engleski

Water chemistry of the Plitvice Lakes from the springs to the Korana River – conditions for tufa precipitation

Tufa is formed by calcium carbonate precipitation from surface freshwater by a combination of physico-chemical and biological processes. In the Plitvice Lakes, situated in the Dinaric karst in central Croatia, the process of tufa formation is very intensive, forming a series of tufa waterfalls and barriers. This process is very sensitive to physico-chemical or biological changes in the water and it could be disturbed or even stopped in the case of water pollutions or change of conditions of tufa precipitation. In this study we have investigated the physico-chemical conditions of tufa precipitation and possible contamination of the water in the Plitvice Lakes. Surface water samples were collected in the period 2003-2007 at 18 sampling points in different seasons. The sampling points include 3 springs, 8 lakes, tributaries and Korana River which outflows from the Plitvice Lakes. Some of the sampling points are characterized by the process of eutrophication in the form of an intense plant growth in the lakes. Temperature, pH values, conductivity and concentration of dissolved oxygen were measured in situ and in the laboratory of the following species were measured: alkalinity, CO2, dissolved organic carbon (DOC), kations Ca2+, Mg2+, Na+, K+, Sr2+, anions Cl-, F- and SO42-, nutrients (ammonium, nitrates) and trace elements (Al, Cd, Cr, Mn, Ni, Pb, Zn, Ba and Cu). The measurements of the depth profile of the lake waters in two biggest lakes, Prošće and Kozjak, up to 25m and 45m water depth respectively, were performed during the period of stratification. The nutrients and the DOC concentration were also determined in the percolating waters in 3 different locations in the wooden area of the Plitvice Lakes. The results of physico-chemical parameters responsible for calcite precipitation in form of tufa and/or lake sediments have shown the following: 1) The spring waters are characterized by a constant temperature (mean 7.3 - 7.8 °C) and pH values (7.5 – 7.8) during the year, with slight differences in chemical composition between the three main springs depending on the recharge area. The Isat values are close to equilibrium of calcium carbonate solution (1.2 – 1.8) and no tufa precipitation was observed ; 2) In the lake waters, where the process of tufa precipitation is very intensive, steadily increase of temperature and pH in downstream direction was observed (mean 8.8°C and pH 8.1 in Matica to 13.0°C and pH 8.4 in Novakovića Brod Lake). The steadily decrease of alkalinity, CO2 and Ca concentrations in downstream direction, as well as some seasonal fluctuation, are the results of the calcium carbonate precipitation in this area. Isat in the lake waters, ranging between 5.7 and 7.3, show that the waters are supersaturated with calcium carbonate ; 3) Water of Sartuk tributary is supersaturated with calcium carbonate with Isat 11.4, whereas tufa does not precipitate, which can be attributed to the higher DOC concentration which inhibits calcite precipitation. The results of the measured parameters as an indicator of pollution in the surface waters of the Plitvice Lakes indicate that the waters are clean and oligotrophic and without significant anthropogenic pollution. In a few cases a slight increase of nutrients (ammonium, nitrates) was observed in the springs Crna Rijeka and Plitvica and in the streams of Matica and Rječica, which can be explained by the catchment area of groundwaters and the surface streams. It has also been proven by the composition of the percolating waters where concentration of nitrates and DOC are much higher than in surface water, that the input of nutrient salts to the lakes is mainly of natural origin. The measurements of water depth profile in both lakes, Prošće and Kozjak, showed the thermocline between 5 and 20 m with the ologotrophic characteristic in all profiles. A little lower concentration of dissolved oxygen in hypolimnion of Prošće Lake then in Kozjak Lake is due to a higher concentration of organic matter transported to the Prošće by Matica stream. Concentrations of trace elements (Al, Cd, Cr, Mn, Ni, Pb, Zn, Ba and Cu) in all waters are very low and do not indicate any anthropogenic pollution. The present status of the water quality was compared with some parameters measured approximately 20 years ago in the Plitvice Lakes water, in the period 1982-1985 (1). The results from the same locations (springs, lake waters and Korana River), show that most of the parameters including pH, alkalinity, concentrations of CO2, Ca, Mg, have very similar values in both periods and no change was perceived. The main change was observed in lake water temperature which increased in average for 1-2 °C for the last 20 years. The temperature of spring waters and Korana River has not changed. The increase of lake water temperature is in correlation with the steadily increase of air temperature observed in the Plitvice lakes area as well as in Zagreb-Grič meteorological station. Measurements of the air temperature for period 1984 - 2008 showed the average increase of ~0.05 °C/year in the Plitvice Lakes area and ~0.06 °C/year for Zagreb-Grič. The increase of air and water temperature could be the consequence of the global warming observed in the last decades in the World, although the observed period of the last 20 years is relatively short and further monitoring is required to support this statement. The observed increase of water temperature in the Plitvice Lakes in the last 20 years influenced to increasing the Isat values in the lakes. Isat values for period 1982 - 1985 in the lake waters ranged between 4.7 and 5.8, and for period 2003 - 2007 between 5.7 and 7.3. This increasing of saturation index of calcite in the Plitvice Lake waters showed that the process of tufa precipitation is not disturbed with temperature increasing, it is even enhanced. But in the same time the increase of lake water temperature could affect the increase of the primary productivity in the lakes and higher evaporation of the lake water. These changes could also influence to the increase of eutrophication processes in the lakes what is already in progress in some lakes. Regarding to all water analyses we can conclude that the Plitvice Lakes water are very clean and oligotrophic with low primary productivity. Local anthropogenic influence/pollution has not been observed but some indication of influence of global climate change was observed.

water chemistry; tufa precipitation; Plitvice Lakes

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