Anthropogenic factors, not hydrometeorology, explains plastic pollution variability in the Odaw river

Variations in macroplastic transport are often linked to hydrometeorological conditions (wind, precipitation, and discharge). However, due to the predominant focus on these hydrometeorological factors as the main driving forces, most research overlooks the impact of anthropogenic factors, such as mismanaged plastic waste (MPW) on plastic transport variability. Here, we investigate the roles of both hydrometeorological and anthropogenic factors on plastic pollution. We collected field data on floating, riverbank, and land litter (macroplastics) between December 2021 to December 2022 at 10 bridge locations along the Odaw river. We tested seasonality in plastic transport/density with the Mann-Whitney U-test. Furthermore, we used multiple regression analysis to evaluate the combined effect of all the hydrometeorological variables (rainfall, discharge, and windspeed) on macroplastic transport. Additionally, we correlated peaks in plastic to peaks in discharge, wind speed, and rainfall, defined with the 90th percentile of a distribution as the threshold. Finally, we correlated the spatial variation in plastic transport/density with MPW and population density. Contrary to previous studies, our results showed no seasonal differences in plastic pollution within the Odaw catchment. Additionally, only weak to no correlations were found between plastic transport and the hydrometeorological variables. Overall, only 14-18% of the plastic peaks corresponded to the hydrometeorological peaks. More of these plastic peaks were associated to windspeed peaks. However, a strong correlation was observed between MPW and plastic transport and riverbank/land plastic density. Therefore, we hypothesize that anthropogenic factors are more important than hydro meteorological factors in plastic pollution variations. Our study emphasizes the need to holistically study the role of both anthropogenic and hydrometeorological factors in explaining plastic transport and retention dynamics at a river basin scale. This insight is vital for developing effective interventions to address plastic pollution in river catchments.