Identifying Environmental Factors Influencing the Concentration, Fluxes and Spatial Distribution of Microplastic Pollution in an Urban River: A Case Study of the Klip River, South Africa

Urban freshwater systems are increasingly recognised as critical conduits for microplastic pollution, however, the interactions between land use, hydrology and microplastic transport remain poorly understood, particularly in developing countries. To address this gap, we investigated the relationships between catchment characteristics and the temporal and spatial variations in microplastic concentrations and fluxes in an urban river in South Africa. A microscopy approach was used to enumerate microplastic particles in 12 monthly surface water samples from six locations during November 2020 – October 2021. Microplastic particles dominated by fibres were observed in all selected sampling sites with monthly mean concentrations ranging from 0.18 to 2.90 particles L⁻¹. Land cover has a significant impact on microplastic inputs; compared to formal residential areas, informal settlements showed increased concentrations. Microplastic fluxes accumulated moving downstream through the catchment. Microplastic flux calculated using modelled runoff was lowest upstream at site 1 (9.69 × 10⁸ MP/y) and peaked at the furthest downstream site 6 (4.41 × 10¹⁰ MP/y). However, measured flow data from Site 6 indicate that actual flows, including return flows from densely populated urban water systems, are almost ten times greater than runoff modelled from precipitation data. Hence the actual flux at site 6 is 4.34 × 10¹¹ MP/year. Rainfall patterns had a significant role in shaping monthly variations in microplastic concentrations and fluxes. Although wetlands were expected to retain microplastics, only one wetland-associated reach showed a decline in fluxes. Further characterisation of wetland types (e.g., channelled vs. unchannelled valley bottom) is required to improve understanding of their differential capacity for microplastic retention and to account for the observed spatial variation. The findings of this study show that seasonality, precipitation, land cover, runoff and wetland presence significantly influence the pervasiveness of microplastic pollution in urban river catchments. Furthermore, this study contributes to the limited freshwater microplastic literature in developing countries and provides insights to guide waste management, integrated natural resource management and wetland conservation strategies in similar contexts.