Microplastic contamination of riverbeds: Tracing the sources and tackling the problem

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Research by Professor Jamie Woodward and colleagues from The University of Manchester’s Department of Geography has shed vital light on the accumulation of microplastic particles on and in riverbeds. This occurs after the release of untreated wastewater into rivers with flow rates too low to disperse the contaminants downstream (dry weather spills). This research was the first to link the microplastic pollution of rivers to the sewage dumping scandal. An assessment of water quality in Britain’s rivers by the UK Government’s Environmental Audit Committee and a report by the All-Party Parliamentary Group (APPG) on Microplastics drew heavily on the research of Woodward’s team. Their recommendations include a UK-wide survey of microplastic river pollution using the research team’s riverbed sampling methodology.
Microplastic particles, in the form of beads, fragments, and fibres, and mostly smaller than 1mm, are released into watercourses across the UK during discharges of inadequately treated wastewater. Untreated wastewater from wastewater treatment works and combined sewer overflows (which release a mix of storm water and untreated human and industrial waste) should occur only during exceptional storms when urban runoff and water flow in rivers is high, diluting the wastewater and dispersing contaminants downstream. However, research by Professor Jamie Woodward and his colleagues at The University of Manchester, UK, shows that the accumulation of microplastics on riverbeds, which they attribute to ‘dry weather spills’, takes place when untreated wastewater is discharged into rivers when river flows are too low to disperse the contaminants downstream.

Riverbeds are biologically diverse habitats where many animals live, feed, and reproduce. As riverbed-dwelling animals feed, they ingest microplastics. These animals are then eaten by those further up the food chain, leading to bioaccumulation of microplastics within fish, birds, and mammals. The full extent of the impact of riverbed microplastic contamination on the health of freshwater aquatic ecosystems and the animals reliant on them is unclear; however, the presence of microplastic particles in the gastro-intestinal tract, and the release of pollutant chemicals into other parts of their bodies, could cause serious harm.

A typical microplastic assemblage including fibres, fragments, and microbeads from the bed of the River Tame in Greater Manchester. This assemblage came from a heavily contaminated reach (Site 8 on the map). Photo Credit: Image by Jiawei Li.

Microplastic contamination in UK rivers

In 2015, Dr Rachel Hurley, Professor Jamie Woodward, and Professor James Rothwell conducted a regional survey of microplastic contamination of riverbeds across the upper Mersey and Irwell catchments within England’s Greater Manchester area. The rivers sampled included the Irwell, Roch, Croal, Tonge, Irk, Medlock, Mersey, Tame, Etherow, and Goyt. The team sampled riverbed sediment at 40 reaches on ten rivers, including urban, semi-urban, and rural catchments. This survey detected microplastic contamination at 39 of the 40 sites, with five riverbed hotspots (located on the Roch, Tonge, Mersey, Tame, and Etherow) where microplastic concentrations exceeded 15,000 particles per kilogram (/kg) of fine-grained riverbed sediment.

Sampling at all 40 sites was repeated in 2016 after a winter of flooding. Microplastic contamination decreased at 28 sites – 18 by an order of magnitude – demonstrating effective catchment-wide flushing of microplastic contaminants from the riverbeds and their transport downstream. This work raised an important question. If flood events can cleanse riverbeds of microplastics, how do they become contaminated in the first place when untreated wastewater is only supposed to be released into rivers during period of exceptional rainfall when runoff is high and rivers are in spate?

“High concentrations of channel bed microplastics are one of the clearest indicators of poor wastewater management.”

In July 2019 a very large flood took place on the River Tame where the team knew that microplastic contamination was a particular problem. The channel-scouring impact of this flood allowed the team to design a field experiment to explore the post-flood build up of microplastics on the river bed and to examine the geography of contamination in the river catchment. After 34 days of drier weather when the Tame’s flow was low, and untreated wastewater discharge is prohibited by the Environment Agency, the team sampled 14 riverbed sites along a 31.6km reach of the Tame.

Jamie Woodward demonstrating sampling of riverbed microplastics to the Environmental Audit Committee of UK Parliament during the field visit to the Windrush Catchment in July 2021. The EAC Chair (Rt Hon. Philip Dunne) is holding the bulk sample container. Photo taken by Adrian Arbib.

The team undertook this extended sampling of the Tame to generate a more detailed picture of microplastic accumulation on riverbeds from the rural headwaters through to the urban reaches of the river during a sustained period of low water flows after a flooding event. The riverbed samples showed microplastic contamination ranging from 2,400 to 138,400 (average 29,600) particles/kg of fine bed sediment. Contamination was greater in the urbanised reaches, with three sites recording extreme contamination (more than 50,000 particles/kg of fine bed sediment). The sites with high contamination showed very different microplastic assemblages and this pointed to discrete inputs of untreated wastewater. Woodward et al concluded that the rapid build-up of microplastics in riverbeds after a flushing event is primarily due to dry weather spills of wastewater, and that the high concentration of channel bed microplastics is one of the clearest indicators of poor wastewater management.

Research informing political debate

The UK government’s Environmental Audit Committee (EAC) undertook an assessment of water quality in Britain’s rivers in 2021. Woodward submitted written evidence for consideration by the committee. Along with a summary of the main findings of The University of Manchester team’s research, Woodward made four recommendations to the committee:

1. Tighten the regulations around the discharge of untreated wastewater into rivers to reduce microplastic transfer to riverbeds and the oceans
2. Improve monitoring by including riverbed sampling
3. Increase regulation around plastic microbead use and disposal
4. Commission a UK-wide survey of microplastic pollution of riverbeds.

