Hungary [antikvár]

A GENERAL REVIEW OF THE THERMAL WATER-BEARING FORMATIONS IN HUNGARY According to a conventional rule of the water management geologic formations producing sustained (safe) yield of thermal water with higher than 30 degrees centigrades are called thermal water reservoirs. The availability of thermal water is determined by the common occurrence of a properly good water-yielding capacity along with a proper formation (reservoir) temperature. The territory of Hungary is geothermally highly favorable. The average geothermal gradient is of 50 °C/km which is accompanied with a heat flow of 90 to 100 mW/m2. It is a greater value than the world average. It means that with the exceptions of mountainous regions the average formation (reservoir) temperature at 1000 m depth amounts 60 °C, whereas at 2,000 m depth reaches 110 °C while annual mean temperature at the ground surface is of 10 °C. The anomalies of the geothermal gradient are higher at the near-surface than deeper which means that a relatively homogeneous heat flow distribution is affected by the heat conduction anomalies of the different rocks and by heat convection caused by ground-water flows (Fig. 1). The thermal water-bearing and yielding geological formations can be classified into two great groups: - fractured—jointed—fissured basement and bedrocks (mainly carbonate rocks) - clastic porous basin sedimentary rocks. The first group involves thermal water reservoirs consisting of carbonate rocks (limestones, dolomites, marls) of Mesozoic age from where natural thermal springs are issuing onto the surface along the border of mountainous areas. These thermal springs are well-known from older historical periods (e.g. in Budapest, Hévíz, Harkány). These springs are recharged from karstic mountainous areas where precipitation infiltrates downward and after a long travel (after many thousands years of residence time) will be ascended and discharged along tectonic zones at the base level. The heat-collecting areas of springs are coinciding with the infiltrating zones and with the border areas of the nearby basins overlain by impervious formations. Along these zones karstic water is descending and being warmed up it draws simultaneously a certain amount of heat from the subsurface environment. The first thermal water wells were drilled around these natural thermal springs in the past century.