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Page 107 The current groundwater extraction system is composed of two pumping wells along the northern boundary of the landfill. Based on results from the downgradient wells and capture zone model completed as part of the updated CAP,these wells do not provide complete capture of the contaminant plume. Additional extraction wells have been recommended to achieve some degree of hydraulic containment over the current plume (CDM, 1999B). Concentrations in some key downgradient monitoring wells,such as MW-I 1 and MW-13,showed concentrations at higher levels of PCE than in the data analyzed in the 1994 EIR- A monitoring well installed further downgradient after the 1994 EIR(MW-14)also has shown consistent detection of PCE above the drinking water MCLS. The revised CAP design recommends additional monitoring and groundwater extraction wells that will extract greater volumes of water than anticipated in the 1994 EIR. The larger extent of the contamination in the groundwater could result in more downgradient extraction wells coming on line which,in turn,could adversely influence local water supply well now. In addition, even though less groundwater is currently extracted than the design capacity outlined in the 1994 EIR,the added potential volume estimated in the 1999 study would result in additional off-gassing from the air stripper and more frequent change out of the activated carbon vessels. These impacts would be significant and mitigable with adoption of the fallowing measure. Mitieation K.6: Continue implementation of the current RWQCB Order and the complete implementation of recommendations in the April 1999 CAP. The new data to be collected should address the potential impacts to the local supply wells. The siting of the new extraction wells should be modified as appropriate to minimize impact to supply wells. Activated carbon vessels should be changed as necessary to serve increased groundwater extraction levels. Impact K.7: Private irrigation wells located downgradient of the Austin Landfill are at risk from the VOC-contaminated groundwater plume. ,,, In October 1995,'a survey of private water wells used for domestic of irrigation was completed. Seventeen irrigation or domestic water supply wells,varying from depths of 120 to 480 feet bgs,were i reported within a 1-mile radius of the site. Of these 17 wells, 0 appeared to be located upgradient or ! transgradient of the landfill(one of which is owned by the City of Stockton and used for dust suppression at the landfill). The four remaining wells,two of which are located at the downgradient California Youth Facility, are at risk from the VOC-contaminated groundwater plume emanating from the Austin Landfill. Two of the closest,relatively shallow,downgradient wells owned by private parties had detectable VOC concentrations above MCLs in 1998(Miller,personal communication,August 1999). Groundwater contamination from the Landfill is generally thought to be limited to mainly the unconfined aquifer sequence within the Victor formation. As a result of the contaminated domestic wells,the City of Stockton entered into an agreement with the two property owners to indefinitely supply the households with bottled water. These impacts would be significant and mitigable with adoption of the following measure. continue to test all known water supply Mitigation K.7: The landfill operator should co Pp Y wells within the area of the mapped and projected groundwater plume. Where detectable VOCs are reported,bottled water should be supplied as is currently being done for two affected households. Conclusion The change in existing conditions, impacts and development of appropriate mitigation measures related to potential health and environmental concerns from hazardous materials at the Austin Road landfill is GRASS'ETTI ENVIRONMENTAL CONSULTING