The transport mechanisms of plastic pollution in rivers are currently poorly understood, hindering our ability to accurately monitor plastics, predict their fluxes, and ultimately intercept them. However, it has recently been shown that turbulent transport models designed for natural sediments, such as the Rouse model, can be used to quantify the vertical distribution of suspended plastic pollution in rivers with an uncertainty of within ±10%, despite differences in bed load or surfaced transport. These models are based on the ratio of the river’s shear velocity to the plastic’s settling (or rising) velocity, which has been shown to be represented by mono- or multi-model probability functions, due to variation in plastic shape, size, density and biofilm colonisation. In order for a Rouse-based model to be applicable as a method of monitoring plastics, and quantifying their concentration and transport in real rivers, a database of the settling velocities of the most commonly occurring macroplastic pollution in rivers, and their probability functions, is needed.

This study aims to explain the settling/rising velocities of the most commonly observed riverine plastics and to describe their full statistical functions. This includes calculating each plastic’s mono- or multi-model settling/rising velocity probability distribution. To achieve this, a state-of-the-art 2 × 2 × 2 m³ settling tank and a high-speed, synchronous multi-camera set up, with an automated plastic detection routine, will be used to describe the dynamics and calculate the settling/rising velocities and full probabilistic functions of the most prominent macroplastic items found in rivers. The plastic samples used in experiments will represent categories within the River-OSPAR litter index, which is an index used to classify plastics pollution by their type, size and material. This includes categories such as plastic cups, food wrappers, and cigarette filters. The data generated from these experiments can be used to shape Rouse-like transport models that can predict the vertical positioning and the concentration profiles of River-OPSAR plastics in the suspended layers of rivers, thus supporting the development of more accurate monitoring strategies for plastics in rivers and improving plastic quantification methods.

How to cite: Lofty, J., Valero, D., and Franca, M.: Determining the settling and rising velocities of the top polluting macroplastics in rivers , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3136, https://doi.org/10.5194/egusphere-egu25-3136, 2025.

River networks are the major pathways for microplastic (MP) transport from terrestrial environments to oceans. However, the ability to quantify the water–sediment exchange of MPs, locations of deposition, and the time scales over which burial occurs is limited and thus often our estimation of where MP deposit is biased. To fill this gap, previous work on processes that control MP deposition will be briefly reviewed, with the aim of enhancing our understanding of the dynamic interplay between flow, sediment transport, and MP deposition. Detailed studies on MP deposition onto surficial sediment show that MP transport can be explained by the shear stress theory, hyporheic exchange, and bioturbation. Nevertheless, these processes cannot fully explain the observed distribution of MPs in deeper river sediments. It is proposed that bedform movement, channel reworking, bar formation, and aggradation/degradation at the river network scale should be included when estimating MP deposition. Results from flume experiments and a numerical model will be shown to explain potential processes that can lead to the burial of MP beneath the moving streambeds, which provides a mechanism for long-term accumulation and may explain resuspension events characterized by high MP loads during floods. It is argued that incorporating data on MP distribution in riverbeds with fluvial geomorphological and particle transport models will improve the current evaluation of MP transport in river networks.

How to cite: Arnon, S., Sturm, V., Peleg, E., and Teitelbaum, Y.: Leveraging Sedimentary Process Insights to Enhance Understanding of Microplastic Deposition in Rivers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2055, https://doi.org/10.5194/egusphere-egu25-2055, 2025.

Microplastic (MP) is ubiquitously found in aquatic environments and poses a significant environmental challenge. However, what controls MP deposition and burial in river networks is unclear, especially when sediments are in motion. This study addresses this gap by examining the impact of streambed motion and particle size on microplastic deposition in sandy streambeds. Experiments were conducted in a stainless-steel flume (650 cm x 20 cm) filled with 25 cm of silica sand (D₅₀ = 0.6 mm) and water (depth = 12 cm). A centrifugal pump circulated the water and maintained a constant stream water velocity. During the first experiment, the velocity of the water was 0.53 m/s, and the streambed celerity was 4 m/hr. The second experiment was conducted under stationary bedforms, with a water velocity of 0.15 m/s. Polypropylene (PP) fibers at lengths of 25 μm, 100 μm, 200 μm, and 2000 μm, and carboxylated Polystyrene (PS) microspheres (diameter of 0.5 μm, 1 μm, and 5 μm) were added to the stream water and their concentration in the water was measured over three days. The deposition of the MP was inferred from the decline of MP in the streamwater. A control experiment was conducted by repeating the same experiments but without sediments. After the relatively fast initial decline in MP, further reduction in MP concentrations in the water occurred due to deposition. Different deposition dynamics were observed for fibers and microspheres. Buried MP particles were partly resuspended during the scouring of the ripples during their movement. It was found that PP fibers 25 μm and 0.5 μm spheres were more mobile in the sediment than longer fibers and larger spheres, respectively. We explain their higher deposition than larger particles by a potential advective movement through the porous media, leading to their transport below the scour zone. PP fibers ≥ 100 μm were immobile within the sediment, and thus, their deposition was only due to burial by the ripple motion. In my presentation I will be comparing the results from the moving bed experiment to the stationary bed experiment and highlight the effect of bed motion. Our results hint at the significant influence of moving sediments on MP and the importance of considering MP size for catchment-scale modeling to predict MP fluxes to oceans. Deposition locations are also likely to be affected by bed motions and thus should be considered when developing effective sampling strategies.

