Effect of Physical Characteristics and Hydrodynamic Conditions on Transport and Deposition of Microplastics in Riverine Ecosystem

Abstract

Microplastic disposal into riverine ecosystems is an emergent ecological hazard that mainly originated from land-based sources. This paper presents a comprehensive review on physical processes involved in microplastics transport in riverine ecosystems. Microplastic transport is governed by physical characteristics (e.g., plastic particle density, shape, and size) and hydrodynamics (e.g., laminar and turbulent flow conditions). High-density microplastics are likely to prevail near riverbeds, whereas low-density particles float over river surfaces. Microplastic transport occurs either due to gravity-driven (vertical transport) or settling (horizontal transport) in river ecosystems. Microplastics are subjected to various natural phenomena such as suspension, deposition, detachment, resuspension, and translocation during transport processes. Limited information is available on settling and rising velocities for various polymeric plastic particles. Therefore, this paper highlights how appropriately empirical transport models explain vertical and horizontal distribution of microplastic in riverine ecosystems. Microplastics interact, and thus feedback loops within the environment govern their fate, particularly as these ecosystems are under increasing biodiversity loss and climate change threat. This review provides outlines for fate and transport of microplastics in riverine ecosystems, which will help scientists, policymakers, and stakeholders in better monitoring and mitigating microplastics pollution.

1. Introduction

Nowadays, plastics have been widely used because of their low cost, durability, and resourcefulness in socio-economic sectors, like fishing, industry, tourism, and more [1,2,3,4]. Plastic pollution gained attention of the scientific community and has been documented across the globe [5]. Plastic particles with a size of less than 5 mm are known as microplastics which have been studied extensively in different ecosystems, such as marine [6,7,8], wetlands [6,9], rivers [10,11,12], groundwater [13,14,15], sub-surface system [14,16,17], atmosphere [18,19], soil [14,20,21], and remote mountain [22,23,24,25]. Microplastics are sub-categorized as primary and secondary depending upon their origin [26], such as products from cleaning, cosmetic hygiene, paints, detergents, personal care, etc., are considered as primary microplastics; whereas degradation of water from soda bottles, fishing nets, and plastic bags, large-sized plastics into micro-sized are regarded as secondary microplastics [27,28]. Regardless of origin, ample quantities of plastics are inevitably available in natural ecosystems due to the degradation of large plastic particles into smaller micro- or nano-sized plastics [29,30]. Global production aggregates to 6,300 million tons of plastics recently, out of which 79% are disposed into landfill sites and in the aquatic environments [31]. Almost 2.41 million tons of plastics are transported to oceans via rivers annually, projected to increase in the coming decades [32]. Microplastics in freshwater environments have increased attention globally due to their high concentration are being transported and ending up in oceans. For example, 0.33 million tons of plastics had been harvested from the Yangtze River, China [32], and 0.12 million tons of plastics were transported via River Ganga, India, per year [17,33,34]. Hence, substantial knowledge is necessary to understand potential implications of physical, chemical, and biological alteration in microplastic properties and their ecological problems in riverine ecosystems.

To estimate the transport process accurately, it is necessary to understand how plastic particles are carried and deposited in freshwater ecosystems (such as rivers, wetlands, lakes). To date, several field investigations have been conducted across the global freshwater ecosystems [9,11,12,17,35,36,37,38,39,40], however, limited investigations have been reported so far for exploring the transport of microplastics in different ecosystems [41,42,43,44,45,46]. Lagrangian numerical simulation model has been used to determine the concentration of microplastics in the marine environment [47,48,49]. Still, the riverine transport processes have been poorly understood, except for mapping the small particle concentration in the rivers. Though the physical transport of microplastics in the aquatic environs are quite challenging and complicated [50,51,52], therefore, during the discharge from the river to oceans, microplastics are not only transported but also settled down to the river bed by either accumulation or remobilization mechanism depending upon flow rates [53,54]. Before addressing the transport mechanisms, one question emerges: how plastic degrades into micro- and nano-plastics in the riverine ecosystem. Plastic debris experiences natural interactions, reactions, and translocation; and degrades into macro-, micro-, and nano-plastics in the natural environment through mechanical abrasion, thermal and ultraviolet (UV) degradation, along with photo-degradation, biodegradation, and friction (e.g., mechanical force) [55,56,57,58]. Polymeric chemical composition is governed by plastics density which changes over time due to the aging and weathering process or biofilm growth [26,59,60,61]. As a result, microplastics alterations occur in terms of their physical properties and influence their sinking rate in riverine ecosystems.

In the aquatic ecosystems, the deposition and transport of microplastics in sediments and water profile are determined by hydrologic characteristics, rainfall or storm events, watershed pattern, vegetation, hydraulic conditions, microplastics physical properties (size, shape, and density), and hydrodynamics behavior of microplastics [62,63,64]. Microplastic particles also possess intrinsic properties depending upon their physical properties, such as size, shape, and density [65,66]. Most of the commercial plastics have densities in the range of 0.85–1.41 g cm⁻³ [5,51,67]. Intrinsic properties of micro-sized particles significantly govern the fate and transport; for instance, polyethylene, polypropylene, polystyrene, and polyurethane with a density less than 1 g cm⁻³ remain suspended in aquatic settings [68]. A high number of low-density polymers remain in suspension due to buoyant force in water columns that get transferred to oceans via rivers. Indeed, plastics of relatively high-density sink directly over the river bed sediments and are subsequently prevalent in the lentic and lotic water systems [11,69,70,71]. The transport behavior of micro- and nano-sized plastics are significantly governed by natural phenomena, such as buoyancy, gravity, and drag forces [72,73,74]. Gravity and buoyancy are subjected to be negligible for fine particles (diameter < 2 μm), and the interfacial tension plays a key role in particle movement [75,76]. In addition to this, the relative influence of momentum and force of gravity on plastic particles interpret the deposition and sedimentation mechanisms [77]. Fine particle possesses large surface area and hydrophobicity as compared with coarser particles. Generally, the surface wettability of micro-sized particles also influences the settling velocity and drag coefficient [72]. Thus, the settling mechanisms of microplastics have also been explored in hyporheic zones; therefore, stagnant zones are crucial reservoirs for the long-term microplastics sink, whereas it can be remobilized/resuspended with the higher flow velocity [39,53,54,69,78,79]. In the Inde River, remobilization of microplastics had occurred at the bed substratum for every rise in the flow velocities and water level [54]. Despite the current efforts to address microplastics pollution in riverine ecosystems, the transport behavior of microplastics quantification is poorly understood compared to other nano- or micro-size pollutants present in riverine ecosystems.

Inclusive theoretical models are required for holistic understanding of microplastics pollution due to the transport and sedimentation occurring in the riverine ecosystem. This review will focus, firstly on sources of microplastics, secondly illuminating the effect of environmental conditions on the aggregations of microplastics, third highlighting the governing processes of its fate in the river system, and lastly presenting details on the current development of numerical models of fluid mechanics to analyze the trajectory of microplastics.

2. Sources of Microplastics

Plastics pollution has been acknowledged as the dark side of industrialization for the riverine ecosystems [80,81]. Direct sources of microplastics pollution, such as agricultural runoff [20,82], wastewater treatment plants [83,84,85,86], fishing nets/cages [87], municipal wastewater [88,89,90], washing clothes [84,91,92,93], and urbanization [17,94,95,96], have been documented across the riverine ecosystem [39,63,71,97,98,99,100,101]. Resultantly, several studies have reported diverse concentrations of microplastics in the riverine networks due to local and diffuse sources, poor site accessibility, variations in flow regimes, and channel morphology, e.g., depth and size of the river channel [95,102,103,104]. Increased microplastics concentrations have been observed commonly near an urban area, especially associated with wastewater effluents [86,105,106].

3. Physical Properties of Microplastics Particles

Numerous plastic particles have been deposited in the natural environment; subsequently, the density and texture of microplastics vary substantially, relying on polymeric characteristics and production process. Microplastics are heterogeneous in nature and exhibit distinct behavior, depending on their physical properties, such as particle density, shape, and size [5,55,65,66,107]. Microplastics are found in water columns or floating water due to density differences of the particle and fluid, i.e., buoyancy effect (Figure 1). It has been observed that the density, shape, and diameter of plastic particles can change, either increase or decrease, as a result of fragmentation and degradation of microplastics [44,70,74]. Density, polymeric chemical composition, and shapes of microplastics govern whether microplastics can be buoyant, suspend, or sink in the riverine ecosystems [68,80,108]. High-density microplastics can slide, roll and intermittently bounce at the river bed under low flow velocity, whereas low-density particles can remain in suspended form. As a result, large-sized microplastics particles are more likely to be deposited in the river bed-load sediments [66]. Besides, the influence of water environmental conditions on transport behavior [109], changes in their density, shape, and diameter [26], and surface roughness [110] have not been inspected so far in the riverine environment.

3.1. Specific Density of Microplastics

Particle densities can significantly define the deposition and mobility of microplastics in riverine ecosystems. Commonly, microplastics, in the density range less than the river water, float or remain in suspension, while the higher density microplastics are non-floating particles and tend to deposit over the river bed [69,70,111,112], as shown in Figure 1. Low-density commercial plastics, such as polypropylene (0.85–0.95 g cm⁻³), polyethylene (0.90–0.99 g cm⁻³), polystyrene (0.95–1.1 g cm⁻³) are commonly detected in riverine ecosystems. High-density plastics, including polyvinyl chloride (1.1–1.58 g cm⁻³), polyamide (1.38–1.45 g cm⁻³), and polyethylene terephthalate (1.38–1.45 g cm⁻³) are also present in rivers [5,31,39,113,114,115]. Plastic virgin materials have a density in the range of 0.01–2.3 g cm⁻³; however, at the same time, plastic particle density also varies with residence time in the riverine ecosystem due to aggregation, biofilm development, degradation, and flocculation mechanisms [51,60,61,116]. Similarly, in streams, Hoellein et al. [77] observed that the denser fragments can easily be retained and also resist remobilization due to high flow velocities as compared with polypropylene pellets (low-density plastics). Pellets have a slow response time to change their flow velocity in benthic environments [41]. A recent study revealed non-uniform distribution of microplastics along the Hillsborough River depth profile due to the river hydrodynamics [117]. Inherent density of microplastic particles and hydrodynamic turbulent flow condition of river govern the trajectory pathways for low-density plastic particles [26,52,70,117]. Though, the trajectory and speed of microplastics in the riverine ecosystem are complex because of hydrodynamic conditions. River flow dynamics, depending upon the Reynolds number, either laminar (R_e < 1) or turbulent (R_e > 100) flow can impact the flow dynamics of microplastics with respect to the river depth profile. Figure 1 conceptualized high-density microplastic particles can dominant at the river bottom in laminar flow conditions and low-density microplastics in suspension. However, in the turbulent flow condition, the low-density microplastic particles travel for long-distance but the high-density microplastics can either be in suspension or settle down at the river bottom after certain distance of translocation.

3.2. Shape and Size of the Microplastics

Plastic particles in the riverine ecosystem exist in various shapes, for instance, fibers, pellets, filaments, fragments, films, foams, microbeads, and granules [99,102,115,118,119,120,121]. Films and fibers are common irregular shapes obtained from the degradation of garbage, construction work, washing clothes, and greenhouse poly bags and further degraded due to photo-degradation and mechanical abrasion. In general, low-density fibers and films have high buoyancy and low settling velocity [52,122]. Several investigations have documented that fibers can remain in suspension for a longer time as compare to fragments and spherical beads in the riverine sediments [45,70,104,123]. Hoellein et al. [77] also reported that fragments have high deposition velocity, followed by fibers and then pellets; therefore, fibers and pellets possess a longer transport length of particles. For that reason, wide distribution of fiber plastics is found in silt and clay sediments, although medium-sized sand particles enriched with microbeads and pellets are mainly found in riverine ecosystems [104]. Particle shape and surface area are key parameters for the transport of microplastics, as the high surface area to volume ratio state the aggregation and sink behavior of fibers, films, and foams compared to the larger plastic particles [51,52,77,124]. Hoellein et al. [77] observed that uniform shaped plastic particles, such as polypropylene pellets having the lowest surface area to volume ratio, can travel for long-distance, whereas variable shaped plastic, for instance, fragments have the shortest transport length in the streams. As the spherical particles have less exposed surfaces, the shear resistance is less, i.e., less drag force is generally required for initial movement. At the same time, fibers require high shear stress for initiating the movement, as microplastics are strongly trapped in the sediments strongly. Both erosivity or drag force (which is dependent upon the shear stress) and lift force (which is function of the exposed surface) are commonly dependent on the geometry of the microplastics [74]. Under the same conditions, drag force can effectively roll out the uniform spherical-sized particles, and consecutively lift the fibrous particles [74]. Hence, the characteristics of the shape of microplastics influences erosion, for example, spherical and fibrous microplastics, possess different shear stress and lift force in an aquatic setting.

