Vízügyi Közlemények, 1999 (81. évfolyam)

4. füzet - Magyarics András-Pomogyi Piroska-Pék Tibor: A Kis-Balaton védőrendszer kialakítása, működésének eredményei

A Kis-Balaton védőrendszerének kialakítása, működésének eredményei 641 The primary objective of the Kis-Balaton system was to retain a fraction of the plant nutrient load of the River Zala

, which discharges into the Bay of Keszthely of Lake Balaton, and to halt the accelerating process of deterioration of this bay. This provides more time for making the water pollution control actions, which are specified in the relevant government decrees, before the process of the acceleration of water quality deterioration would become irreversible. The Kis-Balaton reservoir system is being implemented in two phases (Figure /.). Phase 1, construction of the so called Lake Hídvégi

, started in 1981 and the gradual 5-step inundation of the site was completed in 1985. Phase 2, the construction of the Lake Fenéki, has been started in 1984. It is to be completed, as provided by Government Decree 2371/1999, by the end of 2003. The construction plans were modified several times. An intermediate stage, the so called Grove Ingói of 16 knr area, was gradually inundated in 1992 on the NW-ern part. This partial construction was justified by the need for a water body where the outflow water of the Lake Hídvégi, which is rich in algae, can receive further clean up. On the other hand, the area in concern was a marshland with no through flow for centuries and the turning it to a throughflow system was to bring along (as forecasted by the respective research results) initial water quality deterioration, a process that needs time to achieve a stable state. Considering the objectives, the size and the historical background of this water protection system, one may state that it is unique in global scale. Consequently the investigations should be very comprehensive, dealing with all potential impacts and relationships. Several research teams of more than 10 institutions have been dealing with research into the chemical, biological and mass transport processes and with the nature conservation and hydroecological monitoring of the system. In the first period of the operation of the Lake Hidvég the flow conditions were changes and the rich nutrient supply of the initial period was taken up by the macrophytes that had been dominating the area. The consequence was the decay and the thinning of the macrophyte communities, in the sequence of their inundation, and algae (the microscopic primary producers) took over their place. Due to the relative lack of nitrogen those species started to spread (heterocystic cyanobacteria) which are able to take up nitrogen from the atmosphere. Among the most important results of the operation of KBWP system it can be mentioned that the inorganic nutrient load discharged by the River Zala to the Bay of Keszthely of Lake Balaton was substantially reduced right after the first period of the operation of the system (Table IV.). During the 12 years of the operation of Lake Hídvégi some 85,000 tonnes of suspended solids, 345 t total phosphorus and within this 284 t phosphate phosphorus, 10,000 t total nitrogen and 2,700 tonnes nitrate-nitrogen was retained in the lake. (Table V.). The quantity of materials retained is underestimated, since one cannot estimate the loads originating from tributaries of the lake (other than the Zala) and from the direct catchment of the Lake Hídvégi. The high nutrient loads and the efficient retention of nutrients are associated with an intensive primary production of organic matter in the lake. This means high planktonic eutrophication. The available nutrient resources can be several times recycled by the microscopic producers, while macrophyte succession would require longer time. A portion of the second stage of the system, the Grove Ingói, has been in operation for 5 years, retaining further 18,000 tonnes of sediments, about 44 t of phosphorus, 1,200 t total nitrogen and 560 t nitrate nitrogen. Comparing input and output data one can conclude that in this period the phosphate output was 36.5 t higher than the input. Nevertheless the phosphorus budget remains still positive, since phosphate phosphorus is only a fraction (although a significant fraction) of the total phosphorus. Thus, in the five years of operation of the Grove Ingói about 8 t phosphorus must have been retained there, even if the release of phosphate phosphorus was significant. It must be emphasised, however, that water discharge data of the outflow section are rather rough estimates and quality data are also subject to error due to sampling and methodological causes. This means that the mass balance of the Grove Ingói is subject to higher errors than that of the Lake Hídvégi. Objective oriented