Vízügyi Közlemények, 2004 (86. évfolyam)

3-4. füzet - Papp Ferenc: Vízminőség-védelmi kotrás a Balatonon

496 Papp Ferenc increased nutrient loads arriving via the River Zala, which drains the half of the total Balaton catchment into this small bay. Upon the measures carried out in the catchment of the River Zala (sewage treatment and the launching of the operation of the reconstructed wetland called Little Balaton (Kis-Balaton) the external load has been considerable decreased (Figure 2) although the hypertrophic conditions of the Bay of Keszthely remained so for a longer period of time (Figure 5). One of the possible explanations of this situation was that the internal load, the nutrient input from the bottom sediment into the overlying water body, has been increased, as it follows from the basic process kinetics of internal processes. In the Keszthely Basin of the Lake dredging of the loose and nutrient rich upper sediment layer was applied in order to control the internal load. Such operations called "environmental" or "ecological" or "water-quality protection" dredging were successfully applied in other cases as reported by the relevant international literature. In a first attempt silt-traps had been built at sites known as those of the accumulation of the sediment and then the 0.4-0.8 m thick upper layer of these traps were dredged (Figure 4). After 1992 a special Dutch dredging machine was applied, which removes only the nutrient rich upper 10-20 cm layer of the sediment (Figure 5). The dredging operations, carried out in the period between 1979 and 2003 for water quality protection purposes, have removed a total of 5.5. million m

3 bottom sediment from a lake-bottom area of 17.35 km

2 total size. The up to date "thin-layer-dredging" was applied to 75% of the total area but the dredged material amounted to 36% of the total (Table I). The state of the water quality of the Bay of Keszthely has been substantially improved by the mid 1990-ies. The annual mean chlorophyll-a concentration decreased to one-fourth of the highest value during 5 years, stabilising in the lower part of the eutrophic class and near to the mesotrophic one (Figure 3). The paper describes the results of a multi stage research, which was aimed at the identification of the role of dredging in the improvement of the trophic state of the lake. A mass budget model (Figure 6) developed in 1999 was applied to the data of 23 years. The results obtained indicate that among the measures aimed at the improvement of the water quality of the Bay of Keszthely the creation and operation of the wetland system Kis-Balaton was the most significant remedial measure and the sewage treatment programme of the Zala catchment was also an efficient strategy. The dredging of the bottom sediment also had an important, although secondary, role. The dynamic version of the model, developed in 2004, considers the variation of the water level of the lake and also that of the water temperature. This model was able to prove the relationship between the readily bio-available phosphorus content of the water body and the mean value of the chlorophyll-a concentration (Figure 7). The available measured data have proven that this dynamic model is suitable for the annual-scale description of the complicated processes of eutrophication thus enabling the numerical consideration of the efficiency of dredging for water quality protection purposes. The effect of the dredging, made for water quality protection purposes and carried out in the period 1975-2003, can be seen from Figure 8 as the difference of the results of model runs with and without the assumption of dredging. It can be seen from this figure that in the period 19891987 the effect of dredging was small, then in the next 7 years there were nearly no dredging operations, while the significant effect of the sheet-dredging made in the period 1995-2003 can also be well identified from this figure. The dredging had a deterministic role in the best achieving the 2005 target chlorophyll-a level (as defined by the Government Resolution 2035/2001) in the Bay of Keszthely among the other four basins of the Lake (Figure 10). In order to maintain the favourable conditions of the year 2004 the bottom sediment of the lake should be regularly measured in the sites assigned for such investigations (Figure 9) and if the evaluation indicates so the dredging of the areas of sediment accumulation must be restarted.