SU0012793 - Laserfiche WebLink

Home Browse Search

Page 103 plume extending east of Austin Road(see Figure K-3). The contaminant plume is moving at a similar rate,approximately 4 feet per day, as the general groundwater flow rate. This suggest 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 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 Iimit the contaminant plume from expanding in a downgradient direction. Groundwater Quality and TCE in the 1998 monitoring E The maximum concentrations reported for the key contaminants PCE g were 69 and 48 pg/L,respectively(CDM 1999A). This compares to the maximum concentrations of 210 µg/L PCE and 39pg/L TCE reported in the 1994 EIR. The existing landfill has had a groundwater monitoring program since 1977,and there are 13 monitoring wells (MW 1 through MW 13)and two water supply wells on-site. Ten of the monitoring wells are downgradient of the landfill (MW 1,MW4,MW6,and MW7 through MW 13), and three wells are located upgradient(MW2,MW3, and MW5). One of the water supply wells(MSW)has been decommissioned in compliance with Waste Discharge Requirements Order Number 90-122(because the well was 4 contaminated). Groundwater samples from production wells,monitoring wells,and hydropunch holes +��1 have been tested for VOCs. Chlorinated hydrocarbons are the only VOCs detected at concentrations greater than established maximum contaminant levels. The City is implementing a Corrective Action Plan(CAP)approved by the RWQCB. The CAP design includes the installation of a groundwater extraction and 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 contaminant PCE is the most commonly detected compound in the groundwater. The City of Stockton committed to supplying bottled water to all downgradient well owners where there has been a detection of any VOCs above the drinking water standards. This has been performed since 1997 when downgradient wells 7899A and 8106A first detected PCE above 5 µg/L. The.concentration in these wells has subsequently shown less than 5 µg/L but the City still provided bottled water to those residences in case of future contamination. Groundwater quality appears to have improved when maximum concentrations in wells are looked at. Table l shows the maximum concentrations reported in the 1989 SWAT. In the 1999 CDM study,the maximum PCE concentration was down to 69 pg/L in monitoring well MW-11,with TCE down to 30 gg/L. However,the apparent improvement of the groundwater is likely the result of the plume migration and dispersion rather than mitigation. GRASS'ETTI ENVIRONMENTAL CONSULTING