Diameter of plastic particles also affects the transport and sedimentation under different river dynamics. Great contrast in settling velocities of microplastics of varying particle diameters was observed. Thus, the settling and rising velocities of microplastics particles significantly determine the erosivity of microplastics in addition to natural sediments [40,70]. Both settling and sedimentation of microplastics are dominantly influenced by the gravitational force [118]. In the Rhine River, it has been observed that the settling of microplastics is associated with river bed slope, i.e., an increase in flow velocity allows microplastics to travel over long distances [95]. However, Hoellein et al. [77] observed a positive correlation between deposition velocity (V_dep) and diameter of microplastics, whereas a negative correlation with the ratio of deposition velocity to settling velocity (V_fall) in streams. For V_dep/V_fall ratio equivalent to 1, deposition of particles occurs under the force of gravity for diameters in the range of ~50 to 500 μm. In contrast, for V_dep/V_fall $\gg$ 1, particles of diameter < 50 μm are retained at high rate, and for V_dep/V_fall $\ll$ 1, particles of diameter > 500 μm experience hydrologic actions, such as momentum and resuspension, both higher than gravitational settlings [77]. As a result, microplastics also experience numerous natural processes during transportation, such as aggregation, degradation, biofouling, flocculation, and ingestion in aquatic environs. Plastic degradation and surface heterogeneity also occur simultaneously, which play a crucial role in the environmental fate of microplastics in aquatic environments [5,107].

4. Influence of Environmental Conditions on Aggregation of Microplastics

Aggregation in rivers, both homo-aggregation and hetero-aggregation, play a key role in microplastics translocation, depending upon the shape, size, and density of microplastics [38,107,125,126,127]. Wang et al. [107] stated that aggregation is a key physico-chemical process that dominates the horizontal and vertical movement of microplastics in the aquatic setting. Besides, horizontal transport via flow velocity, slope, wind, and turbulent forces led to the buoyancy of the settled microplastics in suspension. In vertical mixing, buoyant microplastics can be transported as slow as suspended soil/sediments via floatation in the aquatic medium. Therefore, Kooi et al. [126] mentioned that the settling velocity of microplastics increases due to aggregation. As stated above, the aggregation is governed by plastic properties (e.g., size, shape, density, and aging), also depends upon the environmental conditions of water, such as pH, surfactant, organic matter contents, metal ions, and other toxic chemicals. Therefore, plastics of large size possess higher stability and less aggregation in the water environment as compared to small particle sizes [128]. Microplastics being hydrophobic in nature shows aggregation behavior due to steric effects (e.g., the force of attraction due to surface charge/steric hindrance). Therein, higher ionic strength (Fe³⁺ > Na⁺) also increases the aggregation process [129,130,131].

Microplastics retain the electrostatic force of attraction at low pH (<3), which leads to the aggregation of microplastics [129]. At the same time, for natural water conditions (pH > 6.5 and < 9), disaggregation of microplastics results in constant hydrodynamic diameter due to dominating electrostatic force of repulsion. In addition, due to steric hindrance, humic acid has oxygenated functional groups which encourage microplastics to aggregate at low pH (<4), whereas no effect has been observed at relatively higher pH [129]. Similarly, surfactants behave as a stabilizer at high concentrations in aquatic environs; therefore, the stability of microplastics (e.g., polystyrene) colloidal particles get disturbed and tend to aggregate together [132].

In natural aquatic environments, microplastics aggregates with other constituents, such as organisms, clay sediments, metallic oxides, and so on, which ultimately leads to heterogeneous aggregation of microplastics [107]. Microplastics encourage microbial colonization and biofilm development depending upon the microplastics properties, micro-organisms species, and water environmental conditions [60,61,133]. Miao et al. [60] observed that biofouled polyethylene terephthalate (PET) and polyvinyl chloride (PVC) sink due to increased density, which also increases the settling velocity in riverine and lake ecosystems. Biological degradation, chemical oxidation, and mechanical friction encourage surface heterogeneity by cracks, scratches, and pores formation making microplastics less hydrophobic and buoyant and inducing micro-organisms colonization [134,135,136]. In an aquatic setting, the suspended sediments, clay minerals, and metal oxides also get adsorbed at microplastics’ surface, other than micro-organisms, and form heterogeneous aggregation, leading to the settling of microplastics in water [137,138]. At the same time, microplastics remain suspended on the water surface because of the surface adsorption of suspended particles and sediments [139].

5. Natural Phenomena Governing Transport of Microplastics in River Environs

In an aquatic setting, plastics either float or suspend, which get transported for a long-time span (from days to years). Several studies have modeled the transport of plastic debris in different aquatic ecosystems across the globe using different numerical models, such as Lagrangian, two-way particle tracking, PELETS-2D, ARIANE, Lagrangian–Markov chain, MEDSLIK-II oil spill code, TrackMPD [42,140,141,142,143,144,145], tracers and GPS [146], and more. In addition to marine environments, numerous researches have documented microplastics transport in the rivers, such as the Seine River, Paris [147,148], Yangtze River, China [94,119,149], Venoge River, Switzerland [120], Ganga River, India [17,33], Rhine River, Germany [95], Lawrence River, Canada [104] and Danube River, Austria [150,151]. Microplastics pollution in rivers is highly heterogeneous, and therein, various factors underpin the lack of consistency in their observed abundance pattern [17,33,95].

River hydrological characteristics, such as bed-forms, flow velocity, water-level, tidal current, and overall discharge, influence the flow of plastic debris [68,152]. Similarly, the river morphological characteristics, such as vegetation intensity, slope, dams, barrages, reservoirs, also govern the fate and transport of microplastics in river environs [34,115,153,154,155]. For example, a decrease in river flow velocity decreases the abundance of microplastics [156], and flooding events can potentially increase the transport and aggregation of microplastics [157]. Weideman et al. [158] observed low microplastics abundance in the pre-flooding events, whereas large abundance after storm/flooding at the downstream side. Under horizontal transport conditions, aquatic or riparian vegetation retain microplastics and allow them to settle as flow velocity reduces [154,159].

Nizzetto et al. [66] described that microplastics are transported by river flow regimes and flooding events; however, sedimentation is governed by the shape, size, and density of microplastics. For example, in the Gota River, Sweden, higher density and large-size microplastics settle down at the river bottom, whereas microplastics with densities close to 1.0 g cm⁻³ float over the surface, and therefore, they are transported further to the marine ecosystems [160]. In rivers, the fate and transport of microplastics are governed by the presence of constructed structures such as dams and reservoirs, a large number of microplastics is getting retained due to sedimentation [10,34,155]. In contrast, the low flow zones in rivers (i.e., lower energy zone) retain micro- and nano-sized plastic particles and gradually settled down [11,77,153]. Therefore, artificial structures, such as dams, reservoirs, groynes, and guiding walls reduce the river flow that enhances the settling conditions of microplastics in the river bed [149,161].

Fragmentation and degradation are other major sources for the transport and sedimentation of microplastics in the riverine ecosystem [162]. Fragmentation is encouraged by microplastics degradation, hydrolysis, physical abrasion, and photo-oxidation [163,164]. As flocculation is the main factor of sediment transport [38], it plays a significant role in microplastics transport towards the sink. The river sediments are long-term reservoirs for microplastics accumulation, depending upon the characteristics of overlaying bed materials [79,118]. Sedimentation led to aggregation, interactions with organisms, and biofouling of microplastics. Biofouling enhances the particle density, i.e., more settling, while degradation decreases particle mass, which makes microplastics more buoyant in the river water [165,166]. Fragmentation, degradation, and biofouling influence settling rate of microplastics, in which fragmentation and degradation take longer residence time than biofouling in the river water [160].

Microplastics become suspended and are transported in the aquatic environment, depending on the river morphology and plastic characteristics. Ockelford et al. [79] coined the remobilization of microplastics in fluvial sediments, which drive the transfer of microplastics from the active layer, i.e., the upper layer of the sediment bed from where plastic particles are transferred by the river flow (Figure 2). The active layer becomes completely disturbed during flooding events (e.g., turbulent storm flow) across the rivers and consequently releases microplastics. Ockelford et al. [79] also explained the mechanism through storm hydrograph and excess critical shear stress of the median grain sizes with thresholds (excess critical shear stress, i.e., τ*/τ*_c where τ* is the dimensionless shear stress and τ*_c is the critical dimensionless shear stress for the median grain size, D₅₀), where the sediment bed transforms from the sink to source and source to sink for microplastics with respect to the flood wave. Laminar and turbulent flow induce critical shear stress, in which uneven riverine bottom surfaces can play a significant role in the resuspension of microplastics. At the starting point of the rising limb (τ*/τ*_c up to 0.9, i.e., first threshold), the sediment bed initially undergoes armoring, then stabilizes the bed surface while limiting the microplastics release. As the flow increases, the sediment bed becomes unstable (τ*/τ*_c > 0.98) and discharges a significant quantity of microplastics. Lastly, the final threshold (1.26 < τ*/τ*_c < 1.35) signifies the maximum transport of microplastics, and it occurs at the peak discharge (Figure 2). Therefore, Ockelford et al. [74] also reported that the maximum flux of microplastics occurs during the rising limb or in the early part of the flood. After the peak discharge, the storm flow attenuates exponentially as the active layer depth decreases irrespective of falling limb duration and returns from sink to the pre-flood level, resulting in less microplastics at watershed outlets. In river sediments, remobilization of microplastics is also known as microplastics erosion, depending upon the critical shear stress in-between the range of 0.002–0.233 N m⁻² at higher flow velocities [74,167]. Tolhurst et al. [168] stated that bed-load, resuspension, and deposition of pellets under benthic shear environments are considered under laminar flow conditions. Microplastics experience rolling, sliding or saltation, and suspension at the bed-load depending upon their shear velocity, critical erosion velocity, and depositional shear velocity, respectively.

In general, as stated above, sedimentation due to the influence of gravity force is a dominant mechanism for settling of microplastics. Microplastics transportation in the river is influenced by downward movement and the longitudinal direction of the advective flow of plastic particles [77,95,169,170]. Therefore, in the river bed, microplastics can be accumulated or translocated into the hyporheic zone, which may ultimately be deposited in the aquifer [118].

6. Current Progress on Microplastics Pollution Using Numerical Simulations

In-depth investigation of microplastics movement in the riverine ecosystem is based on mathematical/numerical model of fluid mechanics under different hydrodynamic settlings (Table S1) [11,46,53,66,171,172,173,174,175,176]. The fundamental numerical models can simulate and estimate the transport of microplastic, as these fundamental numerical models are able to analyze the trajectory of microplastics in different aquatic settings. Numerical modelling for monitoring microplastics pollution could be a promising tool in the view of a thorough conceptualization of the riverine microplastics dynamics in a holistic manner. Only a few have provided theoretical background and conceptualized the transport mechanisms for investigating the fate of microplastics in aquatic settings [15,41,70,72,74,121,126,163,177,178] (Table 1). Isachenko [178] proposed a stochastic numerical model for determining the transport, especially terminal velocity depending upon the microplastics properties, such as diameter, density, shape, etc. and river flow characteristics, such as water density and viscosity based upon field observations, as follows:

(1)${\mathrm{V}}_{\mathrm{S}}=\mathrm{f} (d, {\rho }_p, csf, \mathsf{\rho }, v_o)$

where f is the function of terminal velocity, which depends on the independent variables such as particle parameters (d), particle density (ρ_p), and Corey shape factor (csf), water density (ρ), and water viscosity ($v_o$).

6.1. Floatation

Floatation is a typical procedure based on theories of diffusion and sedimentation for separating micro-sized plastics from sediments. For floatation, Stokes’ law validates only when the flow regime follows the laminar flow or convective flux dominates over the diffusive flux. In general, the convective flux and laminar flow for extraction of meso- and micro-sized plastics are valid for floatation, determined by the Peclet number and Reynolds number [179].

