HS2.3.4

Rivers are major pathways of plastics from lands into the Ocean. However, there is still a huge lack of knowledge on how riverine litter, including macroplastics, is transferred into the Ocean. Quantitative measurements of macroplastic emissions in rivers even suggest that a small fraction (0.001 to 3%) of the Mismanaged Plastic Waste (MPW) generated within a river basin finally reach the sea. Instead, macroplastics may remain within the catchment and on coastlines because of complex transport dynamics that delay the transfer of plastic debris. In order to better understand those dynamics, we performed tracking of riverine litter over time. First, hundreds of date-prints items were collected on riverbanks in the Seine estuary. The distribution of their Use-By-Dates suggest that riverine litter may remain stored on riverbanks for decades. Second, we performed real time tracking of floating and sub-floating bottles using GPS-trackers. Between March 2018 and April 2019, 39 trajectories were recorded in the estuary under tidal influence and 11 trajectories upriver, covering a wide range of hydrometeorological conditions. Results show a succession of stranding/remobilization episodes in combination with alternating upstream and downstream transport in the estuary related to tides. In the end, tracked bottles systematically stranded somewhere, for hours to weeks, from one to several times on different sites. The overall picture shows that different hydrometeorological phenomena interact with various time scales ranging from hours/days (high/low tides) to weeks/months (spring/neap tides and highest tides) and years (seasonal river flow, vegetation and geomorphological aspects). Thus, the fate of plastic debris is highly unpredictable with a chaotic-like transfer of plastic debris into the Ocean. The residence time of these debris is much longer than the transit time of water. This offers the opportunity to collect them before they get fragmented and/or reach the Sea.

How to cite: Tramoy, R., Gasperi, J., Colasse, L., Silvestre, M., Dubois, P., and Tassin, B.: Endless journey of macroplastics in rivers: From hours to decades tracking in the Seine River, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6027, https://doi.org/10.5194/egusphere-egu21-6027, 2021.

Plastic accumulation in the marine environment is a major concern given the harmful effects and longevity of plastics at sea. Although rivers significantly contribute to flux of plastic to marine systems, plastic transport in rivers remains poorly understood and estimates of riverine plastic flux derived from field measurements and modelling efforts are highly uncertain. In this study, a new probabilistic model of plastic transport in rivers is presented which describes the main processes controlling displacement to predict the statistical distribution of travel distances for individual items of buoyant macroplastic debris. Macroplastic transport is controlled by retention in temporary stores (or traps) created by vegetation, bank roughness elements and other obstacles. The behaviour of these traps is represented in the model via a series of Bernoulli trials conducted in a Monte Carlo simulation framework. The probability of retention or release from traps is described using physical characteristics such as the type of vegetation, channel width or channel sinuosity index. The model was calibrated using a tracer experiment with six replicates, conducted in a small 1.1 km river reach. For each replicate, 90 closed air-filled plastic bottles were injected at the upstream end of the reach and the location of each bottle was recorded several times over a 24-hour period. Bottles were chosen as ‘model’ macroplastic litter items given their high usage and littering volume. Travel distances were low (the average distance travelled over 24 hours was 231 m and no bottles travelled more than 1.1 km, the length of the study reach) and variable (the coefficient of variation for the replicates ranged between 0.54 and 1.41). The travel distance distributions were controlled by the location and characteristics of discrete traps. The numerical model described the observed travel distance distributions reasonably well (particularly the trapping effect of overhanging trees and flow separation at meander bends), which suggests that modelling plastic transport for longer reaches and even whole catchments using a stochastic travel distance approach is feasible. The approach has the potential to improve estimates of total river plastic flux to the oceans, although significant knowledge gaps remain (e.g. the rate and location of plastic supply to river systems, the transport behaviours of different types of plastic debris in rivers and the effectiveness of different traps in different types of river system).

How to cite: Newbould, R., Powell, M., and Whelan, M.: Macroplastic Debris Transfer in Rivers: A Travel Distance Approach, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4498, https://doi.org/10.5194/egusphere-egu21-4498, 2021.

