Freshening of salinized groundwater in Gdansk Quaternary aquifer

The main Quaternary aquifer in the Gdansk region connects directly with salt waters of the Martwa Wisla River and the Gulf of Gdansk. This aquifer has been used for many years by large municipal and industrial intakes located on the marine lowlands, with maximum output 6236 m(3)/h in 1985. Owing to an intensive exploitation, groundwater in the area of the Old Gdansk has been salinized by intrusion of salt water from the Martwa Wisla River. Concentration of chlorides in some wells reached 2000-3000 mgCl/dm(3), leading to degradation of groundwater resources in this area. As a consequence the "Grodza Kamienna" groundwater intake had to be put out of operation. Salt water intrusion from the Gulf of Gdansk was provoked in 1970s and 1980s by overexploitation of groundwater resources by three municipal intakes situated on the marine terrace. Concentration of chlorides in some piezometers of the "Czarny Dwor" groundwater intake amounted to 700-1000 mgCl/dm(3), however in the pumping wells of the intake it never exceeded the drinking water standards. In 1990s groundwater pumping rates in both areas were lowered. As a result, the chloride concentration decreased and the resources started to recover. Current observations prove that groundwater of the main Quaternary aquifer is freshening and salt water intrusion has stopped. However, elevated concentrations of chlorides are observed in the shallow aquifer, due to anthropopressure.