The ability to quantify percolation through a soil profile is one of the important considerations for geoenvironmental systems. Reliable estimates of percolation through natural soil deposits help in determining local groundwater recharge rates. For landfills, accurate measurement of percolation through the cap is necessary for permitting earthen final covers. Even though percolation is generally the smallest component among water balance parameters, quantifying its magnitude is environmentally critical and key in evaluating the overall hydraulic performance of final covers. Direct estimation of percolation through a soil cover is typically achieved using pan lysimeters which consist of a drainage layer underlain by an impermeable geomembrane liner. The presence of this hydraulic barrier in lysimeter, which is used to facilitate the collection and measurement of percolation, alters the hydraulics of the system. This dissertation aimed to evaluate the difference in hydraulic performance of a lysimeter versus actual earthen cap with underlying landfilled waste. Two uncompacted and one compacted field-scale earthen cap test sections were built and instrumented at a landfill near Detroit, Michigan to investigate the hydraulic difference between an actual cap (underlain by waste) and corresponding lysimeter which was used to directly measure percolation. Lysimeter pans were installed in the middle of each test sections and the instrumented area was expanded upslope and downslope of the lysimeter to monitor the soil water storages within and beyond the lysimeter footprint. About 35 sensors were installed in each of the test sections to monitor water contents, water potentials, soil temperatures, water levels, and gas pressures. The field results show soil water storage values for the uncompacted test sections that were underlain by waste were typically greater than those for the corresponding lysimeters. For the compacted test section, there was no significant difference between the soil water storage for the actual cap and the lysimeter. Using the single porosity numerical models UNSAT-H and Vadose/W, the field measured percolation in the lysimeter as well as the variation in soil water storages were predicted with an acceptable accuracy for the compacted test section. The presence of macropore flow through large clods in uncompacted test sections is not accounted for in these single porosity models. A numerical analysis showed that when a lysimeter underestimates the soil water storage of an actual earthen cap, it corresponds to greater actual percolation across the interface between the soil cover and the underlying waste. A lysimeter overestimates percolation because the infiltrated water drained into the lysimeter is immediately removed and is therefore not available for removal by evapotranspiration. Field-scale simulations also showed that the magnitude of capillary barrier effect introduced by the drainage layer in the lysimeters is negligible when the saturated hydraulic conductivity of the soil cover is equal to or less than 10^-5 cm/s.
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