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Austin Road Landfill Section 3 Contaminant Plume Characterization Report Field Investigation Results The northeasterly direction of groundwater flow discussed in Section 3.2 is generally consistent with the distribution of VOC contaminants in the aquifer system north of the ARL. The highest VOC concentrations are detected in wells located closest to the ARL and VOC concentrations decrease to the north and northeast. The VOC trends observed are consistent with a continuous source which originates beneath the ARL. The VOCs also appear to migrate through increasingly - deeper sandy zones within the saturated soils with increasing distance away from the ARL. This is likely due to the position of the permeable sand units north of the ARL,to deep groundwater pumping in areas north and east of the site which induces a well-documented downward component to groundwater flow,and to recharge from surface irrigation water which infiltrates into the aquifer. Groundwater and any dissolved compounds,such as VOCs,present north of the ARL will tend to migrate through an aquifer preferentially in the more permeable deposits. Water quality results were examined for any temporal relationships that might exist,particularly regarding the effectiveness of plume source control provided by the extraction wells EW-1 and EW- 2. Wells MW-1 and MW-4 located along the northern boundary of the ARL,and wells MW-10 through MW-14 located downgradient of the ARL,had VOC data from which evaluations were made. Although there is much variability in results from quarter to quarter,these wells generally show decreases in VOC concentrations with time. Appendix E contains plots of PCE concentrations with time for all monitoring wells that have been included in the monitoring program at the ARL. Maps of the groundwater surface since late 1996 have shown a trend of groundwater flow being more to the north than the northeast away from the ARL,with the June 1997 groundwater contour map even showing a northwest direction of flow. The shift in groundwater flow direction from the northeast toward the north and northwest since late 1996 may be related to operation of extraction wells EW-1 and EW-2. This system began consistent pumping in late 1996 or early 1997. Pumping creates cones of depression in the aquifer in the vicinity of the pumping wells and flattens the hydraulic gradient in downgradient areas. Current pumping data indicates that well EW-2 to the west pumps at about 150 gpm while EW-1 pumps at about 40 gpm. As shown by the capture zones presented in Section 3.4,if aquifer properties are constant then the cones of depression and area of influence of pumping is proportional to the pumping rate of individual wells. The aquifer surface therefore is expected to be drawn down more by well EW-2 than EW-1. If the historical pumping - rates of wells EW-1 and EW-2 have been similar to current rates then this pattern of aquifer drawdown could be well established. An effect of the differential pumping of wells EW-1 and EW-2 is that,north of the ARL,the direction of groundwater flow may have been shifted towards the north due to operation of the extraction well system. It should be noted that other factors could have contributed to a more northerly shift in the direction of groundwater flow. These could include increased pumping from the CYA wells and/or other wells located north and west of the ARL,a decrease in agricultural and/or municipal pumping in regions east and northeast of the ARL,or an increase in recharge in areas immediately east of the ARL and CYA properties. The groundwater and soil sampling results collected north of the ARL(Section 3.3)suggest that biological attenuating mechanisms play a role in contaminant fate and transport of VOCs downgradient of the ARL. PCE is the predominant VOC compound detected offsite in groundwater and is assumed to be a source compound present in landfill leachate that entered the groundwater system beneath the ARL. TCE and cis-1,2 Dichloroethene(cis DCE)have also been detected in the aquifer,but at lower concentrations than PCE(Figure 3-4 and Table 3-1). These are CDM Cam Dresser&McKee 3-8 P CS 010778 W.NREPORTS\STOCKTON\ARLPLUME.981SEC3.WPD Cita of Stockton 190855-0006