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Acute riverine microplastic contamination due to avoidable releases of untreated wastewater

Nature Sustainability volume 4, pages 793–802 (2021)Cite this article

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Abstract

Wastewater discharge to rivers is a controversial practice that compromises water quality, aquatic habitats and human health worldwide. Here we show how untreated wastewater laced with microplastics and raw sewage is routinely discharged into UK river flows that are too low to disperse the microplastics downstream. These ‘dry weather’ spills lead to acute microplastic contamination of river bed habitats. Many aquatic fauna feed in the benthic zone, the quality of which affects the entire riverine ecosystem. All microplastic types accumulate to high concentrations on the channel bed until flushed downstream by floods. These findings pose fundamental questions about the sustainable management of urban wastewater. Treating the wastewater would shut down the major source of microplastic fragments and microbeads in such rivers and prevent their transport to the ocean. Riverine microplastic transport is primarily partitioned between: (1) continuous transport at low concentrations of synthetic fibres from treated wastewater effluent; and (2) episodic flood-driven transport of the full microplastic assemblage entrained from contaminated channel beds. Focusing only on the buoyant non-flood microplastic load can produce highly unrepresentative assessments of riverine microplastic contamination. Climate warming and urban population growth will intensify the microplastic burden on many river ecosystems as summer baseflows decline and wastewater fluxes increase.

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Fig. 1: Sampling periods and the flow record of the River Tame from July to December 2019.
Fig. 2: Microplastic concentrations and assemblages on the channel bed of the River Tame.
Fig. 3: Microplastic concentrations and assemblages in three environments at the same sites.
Fig. 4: CSO outfall and channel bed microplastic contamination.
Fig. 5: Discharge of raw sewage and untreated wastewater into rivers and coastal waters across England in 2020.

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Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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Acknowledgements

We thank N. Scarle from the Cartographic Unit in the School of Environment, Education and Development at The University of Manchester, who drew all the diagrams. We also thank colleagues in the Department of Geography laboratories at The University of Manchester for technical support. K. Murnane assisted with the analysis of the effluent samples. We are grateful to the Environment Agency for providing river flow data for the River Tame.

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Authors and Affiliations

  1. Department of Geography, The University of Manchester, Manchester, UK

    Jamie Woodward, Jiawei Li, James Rothwell & Rachel Hurley

  2. Norwegian Institute for Water Research, Oslo, Norway

    Rachel Hurley

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  1. Jamie Woodward
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Contributions

J.W. designed the study and wrote the paper. J.W., J.R. and J.L. carried out the field sampling. J.L. carried out the laboratory work and quantified the microplastics. R.H. carried out the FTIR analysis. All authors discussed the results and commented on the manuscript.

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Correspondence to Jamie Woodward.

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The authors declare no competing interests.

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Peer review information Nature Sustainability thanks Caroline Gauchotte-Lindsay and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Microplastic contamination and microplastic assemblages at 40 sites on river channel beds around Manchester.

Note the catchment-wide decreases in microplastic storage on channel beds following the winter flooding of 2015/16. These catchments drain an area of about 1500 km2. Samples were collected from the 10 rivers in spring and early summer under low flow conditions2. Microplastic concentrations for the Denton hotspot on the River Tame actually increased between sampling periods from 48,300 to 72,400 microplastic particles per kg of fine bed sediment (FBS). While the pre-flooding assemblage at Denton was dominated by microbeads and fragments in roughly equal quantities, the acutely contaminated post-flooding bed assemblage was 98% microbeads. This shift in composition demonstrated that channel bed flushing did take place in this reach, but also that it was quickly followed by an influx of microbead-rich wastewater from a local point source.

Extended Data Fig. 2 Industrial sites in the River Tame catchment that form potential sources of microplastics.

Note that this map is not exhaustive. The urban area is shown in grey. Industrial premises are concentrated in the river corridor between Stalybridge and Denton. Domestic wastewater is also an important source of microplastics. Wastewater from both industrial and domestic sources is processed in wastewater treatment plants but some industrial premises have consents to discharge directly to the urban drainage system or the river itself. Synthetic microbeads are used in a range of processes including blast cleaning and shot blasting. The 14 sample sites along the main river from 2019 are also shown along with the Denton Hotspot (DH) between Sites 6 and 7 that was sampled in 2015 and 2016. Base map redrawn from https://nrfa.ceh.ac.uk/data/station/info/69027.html.

Extended Data Fig. 3 Microplastic hotspots and the location of combined sewer overflows (CSOs) and wastewater treatments works (WwTW) along the main channel of the Tame.

CSOs are clustered in the main built up areas. CSO location data from United Utilities and our mapping. Hotspots are sites with a total microplastic concentration >15,000 particles per kg of fine bed sediment. The most heavily contaminated reaches (DH, 8, 9, 10, and 11) are located immediately downstream of wastewater treatment works or clusters of CSOs or both. The 2016 data are shown for the Denton hotspot (DH). In our 2015 survey of the wider region we identified 5 urban contamination hotspots in 10 rivers (Extended Data Fig. 1). Applying this threshold in the River Tame now produces 7 hotspots at sites 5, 6, 8, 9, 10, 11 along with the Denton reach from 2015/2016 (Fig. 2). It is very likely that more detailed sampling in the urban reaches (for example the cluster of CSOs between Sites 10 and 11) will identify further contamination hotspots. Base map redrawn from https://nrfa.ceh.ac.uk/data/station/info/69027.html.

Source data

Extended Data Fig. 4 An effluent release into low flows observed on the River Tame at Site 9.

a, This spill took place early (09:30) on Sunday 22 September 2019 with the river at low flow. b, The river water is heavily discoloured (milky silver grey) and the bed obscured even though water depth in the cylinder is <20 cm. The river water gave off a sweet pungent odour. This site is 150 metres downstream of the outlet from Ashton Wastewater Treatment Works (Fig. 2). The reaches immediately upstream of the WwTW were clear at this time. c, The channel bed of the Tame under normal low flow conditions. Site 9 yielded very high concentrations of microplastics on the channel bed and the highest concentrations of fibres in the bulk water sample (Fig. 3 and Supplementary Table 2). The composition of water sample 9 suggests that some fibres are retained in suspension even at low flow, but the bulk of the microplastic load is deposited on the channel bed in the reaches immediately downstream of their point of entry.

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Woodward, J., Li, J., Rothwell, J. et al. Acute riverine microplastic contamination due to avoidable releases of untreated wastewater. Nat Sustain 4, 793–802 (2021). https://doi.org/10.1038/s41893-021-00718-2

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