Plastic pollution in riverbeds fundamentally affects natural sand transport

Rivers link terrestrial and ocean environments, distributing fresh water, nutrients, and 9 sediment to diverse ecosystems. Over the past 50 years, rivers have become increasingly


Introduction 28
Plastics, and their derived products, are pervasive within earth surface systems globally; they 29 are in the air that we breathe (Gasperi et al., 2018;Bullard et al., 2021), in agricultural soils 30 (Liu et al., 2018;Corradini et al., 2019), in aquatic biota (Rezania et al., 2018;Zhu et al., 2019), 31 throughout fresh and marine waterbodies (Lee et al., 2015;Hurley et al., 2018;Pohl et al., 32 2020), and in the deepest abyssal trenches (Peng et al., 2020;Kane et al., 2020). Rivers are the primary terrestrial conduits of plastic, carrying and delivering an estimated 12 M metric tonnes 34 of plastics to coastal and marine environments each year, with this number set to at least double 35 by 2030 (Jambeck et al. 2015). Yet, despite advancements in understanding how plastic travels 36 in rivers (van Emmerik and Schwarz, 2019;Liro et al., 2020;Tasserson et al., 2020), our 37 understanding of its impact on broader sediment transport processes and sedimentary systems 38 more generally is in its infancy (Gabbott et al., 2020), which has implications for environmental 39 monitoring, representative sampling, landscape evolution and sedimentary geology. Plastic has a wide range of physical properties, morphologies, and characteristics (GESAMP,42 2015), such that particles less dense than fresh water will travel floating on or near the water 43 surface, whereas denser particles will travel closer to the riverbed, or even bounce or roll along  (Ockelford et al., 2020;Liro et al., 2020;van Emmerik et al., 2022). 48 Observing and sampling riverbed processes in natural environments without disturbing 49 them is challenging, and thus monitoring and characterising the effect of plastic on 50 riverbed transport processes is difficult. Therefore, we designed and undertook physical 51 laboratory experiments (see Methods) to explore whether plastic influences river transport 52 mechanisms and focused on representing small urban stream systems, which are 53 commonly highly contaminated with plastic due to their proximity to pollution sources 54 (Meijer et al., 2021).

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Through contaminating sand dunes with plastic in a recirculating laboratory flume tank, we 78 test how the inclusion of plastic particles in riverbed sand dunes impacts morphologies and 79 migration processes. We explore how and where plastics interact with bedform dynamics, and 80 how these interactions impact a host of processes known to be important for sediment transport 81 and hydrology, as well as the resulting sedimentary deposit. We seek to understand what the 82 results mean for the fate and distribution of plastic and demonstrate important implications for 83 representative sampling of plastic in riverbeds, as well as understanding the controls on where 84 plastics may be concentrated in certain environments. We advocate that our results offer 85 fundamental insights on what we suggest is a new sub-branch of sedimentology: sediment and 86 plastic particle interactions, set to be increasingly relevant as earth scientists prepare to 87 describe, interpret, and understand Anthropocene landscapes.
4 Sand (median sediment size D50 of 0.35 mm) and 13 different types of plastic were put into a 101 recirculating laboratory flume tank under a constant flow discharge (of 0.05 m³/s) to observe 102 interactions between the plastic and sand under transport conditions (see Methods for full 103 details). The size and density of the plastic and the sand defines their mobility, thereby their 104 erosion and entrainment thresholds. The plastics used in this experiment had a lower density 105 than sand (Table 1), however, some had larger sizes and therefore comparatively lower 106 mobility, leading to a range of complex interactions. The results indicate that, for the flow 107 conditions in the flume tank and the particles used, their behaviour and interaction with the 108 sand bed changed at RD~1.6, (where RD is the particle diameter multiplied by submerged 109 specific gravity). Particles with RD >1.6 were found to be less mobile and more readily 110 incorporated into the sand dune, than particles with RD <1.6, which had greater mobility and 111 were less readily incorporated into the sand substrate (Table 1).  Cigarette filter tips aligned with the flow such that they settled and did not roll down the lee 150 slope, hence forming obstacles that presented zones of sheltered deposition down slope. As 151 such, filter tips were commonly accompanied by a tail of plastic particles (Fig. 1Biii), 152 highlighting the importance of plastic shape on their behaviour and the characteristics of the Plastics with RD>1.6 were more readily incorporated in the lee slope, as they were able to hold 161 their position between occurrences of sand avalanche, which subsequently secured the plastic 162 particle in the deposit. Finally, plastic with similar mobility to the sand grains, such as the 163 thermosetting plastic fragments (Table 1), were found to settle out of the recirculation zone 164 steadily, punctuated by sand avalanches, generating a distinctive diagonally layered deposit 165 characteristic seen throughout the deposits (Fig. 3).

