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t ' highest VOC concentrations are present within a sand layer which extends over an interval of approximately 80 to 104 feet bgs (CDM, 1999A)., The investigation results indicate that contaminant concentrations decrease at distances from the source,with the plume migrating in deeper lithologic units at distance from the site. The plume shape reflects the north to northeast groundwater flow direction, with the leading edge of the plume extending east of Austin Road. The contaminant plume is moving at a similar rate, approximately feet per day,as the general groundwater flow rate. This suggests that VOC 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 water recharge. The general groundwater flow direction, with unimpacted groundwater and with surface wat e. g prior to pumping from extraction wells, 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 indicate that the local groundwater i gradient is 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. The maximum concentrations reported for the key contaminants PCE and TCI= in the 1998 monitoring were 69 and 48 pg/L, respectively (CDM, 1999A). This compares to the maximum concentrations of 210 pg1L PCE and 39 pg/LTCE reported in the 1994 EIR. i The existing landfill has had a groundwater monitoring program since 1977, and there are 13 monitoring wells and two water supply wells on the site: Ten of the monitoring wells are downgradient of the landfill, and three wells are located upgradient. One of the water supply wells has been decommissioned in compliance with Waste Discharge Requirements Order Number 90-122 (because the well was tion wells, monitoring wells, and hydropunch holes have contaminated). Groundwater samples from producr re the only VOC's detected at concentrations greater been tested for VOC's. Chlorinated hydrocarbons a than established maximum contaminant levels. The City is implementing a Corrective Action Plan (CAP) 1 . P design includes the installation of a groundwater extraction and approved by the RWQCB. The CA treatment system to mitigate the detected chlorinated hydrocarbons (R. W. Beck and Associates, 1993). The quality of groundwater in the vicinity of the Austin Road Landfill has been discussed in the 1994 EIR and in numerous reports on file with the City: A recent comprehensive report on the groundwater quality associated with the landfill is the CDM January 1999 Plume Characterization Report and the April 1999 Groundwater Corrective Action Report..The contaminantPCE is the most commonly detected compound in the groundwater. plying bottled water to all downgradient well owners where there has The City of Stockton committed to sup1" been a detection of any VOC's above the drinking water standards. This has been performed since 1997, j when downgradient wells first detected PCE above 5 pg/L. The concentration in these wells has subsequently shown less than 5 pg/L, but the City still provided bottled water to those residences in case of future contamination. In April 1999,the City of Stockton completed a Groundwater Corrective Action Engineering Feasibility Study Report (GCE) to evaluate the range of groundwater cleanup alternatives in terms of protection to the environment and consistency with regulatory requirements. In addition, as described above, leachate from part of the landfill's north face has been added to overall leachate collection quantities. Landfill leachate has been collected in a 12,000-gallon storage tank and removed when near full for off-site treatment at a licensed TDS facility. In the fourth quarter of'1998, the most recent data available, 11.27 tons or 3,100 gallons of leachate were removed from the 12,000-gallon storage tank for off-site treatment. f The GCE was prepared as part of'the RWQCB Order and addresses the requirement of both the RWQCB which include the Waste Management Board requirements. and California Code of Regulations, Title 27, _. _ . . .. i GP-99-7, ZR-99-6, UP-99-17, ER-99-21City of Stockton l San Joaquin County page 16 ; Community Development