Palaeohydrogeological reconstructions of the North Lincolnshire Chalk, UK, for the last 140 000 years

The influence of climatic change on hydrogeology is investigated by studying the evolution of the North Lincolnshire Chalk aquifer system during the last 140 000 years. A numerical modelling approach is adopted using a density coupled, finite difference model to reconstruct past groundwater environments. Two major controls on groundwater circulation are considered: the position of sea-level, which determines the base level condition, and the amount of recharge associated with different climates. The Quaternary history of the region is used to define aquifer boundary conditions, and climate analogue data are used to determine likely recharge amounts. Model results cannot be compared against groundwater flow volumes or head, other than for the present-day situation, and so hydrochemical evidence is used to constrain the model simulations. The main finding is that recharge to the Chalk aquifer during the last 140 000 years has been restricted to periods when the climate and sea-level position have been similar to those of the present day. The Devensian cold period experienced very minor recharge, with no Chalk permeability development. Forest clearance since about 5000 years ago is predicted to have permitted an increase in the volume of freshwater recharge, which increased the rate of Chalk permeability development. The presence of saline water at depth in the aquifer is argued, on the basis of the model behaviour, to be contributed by a component of upward cross-formational flow.