Which benefits of fluvial annex sediment analysis for quantifying and tracing industrial pollution along the Saône River?

In the context of river ecological restoration in Europe and in order to achieve a “good” ecological and chemical status in watercourses, it is essential to quantify the volumes of contaminated sediment accumulated in fluvial annexes, which may be remobilized during floods or human interventions. These assessments not only allow the evaluation of ecotoxicological risks, but also help to assess the ecological functions associated with reconnection to the main channel. The Saône River (France), the main tributary of the Rhône River in terms of hydro-sedimentary contributions, has been little studied from this perspective, despite numerous developments (dykes) that have profoundly altered the lateral connectivity of its main channel. A more in-depth knowledge of the Saône River is therefore clearly needed  to guide effective and safe ecological restoration actions.

The volumes of sediments accumulated in three fluvial annexes distributed along the Saône River were estimated by combining ground-penetrating radar (GPR) transects with sediment cores sampling. These sediment archives were characterized (grain-size, organic matter content, trace metal content) to reconstruct the temporal trends of metal accumulation, based on ¹³⁷Cs and ²¹⁰Pb dating. Depending on the site, these sediment sequences provide six to eight decades of records, extending back to the 1940s for the longest. These data allow quantification of contaminants stocks (trace metals) and estimation of the annual load of contaminated suspended matters by the river in each site.

The study sites exhibit contrasting morphologies and varying levels of lateral connectivity with the main channel. These differences influence the sediment storage volumes within the fluvial annexes, ranging from 8,000 m³ to 100,000 m³. These results reveal metal enrichment since the 1940s, with a clear and well-documented increase in Cd, Cu, Pb and Zn during the post‑World War II economic expansion (1950s), reaching maximum concentrations during the 1970s. Their concentrations subsequently declined in the 1990-2000s before stabilizing at lower plateau values.

A specific feature concerns the contamination history of Ag, most likely driven by the photographic industry, which presented a three-phase pattern: (i) regular increase in the 1970s and 1980s, (ii) successive peaks between 1986 and 1994, and (iii) a marked decline in the late 1990s-2000s, with the decline of silver. This typical signal was observed at all studied sites along the river, despite hydrological connectivity differences. These sedimentary record complement monitoring data, especially for trace metals that were difficult to quantify in the past. This study highlights the major influence of historical contamination sources that released polluted sediments at the basin scale over several decades. This reconstruction also has national-scale implications and complements records obtained by other research works (such as on the Seine or Garonne rivers), highlighting the extent and persistence of pollution linked with photographic product manufacturing in Western Europe before 2000. Together, these results provide an integrated understanding of sediment dynamics and contamination, offering key insights for future river management and restoration strategies.