Effects of urbanisation and a wastewater treatment plant on microplastic densities along a subtropical river system

Global freshwaters are increasingly threatened by pollutants emanating from human activities around watersheds. Microplastic pollution is an increasing problem for rivers worldwide, potentially threatening ecological integrity, ecosystem services and human health. We present quantifications and characterisations of sediment microplastic pollution in a subtropical river system in southern Africa, and relate distributions to wastewater treatment works, abiotic variables and urban environments. We additionally apply several diversity indices to decipher how microplastic types differ across the river system seasonally. Over two thousand microplastic particles were found across five sites and three seasons in the river system, comprising microbeads of various colours and microfibres. Microplastic concentrations were highest and most diverse in the hot-wet (mean range 76.0 ± 10.0-285.5 ± 44.5 microplastic kg) season as compared to the cool-dry (16.5 ± 4.5-27.0 ± 5.0 microplastic kg) and hot-dry (13.0 ± 4.0-29.0 ± 10.0 microplastic kg) seasons, and were mostly dominated by microfibres. However, no clear patterns were found in relation to wastewater treatment operations spatially, or in relation to abiotic variables in the river system. This study therefore finds a diverse range of microplastic types widely distributed in the river system that differ across seasons. Our results provide important, novel insights into plastic pollution in an understudied area of the Global South, and point to extensive pollution from sources outside of wastewater treatment works.