Microplastic trapping in dam reservoirs driven by complex hydrosedimentary processes (Villerest Reservoir, Loire River, France)

Dam reservoirs can strongly influence the spatial distribution of sediment pollution by microplastics (MP). The Villerest reservoir (Loire River, 36 km long) is a good candidate to study the relationship between MP pollution and hydrosedimentary processes. Sediments were collected from the dam-controlled river section and from 3 km downstream. Geomorphological and sedimentological analyses were performed and microplastics were analysed using µFTIR imaging (polymer identification for particle sizes ≥ 25 µm). This paper highlights strong MP levels (on an order of 10 items/kg dw) over the section characterized by fine-grained sediments (FGS). In coarse-grained sediments (CGS), at the upstream part of the reservoir and downstream of the dam, levels are one order of magnitude lower. FGS are indicator of long-time settling processes. Such conditions lead to foster the MP trapping as low-density suspended materials in the water column. CGS deposits originate from the river bed load. These sediments are transported in high-velocity and high-turbulent flow conditions. Moreover, post-depositional reworking of the finest fraction can occur according to hydrofluctuations. Here are adverse conditions for the MP trapping. The polymer diversity is also higher in FGS than in CGS. However, the range of plastic particle sizes is similar in FGS and CGS and is not related to the sediment grain-size distribution. Moreover, in both FGS and CGS, the polymer abundance is not correlated with the grain-size distribution or with the organic matter content. In the reservoir context, a change in the polymer partition appears over the FGS section in the downstream direction, depending on the polymer density. From a fundamental point of view, this work contributes to improving our understanding of the key role played by hydrosedimentary processes in MP repartition. These findings also have operational scopes, providing significant elements to advocate for a better consideration of MP pollution during sediment management operations.