Microplastic contamination of river beds significantly reduced by catchment-wide flooding

Rachel Hurley*†, Jamie Woodward*†, James J. Rothwell†

†Department of Geography, The University of Manchester, Manchester, M13 9PL, UK

Microplastic contamination of the oceans is one of the world’s most pressing environmental concerns. The terrestrial component of the global microplastic budget is not well understood because sources, stores and fluxes are poorly quantified. We report catchment-wide patterns of microplastic contamination, classified by type, size, and density, in channel bed sediments at 40 sites across urban, suburban and rural river catchments in northwest England. Microplastic contamination was pervasive on all river channel beds. We found multiple urban contamination hotspots with a maximum microplastic concentration of approximately 517,000 particles m-2. After a period of severe flooding in winter 2015/16 all sites were resampled. Microplastic concentrations had fallen at 28 sites and 18 saw a decrease of one order of magnitude. The flooding exported ~70% of the microplastic load stored on these river beds (equivalent to 0.85 ± 0.27 tonnes or 43 ± 14 billion particles) and eradicated microbead contamination at 7 sites. We conclude that microplastic contamination is efficiently flushed from river catchments during flooding.

All of the world’s oceans are contaminated with microplastics1,2. Recent estimates suggest

there are 4.85 trillion microplastic (<5 mm) particles in the global ocean2. Microplastics can

be ingested by marine organisms and have the potential to bioaccumulate and threaten

ecosystem health3. They are also pollutant vectors – once ingested they can transfer harmful

organic chemicals into the food chain4–6. Given their pervasive and persistent nature7,

microplastics have become a global environmental concern and a potential risk to human

populations8. Quantification of the various sources of plastic debris in the oceans is not

straightforward. However, it has been estimated that fluxes from the terrestrial environment

contribute between 64 and 90% of the plastic debris in the oceans9-11. Reconnaissance studies

have shown elevated concentrations of microplastics in river catchments12-22, but there has

been no systematic empirical assessment of patterns of microplastic contamination or the

fluvial processes governing their distribution. There is therefore a pressing need to quantify

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the sources, stores, and fluxes of microplastics in river catchments to better understand their

behaviour and the transfer of microplastics to the marine environment.

Fine-grained fluvial bed sediments and the sediment-water interface are zones of intense

biological activity23 providing important habitats and food sources for a range of organisms.

Even low density plastic particles such as polyethylene and polypropylene, that normally

float in freshwater, can become incorporated into the channel bed as a result of biofouling24

or other processes25. If the bed sediment matrix is contaminated with microplastics it poses a

significant threat to the health of the entire riverine ecosystem26. Studies of coastal and large

lake environments close to river outlets have pointed to densely-populated urban

environments as important sources of microplastics27. Whilst fluvial flooding must play a key

role in microplastic transfer, existing work on the role of floods has focused solely on coastal

locations beyond the river catchment28-31. There is therefore a clear need to better understand

the fundamental processes associated with microplastic storage and transfer during fluvial

flooding.

Here we quantify microplastic contamination within the fine-grained bed sediments of 10

rivers in northwest England that drain to the Irish Sea (Supplementary Fig. 1). These rivers

are single thread meandering channels with sandy gravel beds and cohesive banks typical of

many throughout the UK. They form part of the upper Mersey (734 km2) and Irwell (793

km2) catchments in the wider Manchester conurbation – the second most populous (2.55

million people) urban area in the UK. We sampled the top 100 mm of bed sediment matrix at

40 sites across these catchments under low flow conditions. Microplastic concentrations were

determined following a density extraction procedure that included the first quantification of

seawater-buoyant microplastics in freshwater sediment. Fourier Transform Infrared (FT-IR)

spectroscopy was used to verify the microplastic identification procedure (Methods). The

detailed particle size characteristics of the microplastics were also determined. We then

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calculated channel bed sediment microplastic storage across the entire river network by up-

scaling microplastic concentrations in all reaches as a function of total sediment storage and

channel bed area. To quantify the impact of flood-related processes upon the mobilisation and

transfer of bed sediment microplastics, we resampled the same 40 sites following a major

period of flooding in winter 2015/16 that extended from early November to early March. This

included the 26th December (Boxing Day) flood – the largest so far recorded in the Irwell

catchment.

