Microplastics from the Post-Flood Agricultural Soils of Thessaly (Greece) Entering the NW Aegean Sea: A Preliminary Modeling Study for Their Transport in the Marine Environment

Abstract

The dispersion of microplastics in the sea is an emerging and crucial environmental problem. In this preliminary study, the hydrodynamics of microplastics transferred from flooded agricultural areas to the sea was assessed. The Daniel storm in 2023 in region of Thessaly, Greece, initiated the transfer of plastic debris via the Pinios River, which subsequently discharged to the coastal basin at the south area of Thermaikos Gulf (NW Aegean Sea). Field sampling and laboratory measurements of microplastics collected at the mouth of the Pinios were conducted. The dispersion of microplastics discharged by the Pinios River is subject to the dominant wind conditions over the area, which in turn determines the water circulation in the NW Aegean Sea. Thus, a hydrodynamic model was initially applied, followed by a transport model for the study of the dispersion of the microplastics. The models were applied for SW and NE winds and indicated that the majority of microplastics with a settling velocity 0.1 m/s accumulate in areas close to the river’s mouth or lateral coastal zones; however, under the influence of SW winds, minor quantities tend to reach the east coasts of the Thermaikos Gulf, while massive quantities are transported away from the river’s mouth in case of microplastics floating on the sea’s surface.

1. Introduction

The advection and dispersion of particulate matter in aquatic ecosystems is a significant focus of scientific research due to the critical role of this particulate matter in shaping environmental quality and consequent biological and ecological effects. In the Hellenic seas, particular emphasis has been given to the dispersion of sand and clay particles and there has been a focus on the research of sediment transport in marine environments [1,2,3]. Additionally, the dispersion of biological materials such as harmful algal blooms has been investigated by Savvidis et al. [4].

Moreover, several recent studies, focused on plastic and microplastic debris, emerging environmental pollutants, and their dispersion in various regional marine environment [5,6,7]. Ryan et al. [5] assessed various methodologies for monitoring plastic debris abundance in marine environments. Their research emphasized the importance for quantification and identification of plastic waste discharged from ships, riverine plastic levels, and stormwater runoff, as these are primary sources introducing plastic waste into marine systems. Furthermore, climate change—manifesting as increased storm activity, rainfall, and acid rain -has been identified as a factor exacerbating the abundance and transport of microplastics in aquatic ecosystems [8,9].

Uzun et al. [6] conducted a comprehensive classification and review of the existing microplastics modeling studies, categorizing them by environmental domain, modeling methodology, and input–output dynamics. Their findings highlight that hybrid modeling approaches—particularly those integrating hydrodynamic and process-based models or combining hydrodynamic and statistical models—yield more robust and reliable results. They further underscore the necessity for a deeper understanding of the physicochemical properties of microplastics and the environmental processes influencing their fate and transport in aquatic systems. Mitchell et al. [7] applied a coupled hydrodynamic and Lagrangian particle-tracking model to predict microplastic transport in South Australian waters. Their results identified characteristic circulation patterns and potential microplastic accumulation zones, particularly within local marine parks.

A study by Simantiris et al. [10] investigated the dispersion of microplastics discharged from various coastal locations along the western Greek coast into the northeastern Ionian Sea. The study utilized a Lagrangian particle-tracking numerical model informed by oceanographic data and simulated microplastic transport over a period of two years. The aforementioned study is of particular importance as it relates dispersion patterns to the potential threats microplastics pose to regional biodiversity. Lei et al. [11] focused on microplastic pollution in sediment-laden river systems, analyzing their ecological impact on fish species. The study found that toxic polyvinyl chloride (PVC) polymers were major contributors to environmental risk, particularly in the middle and lower reaches of the Yellow River, where the ecological risks posed by small microplastics warrant serious attention. In another recent study, Terzi et al. [12] assessed the abundance and characteristics of microplastics discharged by 29 rivers flowing into the southern Black Sea, identifying riverine input as a primary pathway for marine pollution. The findings highlight the urgent need for strategic interventions to safeguard marine ecosystems. Golubeva and Gradiva [13] modeled various marine plastic pollution scenarios using a Lagrangian particle-tracking model over a five-year period. Their research explored the potential distribution of microplastics transported by Siberian Rivers into the Kara Sea shelf. Soegianto et al. [14] conducted a study on the physicochemical properties of microplastics and associated heavy metal contaminants in green mussels (Perna viridis), finding at both surface level and 6 m depths in the Bekasi estuary, West Java, Indonesia.

