Abstract

This study was initiated to test the extent to which the cation exchange capacity of marine clay-rich units, through which fresh water has been recharged, controls the composition of pore waters in an aquifer. The semi-confined Ledo-Paniselian (Eocene) aquifer in Flanders is recharged in elevated regions, where it is covered by Bartonian Clay. These sediments were initially laid down in marine conditions. The effects of recharging fresh CaHCO₃ water by downward flow through the overlying clay within the aquifer were modelled using PHREEQM. In the aquifer, progressively fresh waters are found in an upstream direction, which indicates a reduced influence of the initial marine conditions. Different stages of cation exchange produce a chromatographic sequence of groundwater types. Hydrogeological and hydrogeochemical evidence suggests the existence of preferential, faster flow pathways for recharge groundwater flow through the Bartonian Clay overlying the aquifer. These conclusions are indicated by differences between the observed and predicted features of the aquifer. It is necessary to invoke hydraulic conductivity values of 10⁻⁹ m s⁻¹ rather than 10⁻¹⁰ m s⁻¹ (measured values) for the Bartonian Clay so that the predicted head distributions in the aquifer match the observed heads. Furthermore, the measured exchangeable cations in clay collected from cores in the recharge area show less freshening than is predicted from the model to produce either the observed groundwater type within the aquifer in the recharge area or the overall distribution of groundwater types in the aquifer. Given the discrepancy between the observed and calculated state of freshening it is assumed that the analysed flow line in the core must be part of a slower flow pathway.