Woodward also gave a presentation and field demonstration to the EAC during a field visit to the River Windrush in Oxfordshire in July 2021 (see photo). Results from the research conducted by Woodward’s team was referred to extensively by the EAC in their report published in January 2022, during debates of the Environment Bill in the House of Lords, and in the first report of the All-Party Parliamentary Group (APPG) on Microplastics, published in September 2021.

Microplastic contamination of the river bed at 14 sites in the River Tame catchment in summer 2019. Note the variety of microplastic assemblages in the most heavily contaminated urban reaches. Woodward et al. (2021) Nature Sustainability www.nature.com/articles/s41893-021-00718-2

The Manchester team’s research findings highlighted in these policy discussions included: the much higher microplastic contamination of urban and suburban riverbeds versus rural headwaters; the major contribution of plastic microbeads in riverine contamination despite their ban in cosmetic products in the UK; the breach by water companies of the legal requirement for discharging untreated wastewater into rivers only after heavy rainfall; the direct link between dry weather spills and the high level of microplastic contamination in riverbeds; and the need for wider riverbed sampling to assess the quality of river habitats across the UK.

Research results informing legislation

In their report, the EAC made 43 recommendations covering six principal areas: monitoring of water quality; rivers fit to swim; agricultural pollution; sewage pollution; surface drainage and urban pollution; and river restoration. The research conducted by Woodward and his team directly informed two key recommendations:

We [the EAC] recommend that the Secretary of State for Environment, Food and Rural Affairs commission, in conjunction with the devolved administrations, [conduct] a UK-wide survey of emerging pollutants and microplastic pollution of river environments, including an assessment of their potential impact on aquatic ecology.

We recommend that the Environment Agency reclassify significant sewage spills from storm overflows into watercourses in dry weather as pollution incidents, irrespective of permit compliance.

“Investing in wastewater storage and treatment capacity and microplastic capture technology are priorities.”

In addition, the report by the APPG on microplastics identified two problems highlighted by Woodward’s team’s research: microplastics settling into riverbeds and remaining there for weeks or months, being ingested by aquatic creatures before being flushed downstream during flooding events, and the link between unlawful discharges of untreated wastewater by water companies and microplastic ‘hotspots’ in riverbeds.

The APPG report recommended monitoring of microplastics in sewage sludge as part of baseline monitoring and suggested the Environment Agency were given overall responsibility for monitoring microplastic release into the UK’s rivers and waterways. The APPG also stated that they supported the request by The University of Manchester team to lead a UK-wide survey of the microplastic burden on the UK’s riverbeds, using the riverbed sampling methodology they outlined. The parliamentary group went on to call for water companies and water authorities to ensure wastewater from treatment plants is thoroughly treated as it is ‘currently the best available means to reduce microfibre plastic release into UK rivers and our seas’. They also highlighted the continued problem of microbead use during industrial processes as demonstrated by the high concentrations in riverbeds observed by Woodward’s team.

Jamie Woodward, Rachel Hurley, and James Rothwell presenting their findings at the BlueDot Festival in 2018.

Microplastic legislation in practice

Several countries have banned microbead use in cosmetics and personal care products to help curb the flow of microplastics into waterways, including the UK, the USA, Canada, Sweden, France, The Netherlands, Australia, and New Zealand. In addition, the APPG recommended that all washing machines are fitted with microfibre filters.

Woodward stressed that tightening effluent discharge regulations and investing in wastewater storage and treatment capacity and microplastic capture technology, are priorities if microplastic contamination of rivers and oceans is to be curtailed. The disposal of sewage sludge laced with microplastics and other contaminants is also a key challenge.

In the UK, water companies are currently under investigation by Ofwat and the Environment Agency. The EAC said ‘we welcome the investigation recently launched […] into permit compliance at sewage treatment works, following evidence presented to us indicating that the number of permit breaches may be far higher than disclosed by water companies under self-reporting arrangements.’

What are the next steps for your team in terms of research into microplastic pollution of aquatic environments?
We are developing our river-based research in several areas including the use of channel bed microplastic assemblages as part of river habitat assessment and to identify reaches where wastewater management falls below acceptable standards.

 

References

DOI
10.26904/RF-142-2957172723

Research Objectives

Professor Jamie Woodward studies microplastic contamination in UK rivers to better understand the geography of contamination, how microplastics enter river channels, where they are stored, and where they pose the greatest ecological threat. A short animation summarising this research can be found here: www.manchester.ac.uk/discover/news/animation-highlights-importance-of-microplastics-research/

Funding

The University of Manchester

Collaborators

  • Dr Jiawei Li
  • Professor James Rothwell
  • Dr Rachel Hurley

Bio

Jamie Woodward is Professor of Physical Geography at The University of Manchester. His research explores rivers, the materials they transport, and how humans interact with river catchments – now and in the past. He is a Fellow of the British Society for Geomorphology and the Royal Geographical Society.

Jamie Woodward

Contact

E: jamie.woodward@manchester.ac.uk
Twitter: @Jamie_Woodward

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