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i i Draft Environmental Impact Report Page 1V.F-18 Forward Inc. Landfill Expansion k Most of the wells on the perimeter of the landfill do not detect contamination. However, these wells are generally screened at intervals that are relatively shallow or relatively deep (i.e., above i : or below the shallow sand layer present between 80 and 100 feet bgs). MW-2, MW-3, and MW- 4 are screened between 58 and 80 feet bgs, while MW-6 and MW-7 are screened in the deep sand layer between 142.4 and 169.5 feet bgs. Historically, MW-5, which is also screened in the F' shallow sand unit,but is located on the upgradient edge of the landfill, has also detected PCE and TCE. The MW-6 and MW-7 screen intervals are below the thick layer of clay observed in well MW-6 from 117 to 115 feet bgs. This clay layer likely serves as an aquiclude or aquitard, preventing or inhibiting contamination from moving vertically into the deeper sand layers in F the area beneath the landfill. The absence of this clay unit in areas north of the landfill may allow downward migration of constituents to occur. The downgradient private wells (8106A and 7898A)owners, where there have been detections of the VOCs PCE and TCE since 1997, have bottles water supplies. Q2-2008 analyses of these wells reported 2.6 µg/L PCE in well 7898A and 5.3 pg/L PCE in well 8106A. The drinking water standard for PCE and TCE is 5 pg/L and the WQO for PCE and TCE is 0.6 ug/L and 0.7 pg/L,respectively. The furthest downgradient area the VOC plume as affected is the NCYCC wells. Low levels(<5 ug/L TCE)of VOCs have been detected in wells CYA-1 and CYA-2. Well CYA-3 has reported VOCs exceeding drinking water standards. North of the MW-14 location,the plume appears to be moving downward within the aquifer system, as contamination has been measured in deeper samples but not in shallower ones. F. VOCs were not detected in any of the samples collected by CDM between 65 and 90 feet bgs, suggesting the absence of contamination at these shallower depths. However,VOCs were idetected in some of the hydropunch samples collected at depths from 140 to 180 feet bgs by FCDM (1999). The RWQCB water quality objectives (WQOs) for the VOC plume are more stringent than the drinking water standards, as shown in Table IV-F1. The highest concentrations of VOCs F associated with the plume recorded in Q2-2008 were recorded in monitoring well AMW-1 at 25 ug/L PCE. Well AMW-1 is located along the northern boundary of the Austin Landfill boundary. The two private wells along Austin Road are more at risk,both because of the uncertainty of p g Y total depth and depth of perforated screen(no records were available with information on the installation dates, depths, or screen intervals of these wells): The plume moves deeper and becomes more diluted as it moves further offsite,but is still being detected above the WQOs in the NCYCC wells 1,2 and 4 between about 5,500 and 6,000 feet downgradient of the landfill. The maximum depth of detectable VOCs has been reported at 180 feet below ground surface at F. the hydropunch sample ARL-313 collected along Austin Road south of the private wells 7898A and 8106A,just north of the former Austin Road landfill boundary in 1995, as shown in Table IV.F-1. Recent samples also show detectable VOCs, although at a reduced concentrations. The Fplume extends outward (northeast) and downward as it moves beyond the landfill boundary, with VOC concentrations being reduced through dilution and attenuation beyond the relatively small capture zone of the two production (extraction) wells EW-1 and EW-2 along the landfill F. boundary. Observing maximum contaminant concentrations in wells over time, groundwater quality appears to have improved, although whether this improvement is a result of groundwater cleanup actions and landfill gas recovery lessening the input into the groundwater is not known. The maximum PCE concentration in 1999 was 210 pg/L PCE in monitoring well MW- 11. This was reported to be reduced to trace levels of PCE at 0.78 ug/L in the most recent Q2- F