Microplastics (MPs) have emerged as pervasive contaminants not only in aquatic ecosystems and terrestrial environments, including agricultural soils, but also in the tissues of the human body. Recent studies have highlighted the alarming infiltration of MPs into agricultural systems, raising concerns about soil health, food safety, and long-term ecosystem stability. One major pathway for the entry of microplastics into agricultural lands is through the application of sewage sludge (biosolids), which is widely used as a fertilizer. Studies estimate that between 63,000 and 430,000 tons of microplastics are added to European farmlands annually through sludge application [15]. These particles may persist in the soil, alter its structure, and affect microbial communities [16]. Plastic mulching, a common practice for improving crop yields and conserving soil moisture, is another source of microplastics. Over time, the degradation of plastic mulch films contributes to the accumulation of MPs in topsoil layers [17]. Furthermore, irrigation with contaminated water and atmospheric deposition have been identified as supplementary sources of MPs in agricultural systems [18]. The implications of microplastic pollution in agriculture are multifaceted. MPs can influence soil properties such as bulk density, water retention, and microbial activity [19]. There is also growing concern about the potential uptake of micro- and nanoplastics by plants, which may lead to trophic transfer and human exposure through the food chain [20].

Flooding events, exacerbated by climate change, are increasingly recognized not only for their immediate damage but also for their role in redistributing pollutants across terrestrial and aquatic environments. Among these pollutants, microplastics (MPs) have emerged as significant contaminants whose concentrations often spike following flood events. Floodwaters mobilize and redistribute microplastics from various sources, including urban runoff, agricultural fields, industrial zones, and landfills. During flooding, MPs are washed from surfaces and sediment reservoirs into rivers, lakes, and coastal zones, increasing their spatial dispersion. According to Gago et al. [21], high-flow conditions significantly increase microplastic transport in fluvial systems due to erosion and surface runoff. Furthermore, floods can resuspend previously deposited microplastics in riverbeds and floodplains. Mani et al. [22] found that extreme flooding in central Europe led to a measurable increase in microplastic concentrations downstream, suggesting that such events can remobilize legacy pollution. In urban areas, overflow from combined sewer systems during floods introduces additional plastic debris and microplastics into natural water bodies [23].

Post-flood environments also display elevated concentrations of MPs in soils and sediments. Luo et al. [24] documented significant increases in microplastic accumulation in riparian zones after flood events in China, emphasizing the role of floods in extending the environmental footprint of plastic pollution into previously less-impacted areas.

The ecological consequences are still being explored, but initial studies present that post-flood surges in MPs can impact aquatic life through ingestion and habitat alteration. These events may also increase human exposure risks in agricultural and residential floodplains [25]. Flooding acts as a catalyst for the mobilization and redistribution of microplastics, intensifying their presence in both aquatic and terrestrial systems. As flood events become more frequent and intense, understanding their role in microplastic dynamics is crucial for developing effective mitigation strategies.

This study focuses on the broader region of the Thermaikos Gulf and a substantial portion of the northwestern Aegean Sea, where microplastics originating from post-flood agricultural soils in Thessaly are transported and ultimately discharged via the Pinios River (Figure 1).

In the greater study area, specifically in the terrestrial zone of Thessaly plains, west of the Thermaikos Gulf, and NW Aegean Sea, widespread structural damage caused by intense rainfall, landslides, and water intrusion led to significant solid waste generation. Proximity to rivers, building age, and construction materials influenced damage severity. Initially, waste consisted largely of debris, vehicles, and agricultural tools, but over time it shifted toward household items, plastics, and electronic waste. Receding waters revealed agricultural lands covered in debris and plastics, while riverbanks became informal dumping sites. Waste near rivers poses environmental risks due to the potential leaching of hazardous compounds into the soil and water.

Beyond the widespread application of agrochemicals—linked to intensive agricultural practices—the region has experienced significant environmental degradation due to groundwater overexploitation, deforestation, and severe soil erosion. Additionally, the Pinios River is subject to further anthropogenic pressures, including semi-industrial operations such as olive oil processing facilities and slaughterhouses, as well as hydromorphological interventions including dam construction and river channelization. It also receives treated municipal wastewater from the urban centers of Larissa and Trikala, which have ~165,000 and ~79,000 inhabitants, respectively [27]. Notably, the river system was heavily impacted by storm Daniel, which affected the Thessaly region on 4 to 8 September 2023; the 200–650 mm of rain during this short period [28,29] exacerbated pollutant mobilization and sediment transport. Given these multiple stressors, the Pinios River represents a compelling case study for evaluating the cumulative impacts of land-based anthropogenic activities on microplastic pollution in fluvial and marine systems, particularly with respect to the Aegean Sea and the broader Mediterranean basin. Hence, the study incorporates a coupled hydrodynamic and particle transport modeling framework. The simulations were complemented by both field and laboratory investigations, which included the identification of microplastic types collected near the Pinios River mouth and the experimental determination of their settling velocities. The regional seawater circulation in the broader Thermaikos Gulf is primarily driven by wind-induced forces [30]; tidal influences are quite weak [31] and are not taken into account in the present study. Moreover, riverine runoff in the area has markedly declined following the implementation of hydraulic infrastructure projects—including dam construction—that began in the mid-1980s [31,32].