${\mathrm{P}}_{\mathrm{e}}= \frac{vL}{D}\gg 1$

${\mathrm{R}}_{\mathrm{e}}= \frac{v\rho d}{\eta } 0.1$

where P_e represents the Peclet number, v is convective velocity, L denotes the characteristics length for floating plastic particles, and R_e highlights the Reynolds number.

6.2. Drag Coefficient and Settling Velocity

Drag coefficient and settling velocity are two fundamental properties for relating the plastic particle movement in the aquatic setting [72,74]. The terminal settling velocity of fine particles is estimated by balancing the net force from gravitational, buoyancy, and fluid drag forces equal to zero. Then, fluid drag (F_D) can be expressed in drag coefficient and terminal settling velocity for fine particles. Stokes [180] provided a theoretical model for terminal settling velocity (V_T) and drag coefficient (C_D). However, Stokes’ law did not include the effect of hydrophobicity on plastic particles, and it is valid for a very low Reynolds number (R_e < 0.1), assuming the force of inertia can be ignored, which correlates the drag coefficient and terminal settling velocity for fine particles only. For higher Reynolds numbers, correlations between drag coefficient and terminal settling velocity cannot be established theoretically. Therefore, empirical correlations have been established for a wide range of Reynolds numbers to evaluate the influence of surface hydrophobicity by considering the effect of gravity and buoyancy, including interfacial tension (i.e., surface free energy) of particles. Modified Stokes’ law, considering surface wettability, confirmed that the hydrophobicity has a significant impact on the settling velocity of particles, only when d* = (Δg/v²)^1/3d < 2 and R_e < 0.35. Moreover, higher hydrophobicity of glass beads resulting in large settling velocity with low drag coefficients caused by micro-fluidicity for the same particle diameter [72]. Similarly, Waldschläger and Schüttrumpf [70] provided adjusted formula (as shown in Table 1) for settling and rising velocity under different drag coefficients associated with Reynolds number, which can be suitable for different shapes, like foams, fragments, and pellets. Interestingly, no such distinctions in their shape have been observed for settling and rising velocities observed for micro- and macro-plastics in the range of 0.16 to 3.52 cm s⁻¹ and 0.18 to 19.85 cm s⁻¹, respectively. Therefore, observed slower velocities of rising and settling might be due to changes in surface properties, such as high roughness and increase in the surface area resulting from microplastics weathering [74]. In benthic environments, the settling velocities for pellets are in the range of 20 to 70 mm s⁻¹; pellets of higher density have faster settling velocities [41]. Further, Ballent et al. [42] also observed the sinking velocity of 28 mm s⁻¹ for high-density plastic particles with an average size of 4.7 mm. Therein, Chubarenko et al. [51] observed that heavy microplastics takes more than 18 h to settle in the marine environment, whereas low-density plastic particles, polyethylene fibers take 6–8 months for sinking in the euphotic zone, and spherical particles can be retained on the water surface for 10–15 years. Whereas, in the laboratory experimental investigation, settling velocities for different density microplastics are in the range of 1 to127 mm s⁻¹ [43,45,181]. For rising velocities, microplastics sampled in North Atlantic subtropical region have positive buoyancy of velocity, 1 to 43 mm s⁻¹ [177,182,183].

6.3. Terminal Velocity of Microplastic

Terminal velocity describes the transport behavior of particles (e.g., either rising or falling) under steady-state or stagnant water conditions, assuming the total forces acting on particles from all directions are properly balanced [178]. Terminal velocity of particles depends upon the difference between the density of particle and the fluid, as rising velocity (i.e., positive buoyancy) is equal and opposite to settling velocity (i.e., deposition) of the particles. According to Stokes’ law, terminal particle velocity is valid for laminar flow, i.e., low Reynolds number (R_e < 1) [185]. Isachenko [178] simulated that rising and settling velocities for plastic particles are 10 cm s⁻¹ and 15 cm s⁻¹, respectively, using the Dietrich formula (as mention in Table 1). It was also predicted that the terminal velocities are close to the above-estimated value for spherical microplastics particles, whereas there would be huge discrepancies for irregularly shaped microplastics [43,45].

6.4. Shields Parameter

The minimum flow velocities required for the first microplastics movement are described by critical shield shear stress, which depends on the fluid density. In the Shield diagram shown in Figure 3, Waldschläger and Schüttrumpf [74] assumed uniform microplastics eroded as rolling, sliding, and saltating (i.e., intermittent bouncing) conditions with natural sediments after achieving critical shear stress under bed-load transport. Microplastics associated with sediment grains encourage erosion depending upon the critical shear stress. For uniform grains, critical shear stress was observed as 0.002–0.233 N m⁻² for eroding the microplastics, whereas it is difficult to predict erosion behavior for non-uniform sized grains. On the far side, the hiding exposure effect shows significant erosion as large grains are possibly exposed via laying over the smaller-sized microplastics. Critical shear stress is strongly influenced by the density, diameter, and shape of microplastics which is expressed as Shields Parameter [42]. Considering densities and shape into account, high-density plastic particles have lower Shield parameters and higher Reynolds numbers. Contrastingly, low-density plastics have lower Reynolds numbers (R_e < 1) and possess higher Shield parameters (>0.1), resulting in no motion in the Shield diagram. Therein, with increased particle diameter, Reynolds number also increases and influences the erosion ultimately [74].

7. Conclusions and Recommendations for Future Research

Plastic pollution is a global and pervasive problem in the riverine ecosystem. This review has highlighted the fate of microplastics in the river ecosystem and the processes governing their transport and sedimentation. There is a critical need to further unravel the role of climatic and realistic field conditions on transport and sedimentation of microplastics in ecologically sensitive systems such as rivers, wetlands, and so on. It is important to account for empirical investigation under the influence of environmental conditions, biofilm colonization, and particle dimensions. We found that the microplastics interact well with natural ecosystems, and thus feedbacks that govern their fate and transport, particularly in terrestrial and aquatic ecosystems, are under increasing threat from development and climate change. We have identified the key gaps in our current understanding that warrant further exploration and research. Therefore, the overall recommendations for future research are specified as follows:

• Spatial and temporal mapping for storage and transport of microplastics is needed to understand the extent of microplastics pollution in the river streams under varying climatic and realistic conditions [77,155,186];

• In river flow conditions, it has been observed that microplastics transport vertically down depending upon the density and shape of microplastics in the water profile [187,188]. However, impacts on biogeochemical cycle and plastics dynamics were not considered until now for determining the microplastics transport;

• The current concern needs to quantify the effect of rheological behavior and viscosity under Newtonian and non-Newtonian fluid conditions on settling velocity and drag force of microplastics [189];

• So far, the influence of environmental conditions, such as temperature cycle, especially cold or warmer temperature, on the transport of nano- and micro-plastic in natural environmental conditions is unknown;

• It is recommended to consider the concept for hydraulic jumps using a Froude number on the transport of microplastics in riverine ecosystems;

• The transport of microplastics in the water column should be assessed along with the concurrent movement of nutrients and other pollutants that mimic the riverine environment;

• Further research on microplastics may affirm insights into how much time a particle takes to remain in suspension and how the vertical distribution of a particle occurs in the riverine ecosystem under different laminar and turbulent flow conditions;

• Biofouling and colonization sensitivity of different microplastics need to be investigated in relation to organisms and permanence of plastic in the riverine and other ecosystems, which should be supported with best statistical tests to compare accumulation areas.

Microplastics are heterogeneous hazardous compounds and possess distinct behavior because of their density, shape, and size in the riverine environs. Therefore, quantifying and predicting the fate and transport of microplastic particles in riverine ecosystems allows scientists, researchers, ecologists, environmental conservationists, hydrologists, policymakers, and other stakeholders to understand plastics pervasive problems at the regional and global scale and can be helpful in monitoring microplastics in aquatic ecosystems.

Author Contributions

Conceptualization, R.K. and P.S. (Prabhakar Sharma); writing—original draft preparation, R.K., and A.V.; supervision, P.S. (Prabhakar Sharma); writing—review and editing, P.S. (Prabhakar Sharma), P.K.J., P.S. (Prabhakar Singh), P.K.G., R.C., and P.V.V.P.; visualization, P.S. (Prabhakar Sharma), P.K.J., P.S. (Prabhakar Singh), R.C., and P.V.V.P. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

All authors thank their organization for supporting their research activities.

Conflicts of Interest

The authors declare no conflict of interest. The views expressed here are of authors and not their representing organizations.

Abbreviations

V: Convective velocity, m/s; d: Diameter of the plastic particle, m; a: Centrifugation m/s²; g: acceleration due to gravity, m/s²; η: Dynamic viscosity, Pa.s; $$v_o$$: Kinematic viscosity, m²/s; $$V_T$$: Terminal velocity, m²/s; D: Diffusion coefficient; csf: Corey shape factor, according to Corey [190]; k: Boltzmann constant, T: Absolute temperature (K); P_e: Peclet number; τ_o: Shear stress, N/m²; R_e: Reynolds number; F_D: Drag force, $$C_D\mathsf{\pi }{d}^2{\rho }_l{V}_T^2/8$$; F_G: Force of gravity, $$\mathsf{\pi }{\rho }_s\mathrm{g}{d}^3/6$$; C_D: Drag coefficient, $$4\left({\rho }_s-{\rho }_l\right)gd/\left(3{\rho }_l{V}_T^2\right)$$; L: Characteristics length for floating plastic particles, m; ρ_l: Density of the solution, kg/m³; ρ_s: Density of the solid particle, kg/m³; Δρ: Difference in the density of particle and separation solution, kg/m³; $$E_{Ol}$$: Dimensionless number, relationship between gravity, buoyancy and interfacial tension; $${\gamma }_{sl}$$: Surface free energy of the particle, J/m²; $$K_z$$: Vertical turbulent mixing coefficient; U_z: Vertical mixing movement; V_*: Dimensionless terminal velocity; ξ: Random coefficient (vary from −1 to 1); θ: Shield parameter.

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Figures and Table

Figure 1. Schematic representation of natural phenomenon occurring for microplastics transport and sedimentation in rivers (VS denotes settling velocity, VRV is river velocity, and Re is Reynolds number).

Figure 2. Proposing hydrograph for microplastics transport in river ecosystem and their relationship with various shear stresses of plastic particles.

Figure 3. Relationship between dimensionless shear stress and Reynolds number for analyzing microplastics transport in riverine ecosystems.

Table 1

Theoretical numerical models for understanding transport of microplastics.

