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

Abstract

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−1) season as compared to the cool–dry (16.5 ± 4.5–27.0 ± 5.0 microplastic kg−1) and hot–dry (13.0 ± 4.0–29.0 ± 10.0 microplastic kg−1) 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.

Gastric Microplastics in Clarias gariepinus of the Upper Vaal River, South Africa

“…Plastic and microplastic research in Africa remains limited compared to other continents, but recent efforts to highlight microplastic pollution in Africa has been made (Naidoo et al, 2015;Nel and Froneman, 2015;Biginagwa et al, 2016;Lourenco et al, 2017;Nel at al., 2017;Nel et al, 2018;Akindele et al, 2019;Dalu et al, 2019;Mayoma et al, 2019;Toumi et al, 2019;Weideman et al, 2019;Weideman et al, 2020;Dahms et al, 2020;Alimi et al, 2021;Dalu et al, 2021;Deme et al, 2022;Masiá et al, 2022). In South Africa, research has been conducted on microplastics in the Vaal River system, determining microplastic concentrations in water, sediment, and the ability for longdistance transfer of microplastics (Weideman et al, 2019;Weideman et al, 2020).…”

Microplastic Pollution in Surface Waters of Urban Watersheds in Central Texas, United States: A Comparison of Sites With and Without Treated Wastewater Effluent

“…Additional findings suggest that increased urbanization and wastewater treatment plants serve as primary sources of microplastic pollution within freshwater systems (Schmidt et al, 2018;Talbot and Heejun, 2022). An investigation of a subtropical river in southern Africa, however, did not find a clear spatial distribution pattern of microplastics relative to a wastewater treatment plant (Dalu et al, 2021). A recent field sampling study of an urban creek in Illinois concluded that concentrations of microplastics in urban streams may be orders of magnitude greater than in larger rivers (Noseworthy et al, 2021).…”

Classification and distribution of freshwater microplastics along the Italian Po river by hyperspectral imaging

“…In particular, rivers are a major pathway of microplastics to the marine environment (Faris and Hart 1994 ; Allsopp et al 2006 ; Guerranti et al 2017 ; Lebreton et al 2017 ; Schmidt et al 2017 ; Li et al 2020 ) due to human activities, such as industry (Karlsson et al 2015 ), agriculture (Corradini et al 2019 ; GESAMP 2013 ; Huang et al 2021 ) or wastewater treatment plants (Kay et al 2018 ; Conley et al 2019 ; Dalu et al 2021 ). It has been estimated that between 1.15 and 2.41 million tons of plastic waste enter the ocean each year from rivers (Lebreton et al 2017 ), with 20 rivers (15 from Asia, 3 from Africa and 2 from South America) responsible of about 67% of the total world pollution.…”