Systematic Design of Methane Migration Control Systems

This paper presents a systematic methodology for design of landfill methane migration control systems. The key to the methodology is that it tests for deficiencies in the conceptual model (working hypothesis) of methane migration, correcting potential design flaws in the landfill methane control system before commencing design and construction. The testing is accomplished by (1) a field investigation of gas well extraction tests and monitoring of transient and steady-state pressures and methane concentrations in the landfill and surrounding soils; (2) constructing a site-specific numerical model of the landfill gas flow and transport consistent with the conceptual model’s structure and pneumatic properties, and (3) calibrating the numerical model by modifying landfill gas (LFG) generation rates and model properties within reasonable limits to provide a best fit to the field data. Insights developed during both the field investigation and the calibration process allow identification and correction of deficiencies in the conceptual model, and, ultimately, in the conceptual engineering design. The gas flow numerical model resulting from the calibration process is used to simulate the performance of a methane migration control system and optimize the control system’s performance and costs. This improved approach is illustrated by methane control system projects recently conducted at three unlined landfills. These landfills include the City of Cairo 6 Avenue Municipal Solid Waste Landfill, located in Grady County, Georgia, the Buckhead Mesa Municipal Solid Waste Landfill, located near Payson in Gila County, Arizona, and the Decatur County SR309 Municipal Solid Waste Landfill, located in Bainbridge, Georgia. Each site investigation included a review of landfill construction and waste disposal history, installing and/or utilizing selected boreholes as gas monitoring wells or gas extraction wells, monitoring the wells’ and probes’ gas pressures and methane concentrations, and conducting tests to determine pneumatic parameters. Vertical and horizontal gas permeabilities were obtained by gas well extraction tests or, where estimates of LFG generation were needed, by a combination of gas pressure monitoring and gas well extraction tests. These pneumatic data were interpreted using analytical and numerical models. The resulting calibrated three-dimensional numerical gas flow and transport models were then used to develop and optimize conceptual engineering designs of methane control systems and assess their performance in meeting methane control objectives. INTRODUCTION The design of a landfill-methane migration control system is done by a sequential trial-and-error approach that includes developing a conceptual model of the affected site’s methane (LFG) flow system; installing monitoring wells to delineate the methane problem; designing, constructing and operating a methane control system composed of LFG control wells, trenches, or subsurface barriers; monitoring methane to determine effectiveness of these remedial control measures; modifying the control system to correct for insufficiencies in methane control; repeating methane monitoring to determine effectiveness of modified control methods; and so on. The problems with this approach are its reliance on a conceptual model that is untested and, therefore, nearly always deficient in some significant way; the rounds of monitoring and system modification required to correct the design of the methane control system; and the consequent delays and extra costs involved in accomplishing the desired level of methane control. Identifying and correcting flaws in the conceptual model and conceptual design before commencing full engineering design and construction will eliminate costly rounds of monitoring and redesign, as will be described in this paper. Testing of the conceptual model of methane migration is accomplished by (1) A field investigation comprised of installing monitoring probes and soil vapor extraction (SVE) wells in the landfill and surrounding soils, monitoring transient and steady-state pressures, and performing soil vapor extraction (SVE) well tests. (2) Constructing a site-specific gas flow-andtransport, numerical mo-del of the landfill that represents its structure and pneumatic proper-