Unsaturated Hydraulic Conductivity of Landfilled Waste

Unsaturated flow modelling of landfill hydraulics is a potentially valuable tool for the prediction of leachate discharge rates, design of leachate control systems and simulation of biodegradation in engineered landfills. Such an approach requires information on the moisture retention and hydraulic properties of waste, usually in the form of empirical functions such as those presented by van Genuchten (1980). However, there is a lack of information on moisture retention and conductivity properties. Recently, Kazimoglu et al. (2003, 2005) reported on moisture retention in MSW and presented a laboratory-scale method by which retention properties can be obtained. As with conventional inert soils, laboratory determination of unsaturated hydraulic conductivity is difficult and recourse is often made to analytical methods. The aim of this paper is to compare the unsaturated hydraulic conductivity of MSW obtained using Passioura's (1976) one-step outflow test method with predictions using van Genuchten's model. Good agreement is observed between the predictive model and experimental method for unsaturated hydraulic conductivities at low moisture contents but there is poor agreement at high degrees of saturation. This latter discrepancy is attributed to the difficulty of measuring retention properties of large pores at low suctions and the applicability of the van Genuchten model to such a material.