Estimation of water movement in a closed landfill by tracer test in gas vents and the change in leachate quality

Leachate accumulation was found in Nakazono landfill, in Asahikawa, Japan because of an insufficient leachate collection and drainage system. In order to lower leachate level in the landfill and to promote stabilization of waste, many passive gas vents were installed as well as leachate collection vaults. This study aimed to evaluate the distribution and movement of leachate in the landfill by determining leachate level and tracer test in the gas vents. The water level varied widely among the gas vents and depended mainly on the original ground level and the depth of the vent. Leachate velocity also showed a wide variation. The leachate velocity was high in the upper layers of the saturated zone in a gas vent. However, this is only the superficial velocity caused by an inflow from unsaturated layers. A sharp decrease in TOC, which was observed in most of the gas vents by installation of vents, were likely due to the effect of aerobic biodegradation in the unsaturated layer of waste. This effect is limited to the small aerobic zone around the gas vent.