How to cite: Levy Sturm, V. and Arnon, S.: Microplastic deposition in streams under moving bedforms, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-904, https://doi.org/10.5194/egusphere-egu25-904, 2025.

The transport and deposition of floating particles in flowing water is a key mechanism that drives the fate of contaminant, organic materials, and debris along river networks. The ability of human-made and naturally buyout particles to sit on the water surface makes them able to travel long distances. The mechanisms that allow these particles to deposit are closely linked to the hydraulics of the channel and the morphology of the river. Research has shown that particles such as plastics and debris tend to accumulate behind obstacles and recirculation areas, creating accumulation hotspots. The location of such hotspot also depends on the interplay between particle shape and size and flow conditions.  Although the influence of river morphology and flow regime is well acknowledged, the precise interaction between these components remains unclear.

To investigate this relationship, we used a new Eulerian-Lagrangian method based on a 2D depth-averaged flow solver simulating transport and dispersion of floating particles in a typical alpine river floodplain. This method offers a computationally efficient way to track the trajectory of single particles moving onto a flow field. Our approach integrates model simulations with data derived from outdoor and laboratory experiments, where we recorded deposition location of particles of different size, shape and material under different discharge conditions.

The results show that the number of floating particles decreases exponentially with the distance from the release point, with decay rates primarily correlated with the water discharge. We find that the deposition of particles depends on the hydraulics of the channel and the roughness elements in the channel, with particle sizes playing a secondary role. Unsteady flow conditions, namely receding water levels, promote particle deposition on shallow areas and channel shorelines. The use of the particle tracking model allowed us to extend the parameter space investigated experimentally, allowing for an in-depth analysis of the spatial and temporal dynamics of particles transport and deposition during floods.

These results deepen our understanding of transport processes of floating material at the reach scale, providing quantitative evidence on the central role played by channel hydromorphology. Although the effect of particle shape and size is not fully understood, the study can offer valuable insights into the dispersion mechanisms of different floating particles, from plastics to organic materials.

How to cite: Caponi, F., Fink, S., Conde, D. A. S., Hatstatt, A., Guiducci, I. R., Demuth, P., and Vetsch, D. F.: A Eulerian-Lagrangian approach to study the effect of river hydro-morphology on (small) floating particle transport and deposition, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16125, https://doi.org/10.5194/egusphere-egu25-16125, 2025.

Nanoplastics (NPs) are defined as nanoparticles that are intentionally manufactured in this size range or that originate from the unintentional degradation of larger plastic fragments. Nanoplastics (NPs) are defined as nanoparticles that originate from the unintentional degradation of plastic that
breaks down into nanoscale particles. Because of their size, buoyancy, and surface properties, NPs are very mobile. In fact, they can be found in aquatic environments, soils, the atmosphere, and even in the human body. During travel across long distances, NPs undergo several processes, including advection, dispersion, and aggregation. If the first two are considered in conventional transport models, the latter is generally neglected. Aggregation consists of the formation of closely attached NPs that can reach the size of colloids, favoring settling and retention within the soil, thereby reducing the NP migration distance. Developing a model that accounts for aggregation is, therefore, a paramount for accurate transport prediction of NPs in the environment. 
Here we present a mechanistic model of NP aggregation over a broad range of conditions that resemble natural aquatic environments. The model combines the mass conservation equation of the population balance equation (PBE) with the constitutive equations based on the extended DLVO theory. The model was verified with data of the evolution of the hydrodynamic diameter of polyester NPs from the literature and used to predict the behavior of a variety of plastic materials such as polypropylene (PP), polyethylene (PE), and polyvinyl chloride (PVC). The model agrees very well
with the data, and no parameter fitting is required as it is based on the physical-chemical properties of the system, e.g., the zeta potential of the suspension. The results in general show that as pH or salinity increase NPs aggregation becomes more important; whereas, organic material inhibits aggregation. The change of the polymer type may affect the magnitude of the aggregation phenomenon but in all cases the effect of bio-geochemical properties change of the solution stays the same.

