SPATIAL DISTRIBUTION OF HEAVY METALS (Cr, Cu, Zn AND Pb) IN BOTTOM SEDIMENTS OF OXBOW LAKES IN NORTHERN POLAND DIFFERED BY HYDROLOGICAL CONNECTIVITY

The investigation of oxbow lakes sediments conducted in the middle section of the River Lyna valley, in northeastern Poland, showed variability in Cr, Cu, Zn and Pb contents in relation to hydrological connectivity and some physical properties of bottom sediments. The research shows that the analysed ecosystems are characterised by low-to-moderate levels of pollution with the metals. Oxbow lakes with limited water-exchange (lentic type) accumulate heavy metals in their bottom sediments in considerable quantities, what is promoted by anaerobic conditions and decrease in pH. An active exchange of water creates unfavourable conditions for heavy metals to be accumulated in sediments. The maximal accumulation of pollutants characterized the uppermost layer (0-30cm) of a sediment profile, where up to 8 g of Cr per 1 m(2), 1.74 g of Cu, 3.24 g of Pb and 16 g of Zn was stated. Beneath, heavy metals found in fluvial material built most of all of clay showed significantly lower values of the metals, which can play a role as background (uncontaminated) levels of heavy metals. In landscapes of river valleys, oxbow lakes may function as very effective biogeochemical filters contributing to heavy metals trapping.