Processes of macroplastic (plastic particles > 5 mm) storage and remobilization in rivers have been overlooked so far, but are of crucial importance for the estimation of plastic accumulation and transport and associated risks. We present a conceptual model that defines phases of the macroplastic route through a fluvial system and systematizes their main controls. We divided macroplastic route into (1) input, (2) transport, (3) storage, (4) remobilization and (5) output phases. Phase 1 is mainly controlled by humans, phases 2–4 by fluvial processes, and phase 5 by both types of controls. We hypothesize that natural characteristics of fluvial systems and their modification by dam reservoirs and flood embankments construction are key controls on macroplastic storage and remobilization in rivers. The zone of macroplastic storage can be defined as a river floodplain inundated since the beginning of widespread disposal of plastic waste to the environment in the 1960s and the remobilization zone as a part of the storage zone currently influenced by floodwaters and bank erosion. The amount of macroplastic in both zones can be estimated using data on the abundance of surface- and subsurface-stored macroplastic, and the lateral and vertical extent of the zones. A demonstrated diversity of factors controlling the route of macroplastic through a fluvial system requires a broader, transdisciplinary perspective including humans who not only dispose plastic, but are also affected by it both physically and aesthetically, and who may remove it from rivers.

How to cite: Liro, M., van Emmerik, T., Wyżga, B., Liro, J., and Mikuś, P.: Macroplastic storage and remobilization in rivers, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7641, https://doi.org/10.5194/egusphere-egu21-7641, 2021.

Methods to quantify plastic transport in rivers have greatly improved during the past few years. As a first approach, visual counting is currently the simplest way to assess plastic transport with minimal effort and cost. It usually results in underestimations of plastic input into the sea of about one to two order of magnitude when compared to models such as the Jambeck’s approach. The latter shows statistical weaknesses and data availability issues leading to large uncertainties, while visual counting miss the water column compartment and often has a low spatiotemporal representativeness. In order to give another ground-truth estimation of plastic transport able to challenge both models and visual counting, we developed innovative methods based on environmental management data in the Seine estuary (500 m³/s) and the Huveaune River ( 2 m³/s; Marseille, France). First, we used data from institutional cleaning in the Seine estuary that consist in litter collection on riverbanks. Their efficiency was measured based on capture-recapture design. Mass flows of plastic debris were then calculated based on the capture rate over one year, the estimation of the fraction of plastic debris which are never collected (hidden or too small) and the assumption that all plastic debris strand on riverbanks. Second, we used data from bar screens spaced of 3 cm in the Huveaune, a small urban river flowing in Marseille, South France. All the water column is screened, and captured waste are automatically collected in dumpsters. Grab sampling were performed after a dry, a wet and a flood period. The corresponding annual mass flows of plastic debris was then calculated relative to the mean fraction of time corresponding to those hydrological periods over 2017 and 2018. Annual mass flows of plastic debris were normalized to the population in both basins. Although methods were different, mass flows of plastic debris per capita are very similar with 8.5 – 13.6 g/cap/yr for the Seine River and 2.4 – 14.9 g/cap/yr for the Huveaune River. This is one to two order of magnitude lower than the Jambeck’s approach. However, when focusing on the fraction ending into the Sea, bar screens in Marseille enable to decrease the mass flow of plastic debris of about one additional order of magnitude, while cleaning of riverbanks decreases it of about 10%. This is related to the nature of the rivers that calls for different solutions, screening the whole Seine River being a tricky idea. Nevertheless, when normalized to water volume, the Huveaune River is visually much more polluted (16.4–102.2 mg/m³) than the Seine estuary (9.0–14.5 mg/m³). In conclusion, environmental management data can help to estimate mass flows of plastic debris and calls for better consideration. However, they often need an improved scientific framework.

How to cite: Tramoy, R., Gasperi, J., Blin, E., Poitou, I., and Tassin, B.: Use of environmental management data for mass flow estimations of plastic debris in rivers: The Seine River and The Huveaune River, France., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6088, https://doi.org/10.5194/egusphere-egu21-6088, 2021.

Global plastic pollution is affecting ecosystems and human health globally. Proposing solutions and coping strategies for this threat requires a clear understanding of the processes controlling the fate and transport of mismanaged plastics at multiple scales, going from watersheds to regions and even continents. River corridors are the primary conveyor and trap for mismanaged plastic produced within the landscape and eventually released to the ocean. New approaches that apply technological sensing innovations for monitoring plastic waste in aquatic environments can improve observations and plastic waste datasets globally. However, our understanding of when, where, and how to target monitoring is limited, reducing the benefit gained. There is therefore a critical demand for predictions of hotspots (as well as hot moments) of plastic accumulation along river networks globally, in order to optimize observational capacity.     