Global- and catchment-scale patterns of microplastic contamination

By compiling the first global microplastic map and ranking for marine and freshwater

environments (Fig. 1 and Supplementary Fig. 2), we show that the concentration of

microplastics in river bed sediments of the Mersey catchment is higher than in any other

environment – either deposited or in suspension – so far reported. The most contaminated site

(~517,000 particles m-2, Supplementary Table 1) is on the River Tame (146 km2), a few

kilometres upstream of its confluence with the Etherow, which is 50% higher than the

maximum value for the next ranked site of the Incheon-Kyeonggi beach sediments in South

Korea (Fig. 1). The maximum value for the Irwell, on the River Tonge (96.5 km2), is 191,000

particles m-2. The River Irwell catchment ranks fourth globally for all environments (and

second for fluvial systems, Supplementary Fig. 2) with a mean microplastic concentration

(across 26 sites) of 16,000 particles m-2. The top nine ranked hotspots in our global dataset

(Fig. 1) include rivers and beaches within some of the world's most populous urban

environments including Seoul, Hong Kong, and the southern Chinese province of

Guangdong. Fig. 1 also highlights the paucity of microplastic data for rivers in the Global

South.

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Microplastic contamination is pervasive across the river channel beds of the Mersey and

Irwell fluvial network. As Fig. 2a shows, before the winter 2015/16 flooding, microplastics

were detected in 39 out of 40 sites with a very wide range of observed concentrations. Even

rural headwater reaches and first order streams are contaminated with microplastics (300–

4800 particles kg-1). Five distinctive contamination hotspots (mean = 34,800 particles kg-1)

were identified where microplastic concentrations in fine-grained bed sediments exceeded

15,000 particles kg-1 (Fig. 2a and Supplementary Table 2). We have also examined how

microplastic particles in these rivers vary by type and density (Fig. 2c and Fig. 3a,c). Across

the river network there is considerable variability in the concentration of microbeads,

microfibres and microplastic fragments deposited on channel beds (Fig. 3a). In most cases

river reaches are characterised by multiple microplastic types. In the lower Irwell, for

example, one site is dominated by microbeads; whilst immediately downstream the

microplastics in the bed sediments are almost entirely made up of microplastic fragments.

This is followed by a return to microbead dominance at the next site in Manchester city

centre. This downstream variability marks the transition of the lower Irwell from a reach

under the immediate influence of effluent from sewage treatment through a suburban area

and finally to a highly urbanised reach with an abrupt increase in the density of combined

sewer overflows (CSOs). This example illustrates the complex microplastic source variability

encountered in the urban and sub-urban fluvial environment. We have recently documented

the extent of microplastic contamination further downstream in the Salford Quays basins

(where the lower Irwell becomes the Manchester Ship Canal, Fig. 2) and demonstrated that

tubifex worms ingest microplastic fibres and fragments from fine-grained bottom sediments26.

Part of the observed spatial variability in these urban and sub-urban catchments is controlled

by microplastic density. Some microbeads, for example, are high density (1.2 to 1.8 g cm-3)

whilst others we have observed are seawater-buoyant (<1.025 g cm-3) (Fig. 3c). Since the

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ultimate sink for the bulk of the microplastic load in the Mersey and Irwell catchments is the

Irish Sea, it is significant that more than one third (38%) of the total mass of microplastics

stored on these channel beds is seawater-buoyant (Fig. 3c).

Flood-driven microplastic flushing and reorganisation

The mean daily discharge for the River Irwell at the Adelphi Weir gauging station from April

2015 to July 2016 is shown in Fig. 2e. The post-flooding microplastic dataset reveals a

striking pattern of microplastic evacuation from the study catchments (Fig. 2a,b) with the

mean microplastic concentration across the 40 sites falling from 6350 to 2812 particles kg-1

(Supplementary Table 1). Of the 40 sites across the 10 rivers, 28 recorded significant

decreases in bed sediment microplastic concentrations and 18 of these fell by one order of

magnitude (Supplementary Table 2). The Supplementary Information gives full details of the

uncertainty analysis we conducted to establish the significance of these changes. Microplastic

contamination of the River Irwell channel beds was significantly reduced with a mean

decrease of 64% and both urban contamination hotspots (Tonge and Roch) extinguished.

