MONITORING POINTS: EVALUATING THE EFFECTIVENESS OF A LANDFILL GAS MIGRATION CONTROL SYSTEM WITHIN A CONFINED SAND UNIT

During the night of April 21, 2000, a home in Rochester Hills, Michigan, exploded without warning. Fortunately, no one was killed, but the loss was substantial and other residents had to be evacuated from their homes. Subsequent investigation indicated that the explosion was triggered when a pilot light ignited methane that had seeped into the basement from an abandoned landfill nearby. At the same time, at a site north of Kalamazoo, Michigan, the Michigan Department of Environmental Quality (MDEQ) and Malcolm Pirnie Inc., had detected up to 40% methane in soils beneath the homes of a rural, residential neighborhood. The source: another abandoned landfill site, which the State of Michigan had inherited after the owner filed for bankruptcy. The MDEQ held a public meeting and distributed in-home methane detectors to the affected residents while Malcolm Pirnie designed and supervised the installation of an emergency perimeter landfill gas extraction system to intercept and withdraw landfill gas at the property boundary. Seven extraction wells, at a spacing of approximately 150 feet, were connected to a pair of 5 hp centrifugal blowers to establish a continuous vacuum trough in the subsurface to prevent further gas migration from the landfill.A network comprising 27 multi-level monitoring points was installed to monitor and evaluate the effectiveness of the system. During the well installation effort, geologists carefully logged each borehole and selected well screen intervals based on the geology and observed methane concentrations. Malcolm Pirnie established a regular monitoring program, collecting subsurface gas and gauge pressure data using CES-Landtec GEM(tm)-500 and GEM(tm)-2000 instruments. Because of the expanse of the monitoring network and the influence of the site's glacial geology, the evaluation of the pressure data had to consider the effects of barometric changes throughout the monitoring period as well as a lag effect between the changing atmospheric and subsurface conditions. The gauge pressure data were converted to absolute pressure and used to develop isobaric maps and distance-drawdown curves to verify that the system achieved the minimum required effective radius of influence and to assess the effect of barometric changes on the system. Malcolm Pirnie also developed methane concentration isopleths to track the dissipation of the landfill gas plume. After system startup, there was a steady decrease in subsurface methane concentrations, and by the end of the first year of operation, the methane concentrations in soils beneath the homes were non-detect.This case study examines the successful extraction and control of potentially explosive concentrations of landfill gas migrating through the soils beneath a rural, residential neighborhood. Detailed cross sections and isopleth maps illustrate the influence that geology exercises on both the migration and extraction of landfill gas. Clayey surficial deposits created a preferred migration pathway through deeper sand units and created a condition analogous to a confined aquifer in groundwater flow. This geologic condition exacerbated the lateral migration of the landfill gas, but also enhanced the extraction efforts. Interpretation of methane concentration data was straightforward, but particularly in the first few weeks of operation, when it was still indeterminate whether methane concentrations were decreasing because of the extraction system or simply fluctuating over time, careful evaluation of the subsurface pressure distribution was necessary to verify the effectiveness of the system. Because of their timely response to the discovery of the landfill gas, the MDEQ, with the assistance of Malcolm Pirnie, may have averted an unthinkable disaster.