Here, we present a new global flow and transport model for plastic waste in riverine environments. Our model predicts that only a small fraction (roughly 2.5%) of the global mismanaged plastic that entered rivers since the 1950s has been delivered to the ocean by 2020, with an overwhelming majority sequestered in freshwater ecosystems. Furthermore, we predict the patterns of mismanaged plastic accumulation and its residence time depend on (i) the topology and geometry of the river network, (ii) the relative location of plastic sources, and (ii) the relative location and trapping efficiency of flow regulation structures, primarily large dams. Our results highlight the role of rivers as major sinks for plastic waste and the need for targeted remedial strategies that consider the structure of the river network and anthropogenic regulation when proposing intervention measures and sampling efforts.

How to cite: Gomez-Velez, J. and Krause, S.: Identifying global hotspots of plastic waste accumulation along river networks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13848, https://doi.org/10.5194/egusphere-egu21-13848, 2021.

It is currently predicted that rivers deliver as much as 80% of plastic waste into the marine environment, including microplastics (MP) <5 mm in size. Yet, the transfer mechanisms of MP in river systems remain poorly understood. While high flow events are thought to flush more microplastics into marine waters, their overall load may depend on factors such as river morphology, land-use, or local MP sources.

Microplastic concentrations were monitored on a seasonal basis (summer 2019 - winter 2020/2021) across 13 sites located across the R. Thames catchment, UK. Sites were selected to include rural, urban and industrial locations with different hydrological characteristics and proximities to potential MP inputs (e.g. sewage or industrial effluents). At each site, bed sediment samples were manually extracted (n=55 samples), and surface water samples collected in 5 L clean polyethylene bottles (n=22 samples) and using a 500-µm plankton net (n=12 samples). Microplastics were extracted from sediment and plankton net samples using density flotation, whilst bulk water samples were filtered with no prior extraction steps. All samples were visually inspected under a stereomicroscope and their morphology recorded. The chemical composition is to be further investigated using µFTIR as part of future research.

Sediment and water samples likely contained MP from different sources (e.g. in-situ breakdown of plastic litter, sewage effluent), which was reflected in the varying MP shapes and loads observed at the study sites. Microplastic levels ranged from <LoD (limit of detection) to 381 MP·100 g^-1 in sediments, <LoD to 16 MP ·L^-1 in bulk water samples and <LoD to 2 MP·m^-3 in plankton net samples and were highest at sites downstream of known sewage inputs. There was also a clear variation in particle shapes and levels with respect to site, with fibres and fragments representing the dominant MP type present along urban river stretches, and microbeads most abundant near industrial locations.

Microplastic levels varied on a temporal basis in both surface waters and sediments. Increasing river discharge generally had a diluting effect on MP levels observed in the water column (mean levels of 5 MP·L^-1 and 2 MP·L^-1 in summer 2019 and winter 2020, respectively). Mean microplastic levels in sediments also decreased from 15.1 MP·100 g^-1 in the summer to 9.4 MP·100 g^-1 in the winter, although some local increases in microplastic pollution were observed during high flow period, particularly at sites situated in close proximity to reported sewage discharges (e.g. from Combined Sewer Overflows).

This study is one of the first few to report spatio-temporal variations in microplastic contamination of both river water and sediments. Our early findings suggest that variability in MP levels and composition in both media may correspond to local pollution sources, and plastic particles could be released from surface sediments during periods of increased precipitation, even in the absence of flooding. Understanding such patterns in MP flux will be crucial to accurately model plastic loads from terrestrial to marine environment and implement effective mitigation measures.

How to cite: Skalska, K., Ockelford, A., Ebdon, J. E., and Cundy, A. B.: Spatio-temporal patterns in microplastic pollution of surface waters and sediments within the R. Thames (UK) and its tributaries, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8320, https://doi.org/10.5194/egusphere-egu21-8320, 2021.

In spite of the fact that present day Russia makes only a small contribution to the global industry of polymers and polymer products, their production increases steadily, which leads to a rise in the concentration of microplastics (MP) in the surface waters including rivers, where MP enters with surface runoff and waste water. 

Since the problem of microplastics pollution of freshwater bodies in Russia has not received sufficient attention, in July-August 2020 the non-profit foundation “Clean Hands, Clean Rivers” together with the Faculty of Geography of M.V. Lomonosov Moscow State University, conducted a comprehensive hydrological and environmental survey along the entire Volga river, from its source to its mouth. It included water sampling and determination of microplastics, nutrients and heavy metals. The main hydrochemical indicators of river water were also monitored.

