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INTRODUCTION

The distribution and movement of water within a waste landfill are important for two main reasons. First, water represents a major potential pathway for pollution of the surrounding environment. As liquid passes through the waste to emerge as leachate, it dissolves or carries in suspension substances that could cause contamination through interaction with the natural groundwater. Artificial and natural barriers help to prevent this, but it is usually also necessary to control leachate pressures (levels) - usually by pumping from wells or drainage blankets - so as to minimise head differences acting on a sealing layer or a natural barrier boundary. Leachate movement through the waste mass in response to pumping is controlled by the hydraulic conductivity or permeability of the waste. The distribution of permeability within the waste, which will vary as a result of anisotropy, heterogeneity, partial saturation and changes in waste density or effective stress, is also of vital importance. The total and drainable porosity of waste will control the rate of build up of leachate levels in response to infiltration or other water inputs, and the rate of decline when dewatering a site. It is also necessary to understand and control the impact of pore pressures on mechanical stability.

Secondly, water and water flow are essential to achieving landfill completion, i.e. bringing it to a stable, non-polluting state in a controlled way. Municipal solid wastes (MSW) have for the past 50 years contained a high proportion of biodegradable components: water is essential to the biochemical decomposition of organic substances (Pohland 1975; Leckie and Pacey 1979; Klink and Ham 1982). Water flow is also needed for the leaching out of soluble compounds, even in an inert or organically stabilised waste.