How to cite: Kozhaya, M., Prigiobbe, V., and Zhang, D.: Modeling the evolution of nanoplastic particle aggregation in aquatic systems, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3035, https://doi.org/10.5194/egusphere-egu25-3035, 2025.

The widespread presence of microplastics (MPs) in freshwater environments underscores the need to better understand their temporal and spatial dynamics. Investigating the settling velocity (W) of MPs in the water column is crucial for comprehending their transport mechanisms within river systems. Several models have been proposed to estimate the W of this type of pollutant. However, to date, none of them account for the simultaneous presence of suspended sediments. This study aims to address this knowledge gap by conducting laboratory experiments to analyze the W of 12 different types of MPs with various shapes, under both clear and turbid water conditions in a still water tank. For each experimental run trajectories are captured by using high resolution camera and UV lighting to enhance the visibility of MPs. Both vertical and horizontal W components, tilt angles, oscillation frequencies and trajectory angles have been calculated. Appropriate non-dimensional parameter (i.e. Reynolds number (Re), Galileo Number (Ga), Stability Number (I*), Strouhal number (St)) have been used to better describe the MPs hydrodynamics. Results have shown, for the first time, that suspended sediments influence the MPs falling behavior by inducing secondary motions that increase MPs settling velocity. Particularly, the more elongated the MPs the greater the increasing rate of W. Findings have also shown a Gaussian probability distribution of the particle’s lateral position along the water column (with respect to the vertical axis of the tank) suggesting a Fickian-type diffusion of MPs throughout vertical water profile with several implications for their accumulation in calm water environment.

The above findings highlight the importance of including suspended sediment as a key factor in developing MP transport models, due to its significant impact on the mass balance of MPs in aquatic ecosystems.

How to cite: Mancini, M., Serra, T., Colomer, J., Francalanci, S., and Solari, L.: Sedimentation of microplastics interacting with sediment, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16991, https://doi.org/10.5194/egusphere-egu25-16991, 2025.

Microplastics (MPs) may be an important component of suspended particulate matter (SPM) in aquatic environments. These particles can be transported independently or as part of larger aggregates (flocs). Recent studies have highlighted that small microplastics (<160 µm) are predominantly transported within flocs across various aquatic systems. Flocculation notably affects the transport dynamics of MPs, particularly by modifying their settling velocities. This process is especially pronounced in estuarine environments, where salinity gradients, turbulence, high suspended sediment concentrations, and organic matter, creates unique conditions for floc formation and movement. This study investigates the settling behavior of small MPs (50-125 µm) and their mixtures with fine cohesive sediment under laboratory conditions. An optical settling column (System of Characterisation of Agregates and Flocs SCAF) was used to measure the settling velocities of MPs with varying characteristics (shape, size and density), both in clear water and when mixed with fine suspended sediments at concentrations representative of turbid estuaries, under previously agitated and no-agitated conditions. The results reveal that regular-shaped MPs exhibit higher settling velocities compared to irregular ones among larger particles (90–125 µm) with similar  density, while no such difference was observed for smaller particles (50–90 µm), highlighting the varying influence of particle shape with size. As expected, high-density particles settle faster, while larger particles also exhibit increased settling velocities due to reduced drag relative to their mass. The presence of fine sediments further enhances the settling velocities of smaller (50-90 µm) regular-shaped MPs and both smaller and larger (50-125 µm) fragmented MPs, particularly under previously agitated conditions, suggesting the occurrence of aggregation. A preliminary evaluation of several settling velocity models based on the calculation of drag coefficients, suggests that, unlike large microplastics, models conceived for natural particles align closely with observed data.

How to cite: Ruiz-Gonzalez, V., Jalon-Rojas, I., and Defontaine, S.: Evaluating settling velocities of microplastics-sediment mixtures under laboratory conditions , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10844, https://doi.org/10.5194/egusphere-egu25-10844, 2025.

