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Austin Road Landfill Section 4 Contaminant Plume Characterization Report Capture Zone Modeling Analysis A second set of model runs were conducted to further evaluate the existing extraction well system. During this set of capture zone simulations,extraction rates equivalent to EW-2(Q= 150 gpm)were input for both wells. As with the first modeled scenario,under the conditions simulated there was still not complete capture of groundwater migrating from beneath the landfill. Figures 4-3 and 4-4 depict the one-and three-year results. These capture zone results depicted in Figures 4-1 through 4-4 differ from initial capture zone evaluations reported in the site's Corrective Action Plan(EMCON.,1991). This earlier report used very short-duration aquifer tests on wells EW-1 and EW-2 (40 minutes and 4 hours,respectively), used only the pair of monitoring wells adjacent to each pumping well to obtain drawdown data,did not incorporate the existing aquifer hydraulic gradient into the analyses and did not simulate multiple-well influences on the groundwater flow field as did the current analysis. Based on these differences,it is not surprising that the conclusions drawn from the earlier analysis differ from the current,more comprehensive aquifer test and subsequent capture zone analysis. Based on the evaluation of plume capture provided by the existing extraction wells EW-1 and EW-2, the existing groundwater extraction system will need to be expanded using additional extraction wells to effectively capture contaminated groundwater emanating from beneath ARL. The new wells should be located along the service road at the north edge of the waste fill,along a line formed by wells EW-1 and EW-2. Detailed evaluations of alternatives to the existing corrective action system will be conducted in the Engineering Feasibility Study phase of the project. CDM Camp Dresser&McKee CS 010786 4-3 W.\REPORTS\STOCKTOMARLPLUME.98\SEC4.WPD City of Stockton 190855-0006