The salinization of Cenozoic aquifers in NW Poland by ascending Mesozoic brines

The salinization of Cenozoic aquifers caused by brines ascending from the Mesozoic was described on the basis of results of 7747 archival chemical analyses. The zones of confirmed aquifer salinization, defined by chloride concentration exceeding 60 mg/dm^3 and 70 mg/dm^3 (upper limits of hydrochemic background values), within the Pleistocene and Paleogene, Miocene and Pliocene usable groundwater aquifers respectively, occupy an area of 8600 km^2 (33 % of the study area), whilst the zones of potential salinization hazard extend on further 4900 km^2 (19%). The groundwater salinization zones are mostly connected with the uplifted tectonic blocks, salt anticlines and fault zones, the structures allowing upwards directed migration of the Mesozoic brines into the Cenozoic useful aquifers. The salinization development is generally controlled by the flow directions within the active circulation zone and causes migration of brines under pressure along the tectonically produced pathways. The salinity increase affecting groundwater in the Cenozoic aquifers does not result from the recent leaching of the Zechstein salt bod-ies, as they are mostly isolated from the groundwater active circulation system. The ascending diluted brines constitute a potential threat to 4 (Uznam–101 and Wolin–102, Rooecino–103, Dębno–134) from among 20 Major Groundwater Reservoirs and to 17 from among 31 main municipal groundwater intakes (total well discharge >100 m^3/h)