Concentrations within the Mersey were also much reduced with a mean decrease of 81%.

Interestingly, total microplastic concentrations at the River Tame contamination hotspot

actually increased by 50% after the winter flooding and show a ~400% increase in microbead

concentration from 14,000 to 70,600 particles kg-1. (Fig. 2 and Supplementary Table 2).

These data show how hotspots of severe microplastic and microbead contamination can form

very rapidly in urban rivers. The microplastic contamination observed in this reach is

mirrored by elevated levels of heavy metals32 that have been linked by the UK Environment

Agency to significant inputs of industrial effluent. At the other 39 sites, post-flooding

microplastic concentrations were all below 3500 with five sites as low as 200 particles kg -1

(Supplementary Tables 1 and 2).

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Alongside the processes of flood-driven channel bed scour, hydraulic sorting, and

microplastic flushing, we also see a distinctive reorganisation of microplastic types between

the two sampling periods (Fig. 3a,b). Across the Irwell and upper Mersey catchments, there

was a ~40% post-flooding reduction (from 57–19%) in microplastic fragments and a ~45%

increase (from 33–77%) in microbeads (Fig. 3a,b). Even though we observed an increase in

the proportion of microbead contamination at most sites, 21 sites show a significant decrease

in absolute microbead concentrations following the winter flooding (Fig. 3a,b and

Supplementary Table 2). We estimate that across both catchments approximately 7 3.5

billion microbeads were lost from channel bed storage during the winter flooding

(Supplementary Table 3 and 4). This equates to about 1 million applications of facial

exfoliant. Our data show a complete cleansing of microbeads from 7 sites that were

previously contaminated – these include reaches in the headwater catchments of the River

Goyt and upper Irwell (Fig. 2d and Supplementary Table 2). At the same time, it is important

to recognise that a phase of lower magnitude flooding in the Mersey still resulted in

catchment-wide reductions in total microplastic concentrations, as well as total microbead

cleansing in 4 reaches (Supplementary Table 2). Thus, the flushing process associated with

channel bed scour and microplastic particle sorting operates over a range of flood discharges.

Fig. 4 presents size data (long axis) for individual microplastic particles (n=487) from the

Irwell and Mersey sample sets (see Methods). The pre- and post-flooding particle size

distributions are very similar with a D50 of approximately 290 µm in each case. Both

distributions are dominated by microbeads and fragments in the size range 100 to 500 µm.

The particles with the greatest long axes are dominated by fibres and fragments. Interestingly,

the flood-related spatial reorganisation of microplastic types has not significantly modified

the size distribution of microbeads or fragments. Whilst we have not observed clear evidence

for microplastic particle fragmentation over this time period, the associated processes require

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more detailed research. Many of the larger microplastics will be transported as discrete

particles, but a component of the microplastic load will probably be incorporated into

composite sediment particles33.

Catchment yields and microplastic export to the oceans

In a recent review34, it was suggest that the vast majority of microplastics in the oceans

originate from the breakdown of larger plastic items in the marine environment. The validity

of this assertion can only be established via robust empirical assessment of the microplastic

flux from river catchments. Our data show that flood events efficiently flush significant

quantities of microplastics from the continents. During the winter flooding of 2015/16, we

estimate that 0.85 ± 0.27 tonnes of microplastics (equivalent to 43 ± 14 billion particles) were

evacuated from channel bed storage in the upper Mersey and Irwell (Fig 5 and

Supplementary Table 3). In terms of microplastic yield from channel beds over the 2015/16

hydrological year, this equates to 0.87 ± 0.28 kg km-2 for the Irwell (793 km2) and 0.22 ± 0.07

kg km-2 for the Mersey (734 km2) or 0.56 ± 0.18 kg km-2 for the combined catchments (1527

km2). The yield for the Irwell is approximately 4 times that of the Mersey over this period

because of higher magnitude winter flooding and more effective flushing of channel beds

across the drainage network.

