Following the cessation of dewatering in the southern part of the Durham Coalfield in 1974, groundwater levels rose in both the mine workings and the overlying Magnesian Limestone Aquifer (MLA). A pollution plume has since been migrating through the MLA, with sulphate concentrations greatly exceeding potable water standards. Prediction of future plume development requires specification of whether upflow of mine water to the MLA occurs from a small number of point sources or in a diffuse manner (i.e. distributed over most of the area in which the MLA is underlain by mineworkings). A programme of conceptual and numerical modelling has addressed this issue. Mine plans and geological information were used to characterize the flooded mine system and its interface with the MLA. Piezometric and meteorological data were used to constrain finite difference simulations of groundwater flow in the MLA. Flow velocity vectors derived from these flow simulations allowed modelling of plume migration by solution of the advection–dispersion equation. The results of these simulations illustrate that it is necessary to invoke both point and diffuse upflow to explain the observed patterns of pollutant migration to 2003. Predictive modelling of further plume migration indicates that sulphate concentrations are likely to rise to unacceptable levels in the most proximal public supply wells within the next two decades.