Long-Term Hydraulic Performance Evaluation for a Multilayer Closure Cap

Abstract

As part of an evaluation of closure alternatives for a radioactively contaminated facility, the Hydrologic Evaluation of Landfill Performance (HELP) Model was used iteratively with published leakage equations to predict the long-term hydraulic performance of a (degraded) multilayer soil/geosynthetic closure cap. The evaluation included an analysis of the possible effects of various degradation processes on the various components of the cap. Because the HELP Model does not account for the long-term effects of aging on the integrity of a geomembrane (a component of the proposed closure cap), a review of environmental and geomechanical degradation processes that could detrimentally affect hydraulic performance of the geomembrane was conducted. Possible effects of these processes on geomembrane hydraulic properties were estimated, and calculations performed to estimate long-term leakage rates through defects predicted to (eventually) occur in the geomembrane as a consequence of these degradation processes. Additional HELP Model simulations were performed using a range of geomembrane quality parameters until the model-predicted flux through the composite barrier layer of the cap duplicated the leakage rates computed from the leakage calculations. Results indicate that, for the cap investigated in this study, the HELP Model underpredicted long-term percolation rates through the cover barrier components, compared to rates projected using the leakage equations.