Whilst these data represent just one component of the total fluvial microplastic flux from

these catchments, it is important to recognise that Fig. 5 shows the change in channel bed

storage over what is effectively the entire 2015/16 hydrological year (Fig. 2e). Our

identification of significant flood-driven microplastic export from river catchments receives

support from the emerging literature on microplastic deposition on beaches and estuarine

environments at catchment outlets in South Korea28 and India29. It is very likely these

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processes apply to river systems more generally at a range of scales and under different

climate regimes. We estimate that, during the 2015/16 winter flooding, 0.36 ± 0.12 tonnes

(equivalent to approximately 17 ± 5.6 billion particles) of seawater-buoyant microplastics

were removed from channel bed storage across the upper Mersey and Irwell catchments (Fig.

5 and Supplementary Table 4). According to recently-published estimates2, this number of

particles equates to approximately 0.5% of the total floating microplastic burden in the global

ocean (4.85 trillion particles2). This value exceeds 1% if we use our total microplastic particle

flux. Whilst a proportion of these fluxes will eventually be transferred to the marine sediment

store, given the relatively small size of our study catchments, these flux data suggest that the

total microplastic load in the world’s oceans must be far higher than previously estimated. It

is clear that further work is needed to identify the quantities and mechanisms of microplastic

transfer from river catchments and the fate of such particles in marine systems, to better

understand the global geography of microplastic contamination.

Managing microplastics in the Anthropocene

It is now very clear that microplastic contamination of both the fluvial and marine realms

represents a global grand challenge with profound implications across many areas of science

and policy. A key part of tackling the global microplastic problem is effective regulation to

ensure that, in all parts of the world, the multiple sources of microplastics in river catchments

are brought under control. However, this must also be underpinned by an understanding of

fundamental processes, robust monitoring protocols, and the establishment of maximum

acceptable microplastic concentrations – with effective sampling across drainage networks.

This will help establish the full extent of the problem and trajectories of recovery following

microplastic source management. The fluvial flushing process reported here is especially

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significant in light of recent restrictions on microbead use, with legislation passed in the USA

in 201535 and UK in 2018, and with proposals pending in Canada, Australia, and across the

European Union36. With appropriate management strategies that control microbead sources in

river catchments, our findings indicate that this form of riverine microplastic contamination

can be effectively and rapidly reduced.

In light of recent suggestions that microplastics may be useful in a formal definition of the

Anthropocene37,38, it is important to fully understand their role in the global sediment system.

In the period following the Second World War, during what has been called the ‘Great

Acceleration’, the Mersey and Irwell rivers were some of the most heavily polluted

catchments in Europe. Remarkable improvements in water quality and ecological status have

taken place in the past three decades. Throughout the post-war period, however, as in other

river systems around the world, microplastic contamination of river systems has passed under

the monitoring and regulatory radars. Whilst our findings undoubtedly highlight a significant

new challenge for river catchment management in all parts of the world, we also show that

fluvial processes have the capacity to rapidly cleanse river channel beds of microplastic

contamination.

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Acknowledgements

We thank colleagues in the laboratories in the Department of Geography and the School of

Earth and Environmental Sciences at The University of Manchester for help with a range of

analyses. R.H. was in receipt of a University of Manchester President’s Doctoral Scholar

Award which supported this research. We thank Nick Scarle for his assistance with the

figures and the Environment Agency for discharge data.

Author contributions

J.W. initiated the microplastics project. R.H., J.W. and J.J.R conceived and designed the

study. R.H., J.W. and J.J.R conducted the field sampling. R.H. performed all analyses. All

authors contributed to interpretation of the data and writing of the manuscript.

Competing financial interests

The authors declare no competing financial interests.

Additional information

Supplementary information is available in the online version of the paper. Correspondence

and requests for materials should be addressed to R.H. (rachel.hurley@manchester.ac.uk) and

J.W. (jamie.woodward@manchester.ac.uk).

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Figure Captions

Figure 1: Global microplastic (<5 mm) concentrations reported by aquatic and

sedimentary environment. Microplastics mapped as maximum (a) and ranked by average

concentration (b). Alternative units and all sample sizes are given in Supplementary Fig. 2

where all the sources are also listed. Note that in the absence of standardised reporting of

microplastic data, this compilation includes data from studies employing a number of

methods. Concentration here refers to both actual and surface concentration.

