Plastic pollution is omnipresent. Researchers and citizens try by various means, on and under water, to characterize it. Like the VigiePlastic project in the Mediterranean, and the Sédi-Plast project in French rivers.
For several years, the hunt for microplastics has been on. As scientific reports and studies take hold, it appears that this pollution is widespread, from the oceans to the air that everyone breathes. Two recent research projects looked at the extent of this pollution, on the one hand, on the surface of the Mediterranean Sea and, on the other hand, under the three largest French rivers.
Record pollution in the Mediterranean
For more than ten years, Expedition Med (for the Mediterranean in danger), a citizen science association, has been tracking plastic waste floating in the Deep Blue or swarming on its coasts every summer. On board the sailboat The Prettyits eco-volunteers, and the researchers who accompany them, sail between Cannes and Monte-Carlo, along the Corsican coastline and to the coasts of Tuscany. The surface waters of 54 sites are combed using manta-type drag nets, whose very dense meshes capture particles of around a hundred microns or less. And this year, unfortunately, the fishing has been particularly good.
“During the 2024 campaign, we identified areas of large accumulation of plastics inside the Pelagos Sanctuary, a protected marine areanotes Ana Luzia Lacerda, oceanographer at Sorbonne University, based on results presented on November 28. This reinforces the idea that plastic pollution is cross-border and that actions must be taken not only nationally, but also across the entire Mediterranean basin.. » The results of this new campaign Vigie Plastic has in fact measured concentrations of microplastics “exceeding all those previously described in the scientific literature”. In the heart of the Corsican Canal, between the island of Capraia and Cape Corsica, the expedition identified more than two million plastic particles of less than one millimeter per square kilometer (MP/km2), “almost twice as much” than what was observed in 2019 or in the “plastic continent” of the North Pacific.
A “soup” that becomes denser
The “plastic soup” Corso-Ligurian was identified for the first time by French, Italian and American researchers in 2017. In 2019, the Italian branch of the World Wide Fund for Nature (WWF Italy) carried out its own samples there and collected around 1, 25 million MP/km2 (compared to 1.1 million MP/km2 in the North Pacific). This situation would have occurred due to the gyre (or whirlpool) of the Corsica Canal. Through the effect of different currents, it receives plastic waste carried from the Italian coasts, on the one hand, going up to the island of Elba, and coming from the Gulf of Napoule, on the other hand. In the Mediterranean Sea, such gyres generally form “areas of temporary accumulations, but never permanent”unlike the phenomenon observed in the North Pacific. But the fact is that plastic pollution seems to support such accumulation over time.
Number and concentrations of microplastics collected during Expedition Med’s 2024 measurement campaign.© Expedition Med
Within this zone, 9 to 46% of the fragments analyzed by the private laboratory Qualyse measured between 25 microns (μm) and 1.5 mm. As for their nature, 69% of them were mainly composed of polypropylene (PP) and 21% of polyethylene (PE), two polymers used in the manufacture of packaging and single-use objects. “ These alarming figures demonstrate that the plastic concentration zone off Cape Corsica persists and is intensifying, despite efforts to reduce marine pollution.notes Bruno Dumontet, former researcher and founder of Expedition Med. Apart from the Corsican canal alone, the average density of plastic waste collected over more than 1,850 kilometers amounts to 156,000 MP/km2. Enough to feed the scientific literature in return.
Accumulation in the sediments of French rivers
But plastic pollution is not just a recent phenomenon. Although it has been accelerating in recent years, it is not exactly new. What researchers from Gustave-Eiffel University, accompanied by their colleagues from the École des Ponts ParisTech, the Clermont-Ferrand Institute of Chemistry, the CNRS and the École Pratique des Hautes Études wanted to demonstrate , through the Sédi-Plast project. The objective? Make the first recording of microplastics in river sediment archives (where the age of the different layers has been established). Launched in 2020 with funding from the National Research Agency (ANR), the consortium delivered its conclusions on December 3.
Concretely, at the start of their research, the scientists carried out sediment corings at two points, one upstream, the other downstream of large towns, of three rivers: the Seine, the Rhône and the Loire. The collected samples, containing particles of 25 μm or more, were then analyzed by several infrared spectroscopic techniques (playing on the emission or absorption of light and the photoconductivity of the molecules), but also by pyrolysis coupled with spectrometry. of mass (which allows, by altering substances under the effect of heat, to know their composition). And this, for sediment cores dating back to the early 1950s, a short decade before the first traces of plastic pollution observed in the watersheds concerned.
Better understand the history of plastic pollution
Result ? River sediments constitute well “a significant sink for microplastics, with levels between 200 and 800 particles per kilogram of dry sediment”attest the researchers. “For comparison, we only find around ten to a hundred particles per cubic meter of surface water. () The fragments observed are generally small, with the majority of fragments less than 100 µm in size. » The determination of their exact nature remains to be finalized.
However, the researchers do not intend to stop there. Building on these first sedimentological results, they now wish to evaluate the “temporal trajectories” and understand the “historical factors” of this pollution. And, in the meantime, they intend to participate in the development of a new microplastic monitoring tool, based on these first historical data, for use by water agencies and the French Biodiversity Office (OFB). ). “The data generated, the recommendations and the tool developed will be used in the short term to monitor and observe microplastics in waterways, and in the long term, to evaluate public policies”thus advance the researchers.
Article published on December 16, 2024
