Understanding the abundant data from site investigations at Sellafield is considerably aided by having a conceptual model of the groundwater flow system. It also forms the basis for numerical modelling. The idea of conceptual models is explained. In general, a conceptual model identifies the process or processes which cause groundwater movement together with the major limits and boundaries on those processes. The conceptual model for Sellafield is based on processes associated with the Irish Sea Basin competing with the present-day landbased freshwater system. The conceptual model is described in terms of three regimes: the Irish Sea Basin, the Coastal Plain, and the Hills and Basement. The Irish Sea Basin Regime is dominated by basinal processes and contains evaporite-derived brines. The Coastal Plain Regime is a topographically driven system containing freshwater within transmissive sandstones. The Hills and Basement Regime is a transitional regime containing mixed brines and freshwater, driven by gravity near surface and in dynamic equilibrium at depth.
Comparing the conceptual model against measured values of groundwater head and chemistry indicates broad agreement. Closer examination shows the value of understanding and measuring the interfaces between the regimes. Numerical modelling of the system is still at an early stage.