Model	Description	Limitations/Advantages	References
Floatation	Convective velocity V expressed by Stokes’ law:$$V=\frac{d^{2}a\Delta \rho }{18\eta }$$	Assumption: Plastic particles to be spherical in shape;For floatation, Stokes’ law validates only when the flow regime follows the laminar flow and convective flux governs over diffusive.	[179]
Diffusion	Stokes-Einstein equation:$$D=\frac{2kT}{6\mathsf{\pi }\eta d}$$$${\mathrm{P}}_{\mathrm{e}}=\frac{VL}{D}\gg 1$$$${\mathrm{R}}_{\mathrm{e}}=\frac{V\rho d}{\eta }0.1$$	Assumption: Plastics particle to be micro-sized;Convective flux and laminar flow for extraction of meso- and micro-sized plastics are valid for floatation, determined by the Peclet number and Reynolds number.	[179]
Terminal Velocity	Dietrich formula:$${\mathrm{V}}_{\mathrm{T}}=\sqrt[3]{V_{*}\frac{{\rho }_s-{\rho }_l}{{\rho }_l}g{v}_o}$$	The net force on particles is zero.Reynolds number < 1.Particles must follow the principle of physical similarity.	[43,45,178,184]
Drag Coefficient and Settling Velocity	$${\mathrm{F}}_{\mathrm{D}}=\frac{1}{8} C_{\mathrm{D}}\mathsf{\pi }{d}^2{\rho }_l{V}_{\mathrm{T}}^2$$ $$V_T=\frac{g{d}^2}{18\eta } \left({\rho }_s-{\rho }_l\right)$$ $$C_{\mathrm{D}}=\frac{24}{{\mathrm{R}}_{\mathrm{e}}}$$	The terminal settling velocity of fine particles states when the net force of three gravitational, buoyancy, and fluid drag forces is equal to zero.Hydrophobicity is not considered.Stokes’ law is valid for a very low Reynolds number (Re < 0.1).	[72,180]
	Drag coefficient for the following:Settling particles: $$C_{D, sp}=\frac{3}{csf\times \sqrt[3]{R_e}}$$Rising particles:$$C_{D, rp}=\left(\frac{20}{{\mathrm{R}}_{\mathrm{e}}}+\frac{10}{\sqrt{{\mathrm{R}}_{\mathrm{e}}}}+\sqrt{1.195-csf}\right)\times {\left(\frac{6}{P}\right)}^{1-CSF}$$Settling and rising velocity:$$V_x=\sqrt{\frac{4}{3} \frac{d_{equi}}{C_D}}\left|\frac{{\rho }_s-{\rho }_l}{{\rho }_l}\right|g$$	$$\mathrm{For} {\mathrm{R}}_{\mathrm{e}}$$ < 0.5,$$C_{\mathrm{D}}=\frac{24}{{\mathrm{R}}_{\mathrm{e}}}$$$$\mathrm{For} 0.5 < R_e$$ < 103$$C_{\mathrm{D}}=\frac{24}{{\mathrm{R}}_{\mathrm{e}}}+\frac{5}{\sqrt{{\mathrm{R}}_{\mathrm{e}}}}+\frac{2}{5}$$$$\mathrm{For} 103 < {\mathrm{R}}_{\mathrm{e}}$$< 2 × 105$$C_{\mathrm{D}}=0.44$$	[185]
Modified version for Drag Coefficient and Settling Velocity	$$E_{Ol}=\frac{g{d}^2 \left({\rho }_s-{\rho }_l\right)}{{\gamma }_{sl}}$$$$\begin{array}{cc}{V}_T=\frac{g{d}^2}{18\eta } \left({\rho }_s-{\rho }_l\right)& \{e^{\left(-0.3+0.18\theta -0.02{\theta }^2\right)}\hfill \\ & +\left(2.3\theta +1.32\right)e^{-\sqrt{E_{Ol}}/[0.01{\theta }^{\left(-0.99\right)}}\}\end{array}$$$$\begin{array}{c}{C}_{\mathrm{D}}=\frac{24}{{\mathrm{R}}_{\mathrm{e}}}\{e^{\left(-0.3+0.18\theta -0.02{\theta }^2\right)}\\ +(2.3\theta \hfill \\ +1.32\left)e^{-\sqrt{E_{Ol}}/[0.01{\theta }^{\left(-0.99\right)}}\right\}{}^{--}\hfill \end{array}$$Dimensionless diameter, d* = (Δg/v2)1/3d	Valid for d* < 2 and Re < 0.35.Reynolds number to evaluate the influence of surface hydrophobicity by considering the effect of gravity and buoyancy, including interfacial tension (i.e., surface free energy) of particles.Consideration for surface wettability resulting in interfacial tension on particles.	[72]
Turbulent vertical mixing	Using Stokes’ law:$${\mathrm{F}}_{\mathrm{D}}=\frac{1}{8} C_{\mathrm{D}}\mathsf{\pi }{d}^2{\rho }_l{V}_{\mathrm{T}}^2$$$$V_T=\frac{g{d}^2}{18\eta } \left({\rho }_s-{\rho }_l\right)$$$$C_{\mathrm{D}}=\frac{24}{{\mathrm{R}}_{\mathrm{e}}}+\frac{5}{\sqrt{{\mathrm{R}}_{\mathrm{e}}}}+\frac{2}{5}$$Vertical movement:$$U_z=V_T+ \mathsf{\xi }\sqrt{\frac{2K_z}{\Delta t}}$$	Assumption: At a high Reynolds number, terminal velocity is at a steady state.ξ is a random coefficient ranging from −1 to 1 for turbulent conditions.	[52]
ShieldsParameter	$$\theta =\frac{{\tau }_o}{({\rho }_s-{\rho }_l)gd}$$	Assumption: Shields diagram for uniform sedimentsCritical shear stress in the range of 0.002–0.233 N m−2 denotes erosion beginning.Force of lift and drag higher than resistance.	[74]

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/w13192710/s1, Table S1: Sources and effect of environmental conditions on the aggregations of microplastics.

References

1. Windsor, F.M.; Durance, I.; Horton, A.A.; Thompson, R.C.; Tyler, C.R.; Ormerod, S.J. A catchment-scale perspective of plastic pollution. Glob. Chang. Biol.; 2019; 25, pp. 1207-1221. [DOI: https://dx.doi.org/10.1111/gcb.14572]

2. Zhang, Q.; Xu, E.G.; Li, J.; Chen, Q.; Ma, L.; Zeng, E.Y.; Shi, H. A review of microplastics in table salt, drinking water, and air: Direct human exposure. Environ. Sci. Technol.; 2020; 54, pp. 3740-3751. [DOI: https://dx.doi.org/10.1021/acs.est.9b04535]

3. Kedzierski, M.; Frère, D.; Le Maguer, G.; Bruzaud, S. Why is there plastic packaging in the natural environment? Understanding the roots of our individual plastic waste management behaviours. Sci. Total Environ.; 2020; 740, 139985. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.139985]

4. Kumar, R.; Verma, A.; Shome, A.; Sinha, R.; Sinha, S.; Jha, P.K.; Kumar, R.; Kumar, P.; Shubham,; Das, S. et al. Impacts of Plastic Pollution on Ecosystem Services, Sustainable Development Goals, and Need to Focus on Circular Economy and Policy Interventions. Sustainability; 2021; 13, 9963. [DOI: https://dx.doi.org/10.3390/su13179963]

5. Andrady, A.L. The plastic in microplastics: A review. Mar. Pollut. Bull.; 2017; 119, pp. 12-22. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2017.01.082] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28449819]

6. Paduani, M. Microplastics as novel sedimentary particles in coastal wetlands: A review. Mar. Pollut. Bull.; 2020; 161, 111739. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2020.111739] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33091840]

7. Malankowska, M.; Echaide-Gorriz, C.; Coronas, J. Microplastics in marine environment: A review on sources, classification, and potential remediation by membrane technology. Environ. Sci. Water Res. Technol.; 2021; 7, pp. 243-258. [DOI: https://dx.doi.org/10.1039/D0EW00802H]

8. Yang, M.; Chen, B.; Xin, X.; Song, X.; Liu, J.; Dong, G.; Lee, K.; Zhang, B. Interactions between microplastics and oil dispersion in the marine environment. J. Hazard. Mater.; 2021; 403, 123944. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2020.123944] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33264989]

9. Kumar, R.; Sarma, P.; Bandyopadhyay, S. Evidence of microplastics in wetlands: Extraction and quantification in Freshwater and coastal ecosystems. J. Water Process Eng.; 2021; 40, 101966. [DOI: https://dx.doi.org/10.1016/j.jwpe.2021.101966]

10. van Emmerik, T.; Schwarz, A. Plastic debris in rivers. WIRs Water; 2020; 7, e1398. [DOI: https://dx.doi.org/10.1002/wat2.1398]

11. Tibbetts, J.; Krause, S.; Lynch, I.; Sambrook Smith, G.H. Abundance, distribution, and drivers of microplastic contamination in urban river environments. Water; 2018; 10, 1597. [DOI: https://dx.doi.org/10.3390/w10111597]

12. Kumar, R.; Sharma, P.; Manna, C.; Jain, M. Abundance, interaction, ingestion, ecological concerns, and mitigation policies of microplastic pollution in riverine ecosystem: A review. Sci. Total Environ.; 2021; 782, 146695. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2021.146695]

13. Kumar, R.; Sharma, P. Microplastics pollution pathways to groundwater in India. Curr. Sci.; 2021; 120, 249.

14. Kumar, R.; Sharma, P. Recent Developments in Extraction, Identification, and Quantification of Microplastics from Agricultural Soil and Groundwater. Fate and Transport of Subsurface Pollutants; Gupta, P.K.; Bharagava, R.N. Springer: Singapore, 2021; pp. 125-143.

15. Ganesan, M.; Nallathambi, G.; Srinivasalu, S. Fate and transport of microplastics from water sources. Curr. Sci.; 2019; 117, pp. 1879-1885. [DOI: https://dx.doi.org/10.18520/cs/v117/i11/1879-1885]

16. Lusher, A.L.; Tirelli, V.; O’Connor, I.; Officer, R. Microplastics in Arctic polar waters: The first reported values of particles in surface and sub-surface samples. Sci. Rep.; 2015; 5, pp. 1-9. [DOI: https://dx.doi.org/10.1038/srep14947] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/26446348]

17. Singh, R.; Kumar, R.; Sharma, P. Micro-plastic in the subsurface: Extraction and characterization from sediments of River Ganga near Patna, Bihar. Advances in Remediation Techniques for Polluted Soils and Groundwater; Gupta, P.K.; Yadav, B.; Himanshu, S. Elsevier: Amsterdam, The Netherlands, 2021; ISBN 978-0-128-23830-1

18. Zhang, Y.; Kang, S.; Allen, S.; Allen, D.; Gao, T.; Sillanpää, M. Atmospheric microplastics: A review on current status and perspectives. Earth-Sci. Rev.; 2020; 203, 103118. [DOI: https://dx.doi.org/10.1016/j.earscirev.2020.103118]

19. Strady, E.; Kieu-Le, T.-C.; Tran, Q.-V.; Thuong, Q.-T. Microplastic in atmospheric fallouts of a developing Southeast Asian megacity under tropical climate. Chemosphere; 2021; 272, 129874. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2021.129874]

20. Kumar, M.; Xiong, X.; He, M.; Tsang, D.C.; Gupta, J.; Khan, E.; Harrad, S.; Hou, D.; Ok, Y.S.; Bolan, N.S. Microplastics as pollutants in agricultural soils. Environ. Pollut.; 2020; 265, 114980. [DOI: https://dx.doi.org/10.1016/j.envpol.2020.114980]

21. Jacques, O.; Prosser, R. A probabilistic risk assessment of microplastics in soil ecosystems. Sci. Total Environ.; 2021; 757, 143987. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.143987] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33310577]

22. Allen, S.; Allen, D.; Phoenix, V.R.; Le Roux, G.; Jiménez, P.D.; Simonneau, A.; Binet, S.; Galop, D. Atmospheric transport and deposition of microplastics in a remote mountain catchment. Nat. Geosci.; 2019; 12, pp. 339-344. [DOI: https://dx.doi.org/10.1038/s41561-019-0335-5]

23. Free, C.M.; Jensen, O.P.; Mason, S.A.; Eriksen, M.; Williamson, N.J.; Boldgiv, B. High-levels of microplastic pollution in a large, remote, mountain lake. Mar. Pollut. Bull.; 2014; 85, pp. 156-163. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2014.06.001]

24. Napper, I.E.; Davies, B.F.; Clifford, H.; Elvin, S.; Koldewey, H.J.; Mayewski, P.A.; Miner, K.R.; Potocki, M.; Elmore, A.C.; Gajurel, A.P. Reaching new heights in plastic pollution—Preliminary findings of microplastics on Mount Everest. One Earth; 2020; 3, pp. 621-630. [DOI: https://dx.doi.org/10.1016/j.oneear.2020.10.020]

25. Pastorino, P.; Pizzul, E.; Bertoli, M.; Anselmi, S.; Kušće, M.; Menconi, V.; Prearo, M.; Renzi, M. First insights into plastic and microplastic occurrence in biotic and abiotic compartments, and snow from a high-mountain lake (Carnic Alps). Chemosphere; 2021; 265, 129121. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2020.129121]

26. Waldschläger, K.; Lechthaler, S.; Stauch, G.; Schüttrumpf, H. The way of microplastic through the environment–Application of the source-pathway-receptor model. Sci. Total Environ.; 2020; 713, 136584. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.136584]

27. Cauwenberghe, L.V.; Devriese, L.; Galgani, F.; Robbens, J.; Janssen, C.R. Microplastics in sediments: A review of techniques, occurrence and effects. Mar. Environ. Res.; 2015; 111, pp. 5-17. [DOI: https://dx.doi.org/10.1016/j.marenvres.2015.06.007]

28. Xu, S.; Ma, J.; Ji, R.; Pan, K.; Miao, A.-J. Microplastics in aquatic environments: Occurrence, accumulation, and biological effects. Sci. Total Environ.; 2020; 703, 134699. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.134699]

29. Cózar, A.; Echevarría, F.; González-Gordillo, J.I.; Irigoien, X.; Úbeda, B.; Hernández-León, S.; Palma, Á.T.; Navarro, S.; García-de-Lomas, J.; Ruiz, A. Plastic debris in the open ocean. Proc. Natl. Acad. Sci. USA; 2014; 111, pp. 10239-10244. [DOI: https://dx.doi.org/10.1073/pnas.1314705111] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/24982135]

30. Eriksen, M.; Lebreton, L.C.; Carson, H.S.; Thiel, M.; Moore, C.J.; Borerro, J.C.; Galgani, F.; Ryan, P.G.; Reisser, J. Plastic pollution in the world’s oceans: More than 5 trillion plastic pieces weighing over 250,000 tons afloat at sea. PLoS ONE; 2014; 9, e111913. [DOI: https://dx.doi.org/10.1371/journal.pone.0111913] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25494041]

31. Geyer, R.; Jambeck, J.R.; Law, K.L. Production, use, and fate of all plastics ever made. Sci. Adv.; 2017; 3, e1700782. [DOI: https://dx.doi.org/10.1126/sciadv.1700782] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28776036]

32. Lebreton, L.C.; Van Der Zwet, J.; Damsteeg, J.-W.; Slat, B.; Andrady, A.; Reisser, J. River plastic emissions to the world’s oceans. Nat. Commun.; 2017; 8, 15611. [DOI: https://dx.doi.org/10.1038/ncomms15611]

33. Sarkar, D.J.; Sarkar, S.D.; Das, B.K.; Manna, R.K.; Behera, B.K.; Samanta, S. Spatial distribution of meso and microplastics in the sediments of river Ganga at eastern India. Sci. Total Environ.; 2019; 694, 133712. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.133712]

34. Sarkar, D.J.; Sarkar, S.D.; Mukherjee, S.; Das, B.K. Impact and Fate of Microplastics in the Riverine Ecosystem. Contaminants in Drinking and Wastewater Sources; Kumar, M.; Snow, D.; Honda, R.; Mukherjee, S. Springer: Singapore, 2020; pp. 95-115.