The Thessaly region plays a critical role in Greek agriculture, serving as one of the country’s most productive areas. Agricultural activities in Thessaly are fully mechanized across all stages, from sowing or transplanting to harvesting. The region is also a major national supplier of fruits—particularly apples and pears—as well as a variety of vegetables. In September 2023, the “Daniel Storm” resulted in severe damage to the broader Thessaly area. The storm caused extensive destruction to households, infrastructure, and both animal and plant production systems. Significant damage was also recorded in the regional road network. The flooding of landfills led to the uncontrolled dispersion of solid waste, which subsequently accumulated along riverbanks, forming unregulated dumping zones. The floodwaters altered the geographical characteristics of the region, as illustrated in Figure 2. Approximately 72,950 hectares (729,500 acres) were inundated, with 97% of this area comprising arable land. This has raised serious concerns about the region’s agricultural recovery and national food security in the aftermath of the disaster.

2. Materials and Methods

2.1. The Study Area and the Computational Domain

The present study is focused on the greater area of Thermaikos Gulf as well a large part of the NW Aegean Sea, where the microplastics from the post-flood agricultural soils of Thessaly are transported and discharged by the river Pinios (Figure 1 and Figure 3).

Figure 3 illustrates the Thermaikos Gulf, the mouth of the Pinios River, and the surrounding northwestern Aegean basin. The area, highlighted in the upper left frame of the figure, represents the computational domain used for the numerical simulations, the description of the water circulation, and the transport of the microplastics in the marine environment.

Figure 3. (a) The Thermaikos Gulf and Pinios River mouth (marked with the red star) in the greater basin of NW Aegean Sea. The area in the black frame corresponds to the study domain. The map in the upper right corner shows the NW Aegean Sea and Pinios River discharging into the Basin [34]. (b) The grid of the computational domain. The star indicates the river’s mouth (where the microplastics are discharged into the sea).

Figure 3. (a) The Thermaikos Gulf and Pinios River mouth (marked with the red star) in the greater basin of NW Aegean Sea. The area in the black frame corresponds to the study domain. The map in the upper right corner shows the NW Aegean Sea and Pinios River discharging into the Basin [34]. (b) The grid of the computational domain. The star indicates the river’s mouth (where the microplastics are discharged into the sea).

For the discretization of the computational domain, a grid of 140 × 134 cells was used (Figure 3b) with a space step Δx = 1250 m. The finite difference method was used for solving the equations of the model.

2.2. Soil and Water Sampling and Sample Analyses for Microplastics

Sediment and water samples were collected monthly from the estuarine area of the Pinios River between May 2024 and February 2025.

During each sampling, a total of six sediment samples were collected from the Pinios River estuary: three from the mid-beach zone and three from the upper-beach zone. At the same time, three surface water samples (1 L each) were taken from a point in the estuarine area. The samplings were carried out monthly for ten months, resulting in the collection of 60 sediment samples and 30 water samples in total. The sediment samples were collected to a depth of 10 cm using a cylindrical core sampler (Ø15 cm) and transferred to the laboratory. From each sediment sample, an equal volume was analyzed, which corresponded to 1 L of dry sand, with an estimated average density of 1.41 kg/L. A total of 60 samples were collected and analyzed. Samples were hot-air-dried and sieved through a nested set of mesh screens to separate particles into two granulometric fractions: small microplastics (SMPs, <1 mm) and large microplastics (LMPs, 1–5 mm). LMPs were visually identified and counted using the naked eye under consistent lighting conditions. SMPs were isolated through density separation using a saturated NaCl solution. The supernatant was vacuum-filtered using a 20 μm filter. The filters obtained from each sample were examined under a stereomicroscope (SLX-3, OPTICA, magnification 10×) to count the microplastics.

