Little evidence that dams in the Orange–Vaal River system trap floating microplastics or microfibres

https://doi.org/10.1016/j.marpolbul.2019.110664Get rights and content

Highlights

  • Concentrations of microplastics and microfibres were low.

  • Most manufactured items were microfibres.

  • There was no difference in plastic and microfibre concentrations at sites above versus below dam walls.

  • Dams do not appear to trap floating microplastics and microfibres.

Abstract

Rivers can be major sources of plastics into coastal seas, but it is unclear whether dams retain floating plastics, thus reducing the amount reaching the sea. To test if dams trap microplastics, we collected bulk water and neuston net samples from five dams on the Orange–Vaal River system, South Africa. Most manufactured items were microfibres and densities of microplastics were modest (bulk water: 0.21 ± 0.27 items·L−1; net: 0.04 ± 0.16 items·m−2). There was an interaction between dam and season: dams on the densely populated Vaal River had higher microplastic concentrations during dry than wet conditions, whereas the opposite pattern occurred on the less industrialised Orange River. Overall there was no difference in microplastic concentration at sites above vs below dam walls nor was there a strong correlation between microplastic concentration and distance to the wall. Our results therefore suggest that dams do not trap floating microplastics or microfibres.

Introduction

Waste plastics and microfibres are pervasive pollutants of freshwater ecosystems that have been detected in rivers and estuaries in North and South America (Ivor do Sul and Costa, 2013; McCormick et al., 2014), Asia (Lin et al., 2018), Africa (Nel et al., 2018), Europe (Mani et al., 2015) and the United Kingdom (Stanton et al., 2019). Plastic pollution may pose a threat to freshwater organisms and fisheries due to entanglement, smothering or ingestion, and ingested microfibres, despite being mostly of natural origin (e.g. cotton and wool), can release harmful chemicals into the gut of an organism (Ladewig et al., 2015; Stanton et al., 2019). However, research on riverine plastic and microfibre pollution has largely focused on the extent to which rivers are sources of pollution into coastal seas (Blettler et al., 2018). Lebreton et al. (2017) estimated that 1.15–2.41 million tonnes of plastic is released into the oceans by rivers annually. Their model projections are based on seven studies, six of which sampled for floating plastics using manta or neuston nets or stationary conical driftnets with mesh sizes ranging between 0.33 and 3.2 mm. They included the presence or absence of dams in their models and concluded that dams may trap approximately 65% of waste plastics entering rivers (Lebreton et al., 2017). However, they noted that their model did not consider differences in retention rates between dams or among different size classes of plastics because there are few estimates of the extent to which dams trap floating plastics, thus preventing them from being released into the sea (Blettler et al., 2018).

Dams retain large amounts of suspended sediments (Syvitski et al., 2005). For example, the construction of the Hòa Bình Dam in Vietnam reduced the downstream flux of sediments by half (Dang et al., 2010) and the Three Gorges Dam in China traps some 172 million tonnes of sediment each year (Hu et al., 2009). It is thus likely that they also trap some plastics and microfibres, especially polymers with higher densities. Mani et al. (2015) suggest that weirs on the Rhine River may act as sinks for plastics, and Zhang et al. (2015) showed that plastics accumulate behind the wall of the Three Gorges Dam. However, no studies have tested whether dams trap plastic or microfibre particles by comparing samples collected above and below dam walls. Given that there are approximately 2.8 million dams, with more than half of the larger rivers in the world having been dammed (Lehner et al., 2011), quantifying whether dams trap plastic and microfibres will improve our understanding of how they are transported by rivers (Blettler et al., 2018).

Urban centres are major sources of waste plastics entering waterways via direct dumping, stormwater runoff or litter being blown off streets or rubbish dumps (Cable et al., 2017; Moore et al., 2011; Fig. 1). Similarly, wastewater treatment plants (WWTPs), which are generally more concentrated in urbanised areas with large human populations, often release large quantities of microplastics and microfibres (0.025–5 mm) into rivers due to a lack of an effective filtration system to remove particles of this size (Eerkes-Medrano et al., 2015). Given that urban centres are major sources of plastics and microfibres into rivers, it is likely that dams closer to urban areas will have higher levels of plastic and microfibre pollution than dams in more rural areas, especially if dams retain plastics and microfibres within their reservoirs. However, even rural dams may contain high levels of plastics, particularly microplastics and microfibres. Sewage sludge, which is often applied to agricultural croplands as fertiliser, is known to contain high levels of microplastics and microfibres which can be washed into rivers or dams when it rains (Li et al., 2018; Nizzetto et al., 2016). Furthermore, air-borne microplastics and microfibres can be carried long distances from point source areas, resulting in microplastics and microfibres being found in even the most remote habitats (Bergmann et al., 2019; Dris et al., 2016; Zhang et al., 2016). Upon entering a dam, plastic particles or fibres with a higher density than water (1 g·cm−3), such as polyethylene terephthalate (PET, 1.4 g ·cm−3), unexpanded polystyrene (PS, 1.04 g·cm−3) and polyvinyl chloride (PVC, 1.18–1.70 g·cm−3), as well as natural microfibres made of wool (1.3 g·cm−3) or cotton (1.5 g·cm−3), are likely to sink due to the decrease in water velocity and turbulence (DeLassus and Whiteman, 1999). These may accumulate in bottom sediments or exit the dam via the bottom sluices which are often kept open to generate hydropower, for downstream irrigation and to maintain river functioning (Hoellein et al., 2014; Kowalski et al., 2016; Fig. 1). Conversely, plastics with a lower density than water such as low- and high-density polyethylene (LDPE 0.92–0.923 g·cm−3, HDPE, 0.941–0.967 g·cm−3) and polypropylene (PP, 0.903 g·cm−3) may wash up on the banks of the dams where they can be buried by sediment, blown inland or go back into the dam (blown by the wind or washed off if dam levels rise; DeLassus and Whiteman, 1999). In the dry season, when the water levels drop below the level of the dam wall, floating plastics may accumulate behind dam walls (Zhang et al., 2015) while in the wet season, when water levels are high, these floating pieces may pass over dam walls which would result in a more homogenous spread of plastics throughout the catchment area.

The Orange–Vaal River system in South Africa provides an ideal system in which to assess the effect of dams on plastic and microfibre transport down rivers. While the banks along the Orange River are irrigated, there is little urbanisation and human populations within its catchment are relatively low (Fig. 2A). Conversely, its largest tributary, the Vaal River, drains the industrial heartland of South Africa and is densely populated. It is therefore not surprising that the Vaal River (and particularly the Upper Vaal) is one of the most polluted rivers in the country (Wepener et al., 2011). However, little is known about levels of plastic contamination in either the Orange or Vaal rivers. The Orange River is one of the most turbid in Africa and delivers large amounts of sediment to the Atlantic Ocean (Compton and Maake, 2007). It may therefore act as a conduit of waste plastics and microfibres to the ocean, especially if plastics and microfibres entering the Vaal River flow into the Orange River. However, five major dams have been built in this system, two on the Orange River (the Gariep and Van Der Kloof [VDK] dams) and three on the Vaal River (the Grootdraai, Vaal and Bloemhof dams; Fig. 2), which may act as sinks for floating plastics and microfibres. We report levels of microplastic and microfibre (0.025–25 mm) pollution in each of the five dams. We compare microplastic and microfibre abundance at sites above and below the dam walls and consider whether particles accumulate behind the walls by comparing densities at sites close to the walls to those further away. If dams do trap microplastics and microfibres, we predict that microplastics and microfibres will be more abundant at sites above than below dam walls, at sites close to dam walls, and in the dams with densely populated catchments (Grootdraai and Vaal dams) compared to dams with catchments containing lower human population densities (Bloemhof, Gariep and Van Der Kloof dams).

Section snippets

Field data

Bulk water and neuston net samples were collected from each of the five major dams on the Orange and Vaal rivers (Fig. 2, Table S1). The two largest dams are those on the Orange River: the Gariep Dam, which is the largest dam in South Africa, has a storage capacity of 5500 million m3 and the Van Der Kloof Dam holds 3200 million m3 (DWAF, 2019a). The Gariep Dam is the most upstream dam on the Orange River, while the Van Der Kloof Dam is 130 km downstream from the Gariep Dam and 185 km upstream

Bulk water samples

The bulk water samples contained 505 potential microplastics (mean = 0.23 ± 0.27 items·L−1) of which 98% (n = 497) were fibres and only 2% (n = 8) were hard plastic fragments. As noted earlier, most microfibres (>90%) found in rivers (Stanton et al., 2019) and oceans (Suaria et al., subm.) are of natural origin (e.g. cotton or wool fibres) rather than plastic. However, the identification of fibres using μ-FTIR spectroscopy is outside the scope of this study and will be published at a later

Discussion

The annual flux of sediments into coastal seas has been reduced by approximately 1.4 billion metric tonnes·yr−1 due to the trapping of sediments in dam reservoirs (Syvitski et al., 2005). In the Orange River, the current annual flux of sediment into the Atlantic Ocean is estimated to be 25–100 million tonnes·yr−1 less than pre-anthropogenic times (Syvitski et al., 2005), suggesting that the five dams built within the Orange–Vaal River system likely retain large amounts of sediment. However, no

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Anna Blaylock and Thobile Dlamini for their help in the field and Calvin Hartnick for his help in the workshop. Coleen Moloney assisted with the fitting of the statistical models. Willem du Randt, Carl de Kock and their support teams kindly took us out on their boats. We also thank the various landowners who allowed us access to their properties and the border post officials who allowed us to sample from the border post bridges in the Northern Cape, South Africa. The South African

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