Evaluating the hydrological processes governing microplastic contamination in stormwater runoff from an industrial catchment discharging into a detention basin in Lyon (France)

Study region An industrial and commercial catchment discharging into a detention basin located in Lyon, France, representing the outlet point of the catchment. Study focus This study investigates the hydrological processes controlling both inter- and intra-event variability in microplastic (MP) concentrations at the inlet of the detention basin. Five rainfall events were monitored over a one-year period using flow-proportional sampling to quantify the hydrological drivers influencing MP concentrations and fluxes. New Hydrological Insights for the Region The results indicate that the annual MP flux per unit active surface area entering in the detention basin through stormwater runoff was estimated to be 0.86 kg/year/ha, representing approximately 0.2% of the total suspended solids (TSS) load. The first-flush phenomenon was evaluated using the conventional mass–volume approach and was not consistently observed across rainfall events. MP concentrations were not consistently elevated during the initial stages of rainfall events. Panel regression analyses revealed that flow rate and turbidity were positively associated with MP contamination, suggesting shared mobilization mechanisms. In contrast, cumulative volume exhibited a negative association with MP concentration, potentially reflecting dilution effects or progressive source depletion over the course of the events. Overall, this study provides novel insights into MP transport dynamics within industrial urban catchments and highlights the hydrological controls governing both intra- and inter-event variability.