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subsurface air supply were combusting (most or all of these temperature probes are believed to have been destroyed during landfill closure). They recommended engineering considerations for landfill closure to control landfill fires, including a cover system with an impermeable geomembrane and a gas collection system. Parametrix (1992a) recognized that permeable lava tubes could supply air for subsurface combustion even after closure with the recommended engineering controls, and recommended that geophysical surveys be performed to identify lava tubes and such tubes be grout -sealed to prevent air flow. A Landfill Closure/Post Closure Plan was approved in 1993. ERM has not yet obtained a copy of this plan; however, we did review the response to comments on the plan by Parametrix (1993a). The Hawaii Department of Health (HDOH) expressed concerns on the heat resistance of the geomembrane liner system. ERM understands based on discussions with the County of Hawaii that this liner has in fact failed in certain locations due to excessive heat. Parametrix (1993b) stated that during closure, "sealing these [landfill] side slopes off, air will be eliminated from the refuse, thereby shutting off the oxygen supply, and 'smothering' the subsurface fires ... If temperature monitoring indicates that there are areas which are not cooling off as rapidly as others, a barrier trench, filled with low -permeability grout will be installed in native soil adjacent to the elevated temperatures, to effectively block lava tubes in the area." During closure of the landfill in 1992 and 1993, a gas -collection system consisting of perforated HDPE piping within 3 -foot -square, gravel -filled trenches at 150 -foot centers was installed and operated. Due to low methane concentrations in the collected gas stream that were not suitable for combustion treatment, the system ceased operations in February 1993 (Parametrix 1993b). A final cover system with 30 -mil PVC geomembrane liner and 2 feet of cover soil was installed in late 1993. Temperature and gas monitoring probes were installed during Phase II cover installation, and routinely monitored after closure (Parametrix, 1995a). Temperatures above 120°F were observed in several temperature probes; however, landfill gases were not identified at perimeter gas monitoring locations. By September 1995, during Phase 3 Closure Work, temperatures in vertical gas extraction wells reached 182 °F, exceeding material design capabilities (Parametrix, 1995a). Several depressions or sinkholes were identified, with several feet of subsidence and smoke odors. Parametrix (1997a) summarized six quarterly monitoring events from 1995 through February 1997. Although they concluded that the landfill was fairly stable, they indicated that "Although temperatures are fairly stable and not alarmingly high, they do not reflect what may be happening deep in the landfill. With the collection wells and trenches closed, there is no flow ERM 2 KAILUA-KONA LANDFILL/ 0061204 - 3/14/2008