Water samples were taken upstream and downstream of several large cities - Tver, Nizhny Novgorod, Cheboksary, Kazan, and Volgograd. To collect water samples for MP, a specialized device "manta" with nets for filtration at 300 µm was used; further analysis of MP fragments was carried out by the method of differential scanning calorimetry.

The analysis of 34 water samples allowed us to determine the average concentration of MP in the surface water layer of the Volga river which accounted for 0.901 part./m³.

MP particles were found in all samples taken. The concentrations ranged from 0.156 to 4.100 part./m³. The maximum MP concentrations were recorded in large cities downstream of the sewage treatment plants. For Kazan, Tver, Nizhny Novgorod and Volgograd they reached 4.100,  3.769, 1.907, and 1.344 part./m³, respectively. The role of large settlements as sources of MP in the Volga water was revealed.

The minimum MP concentrations were recorded upstream of the large cities showing relatively stable levels of 0.25 part./m³. The lowest MP content (0.156 part./m³) was revealed in the downstream area of the Cheboksary reservoir near Cheboksary. The results of weighing MP particles showed that their average concentration in the Volga water is 0.212 mg/m³.

In each of the investigated samples, particles of three determined fractions - fragments, fibers and films - were found, however, their ratio was not constant. On average, the proportion of fragments and films in the Volga water was 41% and 37%, respectively, and share of fibers accounted for 22%.

How to cite: Lisina, A., Platonov, M., Lomakov, O., Frolova, N., Sazonov, A., and Shishova, T.: Microplastics in the water of the Volga River: the results of a summer 2020 field survey, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8338, https://doi.org/10.5194/egusphere-egu21-8338, 2021.

Rivers and estuaries act as conduits of microplastic transport, linking terrestrial and marine environments: however, it is unclear to what extent estuaries act as sources or sinks for marine plastic waste. In densely populated catchments, microplastic pollution could impact human populations and natural ecosystems including through industry, domestic activities or direct exposure. An investigation into the physical behaviour of microplastic within estuarine systems will allow for a greater understanding of plastic retention and exportation to coastal and offshore environments. A high resolution 3D model (Delft D-Flow FM) of the Conwy Estuary (UK) is under development, with current and future projections of microplastic concentrations used to determine local exposure levels, residence times and temporal variability.

The Conwy Estuary (UK) is a well-mixed macro-tidal, embayment type system connecting the Conwy catchment to the North Wales coast and Irish Sea – where waters are used for leisure and aquaculture. Microplastics derived from the catchment population, industry and agriculture are thought to flow into the estuary primarily from the Conwy river network. Because of this, this study will incorporate in-situ samples of microplastic concentrations in river water to be able to predict microplastic levels in the estuary with greater accuracy. Plastic dispersal simulations through particle tracking and water quality monitoring will be undertaken using known concentrations and future projections of microplastic.

The results of the model validation as well as application to plastic dispersal simulations will be presented.

How to cite: Jones, N., Neill, S., Robins, P., and Lewis, M.: Investigating microplastic behaviour in a well-mixed estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8925, https://doi.org/10.5194/egusphere-egu21-8925, 2021.

Freshwater microplastics pollution has been a recent focus. River freshwater microplastics pollution are vital towards freshwater ecosystems as well as have been the prominent source-to-sink conduits to export MPs into the marine realm. Wastewater treatment plants (WWTPs) have been identified as one of the major point-sources. To date, sources-to-sinks comprehensive knowledge are highly limited. This study explored sources-to-sinks microplastics pollution i.e., WWTPs-to-river-to-marine comprehensively. The two rivers i.e., Koya River (KR) and Nishiki River (NR) which are flowing to the Seto Inland Sea (SIS) and the WWTPs effluent samples were collected from selected (n=37) stations in the Yamaguchi prefecture, Japan. Filtration, wet peroxidation, and density separation methods were employed to extract microplastics particles. Polymers were identified via attenuated total reflectance-Fourier transform infrared spectroscopy. The average microplastics abundances were found KR—82.25±67.84 n/L and NR—38.73±24.13 n/L for the river water, and KRWWTPs—79.5±3.5 n/L and NRWWTPs—72.25±23.64 n/L for WWTPs effluents, respectively. The KR were found to be more polluted than the NR. WWTPs effluents were found posing higher abundances than rivers. Significantly higher microplastics concentration were found in the WWTPs downstream stations than other river stations. Characterization revealed that small MPs (<1000 µm) in size, fibers in shape, polymers— polyethylene, polypropylene, polyethylene terephthalate, vinylon were major in both of the WWTPs effluents and rivers. WWTPs influenced river environments by means both of the abundances and microplastics characteristics (shapes-size-polymers). The estimated source-to-sink emission demonstrated a substantial number of MPs discharge into the rivers by the WWTPs (0.007—0.086 billion/day) and rivers-to-SIS marine environments (1.15—7.951 billion/day). The emission represented that the WWTPs were the prominent point-source to cause river microplastics pollution. Rivers were the initial sinks of the Japan land-sourced microplastics and prominent pathways to emit microplastics to the ultimate marine sink i.e., SIS. Large amounts of MPs are being generated on land sources before the plastics wastes degrade into MPs secondarily. The pollution characteristics (shapes-sizes-polymers) indicated ecotoxicological threats to these rivers and the downstream environments. Overall, this study provided an insight of sources-to-sinks pollution, fulfilled the preliminary knowledge gaps of pollution occurring land-sources, fate and loadings. We recommended microplastics pollution control at source. This study will aid in developing microplastics pollution control and management strategies for environmental protection and sustainability in the regional Japan as well as global context upon “thinking globally and acting locally”.

