Main drivers for microplastic records in river sediments since the 1950s: Case study of the Loire river (France)

Microplastics (MPs) are often detected in river sediment, but the processes that lead to their long-term archiving need more investigation. In this study, the evolution of MPs buried in sediments was explored in a river segment with a diversity of deposition conditions. Two cores were collected on a high island -flooded only during overbank episodes- and in a semi-active channel also flooded during moderate-water periods. After a characterisation of sediment (dating, composition, sedimentation rate), 25-5000 μm MP concentrations, size and polymer types were obtained through μFTIR (micro Fourier-transform infrared) spectroscopy. The increase in MP concentrations in the semi-active channel core (2000s-2022) was found to be related to the gradual grain sorting and enrichment in organic content. The no-trend in MP concentrations in the island core (1950s-2022), despite its verified recording capacity for other particle-bound contaminants, was attributed to flood event-driven MP inputs. To sum up, we concluded that (i) MP archiving in riverine sediments was favoured by a high abundance of fine-grained (<63 μm) and TOC-rich sediment, but that (ii) it was conditioned to depositional area determining flooding frequency, and (iii) large quantities of MPs transiting in the river. This study provides a novel insight on heterogeneous MP accumulation processes in riverine depositional environments, which is key for future monitoring studies. These findings participate to a better understanding of MP contamination of river sediments and are a supplementary step in the assessment of sediment cores to give valuable retrospective views on pollution evolution in time.