Limited long-distance transport of plastic pollution by the Orange-Vaal River system, South Africa

Abstract

Much of the plastic waste entering the sea is thought to be transported from land by rivers, yet little is known about the distances over which rivers transport plastic. To address this knowledge gap, we collected surface water samples from the Orange-Vaal River at the end of the wet and dry seasons. The Vaal River drains South Africa’s main urban-industrial centre, whereas the upper Orange River is sparsely populated. Below their confluence, the river flows through increasingly arid regions with very low human populations before entering the Atlantic Ocean. We collected bulk water samples from 33 bridges to test for microplastic and microfibre (0.025-1 mm) pollution and conducted observations for macrodebris (>50 mm). Where possible, we sampled for plastic fragments (>1 mm) using a neuston net. Microfibres and microplastics were found at every site (1.7 ± 5.1 L-1, >99% fibres) and accounted for 99% of the number of items recorded. Microfibres and microplastics were particularly abundant in the lower reaches during the period of low flow prior to the wet season flush. Macrodebris and larger microplastics were orders of magnitude less abundant (observations: 0.0002 ± 0.0007 items·m-2; neuston net: 0.34 ± 0.93 items·m-2). However, at sites where larger items were found, they comprised most of the mass of plastic. Larger plastics were found mostly at sites in the upper reaches of the Vaal River. Our results suggest that, while the Orange-Vaal River system may be a source of microfibres to the Atlantic Ocean, larger plastic items typically only travel short distances. The Orange-Vaal River system therefore does not appear to be a major source of plastics into the Atlantic Ocean, at least under regular flow conditions.