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Ground water is involved in a slow global geological cycle of matter and it is a subject of fast hydrological cycle. It has been involved in formation of rocks from magmatic melts through crystallization, weathering, diagenesis, metamorphism and melting. During the rock cycle water changes its chemical and isotopic composition. Chemical and isotopic composition of natural waters is a result of physico-chemical processes between water, rock and atmosphere and consequent mixing of different ground water and surface water bodies. The interaction is influenced often by biota and today more increasingly by man. Water is a continuum and the separation of genetic types is sometimes artificial and often impossible. In spite of this, the genetic classification of water is useful because it relates water to differences in residence time of water bodies in various geological environments and to various geochemical processes. Tracing the secular evolution of ground water in the context of the geological and hydrological cycles is now possible due to new advances in isotopic geochemistry. Traditional radioactive and stable isotopes such as are supplemented today with data on 11B, 37Cl, 81Br, 44Ca, 3He/4He, 53Cr, 65Cu, 30Si, rare earth elements and noble gases.
Groundwater tracers with a wide dating range (T, 3H/3He, 85Kr, 39Ar, 14C and 4He) enable us to follow secular chemical changes in sedimentary basins. Changes in contents of Rn, F, CO2 and an increase in the ratio 4He/3He of dissolved helium in water during earthquakes indicate relationship between deep tectonic processes and composition of ground water. The common existence of fossil Na - Ca - Cl brines in oil fields, fluid inclusions in minerals and in fractures in old crystalline platforms suggests that the geologically old stagnant waters are result of secular development of the Earth crust. A case of a Na - SO4 brine (140 g/l) in crystalline basement of a Tertiary basin indicates that it is a result of an evolution of Tertiary volcanic volatiles dissolved in a playa lake, subsequent oxidation and evaporation, crystallization of fossil salts in fractures of rocks and final leaching of the salts by present-day meteoric water charged with magmatic CO2. Such a set of complex steps is needed to explain the composition of ground water in fractures of Variscian granite in the Bohemian Massif of central Europe. Prediction of future secular changes in chemical composition of ground water is an important criterion of safety assessment of potential repositories of spent nuclear fuel. A possible future glacial period will influence mixing of ice melt and sea water in the shield of Fennoscandia. A powerful tool to predict the future secular changes is a mathematical modelling of geological and geochemical processes that will influence chemical and isotopic composition of ground water within next one million years.
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