35. Napper, I.E.; Baroth, A.; Barrett, A.C.; Bhola, S.; Chowdhury, G.W.; Davies, B.F.; Duncan, E.M.; Kumar, S.; Nelms, S.E.; Niloy, M.N.H. The abundance and characteristics of microplastics in surface water in the transboundary Ganges River. Environ. Pollut.; 2021; 274, 116348. [DOI: https://dx.doi.org/10.1016/j.envpol.2020.116348]

36. Singh, N.; Mondal, A.; Bagri, A.; Tiwari, E.; Khandelwal, N.; Monikh, F.A.; Darbha, G.K. Characteristics and spatial distribution of microplastics in the lower Ganga River water and sediment. Mar. Pollut. Bull.; 2021; 163, 111960. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2020.111960]

37. Bujaczek, T.; Kolter, S.; Locky, D.; Ross, M.S. Characterization of microplastics and anthropogenic fibers in surface waters of the North Saskatchewan River, Alberta, Canada. Facets; 2021; 6, pp. 26-43. [DOI: https://dx.doi.org/10.1139/facets-2020-0057]

38. Droppo, I.; Ongley, E. Flocculation of suspended sediment in rivers of southeastern Canada. Water Res.; 1994; 28, pp. 1799-1809. [DOI: https://dx.doi.org/10.1016/0043-1354(94)90253-4]

39. Skalska, K.; Ockelford, A.; Ebdon, J.E.; Cundy, A.B. Riverine microplastics: Behaviour, spatio-temporal variability, and recommendations for standardised sampling and monitoring. J. Water Process Eng.; 2020; 38, 101600. [DOI: https://dx.doi.org/10.1016/j.jwpe.2020.101600]

40. Kuizenga, B.; van Emmerik, T.; Waldschläger, K.; Kooi, M. Will it float? Rising and settling velocities of common macroplastic foils. Earth ArXiv; 2021; [DOI: https://dx.doi.org/10.31223/X5TW4B]

41. Ballent, A.; Purser, A.; de Jesus Mendes, P.; Pando, S.; Thomsen, L. Physical transport properties of marine microplastic pollution. Biogeosci. Discuss.; 2012; 9, pp. 18755-18798. [DOI: https://dx.doi.org/10.5194/bgd-9-18755-2012]

42. Ballent, A.; Pando, S.; Purser, A.; Juliano, M.; Thomsen, L. Modelled transport of benthic marine microplastic pollution in the Nazaré Canyon. Biogeoscience; 2013; 10, pp. 7957-7970. [DOI: https://dx.doi.org/10.5194/bg-10-7957-2013]

43. Kowalski, N.; Reichardt, A.M.; Waniek, J.J. Sinking rates of microplastics and potential implications of their alteration by physical, biological, and chemical factors. Mar. Pollut. Bull.; 2016; 109, pp. 310-319. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2016.05.064] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27297594]

44. Kaiser, D.; Kowalski, N.; Waniek, J.J. Effects of biofouling on the sinking behavior of microplastics. Environ. Res. Lett.; 2017; 12, 124003. [DOI: https://dx.doi.org/10.1088/1748-9326/aa8e8b]

45. Khatmullina, L.; Isachenko, I. Settling velocity of microplastic particles of regular shapes. Mar. Pollut. Bull.; 2017; 114, pp. 871-880. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2016.11.024]

46. Liedermann, M.; Gmeiner, P.; Pessenlehner, S.; Haimann, M.; Hohenblum, P.; Habersack, H. A methodology for measuring microplastic transport in large or medium rivers. Water; 2018; 10, 414. [DOI: https://dx.doi.org/10.3390/w10040414]

47. Guerrini, F.; Mari, L.; Casagrandi, R. The dynamics of microplastics and associated contaminants: Data-driven Lagrangian and Eulerian modelling approaches in the Mediterranean Sea. Sci. Total Environ.; 2021; 777, 145944. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2021.145944]

48. Zhang, Z.; Wu, H.; Peng, G.; Xu, P.; Li, D. Coastal ocean dynamics reduce the export of microplastics to the open ocean. Sci. Total Environ.; 2020; 713, 136634. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.136634] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32019021]

49. De Leo, A.; Cutroneo, L.; Sous, D.; Stocchino, A. Settling Velocity of Microplastics Exposed to Wave Action. J. Mar. Sci. Eng.; 2021; 9, 142. [DOI: https://dx.doi.org/10.3390/jmse9020142]

50. Khatmullina, L.; Chubarenko, I. Transport of marine microplastic particles: Why is it so difficult to predict?. Anthr. Coasts; 2019; 2, pp. 293-305. [DOI: https://dx.doi.org/10.1139/anc-2018-0024]

51. Chubarenko, I.; Bagaev, A.; Zobkov, M.; Esiukova, E. On some physical and dynamical properties of microplastic particles in marine environment. Mar. Pollut. Bull.; 2016; 108, pp. 105-112. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2016.04.048]

52. Zhang, H. Transport of microplastics in coastal seas. Estuar. Coast. Mar. Sci.; 2017; 199, pp. 74-86. [DOI: https://dx.doi.org/10.1016/j.ecss.2017.09.032]

53. Hurley, R.; Woodward, J.; Rothwell, J.J. Microplastic contamination of river beds significantly reduced by catchment-wide flooding. Nat. Geosci.; 2018; 11, pp. 251-257. [DOI: https://dx.doi.org/10.1038/s41561-018-0080-1]

54. Lechthaler, S.; Esser, V.; Schüttrumpf, H.; Stauch, G. Why analysing microplastics in floodplains matters: Application in a sedimentary context. Environ. Sci. Process. Impacts; 2021; 23, pp. 117-131. [DOI: https://dx.doi.org/10.1039/D0EM00431F]

55. Zbyszewski, M.; Corcoran, P.L.; Hockin, A. Comparison of the distribution and degradation of plastic debris along shorelines of the Great Lakes, North America. J. Great Lakes Res.; 2014; 40, pp. 288-299. [DOI: https://dx.doi.org/10.1016/j.jglr.2014.02.012]

56. Zettler, E.R.; Mincer, T.J.; Amaral-Zettler, L.A. Life in the “plastisphere”: Microbial communities on plastic marine debris. Environ. Sci. Tech.; 2013; 47, pp. 7137-7146. [DOI: https://dx.doi.org/10.1021/es401288x] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/23745679]

57. Andrady, A.L. Persistence of plastic litter in the oceans. Marine Anthropogenic Litter; Bergmann, M.; Gutow, L.; Klages, M. Springer: Cham, Switzerland, 2015; pp. 57-72.

58. Barnes, D.K.; Galgani, F.; Thompson, R.C.; Barlaz, M. Accumulation and fragmentation of plastic debris in global environments. Philos. Trans. R. Soc. Lond. Ser. B Biol. Sci.; 2009; 364, pp. 1985-1998. [DOI: https://dx.doi.org/10.1098/rstb.2008.0205] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/19528051]

59. Harrison, J.P.; Hoellein, T.J.; Sapp, M.; Tagg, A.S.; Ju-Nam, Y.; Ojeda, J.J. Microplastic-associated biofilms: A comparison of freshwater and marine environments. Freshwater Microplastics; Wagner, M.; Lambert, S. Springer: Cham, Switzerland, 2018; pp. 181-201.

60. Miao, L.; Gao, Y.; Adyel, T.M.; Huo, Z.; Liu, Z.; Wu, J.; Hou, J. Effects of biofilm colonization on the sinking of microplastics in three freshwater environments. J. Hazard. Mater.; 2021; 413, 125370. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2021.125370] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33609862]

61. Miao, L.; Wang, P.; Hou, J.; Yao, Y.; Liu, Z.; Liu, S.; Li, T. Distinct community structure and microbial functions of biofilms colonizing microplastics. Sci. Total Environ.; 2019; 650, pp. 2395-2402. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2018.09.378] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30292995]

62. Yan, M.; Wang, L.; Dai, Y.; Sun, H.; Liu, C. Behavior of Microplastics in Inland Waters: Aggregation, Settlement, and Transport. Bull. Environ. Contam. Toxicol.; 2021; pp. 1-10. [DOI: https://dx.doi.org/10.1007/s00128-020-03087-2]

63. Zhang, L.; Liu, J.; Xie, Y.; Zhong, S.; Yang, B.; Lu, D.; Zhong, Q. Distribution of microplastics in surface water and sediments of Qin river in Beibu Gulf, China. Sci. Total Environ.; 2020; 708, 135176. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.135176]

64. Mani, T.; Burkhardt-Holm, P. Seasonal microplastics variation in nival and pluvial stretches of the Rhine River–From the Swiss catchment towards the North Sea. Sci. Total Environ.; 2020; 707, 135579. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.135579]

65. Wagner, M.; Scherer, C.; Alvarez-Muñoz, D.; Brennholt, N.; Bourrain, X.; Buchinger, S.; Fries, E.; Grosbois, C.; Klasmeier, J.; Marti, T. Microplastics in freshwater ecosystems: What we know and what we need to know. Environ. Sci. Eur.; 2014; 26, 12. [DOI: https://dx.doi.org/10.1186/s12302-014-0012-7]

66. Nizzetto, L.; Bussi, G.; Futter, M.N.; Butterfield, D.; Whitehead, P.G. A theoretical assessment of microplastic transport in river catchments and their retention by soils and river sediments. Environ. Sci. Process. Impacts; 2016; 18, pp. 1050-1059. [DOI: https://dx.doi.org/10.1039/C6EM00206D]

67. Borges-Ramírez, M.M.; Mendoza-Franco, E.F.; Escalona-Segura, G.; Rendón-von Osten, J. Plastic density as a key factor in the presence of microplastic in the gastrointestinal tract of commercial fishes from Campeche Bay, Mexico. Environ. Pollut.; 2020; 267, 115659. [DOI: https://dx.doi.org/10.1016/j.envpol.2020.115659]

68. Horton, A.A.; Dixon, S.J. Microplastics: An introduction to environmental transport processes. WIRs Water; 2018; 5, e1268. [DOI: https://dx.doi.org/10.1002/wat2.1268]

69. Pico, Y.; Alfarhan, A.; Barcelo, D. Nano And Mciroplastic Analysis: Focus On Remediation Technologies And Occurrence In Freshwater Ecosystems. Trends Anal. Chem.; 2018; 113, pp. 409-425. [DOI: https://dx.doi.org/10.1016/j.trac.2018.08.022]

70. Waldschläger, K.; Schüttrumpf, H. Effects of particle properties on the settling and rise velocities of microplastics in freshwater under laboratory conditions. Environ. Sci. Technol.; 2019; 53, pp. 1958-1966. [DOI: https://dx.doi.org/10.1021/acs.est.8b06794] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30688437]

