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Austin Road Landfill Section 5 Contaminant Plume Characterization Report Summary and Conclusions 5.4 Contaminant Fate and Transport The contaminant plume is moving at a similar rate,approximately 4 feet per day,as the general groundwater flow rate. This suggests that VCC migration rates are not affected significantly by sorption of contaminants to aquifer materials. The trend of reduced contaminant concentrations away from the landfill is attributed primarily to dilution occurring through mixing with unimpacted groundwater and with surface water recharge. The general groundwater flow direction,prior to pumping from extraction wells EW-1 and EW-2,is to the northeast. The portion of the plume outside the influence of the pumping wells will continue to migrate in a northeasterly direction. Flow conditions under corrective action system pumping operations indicates the local groundwater gradient shifting to a more direct northerly flow direction. This could result in the leading edge of the plume to also shift to the north over time. Water quality at the CYA wells will be monitored over time to determine if they are impacted by the plume. However,continued operation of the groundwater extraction system at the site will help limit the contaminant plume from expanding in a downgradient direction. 5.5 Engineering Feasibility Study The EFS will be conducted based upon the findings documented in this Contaminant Plume Characterization Report. The purpose of the EFS is to develop and evaluate potential alternatives, or modifications,to the existing groundwater remediation system to better achieve corrective action objectives. These will include components addressing additional extraction wells,modifications to the groundwater treatment plant and conveyance system,wellhead protection for water supply wells in the area,and continued groundwater monitoring. CDM Camp Dresser&McKee CS 010791 5-2 W.\REPORTSISTOCKTOMARLPLUME981SEC5.WPD City of Stockton 190855-0006