Microplastics concentrations in freshwater during a flood event, a case study of the Seine river catchment

Despite recent research efforts, many questions still exist about sources of microplastics, especially their transport in freshwater from land to sea. Particularly, the consequences of flood events on microplastics transport are almost unknown although recent papers mention that large quantities could be transferred. A flood event occurred in January 2018 on the Seine river catchment. With a surface area of approximately 75 000 km2, the Seine basin is a densely populated area (16 million inhabitants mainly clustered in Paris metropolitan area). During the three week flood event the microplastic concentrations were assessed in the Seine and compared to low flow period concentrations. Three different sampling sites were monitored: one 100 km upstream Paris (called “Upstream” in the figure) and two others (“Downstream 1” and “Downstream 2”) 20 km and 45 km downstream. Three sampling campaigns have been conducted, one before the peak flow and two after. A 300 μm net and a 80 μm net were used. The 300 μm net was dropped directly in the middle of the river for 5 minutes while 20 L of freshwater was collected and filtered in the 80 μm net. The samples were sieved and digested with a H2O2 30 %. The samples were then filtered and microplastics were counted with a stereomicroscope coupled with an image analysis software. The fragments and a part of the fibers (30% of the total fibers) were analyzed with FTIR spectroscopy. Work is in progress. The microplastics in the 1 mm to 5 mm size range, collected with the 300 μm net, were counted and observed. Highest concentrations are found before the peak discharge for all sites and concentrations can be 5 to 10 times lower after this peak. Previous works found 0.54 fragments/m3 in Paris during low flow periods, the flood event could provoke microplastics resuspension.