Microplastic pollution and risk assessment in surface water and sediments of the Zandvlei Catchment and estuary, Cape Town, South Africa

Microplastic (MP) (plastic <5 mm) pollution in South Africa is widespread but few studies have been done in catchments and estuaries of the country. The aim of this study was to investigate the abundance, characteristics and risks posed by microplastics in the Zandvlei Catchment and Estuary in Cape Town, South Africa. Water and sediment were sampled between 2019 and 2021, during wet and dry seasons, MPs extracted and identified using microscopy and fourier-transformed infrared spectrophotometry (FTIR) analyses. MP abundances were 70.23 ± 7.36 (standard error) MPs/Kg dw in sediment and 2.62 ± 0.41 MP/L in water samples for the study period. Lower reaches of the catchment and upper reaches of the estuary can be considered sinks for MP contamination as these sites recorded higher MP abundances. MPs were mainly transparent fibres smaller than 0.5 mm. Polyethylene (46%) followed by polypropylene (16%) fibres were the most common polymers recorded. Pollution load indices in MPs were categorised as dangerous in both water and sediment. MP polymer risk indices ranged from moderate in catchment sediment to very high in catchment water. Pollution risk indices were categories as dangerous in water (catchment and sediment) and sediment estuary but low in catchment sediment. Ecological risk assessments hence indicated that polymers in water and sediment were mostly dangerous and poses a threat to the ecological health of both the catchment and estuary studied.