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Regionally the fact that QAP is located directly over the low point in the pumping depression centered northeast of Stockton may be significant. The removal of large volumes of groundwater which creates the pumping depression proabably acts to isolate the underlying groundwater by removing contaminants. Groundwater Flow To estimate the impact on groundwater quality of the wastewater disposal operation it is necessary to estimate the horizontal flow of groundwater in the saturated zone. The subsurface flow will intercept the water percolating downward from below the root zone. The resultant groundwater impacts on groundwater quality can be estimated based on continuity calculations. The California Department of Water Resources has compiled extensive data on the geohydrology of the San Joaquin County Groundwater Basin (5) . Specific yield tabulated by depth increment below ground surface is tabulated for the entire groundwater basin. Horizontal hydraulic conductivity can be estimated from the specific yield (6) . Horizontal conductivity at the site is estimated to be 0.5 in./hr. Porosity was assumed to be approximately 45%. The horizontal flow rate was estimated using the Darcy equation q = I. (dh/dl ) with the hydraulic gradient being approximately 10 ft/mi . at the site. The hydraulic gradient was estimated from the contour maps provided by the San Joaquin County flood District. Based on these assumptions it was calculated that the shallow groundwater below the plant is migrating to the northeast at the rate of 230 ft per year. This rate is in conformance with the measured rate of advance of saline groundwater west of Stockton. It has been estimated that the saline groundwater from the Delta has been moving eastward at 130 ft/year (7) . The hydraulic gradient in the area of the saline front is approximately 25% the onsite gradient. The horizontal flow volume of groundwater beneath the site can be estimated based on the velocity obtained previously, cross sectional area and porosity. As mentioned previously, the site specific aquifer system (see Figure 5) shows a localized clay barrier which prevents hydraulic continuity between the ; ' � shallow groundwater and the preferred aquifer in the Laguna formation. Therefore vertical groundwater flow can be ignored when calculating the amount of groundwater potentially mixing with percolate from QAP fields. In order to estimate the impact of QAP operations on groundwater quality, an estimated flow volume below the site has been prepared. It was assumed that the shallow aquifer was confined to the Victor formation with a maximum depth of 150 ft. To account for some dispersion as the infiltrate passes through the unsaturated zone the area of initial mixing with the shallow groundwater was assumed to be 200 acres. At a horizontal flow rate of 230 ft/year approximately 1.65 x 106 ft3/year of groundwater will flow directly beneath the site per year. 1-10 RP033-L