Rivers serve as primary pathways for transporting microplastics from land to oceans, but they also can retain these small particles within their sedimentary deposits. A critical factor in this process is the role of fine cohesive sediments, which can adhere to microplastic particles and form aggregates. These aggregates can be transported as part of the bedload, resulting in enhanced accumulation of both microplastics and sediments. This study explores the mechanisms of microplastic-sediment aggregation and their influence on the transport and deposition of microplastics in rivers through laboratory experiments. A rotating wheel (34 cm diameter, 7 cm depth) was used to keep the mud in suspension and promote aggregation, while a settling column equipped with a high-resolution camera was utilised to analyse floc properties and their settling velocities. Five experiments were conducted, analysing approximately 4,000 flocs per experiment. Four experiments involved mixtures of water, sediment (1 g/L), and 500 µm-long microplastic fibres (MPs-sediment ratio, in weight, of 1:25), using four different plastic polymers: Polypropylene (PP, ρ=0.9 g/cm³), Polyamide (PA, ρ=1.14 g/cm³), Polyester (PES, ρ=1.38 g/cm³), and Aramid (AR, ρ=1.44 g/cm³). A fifth experiment, serving as a control, was conducted to observe floc formation without microplastics. Mud with a median grain size of 40 µm sampled from a real river (Arno River, Tuscany, central Italy) was used as the sediment. The mixtures were subjected to a shear rate of 10 s⁻¹ in the rotating wheel for two hours. After complete settling, flocs were collected and moved into the settling column. The recorded videos were analysed using an image analysis tool to determine floc size, shape, and settling velocity. Results showed that including microplastic fibres within flocs formed larger flocs that settled faster than flocs containing only sediment. Furthermore, while flocs formed from plastic-free mud generally exhibit a rounded shape, flocs containing microfibres display greater variability in shape, with some maintaining a rounded morphology but others exhibiting more elongated forms.

How to cite: Uguagliati, F., Ali, W., Chassagne, C., Waldschläger, K., Zattin, M., and Ghinassi, M.: Flocculation and its impact on microplastic transport mechanisms in rivers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15327, https://doi.org/10.5194/egusphere-egu25-15327, 2025.

Microplastic (MP) particles in the environment are covered by a so-called eco-corona. The eco-corona is made up of natural organic matter (NOM) like biomolecules, humic substances and other natural molecules. NOM substantially changes the surface properties of MP particles and therefore the interaction with other surfaces in the aqueous environment influencing their heteroaggregation behaviour.

Using Colloidal Probe-AFM we studied the interactions of eco-corona covered MP particles with model sand particles on the nanoscale. Measurements were performed in different ionic concentrations to mimic changing environmental conditions. We found that the eco-corona is able to *pull* at the model sand colloidal probe by macromolecular bridging. Simulations verified the stability of these heteroaggregates under flow. With heteroaggregation experiments and following Raman-Imaging we verified the presence and stability of these aggregates on the microscale.

In conclusion, we present macromolecular bridging as an eco-corona mediated heteroaggregation mechanism. It is present at monovalent salt concentrations > 1 mM and dependent on the eco-corona surface coverage. This mechanism is able to contribute substantially to MP particle heteroaggregation in the aqueous environment and explains how MPs cross the buoyancy barrier.

How to cite: Witzmann, T., Ramsperger, A. F. R. M., Liu, H., Schmalz, H., Greiner, A., Laforsch, C., Gekle, S., Kress, H., Fery, A., and Auernhammer, G. K.: How Microplastics Cross the Buoyancy Barrier, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5849, https://doi.org/10.5194/egusphere-egu25-5849, 2025.

Benthic biofilms are known for their ability to stabilise and trap fine estuarine sediments and contaminants. Due to their presence on the sediment surface, benthic biofilms interact with emerging contaminants such as microplastics (MPs) that settle to the bed, potentially influencing MP transport dynamics from land to sea. This study explores the influence of benthic biofilm development on the capture and retention of MPs under flow and how other chemical stressors adsorbed to the MPs, such as heavy metals, may influence biofilm retention of MPs. We hypothesised that i) higher biofilm development would increase MP capture and retention under flow and ii) that heavy metals associated with MPs would negatively affect the biofilm community and its ability to retain MPs.

