Preferential flow through earthen landfill covers

In order to minimize infiltration of precipitation into the waste, final covers are constructed once a landfill reaches its capacity. In and to sub-humid climates, earthen final covers have been permitted. However, formation of macropores (or shrinkage cracks) and flow through relatively large pores can significantly increase percolation through earthen landfill covers during service. Most commonly used water balance models that are used for predicting percolation through earthen cover are based on Richards' equation that simulates only the micropore flow. Hence, a model that can simulate the micropore and macropore flow is required to simulate the long-term hydrology of earthen covers. In this study, validation of Root Zone Water Quality Model (RZWQM) using data collected form an instrumented field-scale test section and development of a model capable of simulating macropore flow using lattice Boltzmann method were carried out.;An instrumented field-scale test section of an earthen landfill cover, made up of 1.5 m thick compacted clay overlain by 0.3 m thick topsoil, was constructed at a landfill located in Detroit, MI and monitored for about four years. Measured annual percolation increased by an order of magnitude during the second and the third year of service. Controlled irrigation tests conducted on the test section confirmed macropore dominated flow through the test section. Estimated effective field hydraulic conductivities of the test section increased by an order of magnitude during the 4th year of service compare to the 1st year of service. Field methane tracer tests confirmed the presence and locations of macropores.;Water balance of the field test section was simulated using the model RZWQM and a commonly used numerical model UNSAT-H. For the first year data, both models simulated percolation relatively accurately. However, the numerical predictions of percolation were not accurate for the second and the third year when the effect of macropores was ignored. The macropore parameters required for RZWQM were calibrated using field irrigation test results. RZWQM predictions using calibrated macropore parameters yielded relatively accurate prediction of percolation for the second and the third years of the field data.;Measurement of macropore flow through clay in lab-scale samples is relatively challenging due to the effect of confining walls and the relatively small size of the sample. A new laboratory technique was developed to consistently fabricate clay samples containing macropores. High resolution X-ray CT images of compacted clay specimens were taken to visualize the 3D structure of macropores. A 3D lattice Boltzmann model (LBM) that can simulate saturated flow through micropore and macropore flow was developed. Verification of the LBM was carried out using analytical solutions. The LBM was validated using laboratory measurement of saturated hydraulic conductivities (ksat) of compacted clay specimens containing macropores. A prediction equation is formulated to predict the rate of flow of an arbitrary shape and tortuous macropore using the flow rate of straight vertical cylinder. The predicted ksat using the proposed formulation and calculated ksat using the LBM matched very well.