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0 0 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, tetrachloroethene (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 gg/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. 4.0 EVALUATION MONITORING PROGRAM FIELD WORK l>� In August 2011, the County submitted an EMP Workplan to further characterize the nature and extent of VOCs migrating from the CHSL. The workplan outlined a program for temporary and permanent well construction, sampling, analysis, and data evaluation that was approved by the RWQCB in late August 2011. A revised EMP workplan that allowed for the use of HydropunchTM methods to collect samples was submitted and Lapproved by the RWQCB in September 2011. EMP field work was conducted in October through December 2011, and is described in the following sections. 4.1 Well Construction Methods In accordance with the EMP workplan, the County's drilling contractor, Gregg Drilling, Inc., drilled three temporary boreholes:MW-9, MW -10, and MW -11 (Figure 3). Boreholes were drilled using eight -inch diameter hollow stem auger techniques under the direct field supervision of a State of California Certified Engineering Geologist and Certified Hydrogeologist. Lithologic samples were collected at five-foot intervals and logged by the field geologist to ensure proper stratigraphic correlation with existing site monitoring wells. Boreholes were advanced through the Recent alluvium and into the Pleistocene Corcoran Clay (Appendix 1). Once the Corcoran Clay was reached, drilling stopped to prepare the HydropunchTM sampler. Attempts to drive the HydropunchTM into the Corcoran Clay were unsuccessful. EMP Report— Phase 1Geo-LOg0C Page ■ S Corral Hollow Sanitary Landfill ASSOCIATES February 1, 2012