Keywords: Abundance, Point-source, Source-to-sink, Riverine microplastics pollution, Wastewater treatment plants

How to cite: Kabir, A. H. M. E., Sekine, M., Imai, T., Yamamoto, K., Kanno, A., and Higuchi, T.: Microplastics from the Sources to Sinks: Assessment of Microplastics in the River Freshwater Environments and Wastewater Treatment Plants, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7975, https://doi.org/10.5194/egusphere-egu21-7975, 2021.

The increase in microplastics (MPs) research has aroused awareness about their presence and polluting potential in aquatic environments. Wastewater treatment plants (WWTPs) have been identified as one of the main paths for these pollutants to reach the environment. The present study is focussed in the WWTPs emplaced within the Guadalete-Barbate river basin. This geographical area comprises a total of 60 WWTPs treatment plants with an inhabitant equivalent (IE) above 250. Within these 60 wastewater treatment plants, there are 38 plants with conventional treatments and an estimated population of over 800,000. The high percentage of population that lives in this basin leads us to think about the quantity of microplastics that are dumped into the environment daily. Therefore, the aim of this research is to study the occurrence and identify the type of microplastics in these facilities, this information is important in order to design treatments that improve microplastics removal and avoid their entrance in the aquatic environment.

How to cite: Franco del Pino, A. A., Albendín, G., Arellano, J. M., Egea-Corbacho Lopera, Á., Martín García, A. P., Rodríguez, R., Quiroga, J. M., and Coello, D.: Occurrence of microplastics at Wastewater Treatment Plants in the Guadalete-Barbate river basin (Cadiz, Spain)., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12985, https://doi.org/10.5194/egusphere-egu21-12985, 2021.

Development of analytical methods for the characterization (particle size determination, chemical identification, and quantification) of the low µm-range microplastic (MPs; 1-10 µm) and nanoscale plastic (NPs; 1-1000 nm) debris in environmental matrices is a quickly emerging scientific field and has gained considerable attention, not only within the scientific community, but also on the part of policy makers and the general public. However, due to the limited sensitivity of the current state of the art monitoring techniques, detection of MPs and NPs in water is one of the biggest challenges for their monitoring, source identification and, ultimately, risk assessment.

As it is evident that no single method will provide all the information required for a complete characterization of MPs and NPs in water, the present work is aimed to give an overview of different complementary analytical methodologies showing considerable promise for the particle size determination, chemical identification, and quantification of MPs and NPs [1]. In addition, results of three case studies will be included to adequately address the smallest fractions in plastic debris size determination, making such approaches worthwhile to be further explored.

The first case study offers a novel method based on the use of inductively coupled plasma-mass spectrometry operated in single-event mode and relies on our previous work where for the first time ever single particle inductively coupled plasma-mass spectrometry based on carbon monitoring was successfully used for the detection, particle size characterization and particle number concentration of polystyrene MPs [2]. The second case study further explore light scattering methods, including nanoparticle tracking analysis or dynamic light scattering, for MPs and NPs particle size distribution and particle number in water. Finally, the capabilities of size exclusion chromatography in combination with online detection techniques such as UV-visible absorption spectrometry will be presented for the particle size determination of smallest fraction of NPs (1-100 nm).

