Thermal water resources in Hungary, their utilisation and protection

DISPOSAL OF USED THERMAL WATERS

Thermal waters cooled in the course of utilisation usually are released to public sewers, drainage canals, sometimes at lakes or storage reservoirs. In many cases the total dissolved salt content of thermal waters or equivalent % of Na is exceeding the limit value below which used waters may be disposed in public sewers without pollution fee.

According to the regulations in force the limit values of substances damaging the public sewers are varying between 1,5-3,0 mg/l depending on the regional categories as far as the total salt content of natural origin is concerned. This value is 1,0-2,0 mg/l in the case of watercourses. As far as watercourses are concerned, above the total salt content also the cases may be fined in which the equivalent % of Na is higher than 45 in thermal waters.

The ultimate recipient of the watercourses is usually one of the great rivers. Thermal waters are cooled in the course of use generally to the extent when they do not require any special treatment because of their temperature. Despite this in many cases the thermal water utilisation and drain systems on safety reasons are constructed with the insertion of a cooling pool for the case of any breakdown of the system, for the time of hydrodynamic measurements or well reparations to make possible the cooling of water below 40 ^o C in such situations as well.

Thermal waters released from the utilisation systems and discharged into surface waters have various impacts on the environment depending on the way of their release. The temperature higher than that of the environment promotes the development of organic materials, the intensive growth of plants and the silting of the channels. Because of the temperature higher than that of the surface waters the velocity of hydrobiological processes increases and dangerous changes in the biological equilibrium can be expected. The solubility of oxygen decreases. The quantity of oxygen stored in the warmer water can run out intensively and the living organisms can perish in large numbers. The extent of these impacts depends not only on the temperature but also on the quality of the thermal water and on the measure of dilution. The processes are aggravated by the occasionally higher ammonium ion content of thermal waters since the increase of free ammonium ion concentration can occur which can be the direct cause of fish death. Discharge into fish ponds is prohibited above the temperature of 25 ^o C and the salt content of 3,0 g/l. Phenol-derivatives in thermal waters give an unfavourable after-taste to the fish even when dilution is high. The high salt content of thermal waters itself has an adverse impact on the ecosystems in surface waters.

High salt content represents great danger also in the case when irrigation water contains thermal water. Water of high salt content and Na % induces a process of ion-exchange in the course of which the place of Ca-ions is occupied by Na-ions. Since the binding energy of Na-ions is the higher, the process can be considered irreversible and leads to the salinization of the soil. The unobjectionable irrigation water contains dissolved salt less than 500 mg/l and less than 35 % Na. In the case of some soils salt content may be as high as 1000 mg/l and the Na content 45 %. It is obvious from the above that usually the cooled thermal waters may not be used directly for irrigation. Their entry into irrigation waters should be obstacled, or it may be allowed only to the extent when the quality of irrigation water can be kept under the limit-value with sufficient dilution.

At the beginning the users chose the simplest way for the disposal of the used thermal waters i.e. they conducted it to the nearest watercourse, channel, lake and/or oxbow. Parallel to the intensive development of thermal water production, the elaboration of well controlled ways of disposal became necessary. Treatment of thermal water, mainly the reduction of their salt content can be realised yet only exceptionally because of the costly way of desalination (e.g. when extracting medicinal salt). Therefore the minimisation of adverse impacts can be implemented with the environment friendly disposal of thermal waters thus avoiding the pollution of soil and groundwaters. In the course of this the pollution limit values prescribed by the joint ordinance of the Ministry of Environment, Ministry of Public Health, Ministry of Agriculture and Regional Development and the Ministry for Transport, Communication and Water Management No. 10/2000 on the limit values necessary for the protection of ground waters and geological formations should be taken into account. (It should be noted that in connection with the adjoinment to the EU there is under elaboration the regulation of the protection of the quality of surface waters, in which a number of limit values will be elaborated relating to the ecology, the water uses, technologies, the peculiarities of the regions and the catchment areas covering a lot of components.)

The Lower Tisza Valley Water Authority as the most interested organ prepared a proposal in 1980 on the disposal of used thermal waters.

According to the regulations in force that time, one of the most efficient way of disposal was the temporary storage of thermal waters.

An other way of disposal mentioned in the proposal was the disposal of thermal waters in the geothermal reservoirs if the waters do not contain any polluting substances originating from human activities. This is more costly both from investment and operational point of view than the temporary storage, however it has advantages also from the side of thermal water production above the primary environmental aspects, as it promotes the maintenance of reservoir energy.