A few words about the "Origins and fate of plastics" section of the Saône to Rhône project

Plastic pollution is an issue that affects the soil and forests of our immediate surroundings, as well as the most remote parts of the ocean. Understanding its distribution is an essential first step in trying to remedy it.

Most of the studies devoted to this issue focus on marine environments. However, a large proportion of the macroplastics (defined as plastics that are visible or larger than 5 mm) that enter rivers do not seem to reach the sea, but accumulate in and around watercourses: riverbanks, bed, vegetation, man-made infrastructure, etc. (Van Emmerik et al, 2022). 

 

​

These trapped plastics can be (re)mobilized, and would be so essentially at the time of floods, during which a greater quantity of debris is observed being transported in large rivers, notably the Rhône (Castro-Jiménez et al, 2019). 

​

 

As for microplastics (invisible to the naked eye, defined by their size of less than 5 mm), which often but not exclusively result from the fragmentation of larger plastics, their distribution does not necessarily follow that of macroplastics. Their transport or retention in sediments also depends on multiple factors: size, plastic shape, polymer, water velocity, sediment type, stream shape ... (Horton and Dixon, 2018)

 

​

Several studies have estimated the flow of micro- and/or macroplastics into the Rhône, a potentially major source of plastic pollution in the Mediterranean. These are field studies or simulations, which give some orders of magnitude, depending on the size of the plastics considered. In addition, the flow of macroplastics peaks shortly after flood periods (Castro-Jiménez et al, 2019). These results suggest the existence of retention zones between upstream and downstream watersheds, for example upstream of dams.

​

​

To our knowledge, there have been no studies of plastic pollution in the Saône. The Saône is a wide, slow-moving river, a major tributary of the Rhône. Developed for navigation, it has nonetheless retained many islands and wastelands, and is less urbanized than the Rhône, making it an interesting field in which to study the impact of the river's shape on the fate of plastics. Its morphology was studied by L. Astrade in the 90s (Amendola & Weingertner, EPTB Saône-Doubs, 2020; Astrade, 1998), who highlighted variations in stream strength and sinuosity. In terms of land use, the upstream part of the river drains mainly agricultural land, while the downstream part is much more urbanized.

 

Where are the major reservoirs of plastics along rivers?

To answer this question, several approaches will be implemented:

• Listing plastics visible from navigation (Cowger et al, 2019)

• Further characterization of a dozen sites: transects to record macroplastics, sediment sampling in search of microplastics (Darmon et al, 2020 ; Blettler and al, 2017).