Figure 2: Concentration of total microplastics and microbeads in channel bed

sediments. Pre- and post-flooding concentrations of total microplastics (a,b) and microbeads

(c,d) at all 40 bed sediment sampling sites in the Irwell and Mersey catchments. The mean

daily discharge record is for the River Irwell at Adelphi Weir (e). Note that this does not

include the Irk and Medlock catchments.

Figure 3: Microplastic concentrations by type and density. Concentrations are shown for

pre- (a,c) and post-flooding (b,d) samples. Note that figure legends refer to both left and right

panels. Summary charts represent mean values for all 40 sites.

Figure 4: Particle size characteristics of microplastics from the pre- (a) and post-

flooding (b) sample sets. Each bar shows the contribution by microplastic type for each size

class. Particles were measured along their long axis. Note that for microbeads all the axes are

the same length.

Figure 5: Change in microplastic load stored on channel beds across the Irwell and

Mersey catchments. The dashed bar is the contribution of the River Tame contamination

hotspot (Fig. 2) for each of the three microplastic parameters. The error bars in each case are

based on the estimates of uncertainty detailed in Supplementary Table 5.

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Methods

Site locations

This study focuses on the upper Mersey (734 km2) and Irwell (793 km2) catchments within

the Greater Manchester region of northwest England. Manchester is a large urban area with a

population of 2.55 million across the metropolitan borough. We have sampled 10 rivers

within this area: the Irwell, Roch, Croal, Tonge, Irk, Medlock, Mersey, Tame, Etherow and

Goyt, in addition to smaller tributary reaches and headwaters (Supplementary Fig. 1). The

drainage network and the extent of the urban area are also shown in Supplementary Fig. 1.

We sampled in 40 reaches across the Mersey and Irwell catchments that were judged to be

representative of these rivers. Sample sites included rural, sub-urban, and urban reaches from

stream orders 1 to 7. Full sample site information and ambient flow conditions are provided

in the Supplementary Data File. The first phase of bed sediment sampling was undertaken

between April and July 2015 (Fig. 2e).

On Boxing Day (26th December) 2015 a large storm affected the northwest of England

leading to widespread flooding. Many river levels reached record heights with 37 of 44 river

gauges in the upper Irwell catchment exhibiting the highest levels since records began more

than 60 years ago39. Peak discharge was over 560 m3 s-1 in the lower Irwell (specific flow:

0.71 m3 s-1 km2). This flooding took place at the end of the 7th wettest year on record40.

Whilst flooding was more intense in the Irwell catchment, there was also widespread – but

much more typical flooding – in the upper Mersey catchment. Peak discharge on the upper

Mersey on the 26th December 2015 was 133 m3 s-1 (specific flow: 0.18 m3 s-1 km2) and this

formed part of a longer phase of flooding in winter 2015/16. Repeat sampling at all 40 sites

was carried out under low flow conditions between May and mid-July 2016 (Supplementary

Data File).

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Bed sediment sampling

Fine-grained river bed sediment samples were collected using the cylinder resuspension

technique reported by Lambert and Walling41. At each site sediments were sampled at 4

locations across the channel bed and combined into a single sample to account for any intra-

site variability. By isolating a known area of river bed, the use of this cylindrical sampling

apparatus permits estimation of fine-grained sediment storage at each site using equation

(1)42:

BSs=C s W v

A

(1)

where bed sediment storage (BSs), reported as g m-2, is calculated as a function of the

sediment concentration associated with the container (Cs, g l-1) and the volume of water

enclosed in the cylinder (Wv, l), divided by the surface area isolated during sampling (A, 0.14

m2).

Samples were collected by inserting a large cylinder (height 690 mm, diameter 420 mm) into

the bed sediment matrix to a depth of 100 mm in order to sample the biologically active zone.

Sediments within the cylinder were agitated into suspension for 10 seconds using a trowel at

0, 2, and 4 minutes. Turbid water samples were decanted into a clean 25 litre polyethylene

container using 1 litre jugs. Samples were transported to the laboratory and stored at 4ºC.