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Stoss side processes 168 As the dunes migrated downstream, the incorporated plastic became exposed on the stoss side 169 and was eroded rapidly due to the lower RD value and the size difference between the plastic 170 and the sand. The lower RD value of plastic compared to sand corresponds to its lower erosion 171 threshold, and thus its higher mobility (Fig. 1C, Table 1). The size difference between the 172 plastic and the sand encourages erosion of the sand around the plastic obstacle due to local flow 173 acceleration. However, it is not necessary for the plastic to be removed from the stoss side of 174 the dune for the dune morphology to be affected, as a scoured pit may form and persist due to 175 the particle size difference alone.
176 177 Yet, when individual plastic pieces are eroded during migration of the bedform, they may be 178 re-entrained via traction, saltation, or suspension, and leave a pit on the stoss side of the dune 179 (Fig. 1Ci). The erosion and rapid re-entrainment of plastic created small depressions, pits, or 180 scours in the stoss side of the sand dune that subsequently migrated to the dune crest and 181 sometimes induced the generation of small super-imposed bedforms along the stoss (Parsons et al. 2005;Fernandez et al., 2006;Fig. 2Cii). The pits were observed to became larger and 183 migrate towards the dune crest resulting in dunes with a more rounded crest, a lower overall 184 height, and lower sand volumes, and thus general flattening of the topography, i.e., diminished 185 dunes (Carling et al., 2000;Figs 1Cii;2Ci-iii). In cases where a plastic lens is eroded, the plastic 186 may be eroded rapidly from the sand in a "burst" of sediment and plastic that is eroded from 187 the dune and re-entrained in the flow (Fig. 1Ci). Such bursts of eroded material were found to 188 lead to a steeper stoss side, which encouraged further erosion of sand and plastic as the dune 189 sought to re-establish equilibrium (Figs 1Ci; 1Dii; 2B). The upper portion of the plastic lens, 190 or indeed the entire lens, may be eroded and the affected dune may continue to migrate in its 191 changed morphology (Fig. 1Dii). in the channel causes a higher ratio of suspended to bedload sand locally, than would occur in 198 an unpolluted setting (Fig. 1A). The repeated occurrences of stoss-side pit formation (Fig. 1Cii) 199 may amount to a more substantial set of consequences however, including instances of dunes cases, the rate of downstream sand transport will increase due to the reduction in dune volume 205 as the pits and scours migrate towards the dune crest (Figs 1Cii; 1Dii). As the crest of the dune 206 becomes more rounded or flattened, the angle of the lee slope may become too shallow to 207 sufficiently shelter sediment for deposition in the lee side, or become entirely indistinct, such 208 that the dune is washed out (Fig. 1Dii). Such change to the riverbed topography was also found 209 to affects dunes downstream of the disruption although this was not investigated in detail Where the stoss side of the dune becomes significantly over steepened, a unidirectional 218 symmetrical dune may occur (Fig. 1Di). In this experiment, the rate of erosion on the stoss side 219 of the dune is increased by plastic removal, whilst the deposition on the lee side is slowed due 220 to plastic movement in the recirculation zone, or the re-equilibrating of the dune. Such a 221 scenario does not always lead to the dune becoming washed out, particularly if sufficient time 222 passes between plastic erosion events allowing the dune to re-equilibrate (Fig 1Dii).

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Alternatively, the dune remnants may remain and later be either reworked or overlain by the 224 next dune migration, particularly if dune celerity is slow. Despite the initial mixing of the plastic into the sand, the plastic became organised and clustered 228 during the experiment, and remained sufficiently abundant on the riverbed to cause multi-229 layered deposition throughout the experiment (Fig. 1Bi). In the resulting deposit, plastic was 230 found to be pervasive, yet there was a profound spatial variability in the plastic to sand ratio 231 both vertically and laterally throughout the deposit (Fig. 3). Larger plastic particles (> 4 mm) 232 were found to be stored in the sediment as lenses, strings, and individual clasts (Fig. 1Bii), whilst the smaller particles were found to have been distributed more generally, though not 234 equally. Thermosetting plastic fragments showed a tendency to form distinguishable layers and 235 lenses, that themselves may contain larger plastic particles (Fig. 3A). The thermosetting plastic 236 fragments additionally highlighted preserved lee slopes, which mark the downstream 237 progression of the dune (Fig. 3B) in cross-sets (Allen, 1982; Fig. 1A). However, some sections 238 of the deposit seem to be devoid of readily visible plastic particles (Fig. 3A), and such plastic-239 limited zones are found to be vertically and horizontally close to plastic-rich lenses (Figs 3A; 240 3B). Additionally, where lenses from past dunes have not been entirely eroded, they may be 241 overprinted by a new migrating dune, such that the overlying dune topography may not align 242 predictably with expected plastic-rich zones (Fig. 3C). morphological transformation was found to increase, which affects bed topography and enhances dune erosion rates. This is critically important because it demonstrates for the first 261 time that plastic can fundamentally change the conditions and sediment transport processes at 262 the riverbed, which will in turn alter its suitability for biotic habitation and impact longer-term 263 river channel evolution.  sedimentology. It is critical that we continue to explore these novel dynamics with more 296 laboratory experiments, field monitoring and computer models to refine our understanding of 297 these newly established processes. As this new chapter of sedimentology emerges, we are 298 able explore further the relationship of plastic with sediment across our landscapes and its 299 long-term consequences.  Table 1

-A table of included plastic particles and their properties
After characterizing the flow, a 1.49 kg mixture with 13 different types of plastics, including 454 particles, fibres, and fragments, were introduced into the flume. The mass of plastic added to 455 the flume only represents 0.12%, by mass, of the total sediment used within the flume system.

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Plastic selected for the study was of different densities and specific gravity (Eq. 1; Table 1) Table 1 shows our observations. To identify 464 the threshold between high/low mobility we computed the product of the particle's submerged 465 specific gravity (Eq. 1) and its equivalent diameter D and took the ratio with respect to sand.

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The next-to-last column in Table 1 shows how much more mobile the corresponding plastic 467 particle is with respect to the sand grains used in the experiment. The threshold between 468 high/low mobility for the flow conditions used lies at approximately RD ~ 1.6.

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The submerged specific gravity is computed as: Where is the density of the particle (see Table 1 for values) and is the density of the fluid 472 (water in this case).

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Equivalent particle diameters for non-spherical particles were computed as: