Large microplastic particles in sediments of tributaries of the River Thames, UK – Abundance, sources and methods for effective quantification
Graphical abstract
Introduction
Since the 1960s plastics have become widely manufactured and used, with global production of plastics reaching 311 million tonnes in 2014, 59 million tonnes of which were produced in Europe (PlasticsEurope, 2016). However, only 17.9 million tonnes were recycled or used in energy recovery processes in Europe in 2014 (PlasticsEurope, 2016). Their inherent durability and longevity which make plastics such a favourable commercial material are also the characteristics that allow them to persist in the environment (Barnes et al., 2009). Degradation of large plastic items can be a very slow process therefore plastics may persist in the environment over long timescales (Andrady, 2011, Hidalgo-Ruz et al., 2012), even in the range of hundreds of years (Barnes et al., 2009). However, despite the wide-ranging use and disposal of plastic products and the recognised abundance of plastic litter worldwide, the importance of understanding the fate and impacts of these plastics within the environment has only recently started to be addressed.
Microplastics, plastic particles < 5 mm in size, are a specific concern given their small scale and potential for widespread environmental dispersal. The first reports of synthetic fibres and pellets as marine environmental contaminants emerged in the early 1970s (Buchanan, 1971, Carpenter and Smith, 1972), however direct research into this field was not pursued until the last decade (Thompson et al., 2004). Since 2004, many studies have investigated the presence and effects of marine microplastic debris (Arthur and Baker, 2011, Faure et al., 2012, Law et al., 2014, Lusher et al., 2015, Van Cauwenberghe and Janssen, 2014). The majority of plastic debris found in the marine environment (70–80%) has land-based sources and rivers are considered an important medium for transfer of this debris (Arthur and Baker, 2011, Bowmer and Kershaw, 2010, Hirai et al., 2011, Jambeck et al., 2015, Sadri and Thompson, 2014, Wagner et al., 2014, Zbyszewski and Corcoran, 2011, Zbyszewski et al., 2014). Comparatively few studies have actually been published on microplastics in freshwater or terrestrial environments, although this field of research is growing with a number of papers recently published on microplastics in freshwater systems (Corcoran et al., 2015, Klein et al., 2015, Lechner et al., 2014, Sanchez et al., 2014, Zbyszewski and Corcoran, 2011, Zbyszewski et al., 2014), with the greatest proportion of microplastic debris in freshwater environments being observed near to industrialised areas (Dubaish and Liebezeit, 2013, Eriksen et al., 2013, Sadri and Thompson, 2014, Zbyszewski and Corcoran, 2011).
Microplastics fall into 2 categories: primary and secondary. Primary microplastics are those which were manufactured with the intention of them being of a micro scale, for example those used in cosmetics or exfoliating scrubs (such as glitter and ‘microbeads’) or virgin pellets used in the plastic production industry. Secondary microplastics are those that have formed as a result of macroplastic degradation, for example breakdown of in situ litter (Andrady, 2011, Barnes et al., 2009, Rillig, 2012, Shah et al., 2008) or the washing of artificial fabrics in the laundry, which can lead to the loss of up to 1900 fibres into wastewater per wash (Browne et al., 2011). Within these categories, microplastics are categorised into 2 size brackets: ‘large microplastic particles’ (LMPP, 1 mm–5 mm) and ‘small microplastic particles’ (SMPP, < 1 mm). Over time, LMPPs may become SMPPs or even nanoplastics, due to degradation within the environment (Andrady, 2011, Koelmans et al., 2015, Lambert and Wagner, 2016).
Sources of microplastic particles to the environment are numerous and varied. Sewage treatment works (STWs) are a critical link in the microplastic transport and distribution web given that many plastic particles including microbeads and synthetic fibres will enter these STWs. If not physically filtered out within the plant itself then they will be discharged to rivers via effluent or incorporated into sludge (Habib et al., 1996, Zubris and Richards, 2005). Sludge may in turn be applied to agricultural land (DEFRA, 2012), leading to direct terrestrial implications, in addition to potential for runoff into watercourses. STW outfalls discharge directly into rivers representing a point source discharge of particles to freshwater environments. Thus, sewage outfalls have been recognised as a likely significant source of microplastic pollution to the oceans (Arthur and Baker, 2011, Browne et al., 2011). Additional sources include degradation of macroplastic debris such as sanitary waste from sewage treatment overflows, plastic packaging, particle runoff from roads in the form of tyre wear particles or parts of vehicles and runoff from land containing degraded litter (Andrady, 2011, Eriksen et al., 2013, Galgani et al., 2015, Hidalgo-Ruz et al., 2012). Another source was recently recognised in the form of polymer composite paints. Due to the low polymer composition of paints, these are likely to be more brittle than pure polymers and therefore break down quickly into smaller particles in the environment (Imhof et al., 2016, Song et al., 2014, Takahashi et al., 2012).
The aim of this study was to investigate the presence, abundance and types of microplastics within tributaries of the River Thames basin (UK). This study investigated the link between two expected and related drivers of microplastic input, sewage effluent input and population density, with the presence of microplastics in river sediments. The River Thames catchment in the UK was selected as the location for our survey as it is the UK's second longest river and the river basin supports many large urban areas, receiving effluent from a population of over 13 million (Bengtson Nash et al., 2006, National Statistics, 2002). Although likely acting as a source of microplastics to the marine environment, the Thames also has the capability to act as a sink for some plastic particles due to flow dynamics: in the Thames estuary (and other estuaries), water near the riverbed has a tendency to flow landward, meaning that some of the debris entering the river may be retained within estuarine sediments (Board, 1973). Sediment was our selected medium for analysis given that microplastics can accumulate in sediments at an order of magnitude higher than in the water column (Hoellein et al., 2016). This indicates the potential for rivers to act as a sink for environmental microplastics. Studies of macroplastic in the Thames have shown there to be an abundance of litter being transported down the Thames (Morritt et al., 2014). To our knowledge, however, with the exception of estuaries this is the first study investigating microplastics in the Thames catchment or indeed any freshwater system in the UK.
Section snippets
Sampling site selection and sample collection
Sampling sites within the Thames river basin were selected based on two variables; average % effluent present in the river as estimated using the Low Flows 2000 (LF2000) WQX (Water Quality eXtension) model (Williams et al., 2009) and population equivalent density as calculated using population within the catchment area (of known area) served by the upstream sewage treatment works (Pottinger et al., 2013, Williams et al., 2009). Selected sites comprised three tributaries of the Thames: the River
Sorting method
The three control filters analysed to assess contamination during processing, contained an average of two fibres per filter paper. These may arise from aerial deposition and from clothing. Compared to the number of fibres found across all field samples (578 total, with even the least polluted site, the Leach, containing 69 fibres), this contamination was deemed to be negligible.
In order to determine the effectiveness of the different sorting methods the proportion of particles recovered in each
Discussion
In terms of quantification method evaluation, the initial sorting and flotation steps combined successfully removed 75% of microplastic particles with the other 25% remaining in the residual sediment. Recovery would have been at 98% if the particles at The Cut site 1 were excluded, as 34% of these could not be floated due to their dense nature. However an initial manual sort by hand and microscope through an amount of dry sediment alone appears to be ineffective, as a maximum of 37% particles
Conclusions
This study is the first to report relative amounts and types of microplastics present across different locations both in the Thames basin, and also in any low-lying river catchment in the UK. Despite the uncertainties and complexities with predicting and analysing microplastic pollution, microplastics were observed at all sites and inference can be made as to sources. While it is clear that the number and types of microplastics observed in this study are not the entirety of microplastic
Acknowledgements
The authors wish to thank Daniel Read and Colin Johnston for their advice on the Raman spectroscopy work, Alexander Walton for his assistance on the fieldwork and Alexander Robinson for his inspired discussions regarding road marking paints. Also Mike Bowes and the Thames Initiative team for their advice on site selection. This work was funded by the UK Natural Environment Research Council through National Capability funding of the Centre for Ecology and Hydrology Pollution and Environmental
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