THE USE OF A VARIABLY SATURATED DUAL-DOMAIN MODEL TO SIMULATE THE TEMPORAL CHLORIDE DATA FROM A MSW LANDFILL IN FLORIDA

A recent study of the temporal changes in leachate chemistry from a municipal solid waste (MSW) landfill in Florida has shown that the concentrations of many conservative parameters, such as chloride and sodium, exhibit decreasing trends as the landfill ages. The release of such contaminants has been suggested by previous researchers to be solely the result of the flushing of the waste by infiltrating water, and that the concentration is linked to the volume of the flushing, with large volumes producing lower concentrations and smaller volumes produce higher concentrations.

The data from the Florida study, however, showed that these decreasing trends were not affected by closure of the landfill, which consisted of the installation of a poly vinyl chloride (PVC) cap that reduced the infiltration of rainwater by 90%. This reduction in water volumes, based on the previous conceptual model, should have produced leachate with higher concentrations of conservative parameters, but the continued decreasing trend suggests that another mechanism may be important.

The Florida landfill was constructed using local sandy soil as daily cover for the waste, which resulted in the encapsulation of the relatively low permeability waste in a material with much higher hydraulic conductivity. This provides preferential flow pathways for liquids to be conducted through the landfill, and suggests that there may be a rapid zone of leachate release (the waste near the cover material) and a slow zone (the interior of the waste cells). The release of chemicals from the slow zone occurs by slow advection and diffusion while transport in the rapid zone is primarily advection. This two-domain conceptual model could explain the temporally decreasing concentrations.

In order to test this hypothesis, the computer model HYDRUS-2D was used to simulate the rapid and slow zones of leachate release. HYDRUS-2D simulates the flow of water in unsaturated soils and incorporates a dual-domain transport formulation. This dual-domain module was used to simulate the release of chloride from a waste cell into the leachate collection system.

The results of the simulations show that the dual-domain module can simulate the chloride data from the Florida landfill, suggesting that conceptual model of rapid and slow release zones warrants further investigation.