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The storage of thermal waters and their chemical composition

In the Carpathian Basin the places storing the subterranean (thermal) waters can be divided into two larger groups.

1. Carbonate, or, with another word, karstic reservoirs, which primarily can be found in limestone and dolomite rocks from the Trias, are typical in mountainous areas. The thermal springs of Hungary, which have been in use from the Roman times, then utilized in the Turkish baths of the age of the Ottoman conquest, too, broke to the surface at the edge of Hungary’s limestone mountains (e.g. Buda, Eger, Hévíz). Our first artificial wells of thermal water were also drilled in similar regions. (e.g. Harkány 1866, Budapest: Margitsziget 1867, Városliget 1868-78).

By the results of geological analyses the so-called thermal karstic waters get their supply from precipitation water in remote open areas of karstic mountains, where the water filters into the rocks and gets enriched with carbon dioxide. After several thousand years of underground storage these waters, enriched with minerals from the rocks, erupt to the surface. Their chemical composition is determined by those carbonate rocks, the crevices and fissures of which these waters filtered through. This is why these waters are of calcium- magnesium - hydrocarbonate character.

„The chemical composition of the water flowing through the crevices and fissures can approach alkaline hydrocarbonate characteristics, due to its contact with mud.  (e.g. Hévíz, Harkány). This water can have sulfate content of significance, too; sulfur might be present in the form of sulfids, which are important medical factors. In closed and deeply lying water reservoirs the concentration of sodium chloride may be high; in certain cases it can reach the salt content of sea water ( 10 g/) e.g.. Rábasömjén) (1)”.

2. Deep water reservoirs consisting of porous rocks (going back to the Pannon and Pleistocene geological epochs) can be found in the Great Plain area and in deeper-lying regions of Hungary. (Figure 1.) These areas came into being 10-12 million years ago from mud and marlite sediments of inner seas and rivers. In the Small Plain and in the southern areas of the Great Plain the the thickness of the sediments is  2-2,5 km. (1).

Figure 1. Thermal water reservoirs in Hungary. Source: kvvm.hu

The water in these thermal water reservoirs, which can be found in Hungary’s basins are of alkaline hydrocarbonate nature. Their deeper, closed areas produce waters with higher salt content (chloride) and very often they can be found in the vicinity of hydrocarbons. (kvvm.hu). From a hydaulic point of view the waters in both types of reservoirs are related to the waters in cold water storage systems. ( Figure 2.).

Figure 2. Types of thermal water reservoirs and the direction of the flow of underground waters. Source: kvvm.hu