Instrumented permeable blankets for estimating subsurface hydraulic conductivity and confirming numerical models used for subsurface liquid injection

Recirculation of leachate into municipal solid waste landfills is routinely practiced to: (1) enhance waste settlement for gain in landfill volume; (2) increase landfill gas generation rate for cost-effective energy recovery; and (3) potentially reduced post closure maintenance and environmental liability period. In order to better design the leachate recirculation system and model hydraulics of landfills, numerical models are used. The key objectives of this dissertation included: (1) confirmation of numerical model predictions; and (2) develop a new method to estimate vertical hydraulic conductivity of the porous medium underlying an instrumented permeable blanket in real-time. A lab-scale physical model of a landfill (85 cm long x 30 cm wide x 55 cm high) was constructed and instrumented to simulate liquid recirculation system consisting of a permeable blanket. A 2-cm thick permeable blanket made up of pea gravel was built in the landfill model. Uniform fine sand, coarse sand and structured heterogeneous sand comprised of homogeneous blocks of fine, coarse and coarser sands were tested in separate experiments as porous media underlying the blanket. The blanket and the porous media below the blanket were instrumented with pressure transducers and water content sensors to monitor the migration of injected water in the blanket and in the underlying sand. Liquid injections in the blanket were carried out at flow rates ranging from 20 to 150 cm3/s in continuous or on/off modes. For continuous injection, it was observed that the initial wetted width of the blanket gradually decreased as the degree of saturation of the underlying soil increased. The entrapped air bubbles influenced the pressure heads in the underlying sand(s) and the blanket. The air pressure exceeded the hydrostatic pressure at any point in the system during the liquid injection experiments. During continuous injection, the air pressures gradually decreased as injected water gradually removed or dissolved the air. The pressure heads in the blanket and the soil water content distributions predicted by HYDRUS-2D and Vadose/W, agreed relatively accurately with the data at steady-state when the entrapped air within the underlying sand was minimal. The single phase unsaturated flow models do not consider positive air pressures in the pores. The vertical hydraulic conductivity of the soil located below the permeable blanket was accurately estimated from an analytical approach termed ACRES (Analysis of Conductivity in Real-time using Embedded Sensors) using pressure heads measured in the blanket at steady-state during continuous injection. The horizontal hydraulic conductivity and the unsaturated hydraulic properties of the soil below the blanket have no effect on the vertical hydraulic conductivity estimated using ACRES. The vertical hydraulic conductivity of waste estimated through ACRES was found to be lower than the waste hydraulic conductivity in the presence of low conductivity cover soil layers in a simulated landfill.;Keywords: Bioreactor landfills, leachate recirculation system, permeable blanket, hydraulic conductivity, landfill model, instrumentation, HYDRUS-2D, Vadose/W.