Sieved sediment (500µm to remove large fauna) was added to small 250mL chambers and inoculated with 30mL of biofilm-rich surface sediment (control-absent, low and high biofilm biomass). Tidal simulation in an outdoor greenhouse promoted biofilm development in chambers over 21 days. High-density MPs (polyamide) MPs were added to the chambers at ‘high tide’ on day 14. These MPs were mechanically- and UV-aged then exposed to heavy metals (control (no metal), copper (Cu) and lead (Pb)) prior to their use. MPs were also fluorescently stained with a non-toxic dye to aid in MP erosion measurements. At the end of the incubation period, intact inner cores were gently removed, flushed and placed level with the bed of a small benchtop recirculating flume and exposed to incremental increases in flow velocity to erode the MPs. All experimental runs were filmed under UV light to fluoresce MPs and image analysis was used to determine the critical erosion threshold for MP motion from the video footage based on the loss of coverage across the core area. The remaining sediment from the chambers was extracted for biochemical analysis. 

Significantly higher critical shear stresses were required to remove MPs from the bed when biofilm was present, while Cu and Pb contamination had minimal effects on MP resuspension. This suggests that benthic biofilms have the potential to mediate MP resuspension dynamics and therefore MP transport from land-to-sea. Comparing our MP erosion thresholds (for PA particles only) against the bed shear stresses from a 2-D hydrodynamic model of the macrotidal Humber estuary, UK, it was found that MP erosion thresholds would be exceed in ~76% of the modelled cells during a 10-day simulation period covering a late-neap, spring, early-neap tidal cycle in the absence of biofilms. However, the presence of biofilm reduced the area of critical shear stress exceedance to ~36%. Without biofilms, MP erosion thresholds would be exceeded in 80% of the permanently inundated cells and 11.3% of the intertidal cells. Again, with biofilms present, MP erosion thresholds would be exceeded in 38% of the permanently inundated cells and just 3.8% of the intertidal cells. These findings can help improve our understanding of MP fluxes across the sediment-water interface in estuaries and provides evidence that the role of benthic biofilms should be included when parameterising MP transport models. 

How to cite: Hope, J., Chocholek, M., Rimmer, J., Baar, A., Thomas, R., Paterson, D., Harrison, L., Fernández, R., and Parsons, D.: The role of benthic biofilms in trapping estuarine microplastics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-4210, https://doi.org/10.5194/egusphere-egu25-4210, 2025.

The fragmentation of plastic in the environment directly influences particle size, buoyancy, transport, and sedimentation dynamics, shaping their fate and interactions within aquatic systems. Notably fragmentation of plastic particles in coastal environments has been observed to be faster than in the open ocean due to high temperature (heat-build up in the sand), high oxygen levels, high exposure to solar radiation (due to little vegetation coverage) and mechanical forces from waves and sediment movements which accelerates the cracking and fragmentation of plastic debris. However, understanding the drivers of fragmentation based on intrinsic properties of plastic—such as brittleness, a key precursor to fragmentation—remains challenging due to the irregular shapes, weathered conditions, and small sizes of environmental samples, which often do not meet the criteria for standardized mechanical testing. Here we present our study where we investigated the fragmentation and brittleness of field-collected plastic particles using a simple laboratory fragmentation test. Our fragmentation test assessed whether beach-sampled plastic particles would break (brittle) or remain intact (ductile) under fixed pressure, enabling the collection of a large dataset (16,322 plastic particles) for statistical analysis of the number of brittle particles on the beach. To further investigate what drives the brittleness of the sampled polypropylene (PP) and polyethylene (PE) particles, we investigated a subsample of PE and PP particles, including both brittle and ductile particles, focusing on their physicochemical traits that might explain their extent of weathering and links to their observed brittleness. We found that the brittleness of sampled plastics strongly correlates with advanced degradation, characterized by very low average molecular weights (as low as 7 kg mol⁻¹) and the presence of oxidation products. Furthermore, we observed that brittle particles were significantly smaller than their ductile counterparts, underscoring the role of fragmentation in shaping the size distribution of plastics on beaches. Additionally, through this fragmentation test, we established, to our knowledge, the first embrittlement criterion for plastics collected from the environment. To further explore fragmentation of plastics in coastal environments, we use a beach swash-zone laboratory simulator to investigate the fragmentation of polymers aged under controlled laboratory conditions. This allows us to correlate the extent of degradation with fragmentation behavior, providing additional insights into the fragmentation processes of brittle plastics in dynamic coastal environments. By highlighting the brittleness of plastic samples on beaches, our study underscores the significant risks of secondary microplastic generation from degraded, brittle plastics in dynamic beach environments. This study also aims to contribute to the overall understanding of the chemical and physical processes that drive fragmentation in aquatic systems, in our case coastal environments, which can aid to inform targeted intervention strategies to prevent microplastics from entering the environment.

