Abstract The Chalk is a strategically important aquifer in the UK, but it is at risk of contamination, with impacts on abstractions, because of extensive outcrop. Assessments of the risk of contamination impacting water supply are commonly based on bulk aquifer parameters, usually reported for abstraction wells, and do not consider the macro- and micro-scale variation at the site and within the Chalk succession. The presence of less permeable layers is significant in restricting contaminant entry into the aquifer, pushing the contaminants into fractures and voids, where their flow is limited by density differences that cause hydrocarbons to smear into micro-fractures. Dense solvents flowing to depth are further impacted by the presence of lower permeability layers within the Chalk, while horizontal flow is affected by fractures and faults. Porewater concentrations of contaminants are limited, by pore throat size and limited connectivity, to low concentrations of the most soluble compounds, but are characterized by electron acceptors that are observed as reduced and oxidized species. The combination of macro- and micro-scale physical structures, combined with natural attenuation, means that initial risk assessments for hydrocarbons and solvents in Chalk can be highly conservative and should be supported by further targeted investigation.

Abstract Six regional recharge and groundwater models have been recently developed of the Chalk and Upper Greensand from Dorset to Kent. Updated Chalk stratigraphy and mapping have improved understanding of geological structure and the development of preferential groundwater flow pathways along hardground horizons. Where shallow dipping folds bring these into the zone of active groundwater flow, extensive ‘underdrainage’ may result in marked differences between surface and groundwater catchments. Hardgrounds and marls are also associated with spring discharges, as are some faults and the clay formations that underlie or confine the aquifer system. Higher specific yield within the Upper Greensand helps support summer baseflow, as do local groundwater discharges from augmentation schemes, watercress and fish farm operations. The aquifer system has been successfully modelled using the ‘variable hydraulic conductivity with depth’ version of MODFLOW. Depths of secondary permeability development have been distributed according to ground and groundwater level data. Interfluve–valley contrasts overlie a base hydraulic conductivity set according to the formation saturated at the water table and enhanced by active hardgrounds. Local parameter overrides may also be needed. The Wessex Basin conceptual and numerical model is described before summarizing similarities and contrasts from the other five regional model areas.