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PURPOSE The purpose of this report is to document the progress of the corrective action measures at Corral Hollow Landfill. BACKGROUND Landfill Closure Groundwater contamination was indicated in well MW-5 in the fourth quarter of 1991. Closure of the landfill was implemented in 1995 as corrective action. This included placing a clay layer(cap)as a component of the landfill closure surface. LFG Migration Monitoring Closure construction included installing perimeter LFG migration monitoring wells at the landfill property boundary per Title 27. Each well is composed of three nested probes,allowing the measurement of soil gas quality at three depths. Measurements at these monitoring wells in 1995 indicated the presence of methane at the property perimeter in excess of the 5 percent regulatory limit. Analyses of the volatile organic compounds (VOCs) in gas samples taken at the perimeter monitoring wells were very similar to those found in the groundwater at monitoring well MW-5 (Letter report-Evaluation of Volatile Organic Compound Impacts to Shallow Groundwater at the Corral Hollow Sanitary Landfill,Emcon,July 26,2000). The similarity between the VOCs in the gas monitoring wells and those detected in groundwater supported the conclusion that the source of the VOCs at MW-5 was LFG,not leachate. Based on this conclusion, an active LFG collection system was proposed and installed as a corrective action against both groundwater contamination and perimeter LFG migration. LFG Collection System The LFG Collection System was designed to prevent LFG movement into the vadose zone to prevent groundwater contamination and migration beyond the perimeter of the landfill. The system became operational in May of 2001. The LFG system was originally installed with 39 vertical wells, connecting piping,and a flare station containing flare,blowers, and controls. Five additional wells were installed in December 2008 in the northern and northeastern edge of the landfill. Each LFG well is provided with a control valve to adjust the rate of collection from the well and sampling ports to measure the quality of LFG collected. A vacuum is induced in the piping at the flare station, allowing LFG to be withdrawn from each well. Collected LFG is burned in a flare to remove VOCs in accordance with the Air Quality Management District Permit to Operate. The system is operated to maximize collection of LFG from the landfill by inducing a negative pressure in the refuse. If the refuse is under negative pressure, LFG will not move from the refuse into the surrounding soil. The LFG wells are adjusted according to measurements made at the perimeter migration monitoring Corral Hollow Sanitary Landfill (-1 Department of Public Works/Solid Waste 1 s'Quarter 2014 Groundwater Monitoring County of San Joaquin—March 15,2014