Surface water samples were collected approximately 4 m from the shoreline, where water samples were retrieved from a depth of ~30 cm using 1.5 L pre-cleaned glass bottles. Each sample was first passed through a stainless-steel sieve to retain microplastics larger than 1 mm. The retained material was then filtered under vacuum and examined under a stereomicroscope to qualitatively and quantitatively analyze the microplastics. In order to avoid any interference from organic material, a thermal needle method was used.

As part of contamination control during the filtration of samples, a preventive check was carried out to assess the cleanliness of the filters before use. Specifically, a representative number of filters was selected from each package (a total of 50 filters out of the 270 used—approximately 20%) and examined randomly and across different batches. The inspection was performed under a stereomicroscope at 10× magnification, and no traces of microplastics were observed on any of the filters.

2.3. Assessment of Settling Velocities of Common Polymers Used in Agricultural Practices

In order to determine the settling velocities of the most common polymers that are used in agricultural practices, a controlled laboratory experiment was conducted. The vast majority of agricultural plastics are composed of polymers like low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), polypropylene (PP), and polyvinyl chloride (PVC), which are more commonly used in applications such as mulch films, greenhouse covers, irrigation systems, and protective nets.

Single-pellet polymers of the aforementioned polymers were used as model polymers for assessing their potential dispersion behavior. Artificial seawater was prepared following standardized salt concentration protocols to simulate the salinity of natural marine environments. The solution was placed in a transparent graduated cylinder (H-34.22 cm, D-6.10 cm) that had been thoroughly cleaned and sterilized prior to use (Figure 4). Six different types of industrial pellet polymers were selected for use in the experimental procedure (Figure 5). These polymer types are widely utilized in industrial applications and were also identified in field-collected environmental samples from the study area. The procedure involved introducing single industrial pellet polymers at the water surface and allowing it to settle freely without external interference. The time required for the pellets to traverse from the surface to the bottom (fixed vertical distance = 34.22 cm) was recorded using video capture and a stopwatch for temporal accuracy. The experimental conditions, including the water temperature (~20 °C) and quiescent state of the fluid medium, were kept constant throughout the trials. Each measurement was repeated ten times for every polymer type to ensure the reliability and reproducibility of the results.

The recorded settling time was used to calculate the settling velocity using the equation

w_s = s/t

(1)

where w_s is the settling velocity, s is the settling distance (34.22 cm), and t is the recorded time.

Figure 4. Graduated 1 L cylinder filled with synthetic sea water.

Figure 4. Graduated 1 L cylinder filled with synthetic sea water.

Figure 5. Microplastic beads used in the experiment (EPS: Expanded Polystyrene, PE: Polyethylene, PP: Polypropylene, ABS: Acrylonitrile Butadiene Styrene, PVC-S: Suspended Polyvinyl Chloride, PVC-H: High-Density Polyvinyl Chloride).

Figure 5. Microplastic beads used in the experiment (EPS: Expanded Polystyrene, PE: Polyethylene, PP: Polypropylene, ABS: Acrylonitrile Butadiene Styrene, PVC-S: Suspended Polyvinyl Chloride, PVC-H: High-Density Polyvinyl Chloride).

Recovery rates were assessed by comparing the saturated NaCl method to the canola oil method by using spiked soils with beads, based on the oleophilic properties of plastics. However, in the recovery tests conducted between the use of canola oil and NaCl, there was no significant difference observed in the recovery rate. The results from the aforementioned experimental procedure were used in the modeling process in order to assimilate the settling process, which affects their eventual dispersion in the marine environment.

2.4. Mathematical Modeling—Description of the Models

A 2D depth-averaged hydrodynamic model was initially applied, followed by a quasi-3D transport model for the description of the microplastic dispersion in Thermaikos basin and the NW Aegean Sea. If the depth of the water column is greater than 100 m (which corresponds to a minimal portion of the study area), the model is applied only to the surface layer up to a 100 m depth, while the rest of the water column is considered to be motionless, which is very close to the real hydrodynamics that concern time periods of some days. The model predicts the velocity field that corresponds to the steady-state conditions for the prevailing winds blowing over the area.

The output of the hydrodynamic model is used then as the input for the next quasi 3D transport model. This transport model is a Lagrangian model and is based on the Random Walk method. According to this method, a large number of microplastics were placed at the mouth of the Pinios in order to be instantly released within the marine environment. The fate of this material was traced over time. Details about such models can be found in the studies by Koutitas and Scarlatos [35], who focused mainly on sediment particles, and more recently by Simantiris et al. [10], whose study concerned the fate of microplastics, as was already mentioned (in the introduction).

