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landfill cover. The most recent surface emissions scan(April 2003) did not find any locations with emissions in excess of the regulatory limit. LFG Collection System Landfill Gas Migration tion An LFG monitoring plan was implemented in 1991. This plan included installing perimeter migration monitoring wells at the landfill property limit to various depths near or below the bottom of the refuse fill. Each well is composed of three nested probes, allowing the measurement of soil gas quality at three depths. Measurements taken during the first monitoring event at these wells indicated the presence of combustible gas at the property perimeter up to 60 percent,which exceeded the 5 percent regulatory limit. The County analyzed for VOCs in gas samples taken from the perimeter monitoring wells on February 20, 1992. Analytical results reported the presence of six (6)VOCs, including, Tetrachloroethene, indicating that the source of combustible gas was LFG. The presence of LFG in the vadose zone and the similarity between the VOCs in the vadose zone and those detected in groundwater indicated LFG as a possible source of the contamination at MW-2. Based on these findings, an active LFG Collection System was proposed as corrective action to mitigate both perimeter LFG migration and groundwater contamination. LFG Collectfon System The LFG Collection System was designed to prevent LFG emissions from the refuse into the vadose zone,thus preventing both perimeter migration and groundwater contamination. The system began operating in October 1996 (Figure 2). The LFG system consists of 82 vertical wells, connecting piping, and a flare station containing flare,blowers, and controls. The LFG wells are installed in the refuse, to a depth near(but not penetrating)the bottom of fill. Each 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 A vacuum is induced in the piping at the flare station, allowing LFG to be withdrawn from each well.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. 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, then LFG will not move from the refuse into the vadose zone. The LFG wells are adjusted according to measurements made at both the perimeter migration monitoring wells and the LFG collection wells. Vacuum at the LFG collection wells is increased until the concentration at the perimeter(soil gas)wells falls below the regulatory limit of 5 percent(to zero if possible). However, excess withdrawal, as indicated by the reduction of methane content in an individual LFG well to below 25 percent, indicates the potential for a subsurface landfill fire. Note that LFG is approximately 50 percent methane at the point of Progress of Corrective Action Measures 3 Harney Lane Sanitary Landfill Revised-August 22,2003 County of San Joaquin l