REDISTRIBUTION OF INFILTRATION BY A LARGE LANDFILL IN A SEMIARID ENVIRONMENT AND EFFECT ON WATER AND CONTAMINANT MIGRATION IN A THICK VADOSE ZONE

A double-lined hazardous waste landfill in the western United States is planned to have final dimensions of about 400 m wide by 500 m long by 10 m below grade. Multiple computer models were developed to support a forthcoming assessment to determine if there is reasonable assurance that the landfill can contain the disposed waste and limit the long-term environmental and human health impacts for at least the next 1,000 years. Among them were two variably-saturated flow models developed using the finite-difference simulator, STOMP. Simulations predict redistribution of infiltration by the landfill potentially causing a complex flow field in which water and contaminants migrating downward through the 85-m thick vadose zone may spread horizontally up to tens of meters. A two-dimensional cross-sectional model of the landfill simulated that, across a range of failure scenarios, infiltration is focused toward the drainage zone of the surficial cap and low points in the base of the lined excavation, whereas infiltration is diminished and soil moisture decreases beneath most of the lined area. These results were applied in a three-dimensional vadose zone fate and transport model by varying infiltration boundary conditions spatially and temporally and by releasing contaminants at the excavation low points. The concentration and more rapid downward migration of contaminants within the release area is partially mitigated by horizontal spreading in response to the resulting matric potential gradient. The flow field differs markedly from results of an alternative conceptual model in which water infiltrates the liner in a more uniform pattern.