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L' 1W 0 n [I C C r L C n I IM C Groundwater in the shallow aquifer has been impacted by VOCs; whereas no VOCs have been identified in the deeper aquifer. VOC impacts to the shallow aquifer were first identified in 1991 in samples from well MW -5. Well MW -8 was constructed in December 2007 to further delineate the extent of the release. VOCs have been detected in samples from MW -8 since construction. Constituents in MW -8 are generally the same as MW -5, although generally in lower concentration. VOCs historically detected in samples from the shallow aquifer wells have typically included dichlorodifluoromethane, trichlorofluoromethane, tetra chloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene, and 1,1-dichloroethane. The total concentration of all detectable VOCs has been less than 30 micrograms per liter (µg/L) during the last five years, with concentrations of PCE periodically exceeding the State of California and Federal Maximum Contaminant Levels (MCL; 5 µg/L) in samples from well MW -5. Inorganic constituent concentrations have remained relatively static over time in samples from the shallow aquifer. The VOCs detected in groundwater samples are similar to those detected in adjacent soil -pore gas monitoring probes, further suggesting landfill -gas related impacts. In response to historical detections of VOCs in samples from wells MW -5 and MW -8, the County installed additional landfill gas extraction wells near the northeastern portion of the landfill. However, the effect of this additional LFG collection is not yet apparent in the water quality at wells MW -5 and -8. The County has recently submitted a proposal to the RWQCB to extend the closure cover over approximately 1.3 acres waste outside of the northeastern edge of the main landfill, and to install two additional landfill gas extraction wells into refuse now in Caltrans property. 2011-0115 CHSL EMP Workplan Revision 1- August 24, 2011 4 Geo -Logic Associates Historical groundwater equipotential data suggest that groundwater in the deeper aquifer flows to the northeast at a gradient of about 0.004 ft/ft, generally parallel to the regional topographic gradient toward the floor of the Central Valley. 2.4 GROUNDWATER CHEMISTRY Groundwater monitoring at the site began in 1988 with the construction of monitoring wells as part of the Solid Waste Assessment Test (SWAT) program, and as outlined in Section 2.3, while nine wells and one piezometer have been installed at the site, seven monitoring wells comprise the current groundwater monitoring network. Groundwater is basis sampled on a quarterly and reported semiannually. n [I C C r L C n I IM C Groundwater in the shallow aquifer has been impacted by VOCs; whereas no VOCs have been identified in the deeper aquifer. VOC impacts to the shallow aquifer were first identified in 1991 in samples from well MW -5. Well MW -8 was constructed in December 2007 to further delineate the extent of the release. VOCs have been detected in samples from MW -8 since construction. Constituents in MW -8 are generally the same as MW -5, although generally in lower concentration. VOCs historically detected in samples from the shallow aquifer wells have typically included dichlorodifluoromethane, trichlorofluoromethane, tetra chloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene, and 1,1-dichloroethane. The total concentration of all detectable VOCs has been less than 30 micrograms per liter (µg/L) during the last five years, with concentrations of PCE periodically exceeding the State of California and Federal Maximum Contaminant Levels (MCL; 5 µg/L) in samples from well MW -5. Inorganic constituent concentrations have remained relatively static over time in samples from the shallow aquifer. The VOCs detected in groundwater samples are similar to those detected in adjacent soil -pore gas monitoring probes, further suggesting landfill -gas related impacts. In response to historical detections of VOCs in samples from wells MW -5 and MW -8, the County installed additional landfill gas extraction wells near the northeastern portion of the landfill. However, the effect of this additional LFG collection is not yet apparent in the water quality at wells MW -5 and -8. The County has recently submitted a proposal to the RWQCB to extend the closure cover over approximately 1.3 acres waste outside of the northeastern edge of the main landfill, and to install two additional landfill gas extraction wells into refuse now in Caltrans property. 2011-0115 CHSL EMP Workplan Revision 1- August 24, 2011 4 Geo -Logic Associates