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' f surface layer. In 2001 a landfill gas (LFG) collection system was installed to mitigate volatile organic compounds (VOCs) impacts to groundwater and prevent offsite migration of LFG. A landfill gas collection system was installed and began operating in December 2001. 1.2.1 Hydrogeology Three distinct alluvial deposits underlie the site. The first deposit consists of semi- consolidated sand and gravel and extends approximately 10 feet below ground surface (bgs). The second deposit in composed of Corcoran clay and extends from approximately 10 feet to 50 feet bgs. The third deposit consists of a clay-sand matrix and extends 50 feet to 250 feet bgs. The shallow aquifer exists on the northeast side of the property and is encountered between 12 and 65 feet below ground surface. A mound beneath the landfill separates the shallow aquifer into two aquifers and creates bi-directional flow to the northwest and southeast. The deep aquifer is encountered at 320 feet below ground surface and groundwater flows to the northeast. 1.2.2 Environmental Monitoring Seven monitoring wells exist which monitor the two aquifers. Three wells (SB-1, MW-4 and MW-5) monitor the shallow aquifer. One well is up-gradient (SB-1) and the other two are down-gradient. Two monitoring wells exist (MW-6 and MW-7) which monitor the deep aquifer. Monitoring well MW-7 is up-gradient to well MW-6. In 1991, VOCs including tetrachloroethene and trichlorofluoromethane were detected in groundwater samples from MW-5 (shallow aquifer) at concentrations of 4 µg/1 and 2 µg/1. In June 1992 concentrations of VOCs including dichlorodifuoromethane, tetrachloroethene and trichlorofluoromethane were reported at 71 µg/1, 2.8 µg/1, and 1.1 µg/1. The presence of VOCs was believed to originate from landfill gas. In 1995, a corrective action plan including closure and final cover construction was implemented. A LFG extraction system was installed and has been in operation since December 2001 as a corrective action against VOC impacts to groundwater and off site LFG migration. VOC detections, since installation of LFG control and collection system, have been reduced. An increasing trend of nitrate as nitrogen and a decreasing trend in pH has noted in MW-4. In 2003, improvements were made to minimize infiltration and reduce ponding. This has had a positive impact on the groundwater quality in this area. Trends of chloride, total dissolved solids (TDS), and electrical conductivity (EC) are also increasing. Chloride, TDS and EC were reported at 90 mg/l, 690 mg/l, and 1218 umhos/cm in October 2005 for MW-5. The trend in pH is decreasing for MW-5 in July 2003 the pH was reported at 7.03 and in October 2005 the pH was reported at 6.94. 73467 2 August 9,2006 LA 21125 Corrective Action Financial Assurance Cost Estimate Corral Hollow Sanitary Landfill