Polluted discharges from perched groundwater systems in mine spoil heaps are a significant cause of water pollution in many parts of the world. Oxidative weathering and dissolution of sulphide minerals, particularly pyrite, which is widespread within Coal Measures strata, frequently results in surface water discharges rich in iron, sulphate, aluminium and acidity. Reducing the availability of oxygen to mineral surfaces should limit the amount of oxidation taking place and, therefore, reduce the concentrationsof contaminants in percolating waters. A pollutant-generating portion of a spoil heap in County Durham, UK, was covered with a clay cap in February 1998, and subsequent monitoring has shown a significant reduction in the concentrations of contaminants in the water discharging from the spoil heap to a nearby constructed wetland treatment system. This allows the wetland to receive better quality water, thereby extending its future life and making it even more sustainable.