Reaction transport modelling of a freshening aquifer (Tertiary Ledo-Paniselian Aquifer, Flanders-Belgium)

The semi-confined Ledo-Paniselian (Eocene) Aquifer in Flanders, Belgium is recharged in the areas with higher topography, where it is covered by the Bartonian Clay. Initial conditions in these sediments were marine. Hydrogeochemical modelling of the groundwater type distribution in the aquifer with a reaction transport model considers recharge of fresh CaHCO3 water by downward flow through the overlying clay. In the aquifer, in an upstream direction, progressively more freshened water types are found. A typical NaHCO3 type occurs upstream of the brackish-fresh water interface. The different stages of cation exchange produce a chromatographic sequence of groundwater types, which agrees well with observations. Cation exchange processes occurring mainly in the percolated clay, result in a Na+ increase, and peaks of K+, NH4+ and Mg2+ in the aquifer, which are spatially separated as a result of groundwater flow. Calcite equilibrium, gypsum dissolution in the clay and sulphate reduction in the aquifer have also been included in the modelling.