The dispersion of the particulate mass discharged by the Pinios River is mainly subjected to the wind conditions that prevail over the area, which determine the water circulation in the extended coastal basin.

Hydrodynamic model. In more detail, a two-depth-averaged hydrodynamic mathematical model, followed by a transport model was applied here for the description of the dispersion of microplastics in the Thermaikos basin. According to the principles of mass and momentum conservation, the hydrodynamic model was based on the following equations:

$${\displaystyle \frac{\mathsf{\partial }U}{\mathsf{\partial }t}}+U{\displaystyle \frac{\mathsf{\partial }U}{\mathsf{\partial }x}}+V{\displaystyle \frac{\mathsf{\partial }U}{\mathsf{\partial }y}}=-g{\displaystyle \frac{\mathsf{\partial }\zeta }{\mathsf{\partial }x}}+fV+{\displaystyle \frac{{\tau }_{sx}}{\rho h}}-{\displaystyle \frac{{\tau }_{bx}}{\rho h}}+{\nu }_h{\displaystyle \frac{{\mathsf{\partial }}^{2}U}{\mathsf{\partial }{x}^2}}+{\nu }_h{\displaystyle \frac{{\mathsf{\partial }}^{2}U}{\mathsf{\partial }{y}^2}}$$

(2)

$${\displaystyle \frac{\mathsf{\partial }V}{\mathsf{\partial }t}}+U{\displaystyle \frac{\mathsf{\partial }V}{\mathsf{\partial }x}}+V{\displaystyle \frac{\mathsf{\partial }V}{\mathsf{\partial }y}}=-g{\displaystyle \frac{\mathsf{\partial }\zeta }{\mathsf{\partial }y}}-fU+{\displaystyle \frac{{\tau }_{sy}}{\rho h}}-{\displaystyle \frac{{\tau }_{by}}{\rho h}}+{\nu }_h{\displaystyle \frac{{\mathsf{\partial }}^{2}V}{\mathsf{\partial }{x}^2}}+{\nu }_h{\displaystyle \frac{{\mathsf{\partial }}^{2}V}{\mathsf{\partial }{y}^2}}$$

(3)

$${\displaystyle \frac{\mathsf{\partial }\zeta }{\mathsf{\partial }t}}+{\displaystyle \frac{\mathsf{\partial }(Uh)}{\mathsf{\partial }x}}+{\displaystyle \frac{\mathsf{\partial }(Vh)}{\mathsf{\partial }y}}=0$$

(4)

where h is the depth of the water column, U and V are the vertically averaged horizontal sea current velocities, ζ is the surface elevation, f is the Coriollis parameter, τ_sx and τ_sy are the wind surface shear stresses, τ_bx and τ_by are the bottom shear stresses, ν_h is the dispersion coefficient according to Smagorinski [35,36,37], ρ is the seawater density, and g is the gravity acceleration.

The time step dt, used for the hydrodynamic model simulations, was taken equal to 10 s.

Concerning the initial conditions, the model starts from a velocity of zero for the water column and develops up to steady-state conditions. As far as the boundary conditions are concerned, zero velocity applies along the coasts, while Somerfield conditions apply to open-sea boundaries.

Transport model. The velocity field produced from the hydrodynamic simulation then allows for the transport simulation. The transport model applied here was based on the tracer method and Random Walk Simulation [1,37,38,39,40,41]. The advection, diffusion, and sedimentation processes of a conservative suspended matter are simulated by this method. According to this method, a large number of particles, representing a particular mass, are introduced in the flow domain through a source. More specifically, in this study, the particles are released instantaneously in the mouth of the Pinios River towards the marine environment. The particles are considered as a conservative substance. Their transport and fate are traced over time and, finally, the particle distribution in marine basin is calculated from the number of particles found at each grid box.

Based on the scientific literature [35], the tracer method contains the following steps:

(a)

The velocity field is determined by a set of values of the velocity components at specific grid points computed by the hydrodynamic part of the model, and local velocities are obtained from these velocity components using interpolation schemes;

(b)

A suitable time step is selected (here, Δt = 600 s);

(c)

The range of the random velocity ± U_r (horizontally) or W_r (vertically) is computed from the following equation:

$$U_r=\sqrt{{\displaystyle \raisebox{1ex}{$2D_h$}\!\left/ \!\raisebox{-1ex}{$\Delta t$}\right.}}$$

(5a)

$$W_r=\sqrt{{\displaystyle \raisebox{1ex}{$2D_v$}\!\left/ \!\raisebox{-1ex}{$\Delta t$}\right.}}$$

(5b)

where D_h is the local horizontal diffusion coefficient, D_v is the local diffusion coefficient, and Δt (dt) is the time step;

(d)

In the case of instantaneous discharge, a specific amount of particles is released at once from the initial source; while in the case of continuous discharge, for each time step, a specific number of particles is released from the source;

(e)

Integration in time is executed and the new coordinates of each particle are computed; the motion of each particle is analyzed considering (i) a deterministic part that concerns the advective transport and (ii) a stochastic part that concerns the transport-spreading due to diffusion processes.

