Quantifying macroscopic flow and transport in the unsaturated zone to address the long-term contaminant burden of waste repositories.

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The usual way of managing solid waste in the UK has been landfill. We have more than 20,000 sites, containing 6 billion tonnes of waste, which are now full up. Landfills can cause major environmental problems, especially when water (from rain or streams) gets in and mixes with the waste to form a liquid called 'leachate'. If leachate escapes into the environment it can pollute ground and surface water, damage eco-systems and contaminate drinking water.Modern landfill sites are containment systems, sometimes called 'dry tombs'. Plastic membranes line the base and the sides to control how much leachate seeps out. A cap reduces the amount of rainfall entering to reduce how much leachate is formed. Leachate which does form is retained at the base, where it can be collected and treated.After landfilling at a site has stopped, it is covered and enters a management phase known as 'aftercare'. During aftercare, leachate needs to be collected and treated for as long as it presents a pollution hazard. Unfortunately, aftercare periods for modern landfills are measured in centuries. The engineered containment system has to keep working for all this time, along with active environmental control systems for gas and leachate extraction or treatment. Extended aftercare periods cause problems for operators, regulators and society, and are unacceptable in terms of sustainability.A unique project in the Netherlands aims to rapidly improve leachate quality at three demonstration landfills so that they can be brought out of aftercare within the next decade. The project also aims to ensure that future emissions of leachate will be acceptably low - for ever, without any human intervention. If the project succeeds, it will lead to much more sustainable and cost effective methods for landfill aftercare.Our research aims to provide some of the science required to underpin the project. It will be undertaken at the de Kragge landfill, where the operator will recirculate leachate and water through the waste. This will flush contaminants out into the leachate, which will then be treated outside the landfill. The success of this type of treatment depends on how the water or leachate flows through the landfill. If the flow is evenly distributed, the waste will be flushed more uniformly than if preferential flow paths allow the liquid to bypass some of the waste. (This is why it is sometimes possible still to read newspapers that have been buried in a landfill for 40 years). The spacing of preferential flow paths is critical. We calculate that if the flow paths are less than 0.5 m apart, contaminants will diffuse out of the waste fast enough to allow clean-up within about a decade. Flow paths that are more than 1 m apart are likely to limit the release of contaminant from the waste to the extent that a landfill might safely be brought out of aftercare before all the contaminant has been removed.Our research will focus on understanding the nature of liquid flow and flow paths within the landfill, and their influence on landfill clean-up. We will install monitoring systems that can differentiate, at a scale of about 0.5 m, between flow occurring in preferential flow paths and flow occurring more evenly within the unsaturated zone of the landfill. Chemical tracers will be injected into the operator's leachate recirculation system, and we will monitor their flow through the waste. After interpreting the tracer data, we will develop and verify a suite of different models that track flow of contaminants and describe landfill clean-up. We will test a range of model concepts against our new data, to identify those that work best. These will then provide a framework for understanding the performance of the Dutch landfill flushing project and for evaluating any residual risks. The models will also provide a scientific basis for optimising the engineering of flushing, and the management of waste repositories worldwide.