71. Wang, J.; Peng, J.; Tan, Z.; Gao, Y.; Zhan, Z.; Chen, Q.; Cai, L. Microplastics in the surface sediments from the Beijiang River littoral zone: Composition, abundance, surface textures and interaction with heavy metals. Chemosphere; 2017; 171, pp. 248-258. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2016.12.074] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28024210]

72. Wang, L.; Zheng, K.; Ding, Z.; Yan, X.; Zhang, H.; Cao, Y.; Guo, C. Drag coefficient and settling velocity of fine particles with varying surface wettability. Powder Technol.; 2020; 372, pp. 8-14. [DOI: https://dx.doi.org/10.1016/j.powtec.2020.05.102]

73. Soames, A.; Al-Anssari, S.; Iglauer, S.; Barifcani, A.; Gubner, R. Effect of wettability on particle settlement behavior within Mono-Ethylene Glycol regeneration pre-treatment systems. J. Pet. Sci. Eng.; 2019; 179, pp. 831-840. [DOI: https://dx.doi.org/10.1016/j.petrol.2019.04.108]

74. Waldschläger, K.; Schuüttrumpf, H. Erosion behavior of different microplastic particles in comparison to natural sediments. Environ. Sci. Technol.; 2019; 53, pp. 13219-13227. [DOI: https://dx.doi.org/10.1021/acs.est.9b05394]

75. Sharma, P.; Flury, M.; Zhou, J. Detachment of colloids from a solid surface by a moving air–water interface. J. Colloid Interface Sci.; 2008; 326, pp. 143-150. [DOI: https://dx.doi.org/10.1016/j.jcis.2008.07.030] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/18684467]

76. Sharma, P.; Abdou, H.M.; Flury, M. Effect of the lower boundary condition and flotation on colloid mobilization in unsaturated sandy sediments. Vadose Zone J.; 2008; 7, pp. 930-940. [DOI: https://dx.doi.org/10.2136/vzj2007.0163]

77. Hoellein, T.J.; Shogren, A.J.; Tank, J.L.; Risteca, P.; Kelly, J.J. Microplastic deposition velocity in streams follows patterns for naturally occurring allochthonous particles. Sci. Rep.; 2019; 9, pp. 1-11. [DOI: https://dx.doi.org/10.1038/s41598-019-40126-3]

78. Fu, Z.; Wang, J. Current practices and future perspectives of microplastic pollution in freshwater ecosystems in China. Sci. Total Environ.; 2019; 691, pp. 697-712. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.07.167]

79. Ockelford, A.; Cundy, A.; Ebdon, J.E. Storm response of fluvial sedimentary microplastics. Sci. Rep.; 2020; 10, pp. 1-10. [DOI: https://dx.doi.org/10.1038/s41598-020-58765-2]

80. Cole, M.; Lindeque, P.; Halsband, C.; Galloway, T.S. Microplastics as contaminants in the marine environment: A review. Mar. Pollut. Bull.; 2011; 62, pp. 2588-2597. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2011.09.025]

81. Hammer, J.; Kraak, M.H.; Parsons, J.R. Plastics in the marine environment: The dark side of a modern gift. Reviews of Environmental Contamination and Toxicology; Whitacre, D.M. Springer: New York, NY, USA, 2012; pp. 1-44.

82. Guerranti, C.; Cannas, S.; Scopetani, C.; Fastelli, P.; Cincinelli, A.; Renzi, M. Plastic litter in aquatic environments of Maremma Regional Park (Tyrrhenian Sea, Italy): Contribution by the Ombrone river and levels in marine sediments. Mar. Pollut. Bull.; 2017; 117, pp. 366-370. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2017.02.021]

83. Schmidt, C.; Kumar, R.; Yang, S.; Büttner, O. Microplastic particle emission from wastewater treatment plant effluents into river networks in Germany: Loads, spatial patterns of concentrations and potential toxicity. Sci. Total Environ.; 2020; 737, 139544. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.139544] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32512293]

84. De Villiers, S. Microfibre pollution hotspots in river sediments adjacent to South Africa’s coastline. Water SA; 2019; 45, pp. 97-102. [DOI: https://dx.doi.org/10.4314/wsa.v45i1.11]

85. McCormick, A.; Hoellein, T.J.; Mason, S.A.; Schluep, J.; Kelly, J.J. Microplastic is an abundant and distinct microbial habitat in an urban river. Environ. Sci. Technol.; 2014; 48, pp. 11863-11871. [DOI: https://dx.doi.org/10.1021/es503610r] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25230146]

86. Estahbanati, S.; Fahrenfeld, N.L. Influence of wastewater treatment plant discharges on microplastic concentrations in surface water. Chemosphere; 2016; 162, pp. 277-284. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2016.07.083] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27508863]

87. Kasamesiri, P.; Thaimuangphol, W. Microplastics ingestion by freshwater fish in the Chi river, Thailand. Intern. J. GEOMATE; 2020; 18, pp. 114-119. [DOI: https://dx.doi.org/10.21660/2020.67.9110]

88. Zheng, K.; Fan, Y.; Zhu, Z.; Chen, G.; Tang, C.; Peng, X. Occurrence and species-specific distribution of plastic debris in wild freshwater fish from the Pearl River Catchment, China. Environ. Toxicol. Chem.; 2019; 38, pp. 1504-1513. [DOI: https://dx.doi.org/10.1002/etc.4437] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30953377]

89. Blair, R.M.; Waldron, S.; Phoenix, V.R.; Gauchotte-Lindsay, C. Microscopy and elemental analysis characterisation of microplastics in sediment of a freshwater urban river in Scotland, UK. Environ. Sci. Pollut. Res.; 2019; 26, pp. 12491-12504. [DOI: https://dx.doi.org/10.1007/s11356-019-04678-1] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30848429]

90. Eriksen, M.; Mason, S.; Wilson, S.; Box, C.; Zellers, A.; Edwards, W.; Farley, H.; Amato, S. Microplastic pollution in the surface waters of the Laurentian Great Lakes. Mar. Pollut. Bull.; 2013; 77, pp. 177-182. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2013.10.007] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/24449922]

91. Jiang, J.-Q. Occurrence of microplastics and its pollution in the environment: A review. Sustain. Prod. Consump.; 2018; 13, pp. 16-23. [DOI: https://dx.doi.org/10.1016/j.spc.2017.11.003]

92. Browne, M.A.; Crump, P.; Niven, S.J.; Teuten, E.; Tonkin, A.; Galloway, T.; Thompson, R. Accumulation of microplastic on shorelines woldwide: Sources and sinks. Environ. Sci. Technol.; 2011; 45, pp. 9175-9179. [DOI: https://dx.doi.org/10.1021/es201811s]

93. Almroth, B.M.C.; Åström, L.; Roslund, S.; Petersson, H.; Johansson, M.; Persson, N.-K. Quantifying shedding of synthetic fibers from textiles; a source of microplastics released into the environment. Environ. Sci. Pollut. Res.; 2018; 25, pp. 1191-1199. [DOI: https://dx.doi.org/10.1007/s11356-017-0528-7]

94. Wang, W.; Ndungu, A.W.; Li, Z.; Wang, J. Microplastics pollution in inland freshwaters of China: A case study in urban surface waters of Wuhan, China. Sci. Total Environ.; 2017; 575, pp. 1369-1374. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2016.09.213]

95. Mani, T.; Hauk, A.; Walter, U.; Burkhardt-Holm, P. Microplastics profile along the Rhine River. Sci. Rep.; 2015; 5, pp. 1-7. [DOI: https://dx.doi.org/10.1038/srep17988]

96. Schmidt, L.K.; Bochow, M.; Imhof, H.K.; Oswald, S.E. Multi-temporal surveys for microplastic particles enabled by a novel and fast application of SWIR imaging spectroscopy–Study of an urban watercourse traversing the city of Berlin, Germany. Environ. Pollut.; 2018; 239, pp. 579-589. [DOI: https://dx.doi.org/10.1016/j.envpol.2018.03.097]

97. He, B.; Goonetilleke, A.; Ayoko, G.A.; Rintoul, L. Abundance, distribution patterns, and identification of microplastics in Brisbane River sediments, Australia. Sci. Total Environ.; 2020; 700, 134467. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.134467]

98. Fan, Y.; Zheng, K.; Zhu, Z.; Chen, G.; Peng, X. Distribution, sedimentary record, and persistence of microplastics in the Pearl River catchment, China. Environ. Pollut.; 2019; 251, pp. 862-870. [DOI: https://dx.doi.org/10.1016/j.envpol.2019.05.056]

99. Alam, F.C.; Sembiring, E.; Muntalif, B.S.; Suendo, V. Microplastic distribution in surface water and sediment river around slum and industrial area (case study: Ciwalengke River, Majalaya district, Indonesia). Chemosphere; 2019; 224, pp. 637-645. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2019.02.188]

100. Grbić, J.; Helm, P.; Athey, S.; Rochman, C.M. Microplastics entering northwestern Lake Ontario are diverse and linked to urban sources. Water Res.; 2020; 174, 115623. [DOI: https://dx.doi.org/10.1016/j.watres.2020.115623]

101. Galafassi, S.; Nizzetto, L.; Volta, P. Plastic sources: A survey across scientific and grey literature for their inventory and relative contribution to microplastics pollution in natural environments, with an emphasis on surface water. Sci. Total Environ.; 2019; 693, 133499. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.07.305] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31377368]

102. Klein, S.; Worch, E.; Knepper, T.P. Occurrence and spatial distribution of microplastics in river shore sediments of the Rhine-Main area in Germany. Environ. Sci. Technol.; 2015; 49, pp. 6070-6076. [DOI: https://dx.doi.org/10.1021/acs.est.5b00492] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25901760]

103. Campanale, C.; Stock, F.; Massarelli, C.; Kochleus, C.; Bagnuolo, G.; Reifferscheid, G.; Uricchio, V.F. Microplastics and their possible sources: The example of Ofanto river in Southeast Italy. Environ. Pollut.; 2020; 258, 113284. [DOI: https://dx.doi.org/10.1016/j.envpol.2019.113284] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32005487]

104. Crew, A.; Gregory-Eaves, I.; Ricciardi, A. Distribution, abundance, and diversity of microplastics in the upper St. Lawrence River. Environ. Pollut.; 2020; 260, 113994. [DOI: https://dx.doi.org/10.1016/j.envpol.2020.113994]

105. Yonkos, L.T.; Friedel, E.A.; Perez-Reyes, A.C.; Ghosal, S.; Arthur, C.D. Microplastics in four estuarine rivers in the Chesapeake Bay, USA. Environ. Sci. Technol.; 2014; 48, pp. 14195-14202. [DOI: https://dx.doi.org/10.1021/es5036317]

106. Horton, A.A.; Walton, A.; Spurgeon, D.J.; Lahive, E.; Svendsen, C. Microplastics in freshwater and terrestrial environments: Evaluating the current understanding to identify the knowledge gaps and future research priorities. Sci. Total Environ.; 2017; 586, pp. 127-141. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2017.01.190]

107. Wang, X.; Bolan, N.; Tsang, D.C.; Sarkar, B.; Bradney, L.; Li, Y. A review of microplastics aggregation in aquatic environment: Influence factors, analytical methods, and environmental implications. J. Hazard. Mater.; 2021; 402, 123496. [DOI: https://dx.doi.org/10.1016/j.jhazmat.2020.123496] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32717542]

108. Anderson, J.C.; Park, B.J.; Palace, V.P. Microplastics in aquatic environments: Implications for Canadian ecosystems. Environ. Pollut.; 2016; 218, pp. 269-280. [DOI: https://dx.doi.org/10.1016/j.envpol.2016.06.074] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27431693]

109. Waldman, W.R.; Rillig, M.C. Microplastic Research Should Embrace the Complexity of Secondary Particles. Environ. Sci. Tech.; 2020; 54, pp. 7751-7753. [DOI: https://dx.doi.org/10.1021/acs.est.0c02194] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32559095]

110. Pauli, N.-C.; Petermann, J.S.; Lott, C.; Weber, M. Macrofouling communities and the degradation of plastic bags in the sea: An in situ experiment. R. Soc. Open Sci.; 2017; 4, 170549. [DOI: https://dx.doi.org/10.1098/rsos.170549]

111. Castañeda, R.A.; Avlijas, S.; Simard, M.A.; Ricciardi, A. Microplastic pollution in St. Lawrence river sediments. Can. J. Fish. Aquat.; 2014; 71, pp. 1767-1771. [DOI: https://dx.doi.org/10.1139/cjfas-2014-0281]