The horizontal positions of the particles are computed from the superposition of the deterministic and stochastic displacements:

$${ x}_i^{n+1}=x_i^n+\Delta x_i^n+\Delta x_i^{n^{\prime }}$$

(6)

where

$$\Delta x_i^n= u_i^n\left(x_i^n, t^n\right) dt$$

(6a)

and

$$\Delta x_i^{n^{\prime }}= u_i^{n^{\prime }} ·\Delta t ·R$$

(6b)

and

$$y_i^{n+1}=y_i^n+\Delta y_i^n+\Delta y_i^{n^{\prime }}$$

(7)

where

$$\Delta y_i^n= v_i^n\left(y_i^n, t^n\right) dt$$

(7a)

and

$$\Delta y_i^{n^{\prime }}=v_i^{n^{\prime }} ·\Delta t ·R$$

(7b)

where $\Delta x_i^n$ and $\Delta {xy}_i^n$ are the deterministic displacements and $\Delta x_i^{n^{\prime }}$ and $\Delta y_i^{n^{\prime }}$ are the stochastic displacements, with $u_i^n\left(x_i^n, t^n\right)$ and ${ v}_i^n\left(y_i^n, t^n\right)$ being the deterministic velocities at time tn at the location $x_i^n$ and $y_i^n$ of the i particle; $u_i^{n^{\prime }}$ and $v_i^{n^{\prime }}$ are the random (stochastic) horizontal velocities at time tⁿ at the locations x_i and y_i, respectively; $u_i^{n^{\prime }}$= $v_i^{n^{\prime }}$ according to Equation (5a), with D_h being the horizontal particle diffusion coefficient; and R is a random variable distributed uniformly between −1 and +1.

Additionally, the description of the particles’ motion in the vertical direction was also analyzed into a deterministic part Δ${\mathit{zs}}_i^n$ due to settling process and a stochastic part Δ$z_i^{n^{\prime }}$ according to the following relations:

$$z_i^{n+1}=z_i^n+\Delta z_i^n+\Delta z_i^{n^{\prime }}$$

(8)

$$\Delta z_i^n=w_s·\mathit{dt}$$

(8a)

and

$$\Delta z_i^{n^{\prime }}=w^{\prime }·\Delta t·R$$

(8b)

where w_s is the particle settling velocity and w′ is the random vertical velocity due to vertical turbulence (according to Equation (5b)), with D_v the vertical dispersion coefficient (here, 0.01 m²/s).

As far as the particle settling velocity is concerned, representative velocity values are reported in bibliography for microplastic particles. These velocity values range between 0 and 0.2 m/s [42,43]. Here, w_s = 0. 1 m/s was considered in the simulations.

As far as the boundary conditions are concerned, the particles that arrive on the sea floor or tend to move over the sea surface are reflected at their previous position. Furthermore, the particles that reach the coastal boundaries also reflect to their previous position and continue their motion.

The concentrations are then computed from the number of particles that can be found at each grid box. In other words, the spatial particle distribution resulting from the above process can then lead to the computation of the particle concentrations relative to the number of particles contained in each grid box.

2.5. Summary of the Designed Experiments—Laboratory and Modeling and Field Works

Finally, the following

Table 1. Synoptic presentation of the applied experiments and data collection.

Field Works: Soil and Water Sampling	Laboratory Experiments	Numerical Experiments, Hydrodynamic Model Runs, and Transport Model Runs
		Hydrodynamic	Transport
Six sediment samples and three water samples on a monthly basis from May 2024 to February 2025	▪ Assessment of the settling velocities of common polymers used in agricultural practices ▪ Sample treatment and recovery of MPs ▪ Application of stereo/microscopical and thermal needle method for identification and enumeration of MPs	Two hydrodynamic model runs: one for NE and one for SW winds	Ten model runs *. Five runs for the transport of MPs due to NE wind generated sea currents and five runs for transport due to SW wind generated sea currents (1, 5, 10, 15, and 20 days after sudden release of MPs)

* Note: Model runs were realized for settling velocities of 0.1 m/s, 0.05 m/s, and 0 m/s for each one of the above-mentioned cases (i.e., a total of 2 runs for the hydrodynamic circulation and 30 model runs for the transport of MPs).

is given in order to include all the parts of the research, which consists of filed data collection, laboratory experiments, and numerical experiments (model runs).

