Karst water

The hydrogeological system of the Transdanubian Range zone

The hydrogeological information system of the Transdanubian Range

The hydrogeological system of the Transdanubian Range zone

The hydrogeological information system of the Transdanubian Range

Figure 1 Examination of the litho-stratigraphic and structural patterns of karst water bearing formations in Hungary is of crucial importance since one-third of our subsurface water resources are stored in them.
The Transdanubian Range main structural zone is constituted essentially by Mesozoic first of all Triassic carbonate rocks. Upper Triassic rocks the thickest among them make up the most extended continuous karst water bearing assemblage of the country. The karst water resource of the region is widely used as potable water but thermal water springs emerging in the ranges margin are also important like thermal karst springs in Budapest in the NE and the sprig-lake in Hévíz in the SW.
As a result of systematic geological mapping, other baseline research, monographic works and raw material exploration activities a large amount of geological and hydrogeological data has been collected in the area during the recent decades. Hydrogeological interpretation of the data was executed in the 1990s. It is well-known that due to intense mine-water drainage the region found itself in critical state by the end of the 1980s with regard to karst water setting. The latter dropped substantially, the majority of springs became dry (Figure 1) and the yield of the world-famous thermal springs also dropped at a disturbing rate. Owing to these facts and the political and economic change in the country decision was taken to stop mine-water drainage or at least to reduce it substantially. Water exploitation in the former mining area became thus essentially restricted to supplying the established potable water network. Thanks to the large volume of geological and hydrogeological data available the starting rehabilitation process provided an excellent opportunity for karst hydrogeological evaluations.

Examination of the western part of the Transdanubian Range zone

Figure 2 Figure 3 Figure 4 Figure 5 The study of the depression cone associated with the one-time bauxite mine in Nyirád threw light on the widely debated issue of recharge conditions of the spring-lake in Hévíz (Figure 2). The largest medicinal lake of Europe is the most important natural drainage feature of the karst water source in the SW part of the range.
The springs ensuring the recharge of the lake arise in the spring cave formed in Pannonian sandstone overlying the Triassic karst assemblage (Figure 3), where the divers discovering the cave in 1975 detected the inflow of 17.2°C cold and 39.6°C warm water in the eastern and western sides, respectively. The water temperature resulting of their mixture at the caves mouth was 38.8°C. According to the isotope studies focused on the age of the spring water the residence time of the warm and cold components were a couple of tens of thousand years and only some thousand years, respectively. This data show unambiguously that the two systems have different recharge paths.
Below Lake Hévíz a NW-SE structural line can be observed in the Triassic basement resulting in strike-slip displacement. The depression cone formed in the Nyirád mine was confined by similar structural lines (Figure 4). The analyses showed clearly that these lines exhibit excellent transmissibility along their strike, whereas it is quite poor in perpendicular direction.
The recharge site of the cold water is Keszthely Range. Water infiltrating through its karst surface flows to the west then ascends in the eastern side of the spring cave as soon as it reaches the structural line below Lake Hévíz (Figure 5). The recharge scene of the warm water is the Southern Bakony Range. Precipitation infiltrating in the Mesozoic carbonate rocks making up the main part of the Range flow toward SW before reaching the some km wide belt confined by strike-slip faults consisting of impermeable Triassic rocks in the region of the village Nagylengyel. This belt enforces water flow to change its direction to the SE and the water warmed up in its long pathway ascends in the western side of the structural line below Lake Hévíz in the spring cave. It is evident that precipitation falling on the Pannonian fine-grained clastic assemblage directly overlying Triassic karst rocks in Zala Hills is also involved in recharge.

Figure 6 Figure 7 The structural zone extending in the vicinity of Nagylengyel is of critical importance considering karst hydrogeology since it forms a barrier in front of the SW-oriented karst water flow deriving of Bakony. The composition of karst water to the SW of this zone its high solids content bears witness to unambiguously restricted flow volumes (Figure 6) proven by hydrocarbon occurrences in the region (Figure 7).
Instead of the Transdanubian Range itself infiltration zones cropping out in the SW part of the Transdanubian Range main structural zone make up the area of recharge of this region. This fact has crucial importance in terms of decision-making concerning the water management of the area adjacent to Lake Hévíz, as well as of delimiting water bodies in compliance with the regulation of the EU Water Framework Directive.

Figure 8 Figure 9 Figure 10

Examination of the eastern part of the Transdanubian Range zone

The depression cone created by mine-water drainage in the Kincsesbánya region is also controlled essentially by geological and tectonic settings. Its shape elongated in NE-SW corresponds to the general strike direction of the Range. To the south it is confined by impermeable rocks, whereas it is limited by structural lines elsewhere (Figure 8).
The determining role of geological features formations of the Upper Triassic main karst water storing assemblage of different transmissibility and structural patterns can also be observed in the Tatabánya-Mány and Dorog depression zones. As a result of the geological setting the decompression caused by both the Dorog and Tatabánya depressions exerted its influence on the thermal springs in Buda. The role of tectonic lines is also made evident by the fact that springs in the city Tata were dried out as a result of the one-time mine-water invasion in Tatabánya (Figure 9). The same structural line contributed to the swift revival of the Tata springs as soon as mine-water drainage had been stopped. Pressure equalisation between different flow units can be accelerated by the strike-slip fault featuring high transmissibility along its strike that traverses both areas. This happened between the water shafts in Tatabánya and the springs in Tata proved by the far earlier revival of the springs than anticipated.
Hydrogeological surveying is carried out simultaneously with geological mapping in the NE part of the Transdanubian Range zone. In the field each water course and spring is recorded (pH, water yield, temperature, transmissibility) twice a year. Samples were taken from some of the captured springs for chemical and isotope analyses (Figure 10).

Figure 11 Figure 12 A user-friendly database was established upon the collected data (Figures 11 and 12).

Figure 13 All information relevant in the area geological maps, water-level observation wells, data of water exploitation wells were systematised in GIS (Figure 13).

Figure 14 Figure 15 Geological 3D models prepared by means of different geological map series compiled in MÁFI provide a very reliable basis for hydrogeological modelling (Figure 14). Our aim is to introduce the modern method of digital 3D modelling in MÁFI indispensable for hydrogeological modelling which can also be used for other purposes. The Héreg-Tarján basin was selected first as a pilot area for testing this technique (Figure 15).

The summary was prepared by Jocháné Emőke Edelényi