Following settling, the sediments were wet sieved through a 63 µm mesh to separate the

silt/clay and sand fractions. The sand fraction used here was oven dried at 40ºC. The

uncertainty associated with the Lambert and Walling method has been systematically

evaluated by ref43 across a range of fluvial environments in the UK. Based on data collected

from 12 rivers (n=288 samples), they showed the method performs equally well across a wide

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range of bed substrate conditions. They concluded that this method yields reliable reach-scale

estimates of the extent of deposited fine sediment.

Microplastic identification and quantification

Microplastic definitions are largely concerned with the size classes considered. A number of

classes have been suggested, where <5 mm is now mostly commonly applied. Here we use

the 5 mm boundary classification, although the majority of particles observed were <1mm.

A density separation technique was utilised to isolate microplastic particles for quantification.

Three density solutions were used: 1.025 g cm-3, 1.2 g cm-3 and 1.8 g cm-3. Two extractions

were performed for each solution to ensure complete separation of microplastics from

sediment. In line with previous studies24, a saturated NaCl solution (1.2 g cm-3) was used.

However, this fails to isolate polymers with densities greater than 1.2 g cm -3, which can

account for up to 17% of demand for plastic products44. In order to isolate higher density

microplastic particles, we used a 1.8 g cm-3 NaI solution. An additional extraction was

employed at seawater density (1.025 g cm-3 NaCl) to estimate the proportion of microplastics

that may become buoyant upon entry into to saline waters in downstream estuarine or marine

environments.

For all extractions, only sediments >63 µm were analysed since smaller particles cannot be

reliably identified as plastic material through visual inspection24. 10 g of sediment was placed

into 50 ml falcon tubes, which were topped up with 1.025 g cm-3 NaCl solution. Samples

were shaken vigorously for 3 minutes and allowed to settle for 4 hours or until the solution

became clear. Lower density microplastic particles float to the surface of this solution and

can be decanted. The solution was filtered through Whatman 1 filter papers. A second extract

at 1.025 g cm-3 density was applied. The falcon tubes were thoroughly rinsed with the extract

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solution. The process was then repeated sequentially using the denser extract solutions. Filter

papers were transferred to petri dishes and oven dried at 50ºC.

During all stages of sample processing and extraction, aluminum foil was used to cover all

apparatus and samples to prevent contamination. Triplicate vessel, solution and procedural

blanks were created during each round of microplastic analysis to identify any sources of

contamination. A small quantity of particles and fibres (such as cotton fibres) were reported

in the blanks, but none responded to the hot needle test. No microplastic particles were

identified in the blanks at any stage.

Peroxide digests were considered as a means of removing organic material from samples to

reduce obscuration of microplastic particles and the misidentification of natural materials as

polymers. However, initial tests to examine the effects of such digests led to the bleaching, as

opposed to complete removal, of organic material, in some cases concealing organic

structures and producing greater amounts of organic materials that resembled microplastic

fragments. The efficacy of peroxide digestion has been questioned elsewhere45 and these

peroxide digests were not performed on the microplastic samples. Alternatively, a series of

steps were followed to definitively identify and then quantify microplastic particles.

Dried filter papers were traversed systematically using a Zeiss Axio Zoom.V16 stereo

microscope at 20–50x magnification. Microplastic particles were visually identified using the

following criteria:

No visible organic structures24. In cases where the lack of structure was not clear,

particles were transferred to a more powerful light microscope for further

examination

They reacted to the hot needle test46.

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They maintained their structural integrity when touched or moved.

