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rate , and vacuum data collected from the extraction wdl1s ana inlet lines at the pump station . In addition , valve boxes, sa - pling cock valves, and flow control valves are checked for worK- ability. The well field is inspected/monitored on a quarterly basis , or more frequently if indicated by monitoring probe data. Data on gas composition, line pressure, and/or flow rates :: individual extraction wells or at the pump station can reveal tr.e existence of line breaks in the LFG collection system. When identified , the breaks are isolated , and the header lines must :)c excavated and repaired. Isolation of the break can be difficult , requiring assessment of well field performance data , visual ob- servation of the site surface, and knowledge of the site ' s sub- surface conditions . Decomposition of waste materials deposited within a landfill site results in differential settlement of the site surface. Cracks in parking and driveway surfaces are tested for the pre- sence of methane gas prior to being repaired. If methane is detected in excessive amounts , the nearby extraction wells are adjusted to prevent further venting of LFG through the site sur- face . Low areas in the site surface are inspected , recorded on a site plot plan, and periodically brought to grade with asphalt patch to promote proper site drainage . The most visible area of settlement (and the most difficult to repair) is at the building-landfill surface interface. Since both the concession/projection building at the drive-in and the sales/service center at the truck sales site are structurally supported, settlement occurring beneath and adjacent to these structures results in void spaces and cracks in the landfill site surface beneath and around the buildings. Repair of these cracks and voids is typically accomplished by regrading/tamping the area with dirt and/or gravel , followed by paving. T'he resurfaced area is tapered to provide a smooth transition between the asphalt patch and the existing site surface. Repair of cracks and voids and periodic adjustment of ex- traction well flow rates are important to the prevention of subsurface landfill fires in an extraction system. Should exces- sive air be drawn into the landfill through surface cracks , the elements necessary for ignition of subsurface waste materials (i .e. , oxygen, combustible material , and heat ) will be present. Control of subsurface fires is both difficult and costly. In mid-1981 , a subsurface fire was detected beneath a por- tion of the concession building at the drive-in theater site. Smoke and carbon monoxide (CO) were detected in surface cracks at the building-landfill surface interface. As an initial step in controlling/extinguishing the fire , cracks were filled and flow rates. from adjacent LFG extraction wells were adjusted ; all evi - dence of subsurface fire diminished. LFG-74