3. Results

3.1. Microplastics in Soil and Water

The concentrations of microplastics in the Pinios River mouth sediments and water throughout the sampling period are presented in Figure 6. The results from the optical identification of polymers in the Pinios River mouth are presented as a percentage in Figure 7.

The amount of microplastics present in both soil and water are increased in late spring and summer, presenting the highest concentration in July, reaching almost 180 items/m² and 6 items/L, respectively (Figure 6). A proportional increase is observed, enhancing the correlation of the transfer mechanism. Most of the microplastics found are polyethylene and expanded polystyrene polymers (Figure 7A). Fragments, foams, and films were detected at comparable proportions in both soil and water samples (Figure 7B), whereas fibers exhibited a higher prevalence in water samples (Figure 7C).

Polyethylene (PE) and expanded polystyrene (EPS) are among the most prevalent plastic contaminants found in agricultural soils, primarily due to their widespread use in modern farming practices. PE enters soils largely through plastic mulching films, greenhouse coverings, irrigation tubing, and packaging materials, which degrade over time due to UV exposure and mechanical disturbance, leaving behind fragments. EPS, although less directly applied in agricultural settings, can reach soils via indirect pathways such as the breakdown of insulation materials used in rural construction, packaging waste from agricultural inputs, or wind-blown litter from nearby urban or industrial areas. Inadequate waste management, repeated tilling, and irrigation runoff exacerbate the incorporation of these microplastics into the soil matrix. These persistent polymers not only alter soil structure and microbial communities but may also facilitate the transport of agrochemicals, posing a challenge for sustainable soil management.

In agricultural soils, polyethylene (PE) and expanded polystyrene (EPS) undergo a range of physicochemical degradation processes that contribute to their fragmentation into microplastics. Photodegradation, driven by solar ultraviolet (UV) radiation, initiates the breakdown of polymer chains at the surface, particularly in exposed mulch films and plastic residues, resulting in increased brittleness and susceptibility to mechanical fragmentation [44]. Concurrently, thermal oxidation accelerates polymer degradation under the fluctuating temperature conditions common in open-field agriculture. Mechanical forces from tillage, harvesting, and foot or vehicular traffic further fragment weakened plastics into progressively smaller particles. In the case of EPS, its porous structure enhances its vulnerability to physical disruption and facilitates the entrapment of soil particles and organic matter, which can influence its environmental behavior and persistence. Chemical processes, such as hydrolysis and oxidative reactions catalyzed by soil minerals and metal ions, also contribute to polymer chain scission. Although microbial activity has a limited direct impact on PE and EPS due to their recalcitrant nature, biofilm formation on plastic surfaces can mediate localized chemical environments that promote degradation. These synergistic processes ultimately lead to the formation of microplastics that persist in the soil matrix, posing potential risks to soil health and biota.

3.2. Polymer Settling Velocities

The results from the laboratory experiments measuring the settling velocities of microplastic pellets are presented in

Table 2. Settling velocity of industrial pellet polymers measured in the laboratory.

ws (m/s)
PVC-H	PVC-S	ABS	PE	PP	EPS
0.014–0.118	0.01–0.071	0.013–0.056	0.019–0.037	0	0

. Settling velocity, w_s, ranged between 0.01 and 0.1 m/s for PVC-H, PVC-S, PE, and ABS, while a significant portion of microplastics presented zero settling velocity (w_s = 0 m/s), i.e., they are floating particles (PP and EPS), indicating that these types of polymers may only be transported and dispersed on the surface of the water body. Furthermore, concerning the vertical motion, PE microplastics may present either settling or rising behavior according to recent studies [44,45,46].

Polyethylene (PE) and polypropylene (PP) exhibit distinct behaviors in marine environments due to differences in their physical properties and interactions with environmental factors [46]. Both polymers are less dense than seawater (PE: 0.91–0.96 g/cm³; PP: 0.85–0.92 g/cm³) and therefore tend to float upon entering the ocean. However, their long-term buoyancy is influenced by processes such as biofouling, which increases the effective density of plastic particles. PE can be more hydrophobic and thus tends to form biofilms more rapidly than PP, leading to a higher likelihood of eventual sinking [46]. As a result, PE particles may transition to the benthic zone more quickly, while PP fragments often remain suspended or floating for longer periods. These differences in vertical transport have important implications for modeling the distribution and fate of plastic debris in marine systems.

