Assessment of spatial distribution of sediment contamination with heavy metals in the two biggest rivers in Poland

The contamination of six heavy metals (Cr, Ni, Cu, Zn, Cd and Pb) in 42 river sediment samples was investigated in the Oder and Vistula, the two biggest rivers in Poland. This is the first research which considers almost the whole area of Poland (96.3%) to obtain the overall characteristics of heavy metals (HMs), their spatial distribution, pollution levels and their possible sources. The degree of pollution in sediments was calculated by several geochemical indices (geoaccumulation index, enrichment factor, pollution load index, and metal pollution index). Moreover, the potential toxic effects were assessed on the basis of sediment quality standards (threshold effect concentration, probable effect concentration, midpoint effect concentration) and the toxic risk index. Values of the pollution load index and metal pollution index showed that the ecological risk related to the presence of HMs in the river bottom sediments was high in both rivers. It was observed that in most of the samples the concentrations of HMs were under the probable effect concentration value, defined as the limit above which a toxic effect on aquatic organisms can be expected. However, Ni and Zn concentrations also exceeded the PEC level by 11.76% and 16% for the Oder and Vistula rivers, respectively. Assessment of the EF index values for the Oder and Vistula rivers demonstrated that Cr and Zn have generally greater enrichment compared to other heavy metals. Cluster analysis and principal component analysis showed that the spatial distribution of HMs in sediments is mainly related to point sources of pollution, and is modified by the river fluvial process. The direction of pollution distribution in sediments is opposite in the two analyzed rivers. The Oder River shows higher concentrations from downstream to upstream, caused by the presence of point and area sources of pollution. In the case of the Vistula River, pollution decreases from downstream to upstream, which may be an example of sediment deposition in reservoirs located along the river, working modern sewage treatment plants in big cities, and river self-purification processes.