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7 <br />i; <br />u <br />F-1 <br />J <br />Environmental Consultants 4707 Greenleaf Circle 209 545-8490 <br />and Contractors Suite F FAX 209 545-8391 <br />Modesto, CA 95356 www.sesfieldservices.com <br />Y <br />February 13, 2009 <br />File No. 07206051.00 <br />Mr. Dan Johnson <br />County of San Joaquin <br />Post Office Box 1810 <br />Stockton, California 95201 <br />Subject: Operation, Monitoring, and Maintenance of the Landfill Gas (LFG) Migration <br />Control Facilities at the Foothill Landfill, Linden, California <br />IDear Mr. Johnson: <br />This letter provides the status report on the operation, monitoring, and maintenance (O, M, & M) <br />work performed by SCS Field Services (SCS) on the subject system. All activities were <br />performed in accordance with our approved work scope dated October 24, 2006. Below is a <br />summary of O, M & M efforts performed during the month of January 2009. <br />CONCLUSIONS AND RECOMMENDATIONS <br />As of the dates of our testing, the LFG control facilities appeared to be operating satisfactorily <br />and generally meeting the operational criteria. All LFG extraction wells were observed to be <br />receiving adequate vacuum during this monitoring period. Recommendations regarding <br />repair or maintenance activities may be contained in subsequent sections of this report. <br />Please advise SCS as soon as possible regarding implementation of these recommendations. <br />BACKGROUND <br />As you know, the Foothill Landfill site is an active organic refuse disposal site. By way of <br />background, organic materials buried in a landfill decompose anaerobically (in the absence of <br />oxygen) producing a combustible gas, which contains approximately 50 to 60 percent methane, <br />40 to 50 percent carbon dioxide, and trace amounts of various other gases, some of which are <br />odorous. The Foothill property contains a system to control the combustible gases generated in <br />the landfill. <br />The gases produced in a landfill will either vent vertically to the atmosphere or migrate <br />horizontally through subsurface soil to locations on adjacent properties. If the soil surrounding a <br />landfill consists of permeable materials, there is a greater likelihood that the LFG will migrate to <br />off-site locations. <br />If the methane gas component of LFG is allowed to accumulate in a confined area (i.e., utility <br />lines, irrigation valve boxes, vaults, basements, wall spaces, etc.) and is exposed to an ignition <br />source, it can be explosive at concentrations between 5 and 15 percent by volume. At higher <br />u <br />