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2.4 Monitoring Network Groundwater quality at the site has been monitored since 1988. The current groundwater monitoring well network consists of six wells (MW-1 through MW-5, and SB-1; see Figure 2). Two water-bearing zones are monitored. Groundwater in the deep water- bearing zone, which historically flowed to the southeast, is monitored by wells MW-1, MW-2, and MW-3. MW-2 is the background well for this water-bearing zone. However, these wells have been dry since 1991. The shallow water-bearing zone is a perched groundwater zone that is monitored by wells MW-4, MW-5, and SB-1. As noted above, groundwater in this water-bearing zone flows both to the northwest and to the southeast (see Figure 2). SB-1 is not required to be sampled by WDR Order No. 94-259; it was installed to provide the third data point required for groundwater flow characterization. 2.5 Monitoring and Response Actions A SWAT program for the Corral Hollow Sanitary Landfill was conducted in 1988. During the SWAT program and subsequent testing (until December 1991), VOCs were not detected in groundwater samples from the well.consistent with the previous WDR Order No. 89-046, VOC analyses were required only annually during the fourth quarter monitoring event. Upon the first detection of VOCs in December 1991, the County instituted an evaluation monitoring program at well MW-5 with respect to VOCs. To date, none of the VOCs detected in well MW-5 have exceeded primary drinking water standards (maximum contaminant levels [MCLs]). Tabular summaries of the historical data for Corral Hollow Sanitary landfill is presented in Appendix A. The release was evaluated in a phased approach, consistent with state guidelines (State Water Resources Control Board Resolution No. 92-49, Policies and Procedures for Investigation and Cleanup and Abatement of Discharges under Water Code Section 13304 of the Porter Cologne Act). The first phase of the evaluation program was to confirm the VOC results observed in the December 1991 fourth quarter monitoring event, in which trichlorofluoromethane (Freon 11) and tetrachloroethane (PCE) were detected in samples collected from well MW-5 at concentrations of 2 and 4 micrograms per liter(µg/L),respectively. Well MW-5 was resampled on February 12, 1992, for VOCs by Environmental Protection Agency (EPA) Method 624. The analytical results from the February 1992 sampling event confirmed the December 1991 results. Freon 11 and PCE were detected at concentrations of 1 and 3 gg/L,respectively. The results of the evaluation samples were submitted to the RWQCB by the County on March 23, 1992. The second phase of the evaluation program was to sample well MW-5 at a higher frequency. The detection monitoring program in the WDR requires annual monitoring for SAC1N.\PJOM3M390031.9BG-94\s :1 Rev.0 4 0 d8 R /95 0939.003.19 2-5