112. Lenaker, P.L.; Baldwin, A.K.; Corsi, S.R.; Mason, S.A.; Reneau, P.C.; Scott, J.W. Vertical distribution of microplastics in the water column and surficial sediment from the Milwaukee River Basin to Lake Michigan. Environ. Sci. Technol.; 2019; 53, pp. 12227-12237. [DOI: https://dx.doi.org/10.1021/acs.est.9b03850]

113. Enders, K.; Käppler, A.; Biniasch, O.; Feldens, P.; Stollberg, N.; Lange, X.; Fischer, D.; Eichhorn, K.-J.; Pollehne, F.; Oberbeckmann, S. Tracing microplastics in aquatic environments based on sediment analogies. Sci. Rep.; 2019; 9, pp. 1-15. [DOI: https://dx.doi.org/10.1038/s41598-019-50508-2]

114. Hidalgo-Ruz, V.; Gutow, L.; Thompson, R.C.; Thiel, M. Microplastics in the marine environment: A review of the methods used for identification and quantification. Environ. Sci. Technol.; 2012; 46, pp. 3060-3075. [DOI: https://dx.doi.org/10.1021/es2031505]

115. Scherer, C.; Weber, A.; Stock, F.; Vurusic, S.; Egerci, H.; Kochleus, C.; Arendt, N.; Foeldi, C.; Dierkes, G.; Wagner, M. Comparative assessment of microplastics in water and sediment of a large European river. Sci. Total Environ.; 2020; 738, 139866. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.139866]

116. Akdogan, Z.; Guven, B. Microplastics in the environment: A critical review of current understanding and identification of future research needs. Environ. Pollut.; 2019; 254, 113011. [DOI: https://dx.doi.org/10.1016/j.envpol.2019.113011]

117. Haberstroh, C.J.; Arias, M.E.; Yin, Z.; Wang, M.C. Effects of hydrodynamics on the cross-sectional distribution and transport of plastic in an urban coastal river. Water Environ. Res.; 2021; 93, pp. 186-200. [DOI: https://dx.doi.org/10.1002/wer.1386]

118. Frei, S.; Piehl, S.; Gilfedder, B.; Löder, M.; Krutzke, J.; Wilhelm, L.; Laforsch, C. Occurence of microplastics in the hyporheic zone of rivers. Sci. Rep.; 2019; 9, pp. 1-11. [DOI: https://dx.doi.org/10.1038/s41598-019-51741-5]

119. Zhao, S.; Zhu, L.; Wang, T.; Li, D. Suspended microplastics in the surface water of the Yangtze Estuary System, China: First observations on occurrence, distribution. Mar. Pollut. Bull.; 2014; 86, pp. 562-568. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2014.06.032]

120. Faure, F.; Demars, C.; Wieser, O.; Kunz, M.; De Alencastro, L.F. Plastic pollution in Swiss surface waters: Nature and concentrations, interaction with pollutants. Environ. Chem.; 2015; 12, pp. 582-591. [DOI: https://dx.doi.org/10.1071/EN14218]

121. Mintenig, S.; Kooi, M.; Erich, M.; Primpke, S.; Redondo-Hasselerharm, P.; Dekker, S.; Koelmans, A.; van Wezel, A. A systems approach to understand microplastic occurrence and variability in Dutch riverine surface waters. Water Res.; 2020; 176, 115723. [DOI: https://dx.doi.org/10.1016/j.watres.2020.115723] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32220661]

122. Filella, M. Questions of size and numbers in environmental research on microplastics: Methodological and conceptual aspects. Environ. Chem.; 2015; 12, pp. 527-538. [DOI: https://dx.doi.org/10.1071/EN15012]

123. Cable, R.N.; Beletsky, D.; Beletsky, R.; Wigginton, K.; Locke, B.W.; Duhaime, M.B. Distribution and modeled transport of plastic pollution in the Great Lakes, the world’s largest freshwater resource. Front. Environ. Sci.; 2017; 5, 45. [DOI: https://dx.doi.org/10.3389/fenvs.2017.00045]

124. Ryan, P.G. Does size and buoyancy affect the long-distance transport of floating debris?. Environ. Res. Lett.; 2015; 10, 084019. [DOI: https://dx.doi.org/10.1088/1748-9326/10/8/084019]

125. Besseling, E.; Quik, J.T.; Sun, M.; Koelmans, A.A. Fate of nano-and microplastic in freshwater systems: A modeling study. Environ. Pollut.; 2017; 220, pp. 540-548. [DOI: https://dx.doi.org/10.1016/j.envpol.2016.10.001] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27743792]

126. Kooi, M.; Besseling, E.; Kroeze, C.; Van Wezel, A.P.; Koelmans, A.A. Modeling the fate and transport of plastic debris in freshwaters: Review and guidance. Freshwater Microplastics; Wagner, M.; Lambert, S. Springer: Cham, Switzerland, 2018; pp. 125-152.

127. Hotze, E.M.; Phenrat, T.; Lowry, G.V. Nanoparticle aggregation: Challenges to understanding transport and reactivity in the environment. J. Environ. Qual.; 2010; 39, pp. 1909-1924. [DOI: https://dx.doi.org/10.2134/jeq2009.0462] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/21284288]

128. Song, Z.; Yang, X.; Chen, F.; Zhao, F.; Zhao, Y.; Ruan, L.; Wang, Y.; Yang, Y. Fate and transport of nanoplastics in complex natural aquifer media: Effect of particle size and surface functionalization. Sci. Total Environ.; 2019; 669, pp. 120-128. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.03.102]

129. Lu, S.; Zhu, K.; Song, W.; Song, G.; Chen, D.; Hayat, T.; Alharbi, N.S.; Chen, C.; Sun, Y. Impact of water chemistry on surface charge and aggregation of polystyrene microspheres suspensions. Sci. Total Environ.; 2018; 630, pp. 951-959. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2018.02.296]

130. Mao, Y.; Li, H.; Huangfu, X.; Liu, Y.; He, Q. Nanoplastics display strong stability in aqueous environments: Insights from aggregation behaviour and theoretical calculations. Environ. Pollut.; 2020; 258, 113760. [DOI: https://dx.doi.org/10.1016/j.envpol.2019.113760]

131. Cai, L.; Hu, L.; Shi, H.; Ye, J.; Zhang, Y.; Kim, H. Effects of inorganic ions and natural organic matter on the aggregation of nanoplastics. Chemosphere; 2018; 197, pp. 142-151. [DOI: https://dx.doi.org/10.1016/j.chemosphere.2018.01.052]

132. Jódar-Reyes, A.B.; Martín-Rodríguez, A.; Ortega-Vinuesa, J.L. Effect of the ionic surfactant concentration on the stabilization/destabilization of polystyrene colloidal particles. J. Colloid Interface Sci.; 2006; 298, pp. 248-257. [DOI: https://dx.doi.org/10.1016/j.jcis.2005.12.035] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/16457839]

133. Lagarde, F.; Olivier, O.; Zanella, M.; Daniel, P.; Hiard, S.; Caruso, A. Microplastic interactions with freshwater microalgae: Hetero-aggregation and changes in plastic density appear strongly dependent on polymer type. Environ. Pollut.; 2016; 215, pp. 331-339. [DOI: https://dx.doi.org/10.1016/j.envpol.2016.05.006] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/27236494]

134. Liu, Y.; Zhang, J.; Cai, C.; He, Y.; Chen, L.; Xiong, X.; Huang, H.; Tao, S.; Liu, W. Occurrence and characteristics of microplastics in the Haihe River: An investigation of a seagoing river flowing through a megacity in northern China. Environ. Pollut.; 2020; 262, 114261. [DOI: https://dx.doi.org/10.1016/j.envpol.2020.114261] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32120261]

135. Lobelle, D.; Cunliffe, M. Early microbial biofilm formation on marine plastic debris. Mar. Pollut. Bull.; 2011; 62, pp. 197-200. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2010.10.013] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/21093883]

136. Gong, M.; Yang, G.; Zhuang, L.; Zeng, E.Y. Microbial biofilm formation and community structure on low-density polyethylene microparticles in lake water microcosms. Environ. Pollut.; 2019; 252, pp. 94-102. [DOI: https://dx.doi.org/10.1016/j.envpol.2019.05.090]

137. Oriekhova, O.; Stoll, S. Heteroaggregation of nanoplastic particles in the presence of inorganic colloids and natural organic matter. Environ. Sci. Nano; 2018; 5, pp. 792-799. [DOI: https://dx.doi.org/10.1039/C7EN01119A]

138. Dong, Z.; Zhang, W.; Qiu, Y.; Yang, Z.; Wang, J.; Zhang, Y. Cotransport of nanoplastics (NPs) with fullerene (C60) in saturated sand: Effect of NPs/C60 ratio and seawater salinity. Water Res.; 2019; 148, pp. 469-478. [DOI: https://dx.doi.org/10.1016/j.watres.2018.10.071] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30408733]

139. Li, Y.; Wang, X.; Fu, W.; Xia, X.; Liu, C.; Min, J.; Zhang, W.; Crittenden, J.C. Interactions between nano/micro plastics and suspended sediment in water: Implications on aggregation and settling. Water Res.; 2019; 161, pp. 486-495. [DOI: https://dx.doi.org/10.1016/j.watres.2019.06.018] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31229729]

140. Yoon, J.-H.; Kawano, S.; Igawa, S. Modeling of marine litter drift and beaching in the Japan Sea. Mar. Pollut. Bull.; 2010; 60, pp. 448-463. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2009.09.033] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/19889428]

141. Kako, S.i.; Isobe, A.; Kataoka, T.; Hinata, H. A decadal prediction of the quantity of plastic marine debris littered on beaches of the East Asian marginal seas. Mar. Pollut. Bull.; 2014; 81, pp. 174-184. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2014.01.057] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/24559735]

142. Neumann, D.; Callies, U.; Matthies, M. Marine litter ensemble transport simulations in the southern North Sea. Mar. Pollut. Bull.; 2014; 86, pp. 219-228. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2014.07.016]

143. Mansui, J.; Molcard, A.; Ourmières, Y. Modelling the transport and accumulation of floating marine debris in the Mediterranean basin. Mar. Pollut. Bull.; 2015; 91, pp. 249-257. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2014.11.037] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/25534631]

144. Liubartseva, S.; Coppini, G.; Lecci, R.; Creti, S. Regional approach to modeling the transport of floating plastic debris in the Adriatic Sea. Mar. Pollut. Bull.; 2016; 103, pp. 115-127. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2015.12.031]

145. Jalón-Rojas, I.; Wang, X.H.; Fredj, E. A 3D numerical model to track marine plastic debris (TrackMPD): Sensitivity of microplastic trajectories and fates to particle dynamical properties and physical processes. Mar. Pollut. Bull.; 2019; 141, pp. 256-272. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2019.02.052] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30955734]

146. Newbould, R. Understanding river plastic transport with tracers and GPS. Nat. Rev. Earth Environ.; 2021; 2, 591. [DOI: https://dx.doi.org/10.1038/s43017-021-00212-5]

147. Dris, R.; Gasperi, J.; Rocher, V.; Saad, M.; Renault, N.; Tassin, B. Microplastic contamination in an urban area: A case study in Greater Paris. Environ. Chem.; 2015; 12, pp. 592-599. [DOI: https://dx.doi.org/10.1071/EN14167]

148. Dris, R.; Gasperi, J.; Rocher, V.; Tassin, B. Synthetic and non-synthetic anthropogenic fibers in a river under the impact of Paris Megacity: Sampling methodological aspects and flux estimations. Sci. Total Environ.; 2018; 618, pp. 157-164. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2017.11.009]

149. Di, M.; Wang, J. Microplastics in surface waters and sediments of the Three Gorges Reservoir, China. Sci. Total Environ.; 2018; 616, pp. 1620-1627. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2017.10.150]

150. Lechner, A.; Keckeis, H.; Lumesberger-Loisl, F.; Zens, B.; Krusch, R.; Tritthart, M.; Glas, M.; Schludermann, E. The Danube so colourful: A potpourri of plastic litter outnumbers fish larvae in Europe’s second largest river. Environ. Pollut.; 2014; 188, pp. 177-181. [DOI: https://dx.doi.org/10.1016/j.envpol.2014.02.006] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/24602762]