How to cite: Delorme, A., Lebreton, L., Royer, S.-J., Arhant, M., Le Gall, M., and Le Gac, P.-Y.: Towards Understanding Drivers of Plastic Embrittlement and Fragmentation in Coastal Environments, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-7830, https://doi.org/10.5194/egusphere-egu25-7830, 2025.

Plastic pollution is a significant environmental problem of high magnitude, with far-reaching impacts on terrestrial and marine ecosystems. Plastic has various pathways to reach the ocean, with the land-to-ocean route being a critical one. Across both terrestrial and marine environments, plastic pollution threatens biodiversity, disrupts food chains, and accumulates in remote regions. Despite its importance, the mechanisms governing this transition remain understudied, particularly in overland systems. In this study, experiments in the laboratory are used in conjunction with numerical modeling tools to study the hydrodynamics of plastic transport overland under ideal conditions. Laboratory experiments utilized a controlled flume setup that simulates overland flow and analyzes the movement of plastic bottles, considering variations in size, shape, orientation, and weight. Then, numerical simulations were conducted to recreate the laboratory experiments as a simplified finite-element flume domain using a loosely coupled hydrodynamic and particle tracking framework. The framework was then validated against experimental results that proved the model's capability to reproduce the observed transport patterns. After validation, simulations were extended to a real-life-scale idealized domain to investigate the sensitivity of plastic transport to various parameters through a sensitivity analysis. The main results show the dependency of plastic mobility on hydrodynamic forces and particle characteristics. This integrated approach gives insight into overland plastic transport and informs mitigation strategies for plastic pollution in terrestrial environments. Future work will extend these findings to real-world scenarios, evaluating the interplay of rainfall and coastal flooding in plastic mobilization, such as during compound flood events. Thus, this research lays the foundation for developing comprehensive models that enhance our understanding of plastic pollution dynamics and support efforts to protect terrestrial and aquatic ecosystems from the escalating impacts of plastic waste.

How to cite: Oruc Baci, N., Santiago Collazo, F. L., and Jambeck, J.: Tracing Plastic Pathways: Laboratory Validation and Sensitivity Insights in Overland Transport Numerical Models, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3846, https://doi.org/10.5194/egusphere-egu25-3846, 2025.

There is a significant mismatch between the estimated amounts of ocean plastic and the expected plastic ingress by rivers (De Fockert et al., 2024; OECD, 2022). Most plastic transport monitoring data used for global modelling of plastic flux estimations is based on the number of items transported at the water surface (González-Fernández, 2023). However, the amount of the submerged plastic items transport in rivers is not accounted for or is based on approximations (Blondel & Buschman, 2022; Hurley et al., 2023). Notably, submerged plastic transport can exceed surface transport by 4-5 orders of magnitude (Vriend et al., 2023).

Numerical models often oversimplify the macro plastic transport by focusing solely on rising velocity, neglecting physical processes like particle characteristics and free surface interaction (Wickramarachchi et al., 2024). This study addresses these aspects through a detailed experimental investigation on the vertical distribution of near-neutrally buoyant macro-plastics in a controlled laboratory environment to provide validation data to improve the current parametrisation in the particle tracking model Delft3D-PART as part of the Delft3D open source software suite (Stupary et al., 2015).

Experiments were conducted in a  45m long, 1.2m high, and 1m wide flume at Deltares. Large quantities of different types of plastic items were released close to the flume bed 30m upstream of the measurement location, where the position of each item was measured and counted to obtain a vertical distribution. The plastic items used in the campaign were similar to commonly found riverine litter such as bags, foils, cups and spheres, with varying size and plastic type (PP & HDPE). Additionally, the hydrodynamic conditions were varied allowing testing of different turbulent flow conditions.

Acoustic Doppler Velocimeters (ADV) measurements were performed to characterize the flow field and turbulence. Computer vision AI algorithms were used to track the plastic particle positions within the water column, enabling the construction of plastic vertical distribution profiles.

Similar to Valero et al. (2022), the experiment confirmed distinct surfaced and suspended transport layers for near-neutrally buoyant plastics for low turbulent flows. Under low turbulent flow conditions, plastic items concentrated at the free surface, confirming dominance of buoyancy and surface tension effects over turbulent mixing. Within the suspended transport layer, the plastic particles exhibited an inverse Rouse profile. At higher turbulence, the vertical distribution of observed plastics became more uniform for plastic bags, while smaller sized plastics remained well-represented by the inverse Rouse profile. This suggests that classical inverse Rouse theories, which neglect particle size, may not adequately describe the plastic observation profiles of larger sized plastic transport in rivers.

