Using numerical modelling to test the geological and groundwater conceptual understanding of a complex, layered aquifer: a case study from the Fell Sandstone, Northumbria

Groundwater abstractions from the Carboniferous Fell Sandstone, Northumbria, north-east England, provide water supply to the Berwick-upon-Tweed area. Management of these abstractions, totalling 6.5 Ml/day, by the water company along with the regulator for sustainability issues is required. Groundwater abstraction takes place from different sandstone units, which are separated by mudstones, with monitored groundwater heads showing variable responses to system stresses. To improve understanding of this complex system, various activities have been undertaken. Geological mapping and interpretation have been conducted to characterise the nature, geometry, and interconnection of the sandstone units, along with the superficial deposits. Recharge modelling has used to quantify inputs to the system and to understand the long-term water balance. A time-variant model has been implemented to simulate groundwater flow in the sandstone units and to quantify the groundwater balance. The work confirms that the Fell can be split into seven discrete sandstone units, separated by low permeability mudstones, but they are not necessarily laterally connected. There is a range of timescales of groundwater response to recharge events from slow (six months) to very rapid (∼1 day). These findings confirm the complexity of this groundwater system and offer lessons for similar sandstone systems in the UK and worldwide.