Purpose Oxbow lakes, reaches that were cut off from a river during river straightening works, can accumulate during flooding metal-rich suspended sediments transported by a river from mining-impacted source areas and other anthropogenic sources. Additionally, the anthropogenic use of oxbow lakes may significantly impact the sediment-bound metal concentrations. The aim was to determine the dominating effect in the sediments' enrichment in heavy metals. Materials and methods We collected surface sediments (< 10-cm depth) from seven oxbow lakes once connected to the Tisza River (a transboundary river in Central and Eastern Europe). Four of the oxbows were located on the active floodplain, while three oxbows were at the reclaimed side (i.e., outside the flood defense levee). The sediment samples were subjected to total metal analyses (Cd, Co, Cr, Cu, Ni, Pb, and Zn) and sequential chemical extractions. Results and discussion Contamination indices (geoaccumulation index (Igeo) and pollution load index (PLI)) show a moderate but significant metal pollution of oxbow lakes situated on the active floodplain (I geo (Pb) 0.95-1.25, PLI 2.1-2.8) and an overall unpolluted to slightly polluted status of those at the reclaimed side (I geo (Pb) − 0.36-0.51, PLI 1.3-1.7). Additionally, the geochemical fractionation of the target metals showed that Cd and Zn were the most labile (Cd 29-48%, Zn 18-37% of nonresidual proportions), indicating their environmental significance. Conclusions Canonical discriminant analysis of the sediment-bound metal concentrations revealed the prevalent role of the river connection over the anthropogenic use in controlling the metal enrichment of oxbow sediments.

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 15 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 20 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 25 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 re-cultivation of oxbow lakes a close attention to be paid to elements in 30 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.

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