Additional features such as ‘unnatural’ dimensions (i.e. perfectly spherical shape) or colour

(i.e. blue or bright pink) can be used to characterise materials of likely artificial origin, but

should not be solely relied upon to classify particles as microplastics. Only particles which

unanimously passed this visual identification procedure were counted as microplastic and

included in the total microplastic concentrations. Verification of polymer composition was

performed on a sub-sample of the microplastic particles using a PerkinElmer Spotlight 400

Fourier Transform Infrared (FT-IR) spectrometer equipped with a diamond attenuated total

reflectance (ATR) accessory. The spectrum range was set at 4000 to 650 cm -1, with a

resolution of 4 cm-1. The microplastic particles analysed using FT-IR were randomly selected

from across the 40 sample sites in order to incorporate spatial variability. Particles were

selected from across the spectrum of particle characteristics – including density, type, and

size class – for pre- (n=50) and post-flood (n=50) sediment samples. In total, 16 co-scans

were obtained for each particle and the resultant spectra were compared to reference data

from a standard ATR Polymers library. One hundred particles were analysed and all were

verified to be polymer.

Once identified as microplastic, particles were visually characterised as fragments, fibres,

microbeads or ‘other’ and the size, colour and extent of visible biofouling or flocculation into

composite particles33 was also noted. In almost all cases, ‘other’ refers to pieces of glitter

coated in clear plastic and a smaller number of plastic particles that could not be reliably

identified as either specifically engineered or fragmented.

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Particles were carefully extracted from the filter paper using fine tweezers and placed into a

pre-weighed pot. Once examined in full, samples were then weighed for each density

extraction, i.e. three sub-samples per site (the duplicate extracts for each density solution

were aggregated into a single sample at this stage). The counts and weights obtained for the

samples facilitated the quantification of microplastics in a number of ways. In order to permit

comparison between studies, the microplastic concentrations are expressed in several units as

employed in other published studies (Supplementary Table 1). For samples where no

microplastic particles were identified, two further 10 g samples of bed sediment were

analysed to verify the absence of microplastic particles. Only one site in the pre-flood sample

set was found to be free of microplastic contamination. For the low contamination sites, two

further samples were analysed and the results of the three analyses were then averaged.

The size characteristics of microplastic particles were also analysed. A large subsample of

particles from pre- (n = 266) and post-flood (n = 221) samples was selected. Measurement

was achieved using a Zeiss Axiocam 105 colour camera attached to a stereomicroscope.

Dimensions were measured on Zen 2 imaging software following graticule calibration.

Because of the size characteristics of microplastic particles such as fibres, the long axis was

measured to facilitate meaningful comparisons across microplastic types. Note that for

microbeads (which are spherical) all the axes are the same length.

Uncertainty analysis

In order to quantify the uncertainty associated with the microplastic concentrations and flux

estimates, a full uncertainty analysis was performed. A further 12 channel bed sediment

samples were collected from a single river reach at the Urmston site on the Mersey

(Supplementary Fig. 1). A grid of 12 sampling locations (4 m x 3 m) within the reach was

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marked out and bed sediments were collected following the procedure detailed above. The

sediments were processed and analysed for microplastics. This exercise allowed us to

establish uncertainty estimates for both the sediment storage and microplastic analyses in this

study. This included the uncertainty associated with each microplastic type and density. The

full results of the uncertainty analysis are provided in Supplementary Table 5. Uncertainties

associated with each microplastic type allowed us to establish the significance of any

differences between the pre- and post-flood datasets (Supplementary Table 2). Furthermore,

the full assessment of sediment storage uncertainty at Urmston facilitated estimation of the

uncertainty associated with both reach- and catchment-scale microplastic flux calculations

(Supplementary Tables 3 and 4).

Spatial quantification and estimation of the impact of flooding

Reach-scale estimates of bed sediment microplastic storage were obtained from calculations

derived for each site, scaled up to representative river reaches (Eqn. 2):

RSMicroplastic=Ac × BSS × MPc

k

(2)

where reach-scale microplastic storage (RSMicroplastic, mg) is calculated as a function of channel

area (Ac, m2), bed sediment storage (BSs, kg m-2), and microplastic concentration (MPc, mg kg-

1). The result can be converted using the dimensionless conversion factor k to produce output

in kg or tonnes. For microplastic types (such as microbeads), quantification by mass was not

possible due to the weighing of bulk samples consisting of multiple particle types. Hence,

these calculations use microplastic concentrations in particles kg-1 of sediment and produce

an output in number of particles. The calculations were performed for each defined river

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reach and combined to produce totals for the entire upper Mersey and Irwell drainage

network.

