Among the important considerations for geoenvironmental systems is the ability to quantify percolation through a soil profile. Accurate measurement of percolation through the cap is essential for permitting alternative earthen final covers in landfills. Even though percolation is generally the smallest component among water balance parameters for landfill earthen covers, quantifying its magnitude is environmentally significant in evaluating the overall hydraulic performance of final covers. Direct estimation of percolation through a soil cover is typically attained using pan lysimeters. These lysimeters consist of a drainage layer underlain by an impermeable geomembrane liner, which is used to facilitate the collection and measurement of percolation. The presence of this hydraulic barrier in lysimeter, however, alters the hydraulics of the system. Thus, this study aimed to evaluate the hydraulic differences between an actual cap (underlain by waste) and corresponding lysimeter which was used to directly estimate percolation.

A field-scale test section consisting of compacted clay cover was built and instrumented to investigate the adequacy of the lysimeter to measure percolation. Lysimeter pan was installed within the middle of the test section and the instrumented area of the test section was expanded upslope and downslope of the lysimeter to monitor the soil water storages within and beyond the lysimeter footprint where the clay cover was hydraulically connected to the underlying landfilled waste. The numerical model UNSAT-H was used to simulate the water balance of the lysimeter. The lysimeter-validated model was then used to simulate the water balance of the actual cap. Both field data and numerical simulations indicated slightly higher soil water storage for the actual cap than the corresponding lysimeter. A series of numerical analyses 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.