Based on these findings, a Delft3D-FLOW hydrodynamic model was developed and validated against the ADV measurements, in which the particle tracking model Delft3D-PART was adapted to incorporate surface interaction effects based on particle dimensions. This parameterization enables more accurate simulation of riverine plastic concentrations by considering hydraulic dynamics, surface interaction, and plastic dimensions. The improved model parameterization will enhance the accuracy of predicting plastic transport and contribute to the development of effective mitigation strategies.

How to cite: van Batenburg, T., Moreno-Rodenas, A., Bakker, W., Valero, D., Kleissen, F., Buschman, F., Vriend, P., Franca, M. J., and de Fockert, A.: Experimental and numerical investigation of the vertical distribution of macro plastic transport in rivers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18928, https://doi.org/10.5194/egusphere-egu25-18928, 2025.

Microplastics (MPs) are pervasive contaminants, yet understanding their pathways and fate in the marine environment remains unclear. A key challenge is the lack of in-situ, complementary measurements linking MP quantification with oceanographic parameters, particularly concerning submesoscale processes and density fronts. Submesoscale dynamics, including filaments, eddies, and fronts, significantly influence the transport and accumulation of MPs by creating convergence zones and sharp density gradients. Density fronts serve as critical hotspots for MP aggregation, concentrating particles through upwelling and downwelling processes. Despite their importance, these interactions remain poorly studied, emphasizing the need for integrated approaches to directly measure the interplay between MPs and the physical processes that drive their distribution.

This study addresses this gap by utilizing in-situ measurements collected with an autonomous surface vehicle (ASV) in the southern North Sea, simultaneously collecting water samples for MP analysis and key oceanographic data. The ASV simultaneously sampled air, sea surface microlayer, and underlying water for MP analysis. A weather station and conductivity, temperature, and depth (CTD) sensors were deployed on the ASV to further contextualize the distribution of MPs. Additionally, CTD profiles were obtained by an accompanying research vessel to investigate the influence of stratification and temporal dynamics on MP distribution. An acoustic Doppler current profiler measured water current velocities and flow direction.

The measurements underscore the pivotal role of submesoscale fronts and filaments in shaping the accumulation and distribution of MP. Upwelling and downwelling processes at these fronts and filaments concentrated MP up to 30.48 µg MP L⁻¹, and distributed MPs vertically across depth profiles and horizontally across fronts. Wind direction was found to influence the presence of MP in the atmosphere, while wind speeds appeared to enhance heterogeneity in MP composition and concentration within the water.

Submesoscale fronts and filaments are highlighted as key zones for MP accumulation, driven by the interplay of horizontal and vertical water flow linked to ageostrophic circulation. The data provide novel insights into their transport mechanisms in the marine environment.

How to cite: Meyerjürgens, J., Goßmann, I., Albinus, M., Achtner, C., Robinson, B.-T., Held, A., Lehners, C., Gassen-Bertzbach, L., Ayim, S. M., Badewien, T. H., Scholz-Böttcher, B. M., and Wurl, O.: What Influences Microplastic Distribution in the Marine Environment? A Study Highlighting the Role of Fronts and Submesoscale Processes in the North Sea, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17177, https://doi.org/10.5194/egusphere-egu25-17177, 2025.

The environmental pollution from plastics is steadily increasing, reaching 390.7 million tons in 2021 (Plastics Europe, 2022). Between 2% and 5 % of MPs produced worldwide, may ultimately find their way into the ocean, where they accumulate on the deep seafloor (Phuong et al., 2021), infiltrate into hyporheic zones (Mancini et al., 2023) or may remain in suspension in the water column (Zobkov et al. 2019). MPs have been reported not only in marine and coastal areas (Jung et al., 2021) but also in Marine Protected Areas (Zachello Nunes et al., 2023). Consequently, plastic pollution is recognized one of the most serious anthropogenic generated pollutants affecting aquatic ecosystems.

