Occurrence and distribution of microplastics in functionally delineated hydraulic zones in selected rivers, Eastern Cape, South Africa

Microplastic pollution in freshwater ecosystems has garnered increasing attention due to its ecological risks and potential impacts on human health. While numerous studies have addressed microplastic prevalence in rivers, a critical gap remains in understanding their spatial distribution within the geomorphological and hydraulic structure of river channels. This study investigated the occurrence and variability of microplastics across hydraulic units in two urban river systems, the Buffalo and Swartkops Rivers, in the Eastern Cape Province of South Africa. Hydraulic units were categorised into flush (riffles and runs) and sink (pools and backwaters) hydraulic zones, based on in situ measurements of flow velocity, depth, and substrate composition. Flush zones were hypothesised to promote microplastic remobilisation due to higher flow energy, whereas sink zones were expected to favour microplastic deposition owing to reduced hydraulic energy. Samples of surface water (suspended microplastics) and sediment-water mixtures (settled, then resuspended microplastics) were collected across these units in both the wet/hot and dry/cold seasons. Settled microplastics were significantly more abundant in sink zones (1.82 ± 1.98 items/L) than flush zones (1.32 ± 1.49 items/L; p < 0.05), confirming their role as depositional environments. Suspended microplastics were more evenly distributed but slightly higher in flush zones (1.76 ± 1.44 items/L). Seasonal variation was evident, with higher abundances of both settled and suspended microplastics during the wet season, particularly in sink zones, indicating enhanced transport and deposition under high-flow conditions. Fibres and fragments were the dominant microplastic forms, and the smallest size class (0.063-0.5 mm) was most prevalent, especially in flush zones. Polyethylene (PE) and polypropylene (PP) were the most common polymers. Denser polymers such as polyamide, polycarbonate, and polyvinyl chloride occurred exclusively in sink zones, suggesting selective retention linked to particle density and hydraulic conditions. Polymer diversity was also higher in these zones. Effluent-impacted sites showed similar microplastic patterns to other locations, though upstream sites revealed localised variation. These findings underscore the role of hydraulic heterogeneity and seasonal hydrology in microplastic distribution and highlight the need for flow zone-based monitoring approaches in riverine microplastic assessments and management.