M.V. is a senior postdoctoral fellow of the Research Foundation – Flanders (FWO 12ZD120N).

References

[1] Velimirovic M., Tirez K., Voorspoels S., Vanhaecke F. (2020) Recent developments in mass spectrometry for the characterization of micro- and nanoscale plastic debris in the environment, Analytical and Bioanalytical Chemistry, 1-9.

[2] Bolea-Fernandez E., Rua-Ibarz A., Velimirovic M., Tirez K., and Vanhaecke F. (2020) Detection of microplastics using inductively coupled plasma-mass spectrometry (ICP-MS) operated in single-event mode. Journal of Analytical Atomic Spectrometry 35, 455-460.

How to cite: Velimirovic, M., De Wit, J., Jacobs, G., Bolea-Fernandez, E., Rua-Ibarz, A., Voorspoels, S., Tirez, K., and Vanhaecke, F.: Analytical methods for detection and size determination of micro- and nanoscale plastic debris in water, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2533, https://doi.org/10.5194/egusphere-egu21-2533, 2021.

The degree of microplastic dispersal and retention in lakes and oceans critically depends upon the microplastic particle’s density, which can change over time due to microbial growth (biofilm). This experiment tests the mechanism by which initially buoyant microplastics can be lost from the surface layers of a lake and become deposited in sediments. While buoyant microplastics do initially float in water, the growth of biofilm denser than water on the microplastic surface leads to an increase in particle density as a whole. This increase in density results in slower rise velocities of biofouled particles when they are mixed into the water column, and can even lead to sinking of biofouled particles. Both slower rise velocities and particle sinking would increase microplastic residence time in the water body. Through ex-situ experiments on irregularly-shaped polypropylene microplastic granules in an emulated lake environment under overcast light levels, we have found that biofouling alone is sufficient to increase microplastic particle density and lead to sinking for small particles (~125-212 µm) in 18 days and larger particles (1000-2000 µm) in 50 days. These differences in settling onset time would likely lead to size-fractionation of particle sedimentation, where smaller particles are deposited closer to their sources relative to larger particles. Using the measured values of biofilm-induced sinking rates of larger microplastics (1000-2000 µm) and lake residence times, we can describe the fraction of microplastics expected to become deposited after they enter lakes. Our results on terminal velocity change inherent to biofouling provide new information for microplastic transport modelling.

How to cite: Semcesen, P. and Wells, M.: Biofilm-induced sinking of buoyant microplastics in a freshwater environment, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3710, https://doi.org/10.5194/egusphere-egu21-3710, 2021.

Plastic fragments floating on the surface of oceans represent less than 1% of plastic pollution entering these environments annually, with the fate of the remaining plastics largely unknown. There are several removal mechanisms that have been suggested for microplastics (<5mm) including ingestion by biota, biofouling and/or aggregation with organic material leading to flocculation and a change in particle density that can impact trajectory and fate of the material. Furthermore, despite the widespread recognition that rivers dominate the global flux of plastics to the ocean, there is a key knowledge gap regarding the behaviour of microplastics in transport and its pathways from rivers into the coastal zone, especially in regards to how biofilm formation and aggregation influence particle fate. This prevents progress in understanding microplastic dynamics and identifying zones of high accumulation, as well as curtailing the evolution of effective mitigation and policy measures. To predict transport, fate and biological interactions of microplastics in aquatic environments at a global scale, the factors that control these processes must be identified and understood.

A laboratory settling experiment was therefore conducted to recognise how different factors, including salinity, suspended sediment and biofilm formation influence microplastic particle settling velocities, and thus transport. The results presented herein explore the role of biofilms on the generation of microplastic flocs and the impact on buoyancy and settling velocities. Six different polymers were tested and compared including fragments and fibres. Settling velocities were then combined with field flow data from the Mekong River, one of the top global contributors to marine plastic pollution, allowing predictions of areas of microplastic fallout and hotspots. The results also highlight potential areas of ecological risk related to the dispersal and distribution of microplastics across the river-delta-coast system including the ecologically important Tonle Sap Lake. Future work involves further aligned fieldwork within the Mekong River that details the particulate flux and transport of microplastics throughout the vertical velocity profile.

How to cite: Mendrik, F., Fernández, R., Hackney, C., Waller, C., Dorrell, R., Vasilopoulos, G., and Parsons, D.: Controls of bio-modulated flocculation on the fate of microplastic pollution in river-estuary transition zones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-601, https://doi.org/10.5194/egusphere-egu21-601, 2021.