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.