, the existing unlined sections of the Forward Unit and Austin Unit have the highest potential for additional groundwater impacts as the result of a release associated with LFG, characterized by modest concentrations of VOCs. If waste constituents reach groundwater, the natural groundwater flow gradients and the dispersive properties of the specific contaminants would determine how groundwater impacts will migrate from the site. Given the fact that groundwater typically flows in a northerly to northeasterly direction in accordance with the regional gradient, it is anticipated that potential groundwater impacts would be concentrated in the upper groundwater zone of the Victor Formation and would be detected by the existing DMP wells at the Forward and Austin Units. The following discussion identifies the most reasonably foreseeable release scenario that might be expected at the site and the mitigation response measures associated with the scenario for the purposes of establishing financial assurance for future corrective actions. 5.1 Surface Water Mitigation Since creek waters will not come in contact with wastes, and run-on and run-off will be controlled, the worst-case surface water release scenario involves transport of minor volumes of landfill constituents in run-off to the creek. However, considering that wastes will be covered by daily and interim cover soils, and since run-off will be controlled in a drainage system designed for the 1000 - year storm event and will be monitored as part of both the Waste Discharge Requirements and NPDES permits, the volume of impacted water that might be released from the site is expected to be small. In the event that runoff from the WMU impacts creek water, a program would be implemented to investigate how such impacts occur and to identify engineering measures (e.g., leachate seep controls such as additional cover soils or drains) to eliminate the source of impacts. The existing operations budget already provides funding for maintenance of the cover, and drainage control. D:\2014-0012\FA finassur.doa 7 3/10/2014, Rev. 0