Microplastic Pollution in Shoreline Sediments of the Vondo Reservoir Along the Mutshindudi River, South Africa

Rivers are recognized as significant pathways and transportation for microplastics (MPs), an emerging contaminant, to aquatic environments. However, there is limited evidence on how riverine reservoirs influence MPs transport. To fill this gap and provide baseline empirical data and insights to South African context, the current study assessed the seasonal variation in MP densities from sediments collected upstream, within the reservoir, and downstream of the Vondo Reservoir along the Mutshindudi River. We hypothesised that MP densities would be highest within the reservoir, due to the lack of constant flow that would otherwise transport accumulated particles downriver. Additionally, we expected the cool–dry season to be associated with the highest MP densities. As expected, high MP densities were observed within the reservoir (117.38–277.46 particles kg−1 dwt) when compared to the downstream (72.63–141.50 particles kg−1 dwt) and upstream (28.81–91.63 particles kg−1 dwt) sites of the reservoir. The cool–dry season (91.63–277.46 particles kg−1 dwt) exhibited the highest MP densities compared to the hot–wet season (28.81–141.50 particles kg−1 dwt). However, MP densities downstream the reservoir were higher during the hot–wet season (141.50 ± 24.34 particles kg−1 dwt) compared to the cool–dry season (72.63 ± 48.85 particles kg−1 dwt). The most dominant MP particles identified were white, transparent, and black fibres/filaments composed primarily of polypropylene (PP) and polyethylene (PE). This suggests diverse sources of MP particles. No significant correlations were found between water parameters and MP densities across sampling sites and seasons, indicating a widespread and context-independent presence of MPs. These findings contribute to MP studies in freshwater environments and further reinforce the role of sediments as sink for MPs and suggest that riverine reservoirs similar to dams can trap MPs, which may then be remobilized downstream during high-flow periods. Importantly, the results of this study can support local municipalities in implementing targeted plastic pollution mitigation strategies and public awareness campaigns, particularly because the Vondo Reservoir serves as a critical water resource for surrounding communities.