151. Pojar, I.; Stănică, A.; Stock, F.; Kochleus, C.; Schultz, M.; Bradley, C. Sedimentary microplastic concentrations from the Romanian Danube River to the Black Sea. Sci. Rep.; 2021; 11, pp. 1-9. [DOI: https://dx.doi.org/10.1038/s41598-021-81724-4] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33479493]

152. Van Emmerik, T.; Kieu-Le, T.-C.; Loozen, M.; van Oeveren, K.; Strady, E.; Bui, X.-T.; Egger, M.; Gasperi, J.; Lebreton, L.; Nguyen, P.-D. A methodology to characterize riverine macroplastic emission into the ocean. Front. Mar. Sci.; 2018; 5, 372. [DOI: https://dx.doi.org/10.3389/fmars.2018.00372]

153. Corcoran, P.L.; Belontz, S.L.; Ryan, K.; Walzak, M.J. Factors Controlling the Distribution of Microplastic Particles in Benthic Sediment of the Thames River, Canada. Environ. Sci. Technol.; 2019; 54, pp. 818-825. [DOI: https://dx.doi.org/10.1021/acs.est.9b04896] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31884780]

154. Wu, F.; Pennings, S.C.; Tong, C.; Xu, Y. Variation in microplastics composition at small spatial and temporal scales in a tidal flat of the Yangtze Estuary, China. Sci. Total Environ.; 2020; 699, 134252. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.134252] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31629313]

155. Liro, M.; Emmerik, T.v.; Wyżga, B.; Liro, J.; Mikuś, P. Macroplastic storage and remobilization in rivers. Water; 2020; 12, 2055. [DOI: https://dx.doi.org/10.3390/w12072055]

156. Hübner, M.K.; Michler-Kozma, D.N.; Gabel, F. Microplastic concentrations at the water surface are reduced by decreasing flow velocities caused by a reservoir. Fundam. Appl. Limnol.; 2020; 194, pp. 49-56. [DOI: https://dx.doi.org/10.1127/fal/2020/1307]

157. Roebroek, C.T.; Harrigan, S.; van Emmerik, T.; Baugh, C.; Eilander, D.; Prudhomme, C.; Pappenberger, F. Plastic in global rivers: Are floods making it worse?. Environ. Res. Lett.; 2020; 16, 025003. [DOI: https://dx.doi.org/10.1088/1748-9326/abd5df]

158. Weideman, E.A.; Perold, V.; Ryan, P.G. Little evidence that dams in the Orange–Vaal River system trap floating microplastics or microfibres. Mar. Pollut. Bull.; 2019; 149, 110664. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2019.110664]

159. Mudd, S.M.; D’Alpaos, A.; Morris, J.T. How does vegetation affect sedimentation on tidal marshes? Investigating particle capture and hydrodynamic controls on biologically mediated sedimentation. J. Geophys. Res. Earth Surf.; 2010; 115, F03029. [DOI: https://dx.doi.org/10.1029/2009JF001566]

160. Bondelind, M.; Sokolova, E.; Nguyen, A.; Karlsson, D.; Karlsson, A.; Björklund, K. Hydrodynamic modelling of traffic-related microplastics discharged with stormwater into the Göta River in Sweden. Environ. Sci. Pollut. Res.; 2020; 27, pp. 24218-24230. [DOI: https://dx.doi.org/10.1007/s11356-020-08637-z]

161. Watkins, L.; McGrattan, S.; Sullivan, P.J.; Walter, M.T. The effect of dams on river transport of microplastic pollution. Sci. Total Environ.; 2019; 664, pp. 834-840. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2019.02.028]

162. Andrady, A.L. Microplastics in the marine environment. Mar. Pollut. Bull.; 2011; 62, pp. 1596-1605. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2011.05.030]

163. Kooi, M.; Nes, E.H.v.; Scheffer, M.; Koelmans, A.A. Ups and downs in the ocean: Effects of biofouling on vertical transport of microplastics. Environ. Sci. Technol.; 2017; 51, pp. 7963-7971. [DOI: https://dx.doi.org/10.1021/acs.est.6b04702]

164. Song, Y.K.; Hong, S.H.; Jang, M.; Han, G.M.; Jung, S.W.; Shim, W.J. Combined effects of UV exposure duration and mechanical abrasion on microplastic fragmentation by polymer type. Environ. Sci. Technol.; 2017; 51, pp. 4368-4376. [DOI: https://dx.doi.org/10.1021/acs.est.6b06155]

165. Parrish, K.; Fahrenfeld, N. Microplastic biofilm in fresh-and wastewater as a function of microparticle type and size class. Environ. Sci. Water Res. Technol.; 2019; 5, pp. 495-505. [DOI: https://dx.doi.org/10.1039/C8EW00712H]

166. Meng, Y.; Kelly, F.J.; Wright, S.L. Advances and challenges of microplastic pollution in freshwater ecosystems: A UK perspective. Environ. Pollut.; 2020; 256, 113445. [DOI: https://dx.doi.org/10.1016/j.envpol.2019.113445] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/31733965]

167. Alimi, O.S.; Farner Budarz, J.; Hernandez, L.M.; Tufenkji, N. Microplastics and nanoplastics in aquatic environments: Aggregation, deposition, and enhanced contaminant transport. Environ. Sci. Technol.; 2018; 52, pp. 1704-1724. [DOI: https://dx.doi.org/10.1021/acs.est.7b05559] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/29265806]

168. Tolhurst, T.; Black, K.; Paterson, D.; Mitchener, H.; Termaat, G.; Shayler, S. A comparison and measurement standardisation of four in situ devices for determining the erosion shear stress of intertidal sediments. Cont. Shelf Res.; 2000; 20, pp. 1397-1418. [DOI: https://dx.doi.org/10.1016/S0278-4343(00)00029-7]

169. Ji, Z.-G. Hydrodynamics and Water Quality: Modeling Rivers, Lakes, and Estuaries; John Wiley & Sons: Hoboken, NJ, USA, 2017.

170. Hoellein, T.J.; McCormick, A.R.; Hittie, J.; London, M.G.; Scott, J.W.; Kelly, J.J. Longitudinal patterns of microplastic concentration and bacterial assemblages in surface and benthic habitats of an urban river. Freshw. Sci.; 2017; 36, pp. 491-507. [DOI: https://dx.doi.org/10.1086/693012]

171. Xiong, X.; Wu, C.; Elser, J.J.; Mei, Z.; Hao, Y. Occurrence and fate of microplastic debris in middle and lower reaches of the Yangtze River–From inland to the sea. Sci. Total Environ.; 2019; 659, pp. 66-73. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2018.12.313] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30597469]

172. Chinfak, N.; Sompongchaiyakul, P.; Charoenpong, C.; Shi, H.; Yeemin, T.; Zhang, J. Abundance, composition, and fate of microplastics in water, sediment, and shellfish in the Tapi-Phumduang River system and Bandon Bay, Thailand. Sci. Total Environ.; 2021; 781, 146700. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2021.146700] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/33812121]

173. He, B.; Wijesiri, B.; Ayoko, G.A.; Egodawatta, P.; Rintoul, L.; Goonetilleke, A. Influential factors on microplastics occurrence in river sediments. Sci. Total Environ.; 2020; 738, 139901. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.139901] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/32531603]

174. Kataoka, T.; Nihei, Y.; Kudou, K.; Hinata, H. Assessment of the sources and inflow processes of microplastics in the river environments of Japan. Environ. Pollut.; 2019; 244, pp. 958-965. [DOI: https://dx.doi.org/10.1016/j.envpol.2018.10.111]

175. Gerolin, C.R.; Pupim, F.N.; Sawakuchi, A.O.; Grohmann, C.H.; Labuto, G.; Semensatto, D. Microplastics in sediments from Amazon rivers, Brazil. Sci. Total Environ.; 2020; 749, 141604. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2020.141604]

176. Wong, G.; Löwemark, L.; Kunz, A. Microplastic pollution of the Tamsui River and its tributaries in northern Taiwan: Spatial heterogeneity and correlation with precipitation. Environ. Pollut.; 2020; 260, 113935. [DOI: https://dx.doi.org/10.1016/j.envpol.2020.113935]

177. Reisser, J.; Slat, B.; Noble, K.; Du Plessis, K.; Epp, M.; Proietti, M.; de Sonneville, J.; Becker, T.; Pattiaratchi, C. The vertical distribution of buoyant plastics at sea: An observational study in the North Atlantic Gyre. Biogeoscience; 2015; 12, pp. 1249-1256. [DOI: https://dx.doi.org/10.5194/bg-12-1249-2015]

178. Isachenko, I. Catching the variety: Obtaining the distribution of terminal velocities of microplastics particles in a stagnant fluid by a stochastic simulation. Mar. Pollut. Bull.; 2020; 159, 111464. [DOI: https://dx.doi.org/10.1016/j.marpolbul.2020.111464]

179. Wang, Z.; Taylor, S.E.; Sharma, P.; Flury, M. Poor extraction efficiencies of polystyrene nano-and microplastics from biosolids and soil. PLoS ONE; 2018; 13, e0208009. [DOI: https://dx.doi.org/10.1371/journal.pone.0208009] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/30496263]

180. Stokes, G.G. On the effect of the internal friction of fluids on the motion of pendulums. Trans. Camb. Philos. Soc.; 1851; 9, pp. 8-106. [DOI: https://dx.doi.org/10.1017/CBO9780511702266.002]

181. Bagaev, A.; Mizyuk, A.; Khatmullina, L.; Isachenko, I.; Chubarenko, I. Anthropogenic fibres in the Baltic Sea water column: Field data, laboratory and numerical testing of their motion. Sci. Total Environ.; 2017; 599, pp. 560-571. [DOI: https://dx.doi.org/10.1016/j.scitotenv.2017.04.185] [PubMed: https://www.ncbi.nlm.nih.gov/pubmed/28494282]

182. Kukulka, T.; Proskurowski, G.; Morét-Ferguson, S.; Meyer, D.; Law, K. The effect of wind mixing on the vertical distribution of buoyant plastic debris. Geophys. Res. Lett.; 2012; 39, [DOI: https://dx.doi.org/10.1029/2012GL051116]

183. Kooi, M.; Reisser, J.; Slat, B.; Ferrari, F.F.; Schmid, M.S.; Cunsolo, S.; Brambini, R.; Noble, K.; Sirks, L.-A.; Linders, T.E. The effect of particle properties on the depth profile of buoyant plastics in the ocean. Sci. Rep.; 2016; 6, pp. 1-10. [DOI: https://dx.doi.org/10.1038/srep33882]

184. Dey, S.; Zeeshan Ali, S.; Padhi, E. Terminal fall velocity: The legacy of Stokes from the perspective of fluvial hydraulics. Proc. R. Soc. A Math. Phys. Eng. Sci.; 2019; 475, 20190277. [DOI: https://dx.doi.org/10.1098/rspa.2019.0277]

185. Waldschläger, K.; Born, M.; Cowger, W.; Gray, A.; Schüttrumpf, H. Settling and rising velocities of environmentally weathered micro-and macroplastic particles. Environ. Res.; 2020; 191, 110192. [DOI: https://dx.doi.org/10.1016/j.envres.2020.110192]

186. Alimi, O.S.; Farner, J.M.; Tufenkji, N. Exposure of nanoplastics to freeze-thaw leads to aggregation and reduced transport in model groundwater environments. Water Res.; 2020; 189, 116533. [DOI: https://dx.doi.org/10.1016/j.watres.2020.116533]

187. Thompson, R.C.; Olsen, Y.; Mitchell, R.P.; Davis, A.; Rowland, S.J.; John, A.W.; McGonigle, D.; Russell, A.E. Lost at sea: Where is all the plastic?. Science; 2004; 304, 838. [DOI: https://dx.doi.org/10.1126/science.1094559]

188. Doyle, M.J.; Watson, W.; Bowlin, N.M.; Sheavly, S.B. Plastic particles in coastal pelagic ecosystems of the Northeast Pacific ocean. Mar. Environ. Res.; 2011; 71, pp. 41-52. [DOI: https://dx.doi.org/10.1016/j.marenvres.2010.10.001]

189. Mrokowska, M.M.; Krztoń-Maziopa, A. Viscoelastic and shear-thinning effects of aqueous exopolymer solution on disk and sphere settling. Sci. Rep.; 2019; 9, pp. 1-13. [DOI: https://dx.doi.org/10.1038/s41598-019-44233-z]

190. Corey, A.T. Influence of Shape on the Fall Velocity of Sand Grains. Ph.D. Thesis; Colorado State University: Fort Collins, CO, USA, 1949.