Characterization of Macroplastics in Flood Deposits along Contrasting Sections of the Saône River (France)

Rivers transport macroplastics (> 1 cm), which can remain trapped in several parts of the floodplain: banks, islands, lateral channels, infrastructures, and vegetation. Residence times vary and can be up to years. This work focuses on the Saône River, a lowland river, where 19 organic flood deposits were sampled on islands and riverbanks. They are located in four sectors characterized by variations in slope, specific stream power and land-use. Macroplastics were collected, weighed, and characterized visually during the field trip by boat, and then identified using ATR-FTIR. Seventy percent of the 1324 macroplastics were unknown fragments, which suggests that they have undergone river transport and fragmentation processes. Point deposits in calm water areas along the riverbanks accumulate mostly foam-type macroplastics, while diffuse deposits on densely vegetated islands particularly trap film-type macroplastics. Sectors characterized by low specific stream power may concentrate more macroplastics, but with a lower mass. Macroplastics with a higher median mass were observed in the upstream sector, where slopes are steeper. Our results confirm the existence of preferential trapping zones for macroplastics in areas with gentle slopes and dense vegetation. They also highlight the existence of several trapping modes, which could imply variable residence times and microplastic fragmentation rates depending on the storage areas and types of debris considered.