Groundwater chemistry patterns in the phreatic aquifer of the central Belgian coastal plain

► Groundwater quality in the Belgian coastal plain is linked to Holocene geological evolution. ► Mixing, cation exchange, calcite dissolution and oxidation of organic matter are main reactions. ► Old samples (<10th century) show evidence of freshening as well of salinization. ► Old samples are testimonies of a dynamic geological environment. ► Younger samples are characterized by freshening of the aquifer due to impoldering.