Endless journey of macroplastics in rivers
Résumé
Rivers are supposed to be the main pathways of plastics from lands into the Ocean (Lebreton et al., 2017; Schmidt et al., 2017). However, there is still a huge lack of knowledge on how riverine litter, including macroplastics, is transferred into the Ocean. Quantitative measurements of macroplastic emissions in rivers even suggest that a small fraction (0.001 to 3%) of the Mismanaged Plastic Waste (MPW) generated within a river basin finally reach the sea (Emmerik et al., 2019; Schöneich-Argent et al., 2020; Tramoy et al. submitted). Instead, macroplastics may remain within the catchment and on coastlines because of complex transport dynamics that delay the transfer of MPW from lands into the Ocean (Olivelli et al., 2020; Weideman et al., 2020). In order to better understand those dynamics, we released 1L plastic bottles equipped with GPS-trackers in the Seine River. Between March 2018 and April 2019, 39 trajectories were recorded in the estuary under tidal influence and 11 trajectories upriver, covering a wide range of hydrometeorological conditions. Results show a succession of stranding/remobilization episodes in combination with alternating upstream and downstream transport in the estuary related to tides. In the end, 100% of the tracked bottles stranded somewhere, for hours to weeks, from one to several times at different sites. In addition, date-prints items found in historical polluted sites in the estuary confirm that plastic may remain stored on riverbanks for decades. The overall picture shows that different physical phenomena interact with various time scales ranging from hours/days (high/low tides) to weeks/months (spring/neap tides and highest tides) and years (seasonal river flow). Thus, the fate of MPW is highly unpredictable with a chaotic-like transfer of plastic debris into the Ocean, and their residence time is much longer than the transit time of water.