An ecohydrological perspective on heavy metal transport in the Vistula, Oder, and Polish coastal river systems: From catchment to the Baltic Sea

The aim of this study was to quantify the contribution of Poland’s largest rivers, viz. the Vistula, Oder and main Baltic rivers, to the transport of heavy metals (Cd, Pb, Hg, As, Ni, Zn, Cu, Cr) to the Baltic Sea. The analysis draws on ten years of monitoring data (2012–2021) from the Chief Inspectorate of Environmental Protection and the Institute of Meteorology and Water Management in Poland. Multivariate statistical analysis found the presence of heavy metals in the studied matrices to be primarily determined by flow rate, and by the physicochemical properties of water. Significant spatial variation in metal contamination was observed in the Vistula (points W1-W4) and Oder (O1-O4), with the highest loads recorded in the middle reaches, which are particularly vulnerable to urbanization and anthropogenic pressure. The highest mean daily load was estimated for Zn (1446.01 kg/day) at point W4, while maximum Cr (115.56 kg/day), Pb (56.58 kg/day), Hg (0.88 kg/day) and Cd (7.21 kg/day) were noted at point W3 in the Warsaw metropolitan area. In the Oder, the highest load was noted at O2 for Cu (1389.99 kg/day). Among the Coastal rivers, the greatest loads were found in the Rega and Parsęta, where metal concentrations correlated strongly with local physicochemical parameters. This study presents the first long-term assessment of heavy metal transport along the full continua of the largest rivers in Poland, and offers a new insight into the spatial determinants of pollutant migration. It also proposes catchment-scale solutions to reduce heavy metal outflow to the Baltic Sea.