Determination of heavy metal fractions in the sediments of oxbow lakes to detect the human impact on the fluvial system (Tisza River, SE Hungary)

Abstract. Hungary is rich of natural values, but often the condition of them does not meet our expectations. Oxbow lakes and floodplain areas of the South-Plain represent very significant value, not only from landscape, ecological or national tourism point of view, but it is also important from conservation aspect. The scale and ecological risk of contaminations deducted on rivers that are trapped in floodplains, especially in beds of oxbow lakes, can be judged by examining the sediment of the oxbow lakes. Though data from sludge analysis can provide information about the condition of the oxbow lakes, from which long-term processes or potential ruination of oxbow lakes can be concluded.

In autumn 2011 we have collected samples of sediment from seven oxbow lakes of Lower-Tisza, out of these four were located in floodplain and three outside the dam. Following the suggested method of Commission of the European Communities Bureau of Reference we analyzed the sample's arsenic, zinc, cadmium, lead, nickel, cobalt, manganese, chromium and copper content with four step BCR sequential extraction, in this way we defined the acid extractable, reducible, oxidisable and residual phases of the heavy metals can be found in the sediment.

It can be stated in all fractions that the contamination matter content in floodplain oxbow lakes is higher than outside the dam. Arsenic, manganese and zinc are present in the sediment of the oxbow lakes in an easily mobilizable form. Regarding the third fraction of arsenic a high metal proportion (15 % < III. fraction < 55 %) can be observed, whilst examining cadmium and in some cases manganese in specific oxbow lakes a high proportion of fraction one and two can be identified. Lead and chromium are present in a strongly bounded form (IV. fraction > 90 %). During utilization and recultivation of oxbow lakes a close attention to be paid to elements in an easily mobilizable form like arsenic, cadmium, zinc and manganese, as these elements can damage the natural vegetation, respectively can get into the food chain through fishing, irrigation or by outplacing sludge onto plough-land.