Long-term spatio-temporal dynamics of sediment-associated heavy metals in a large river system

Despite the widespread use of geochemical and ecological indices, our understanding of long-term heavy metal dynamics in large river systems is still limited, particularly when it comes to capturing temporal variability and interactions between multiple elements. In this study, we examine patterns of sediment-associated metal contamination and ecological risk in the Vistula River (Poland), using monitoring data collected between 2015 and 2022 from 23 sites distributed along the entire river course. Concentrations of eight metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn) were analysed using a set of commonly applied geochemical and ecotoxicological indices, including Igeo, CF, PLI, PERI and the mean probable effect concentration quotient (mPEC-Q). To better capture changes over time, we also propose a Sediment Contamination Trend Index (SCTI) as a complementary metric that integrates proportional, toxicity-weighted changes in multi-metal contamination. The results show clear spatial variability in sediment contamination, with the highest enrichment observed for Zn, Pb, Cd and Cr. While classical indices indicate moderate to considerable contamination and generally moderate ecological risk, the inclusion of SCTI highlights pronounced year-to-year variability, with alternating periods of increasing and decreasing metal concentrations that are not fully reflected by conventional static approaches. Taken together, the results suggest that combining long-term monitoring data with both established indices and a simple trend-based metric can improve the interpretation of sediment contamination dynamics in large river systems. Such an approach may be particularly useful in supporting environmental monitoring and river basin management under changing environmental conditions.