ABSTRACT:

Mathematical models are useful tools for assisting the design of engineered landfills. Existing landfill models usually assume the soil beneath a landfill is saturated, but with the arid conditions of the Australian and other world climates, this type of model is often inappropriate. The main problem associated with modeling contaminant transport through unsaturated fine-grained soils is that the transport parameters can be very difficult to estimate experimentally. A novel experimental apparatus based on desaturation by osmotic suction can be employed to estimate the parameters in the transport equation for a fine-grained soil. One of the important aspects of this new experimental apparatus is the behaviour of the membrane that surrounds the clay plug. Several sets of experiments were conducted to investigate the influence of the osmotic membrane acting alone. The diffusion of sodium fluoride (NaF) through the osmotic membrane was found to be very pH dependant. A reaction mechanism is proposed to explain the observed results.

INTRODUCTION

Environmental risk control has recently become a very significant and topical issue in our society. Mathematical modeling of landfills plays an important role in our understanding and design of landfills for waste disposal. Existing landfill modeling primarily focuses on transport models that assume the soil to be saturated. But with the arid conditions of the Australian and other world climates, the sub-surface soils are not necessarily saturated. The results of the saturated model transport equations are in doubt for these conditions, and therefore appropriate parameters for unsaturated soils need to be found. In fine- grained soils the practical problem associated with this is that the transport parameters are often difficult to estimate experimentally. In the unsaturated clay, both the diffusion and partition coefficient are a function of the volumetric moisture content. Previous methods have been employed to estimate these parameters.

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