Microplastic dynamics along an extreme flood event in a peri-urban stream

Recent studies have investigated the effects of hydrological conditions on the dynamics of microplastics (MPs) in rivers. However, no clear correlation is consistently observed. In addition, these studies focus on sampling before and after the event, neglecting the dynamics of MPs over the course of the event. Given the increased intensity of extreme events related to climate change, an understanding of fine temporal MP dynamics in rivers is required to better estimate the fluxes and their contribution to downstream contamination. To address this gap and enhance understanding of the impact of hydrological conditions on MP contamination, an intensive sampling campaign was conducted throughout hurricane Kirk. This event caused an extreme flood in October 2024 at the outlet of a small peri-urban catchment (Avenelles, 45 km², 70 km east of Paris). Sampling was performed using a plastic-free in-situ filtration pumping system coupled with in series filtration (300 and 10 µm). Analyses were carried out using Fourier transform infrared µ-spectroscopy (automated imaging and data processing with SiMPle software), enabling the characterization of MP particles down to 25 µm. In addition, total suspended solids (TSS) were characterized alongside the sampling campaign to investigate their potential role in the transport and dynamics of MPs. Throughout this campaign, 18 samples were collected throughout the 6-day flood event, including pre-event conditions, the increasing and decreasing water levels, and post-event base flow. The MP concentrations and flux increased twofold from pre-flood (669 MPs m^-3 and 398 MPs s^-1) to flood peak (10804 MPs m^-3 and 56181 MPs s^-1). Four days after the flood peak, the concentration and flux returned to levels similar to those observed pre-flood (977 MPs m^-3 and 479 MPs s^-1). In addition, the concentrations of MPs and TSS exhibited a similar trend, with a correlation coefficient of 0.66.  These results highlight the limitations of interpreting hydrological effects based on data limited to pre- and post-flood sampling, underscoring the importance of studying the full progression of flood dynamics.