Warunki hydrochemiczne utworów kredowych w rejonie Gdańska

Abstract

W rozpoznanej partii basenu górnokredowego w rejonie Gdańska stwierdzono wody wodorowęglanowo-wapniowe na obszarze Trójmiasta. Na Żuławach gdańskich w wodach kredowych notuje się podwyższone stężenie jonu sodowego i wody wodorowęglanowo-sodowe, a w centralnej partii Żuław lokalnie wody chlorkowo-wodorowęg1anowo·sodowe, a nawet chlorkowo-sodowe o podwyższonej mineralizacji (do 5 g/dm3). Dwa ostatnie typy wód znajdują się w strefach wolniejszej wymiany i są najstarsze. Zróżnicowanie jakości wód kredowych analizowanego rejonu uzależnione jest od ich wieku i zajmowanego miejsca w układzie hydrodynamicznym basenu gdańskiego.HYDROCHEMISTRY OF CRETACEOUS WATER-BEARING SYSTEM IN THE GDAŃSK REGION In the Gdansk region, groundwaters occur in sandy series of Coniacian, Santonian, and Maastrichtian (Cretaceous), forming an artesian basin 4,000 sq. km in area, and locally as fissure waters in an aquifer related to top part of the Mesozoic. Pre-Cretaceous rocks of the platform sedimentary cover yield mineralized waters of CI-Na type, separated from those 9f the Cretaceous aquifer by mudstone-clay series 100 to 300 m thick. Groundwaters of the Gdańsk Basin are of meteoric origin and contain substance dissolved during percolation through Cenozoic sediments. Their average age is estimated at about 2000 years on the basis of rates of exchange in the scale of the basin.In the studied aquifer there were found waters of different types: HCO3 - Ca, HCO3 - Na, and their mixtures. Moreover, waters of Cl-HCO3-Ca type were found in southern zone of the basin, along the line Starogard Gd. - Tczew - Nowy Dwór Gd. In northern part of the basin, there occur HCO3-Ca waters of low mineralization (0.23 - 0.4 g/dm3) and concentration of Na+ ions increases towards the center of the basin, presumably in result of ion exchange in the course of percolation of the waters in recharge zone situated in the Casubian Lake District. Water exchange appears impeded at southern margin of the basin which leads to increase in their mineralization (locally up to a few g/dm3) and concentration of ions CI-, Na+, and F-. Salt and brackish waters occur in the Cretaceous in the Vistula River delta, i.e. in zone of very slow flow or even stagnant waters, stretching along the contact of two reversely .oriented ground water streams flowing from higher situated lake regions. Local presence of salty water in the aquifer may be explained on the one hand in terms of the widely known hypothesis of brine ascension, and on the other -as due to enduration of relic waters of Quaternary marine transgression at the top of the Cretaceous in the Żuławy area. Moreover, there is also possible an influx of relic (synsedimentary?) waters, released in result of compaction of Mesozoic sediments, along southern margin of the basin.