Channel area was obtained using Ordnance Survey Mastermap Water Network data and field

calculations applied in QGIS 2.18.0. All artificial waterways (such as canals) were removed

from the dataset. Additionally, channels above reservoirs were excluded from areal

calculations as the influence of reservoirs on the transport of fluvial microplastics is not yet

sufficiently understood.

Several methods were considered to characterise reach-scale contamination. UKTAG-defined

water bodies47 utilised by the UK Environment Agency (EA) in the context of the Water

Framework Directive (WFD) were deemed to be of insufficient resolution. Three alternative

approaches were evaluated.

Method 1

The first method utilised an average of microplastic concentrations for reaches between sites.

When confluences were included in a defined reach, an average was taken of the three sites

involved. The last sites on each reach were applied in downstream extrapolation and the first

sites on each reach were used in upstream extrapolation. This approach is most commonly

applied in studies of sediment storage change48,49 which assume that reach-scale variability

can be accounted for by using the average of two monitoring sites. However, it was found

that this approach exaggerated the influence of the site-specific contamination hotspots.

Method 2

The second method applied microplastic concentrations to channel area downstream of a site

until another site was encountered. If a confluence point was reached before the next site, the

average of the two sites upstream was taken for the corresponding channel area. Headwater

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catchments were characterised by an upstream extrapolation of the first site encountered. This

method followed the assumption that each site represents a known concentration that persists

downstream. However, this method was found to exaggerate the influence of microplastic

concentrations from headwater sites by propagating their low concentrations into urban areas.

Method 3

The third method followed a similar approach, but considered upstream extrapolation of

microplastic concentrations. The reach below the final site within a catchment was

characterised by downstream extrapolation. This follows the assumption that the degree of

contamination at any point reflects an aggregate of upstream characteristics, an approach

which has been applied in US Environment Protection Agency (EPA) water quality

frameworks50.

After evaluating each approach, Method 3 was selected as it was judged to provide the most

representative definition of reaches and the most realistic expression of contamination

hotspots. The results for each of the methods are provided in Supplementary Table 3. The

export of microplastics associated with flooding in each catchment was calculated as the

difference between pre- and post-flood channel bed sediment loads. Incorporating catchment-

wide changes in sediment storage as described above accounts for the influence of the

mobilisation and redeposition of channel bed sediment and associated microplastics across

the entire study area. Our flux estimate approach yields a conservative estimate of the total

export of microplastics as it does not consider:

the role of sewer overflows active during the flooding where significant quantities of

microplastics may have been released during flood events;

the export of microplastic particles transported directly in suspension in the overlying

water column;

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the accumulation of microplastics under low flow conditions following the winter

2015/16 flooding but prior to the second round of bed sediment sampling;

the contribution from biofouling. Organic matter removal from microplastic particle

surfaces was not undertaken. In this context, it is important to appreciate that during the

particle size analysis (n = 487 particles) of the microplastics we did not observe a

significant contribution from biofouling and the microbeads, which typically

predominate, showed clean, lustrous surfaces. Furthermore, the FT-IR spectra

demonstrated very little evidence of interference from particle coatings despite the

absence of an organic matter removal step. This suggests that particles were not heavily

coated in a biofilm layer during the gravimetric separation. The limited evidence for

biofouling may be explained by the short residence time of microplastic particles on the

stream beds in our study catchments and the potentially abrasive effects of a significant

sand load in these rivers.

Global microplastic contamination map and ranking

We have assembled a global inventory of microplastic studies and present these data in a map

and ranking using the reported concentrations12-22, 28, 51-119. Only concentrations reported for

microplastics <5 mm expressed as a mean or maximum value were included. A range of units

have been used to report microplastic concentrations in the literature. Most commonly, results

are expressed by mass or volume (e.g. particle kg-1, m-3) or by area (particles m-2, km-2). To

allow comparability between studies, units were converted, where possible, to two units:

particles kg-1 and particles m-2. Since a standardised method for analysing microplastics has

not yet been established, the concentrations presented on the map and ranking incorporate

data derived from a range of methodologies – including different particle size classes,

extraction techniques and identification procedures.

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Data availability

All datasets are available from the corresponding authors upon request.

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