A physically based contaminant transport model, POTOMAC (Pyrite Oxidation products Transport: Object-oriented Model for Abandoned Colliery sites), has been developed to simulate the pyrite oxidation process in mine spoil heaps and the subsequent transport of the reaction products. This is believed to represent the first particle tracking model created using object-oriented technology and has proved capable of simulating the large time scales (on the order of centuries) required for this application. The model conceptualises a spoil heap as a series of ‘columns’, each representing a portion of the unsaturated zone, where active weathering and precipitation of secondary minerals takes place. The columns are then connected to a saturated zone, beneath the water table, where the contaminants are transported to the heap discharge. A form of particle tracking, the ‘random walk method’, is used to transport both the oxidant, oxygen, and the products, ferrous iron and sulfate. The subsequent oxidation of ferrous iron and precipitation of ferric oxyhydroxide is incorporated to provide an iron ‘sink’, where iron is effectively removed from the transport process. The application of POTOMAC to a case study, the Morrison Busty spoil heap in County Durham, UK, has produced encouraging results.