Abstract At present there is no suitable method to predict either the longevity of contaminant sources within spoil heaps, or the evolution of their strength over the contaminating life time of the sites. Existing acid-base accounting techniques provide little information relevant to the prediction of field contaminant concentrations and time scales. For robust prediction, the relative rates of contaminant generation and attenuation must be evaluated then extrapolated to the physical scale and environmental conditions of real field sites. Laboratory unsaturated column experiments on colliery spoil from a well-documented site in County Durham have been set up to assess its contamination potential, and the results compared with results from a mathematical model for contaminant release and transport. The random walk method, a form of particle tracking, is used to transport iron and sulphate ‘particles’, released by oxidative weathering of pyrite minerals. The model also includes the oxidation of ferrous iron to ferric iron in an attempt to account for contaminant ‘sinks’, for example where ferric iron spontaneously precipitates as ferric oxyhydroxide and is effectively removed from the transport process. In general, the modelled results compare favourably with the laboratory results and any discrepancies can be accounted for.