Tracking heavy metal contamination in a complex river-oxbow lake system: Middle Odra Valley, Germany/Poland.

Oxbow lakes have received much attention in recent years due to their ecological importance and vulnerability for contamination. Sediment cores collected from the floor of oxbow lakes indicate the increasing contamination of lakes with heavy metals (Cu, Cd, Pb, Zn, Ni, Cr) over the last fifty years, owing to the development of transport and industry. Little is known, however, about how various factors can enhance or impede the migration of metallic contaminants between the pollution source and lake ecosystems. To untangle these complex processes, the metal distribution was studied throughout the waters and sediments of an urban zone-river-oxbow lake system in the Middle Odra Valley, Germany/Poland. As expected the degree of metal contamination is highest (>10 enrichment) at low distance (<1km) from the human source of pollution. Using correlations with highly mobile in water Na+ and Cl- ions, we reveal, however, that due to hydrological factors some metals (Cu, Cd, and Pb) are up to >5 times enriched in the sediments of lakes even >10km distant from the nearest source of pollution. The lakes that are permanently connected with the Odra River or that are frequently flooded are most vulnerable for contamination. Although the metal concentrations are especially enriched in the smallest grain size fractions, this portion of metals seems to be less bioavailable with respect to that accumulated within the larger grain size fractions. Concentrations of metallic contaminants in the bottom sediments of the Middle Odra lakes are generally lower than in other oxbow lakes. Having untangled the variety of processes controlling metal migration in a complex river-oxbow lake system operating on these low metal concentrations, allows us to believe that our approach could become a paradigm for future trace element studies in a variety of similar lowland systems across the World.