Quantifying changes in litter loads in urban stormwater run-off from Cape Town, South Africa, over the last two decades

Eleanor A Weideman; Vonica Perold; Gael Arnold; Peter G Ryan

doi:10.1016/j.scitotenv.2020.138310

Quantifying changes in litter loads in urban stormwater run-off from Cape Town, South Africa, over the last two decades

Sci Total Environ. 2020 Jul 1:724:138310. doi: 10.1016/j.scitotenv.2020.138310. Epub 2020 Mar 30.

Authors

Eleanor A Weideman¹, Vonica Perold², Gael Arnold², Peter G Ryan²

Affiliations

¹ FitzPatrick Institute of African Ornithology, DST-NRF Centre of Excellence, University of Cape Town, Rondebosch, 7701, South Africa. Electronic address: el.weideman@gmail.com.
² FitzPatrick Institute of African Ornithology, DST-NRF Centre of Excellence, University of Cape Town, Rondebosch, 7701, South Africa.

PMID: 32268294
DOI: 10.1016/j.scitotenv.2020.138310

Abstract

To implement effective mitigation measures to stop the flow of litter from land-based sources into the sea, it is important to identify key pollution sources and to monitor litter trends over time. We sampled plastic and other anthropogenic debris in urban stormwater run-off in Cape Town, South Africa, from three catchment areas representing different land-use types: residential, industrial and a mixed commercial/residential area. Sampling was conducted in 2018-19 by placing nets over stormwater outlets during rain events. Organic matter constituted 79% of material by dry mass (industrial: 51 ± 22%, commercial/residential: 86 ± 4%, residential: 88 ± 13%). The nets caught 5-576 anthropogenic litter items·ha^-1·day^-1 (2-377 g·ha^-1·day^-1) with significantly higher densities in the industrial and commercial/residential areas than the residential area. Among anthropogenic litter items, 40-78% were made of plastic (52-64% by mass). Most plastic items were single-use packaging, but industrial pellets washed out of the industrial outlet during every rain event. Compared to a similar study conducted in 1996, the number of litter items decreased by ~20% in the industrial and residential areas but tripled in the commercial/residential area. The proportion of plastics in the litter stream was broadly similar to 1996. We extrapolate that some 60-570 t of plastic are released from Cape Town stormwater outlets annually, which is orders of magnitude less than predicted by global models. It is nevertheless clear that stormwater outlets are a significant source of litter into the sea. Intercepting this waste before it reaches the sea would greatly reduce litter loads in coastal waters around Cape Town.

Keywords: Plastic pollution; Solid waste management; Street litter; Urban run-off.