Legacy‐micropollutant contamination levels in major river basins based on findings from the Rhône Sediment Observatory

For more than half a century, chemical contamination has progressively spread to all the large river basins. Large river outlets integrate multiple anthropogenic pressures in watersheds, making them the largest source of sediment-bound contaminants to continental shelf areas. However, comparing particulate micropollutant contaminations between the large river basins is a challenging task, especially due to the scarcity of long-term river monitoring programs. Here we address this issue, with a focus on legacy particulate micropollutants (polychlorobiphenyls [PCBi], polycyclic aromatic hydrocarbons [PAHs] and trace metal elements [TME]) yields. For this purpose, we employed a bottom-up multiscale approach to chemical contamination in river basins that takes micropollutant yields measured in the Rhone River sub-basins (France) as a benchmark of other large river basins. Data on the Rhone River basin came from a unique 10-year-long monitoring program within the Rhone Sediment Observatory (OSR), and were compared to data gathered on 18 major worldwide river outlets. The Rhone River basin is far cleaner now than a few decades ago, likely due to environmental regulations. At a wider spatial scale, our results depict an overall contamination gradient splitting the most heavily contaminated river basins, located in developing and industrializing low-to-middle-income countries, from the least contaminated rivers located in developed high-income countries. We argue that chemical contamination levels of large river basins depend on their stage of economic development.