Influence of the former marine conditions on groundwater quality in the neogene phreatic aquifer, Flanders

The Neogene Aquifer is situated in the Campine basin in the northeast of Belgium. The groundwater body consists mainly of sands with some clayey intercalations. The Boom Clay forms the lower impermeable boundary of the aquifer. Groundwater flow has been modelled by means of the MODFLOW code. Calculated piezometric levels show good agreement with the observed piezometric heads. The main processes that influence groundwater quality on his way through the aquifer have been studied. The aquifer is well flushed resulting in low amounts of total dissolved solids. In the recharge areas, infiltrating meteoric water mainly influences the chemical composition of groundwater. Pyrite oxidation raises sulphate and iron contents in the top layers. Groundwater mineralisation increases along a flow line, primarily as a result of silicate and calcite dissolution and cation exchange. Large parts of the aquifer are decalcified. Calcareous deposits are present in the deep parts and to the north. The former marine depositional environment has still influence on the present-day groundwater quality. Cation exchange is the most important witness for this marine depositional environment.