MPs can be transported and deposited by turbidity currents from shallow waters to the deep ocean (Pohl et al., 2020). This study contributes to further knowledge about the transport and the depositional patterns of MPs by turbidity currents related to different factors:  the MP shape, the MP density and the sediments’ characteristics. To mimic turbidity currents transporting MPs, lock-exchange flume experiments were performed with sediment contaminated with three types of microplastics: PET fibers, PVC fragments, and melamine fragments. These MPs were selected to represent a range of densities and shapes. The study revealed distinct sedimentation patterns: higher sediment concentrations enhance MP transport, and turbidity currents with finer sediments transported MPs over greater distances, highlighting the important role of sediment in transporting MPs in the propagation of turbidity currents. Further, MP sedimentation patterns varied with MP-particle shape, size, and density, highlighting the crucial role of MP particle properties in determining MP distribution in turbidites. These findings enhance our understanding of the mechanisms controlling the spatial distribution of MPs in marine sedimentary-environments and underscores the importance of considering both hydrodynamic and particle-specific factors when addressing the complex behaviour of MPs.

References

Plastics Europe, 2022. Plastics- the Facts 2022. An analysis of European plastics production, demand and waste data.

Phuong, N.N., Fauvelle, V., Grenz, C., Ourgaud, M., Schmidt, N., Strady, E., Sempéré, R., 2021. Highlights from a review of microplastics in marine sediments, STOTEN, 777.

Mancini M., Francalanci S., Innocenti L., Solari L., 2023a. Investigations on microplastic infiltration within natural riverbed sediments. STOTEN, 904, 167256.

Jung, J.W., Park, J.W., Eo, S., Choi, J., Song, Y.K., Choi, Y., Hong, S.H. and Shim, W.J. 2021. Ecological risk assessment of microplastics in coastal, shelf and deep sea waters with a consideration of environmentally relevant size and shape. Environmental Pollution. 270, 116217.

Pohl, F., Eggenhuisen, J. T., Kane, I. A., Clare, M. A., 2020. Transport and Burial of Microplastics in Deep-Marine Sediments by Turbidity Currents. Environmental Science & Technology, 54(7), 4180–4189.

Zachello Nunes, B., de Oliveira Soares, M., Zanardi-Lamardo, E., Braga Castro, I. 2023. Marine Protected Areas Affected by the most extensiveOil Spill on the Southwestern Atlantic coast. Ocean and Coastal Research, 71(2), e23028,

Zobkov, M.B. , Esiukova, E.E. , Zyubin, A.Y. , Samusev, I.G., 2019. Microplastic content variation in water column: The observations employing a novel sampling tool in stratified Baltic Sea, Marine Pollution Bulletin, 138, 193-205. 

How to cite: colomer, J., soler, M., pohl, F., and serra, T.: Microplastics in turbidity currents: transport and sedimentation, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9865, https://doi.org/10.5194/egusphere-egu25-9865, 2025.

Due to their persistent presence in the environment and the largely unknown long-term impacts on biota, plastic waste has increasingly become the focus of scientific research in recent years. Particularly in the field of microplastic monitoring in rivers, there remains a significant need for research. As methodologies are not yet standardized, and often not representative and comparable, efficient sampling poses a major challenge.

Within the framework of the "Alplast" project, intensive efforts were made to develop a methodology capable of accurately capturing microplastic transport in rivers. Various carrier systems tailored to different river sizes were developed for net sampling, which targets the coarser fractions of microplastics (> 250 µm). In addition, a novel isokinetic pump was designed to analyze finer microplastic fractions from 50 to 250 µm

Given the spatial and temporal variability of plastic transport, multipoint sampling under varying hydrological conditions is strongly recommended. Larger microplastic particles occur less frequently, making it essential to sample large volumes of water to representatively capture this size range. Using net sampling, up to 1,000 m³ of water can be filtered at a single sampling point. However, finer particles, which cannot be captured by the limited mesh size of nets due to clogging especially under turbid boundary conditions, must be analyzed using pump samples. The number of sampling points across the river profile is often limited related to the high costs of analysis.

The newly developed isokinetic pump addresses this gap by measuring flow velocity at the intake area of the sampling device. A control unit regulates the pump speed so that the pumped water volume matches the natural flow in the sampling cross-section. This isokinetic sampling approach offers two major advantages. First, the plastic flow is not disproportionately altered during sampling, and second, a direct weighting of the spatial distribution in flow and concentration is automatically accounted for. This significantly reduces the number of required samples, which is particularly beneficial given the high costs associated with sample analysis.

The new methodology was successfully combined with net sampling and applied at multiple measurement sites. Results demonstrate that this approach enables efficient and representative monitoring of microplastic transport in rivers.

How to cite: Liedermann, M., Pessenlehner, S., Mayerhofer, E., and Gmeiner, P.: Isokinectic pump sampling – a methodoligy addressing the small size ranges of microplastic in rivers, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20227, https://doi.org/10.5194/egusphere-egu25-20227, 2025.