CORRESPONDENCE_2008 (7/08 TO 12/08)

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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. Groundwater flow is apparently bi-directional to the northwest and southeast. The deep aquifer is encountered at 320 feet below ground surface. The flow in this aquifer is to the northeast. Environmental Monitoring Seven monitoring wells have been installed to 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, volatile organic compounds (VOCs) indicating impact by LFG were detected at one well, (MW -5), and have been detected since that time. In 1995, a corrective action plan including closure and final cover construction was implemented. A landfill gas (LFG) collection system was installed in 2001 to mitigate VOCs and to control offsite migration of LFG. This system has been in operation since December 2001, and has controlled off-site migration; however, VOCs continue to be found in MW -5 at low concentrations (less than 10 ppb). An increasing trend of nitrate as nitrogen and a decreasing trend in pH has noted in MW -4. In 2003, improvements were made to improve drainage along the drainage ditch on the eastern side of the property. Since nitrogen has shown a gradual decrease that time. In further investigation of the causes of nitrogen impact, the County investigated the extent of the closure cap, and discovered refuse outside the closure cap within County property and also extending into the adjacent property. Data from groundwater monitoring wells indicate that the LFG impact at well MW -5 is most likely emanating from this additional refuse, which is probably outside the influence of the existing LFG system. Additional LFG collection wells are proposed to increase LFG collection effectiveness in the newly -discovered refuse. These wells are scheduled for installation in November 2008. Run-off/Run-on facilities The Corral Hollow Sanitary Landfill utilizes several methods to control run-off and run-on to the facility. A perimeter road/berm around the waste management unit prevents any run-on to the unit. Run-off is directed into two retention ponds via a mid -slop berm and perimeter ditches. Corrective Action Financial Assurance Cost Estimate 2 Public Works/Solid Waste Div. Corral Hollow Sanitary Landfill September 30, 2008