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t ? Ms. Wendy Arano Project No.: 791291 June 7, 2001 Page 2 The secondary series of deep wells were monitored from 1988 to 1990, until they went dry. The groundwater detected in the secondary series of deep wells was separated from the primary series of shallow wells by over 200 feet of vadose zone in 1990. The deep monitoring wells went dry as a result of falling water levels due to regional groundwater overdraft. The depth to groundwater in the deep zone is not known at this time. The direction of groundwater flow determined by these deep wells was primarily to the northeast. The Corcoran Clay, which acts as perching horizon for the groundwater monitored by the shallow primary wells, does not extend entirely beneath the landfill as shown on Figure 2. The primary series of monitoring wells are installed at the downgradient point of compliance of the landfill. The primary series of wells have been continuously monitored since 1988. Several low to trace levels of volatile organic compounds (VOCs) have been detected in shallow well MW-5. With the exception of a few segregated low to trace level VOCs detected in shallow well SB-1, the other two shallow wells have not shown the presence of VOCs. The secondary series of deep wells did not detect any VOCs from 1988 to 1990. The greatest threat of contaminate release to deep groundwater would have existed while the deep series of monitoring wells were operational. The landfill accepted waste for approximately 30 years before the deep monitoring wells were installed. With the continual drop in deep groundwater levels, the closure, capping, and installation of a landfill gas (LFG) collection system for the landfill that threat has been reduced. LANDFILL CLOSURE AND CORRECTIVE ACTION Investigations undertaken by the County into the source of the shallow VOC groundwater contamination have shown that LFG is the transporting medium for VOC migration from the landfill. The concentration and number of VOCs detected in LFG in the refuse correlates with the concentration and number of VOCs detected in the LFG monitoring probes, and shallow well MW-5. The extent of VOC contamination adjacent to the landfill is very limited. An investigation program was completed in July 2000, which shows that the extent of groundwater impact was localized in the northeastern portion of the landfill around shallow well MW-5. MW-5 is located approximately 40 feet from the landfill refuse. This correlates with the LFG probe data, which also showed that the extent of LFG migration from the landfill was also limited to the northeastern portion of the landfill. \\SACRFPI\COMMON\sacDP\2001\PRJ\PJ7\'791291.1 aa.doc (00006000)