3.3. Results of Mathematical Simulations

The model simulations investigated wind-generated currents and particle dispersion induced by prevailing northeasterly (NE) and southwesterly (SW) winds in the area adjacent to the river mouth, based on recorded data [47].

The NE wind scenario corresponded to the wind conditions prevailing during September 2023, i.e., the period during which the extreme flooding events described above occurred.

The exact numbers of microplastic particles over the cross sections of the river mouth were not measured. So, an arbitrary number of 100,000 MP was used for the simulations, considering that figure representative of the total mass of the microplastic particles discharged by the river.

As discussed in the previous section, the representative settling velocities (w_s) for microplastic particles reported in the literature range from 0 to 0.2 m/s [42,43]. These values are consistent with the settling velocities measured in the laboratory and presented above (Table 1). Furthermore, as previously mentioned, specific types of microplastics exhibit a settling velocity of zero, indicating that these particles are transported (floating) at the surface of the water column.

Based on these field and laboratory findings, numerical experiments were conducted referring to (a) microplastics with a settling velocity of 0.1 m/s or a settling velocity of 0.05 m/s and (b) microplastics that float, i.e., particles with zero settling.

Figure 8 presents the pattern of water circulation (mean in depth currents) for NE constant winds of 10 m/s, while Figure 9, Figure 10, Figure 11, Figure 12 and Figure 13 present the microplastics’ dispersion 1, 5, 10, and 20 days after the mass discharge at the mouth of the Pinios river (for settling velocity w_s = 0.1 m/s). The thick blue arrow in Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17, Figure 18, Figure 19, Figure 20 and Figure 21 represents the wind direction prevailing in the greater area of Thermaikos Gulf and NW Aegean Sea. The red star in these Figures (Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 13, Figure 14, Figure 15, Figure 16, Figure 17, Figure 18, Figure 19, Figure 20 and Figure 21) indicates the Pinios River mouth, as given also in Figure 3. Different colored arrows in the area of the marine environment of Figure 8 and Figure 14 correspond to different sea current intensity. This information is given in the color scale of the bar next to the figure which ranges from blue (low current velocities) to red (high current velocity).

The bars on the right of the above figures describing the dispersion of the particles indicate the number of particles, according to the presumption that 100,000 microplastic particles were instantaneously released at the location of the Pinios River mouth. Following this release, and after a specific amount of time, the dispersion of the microplastics is quantitatively represented by the number of particles found at each grid point (i.e., within the area of a computational cell). In this manner, a count of 20,000 particles corresponds to 20% (20,000/100,000) of the initial total mass of microplastics discharged at the river mouth.

Figure 14 presents the pattern of water circulation (mean in depth currents) for SW constant winds of 10 m/s, while Figure 15, Figure 16, Figure 17, Figure 18 and Figure 19 present the dispersion of particles 1, 10, and 20 days after the mass discharge at the mouth of the Pinios river (for w_s = 0.1 m/s).

Figure 14. Velocity of sea currents due to SW wind of 10 m/s (the bar on the right is in m/s).

Figure 14. Velocity of sea currents due to SW wind of 10 m/s (the bar on the right is in m/s).

Figure 15. Dispersion of particles 1 day after the discharge (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 15. Dispersion of particles 1 day after the discharge (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 16. Dispersion of particles 5 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 16. Dispersion of particles 5 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 17. Dispersion of particles 10 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 17. Dispersion of particles 10 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 18. Dispersion of particles 15 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 18. Dispersion of particles 15 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 19. Dispersion of particles 20 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 19. Dispersion of particles 20 days after the release (and SW wind). The grey scaling of the bar on the right indicates the number of microplastic particles.

The present study and the results of the model simulations focused on the patterns of circulation and dispersion of microplastic particles induced by depth-averaged, wind-generated sea currents. However, the hydrodynamic model presented above is also capable of providing satisfactory estimations of surface currents, as described by Koutitas [37]. Based on this capability, the model was applied to compute surface water velocities, followed by the application of the transport model to simulate the dispersion of particles with zero settling velocity, i.e., microplastics floating on the sea surface.

The results of these simulations indicate that microplastics tend to accumulate and mostly become trapped around the river mouth, which served as the source of microplastic discharge, driven by surface currents generated by northeasterly (NE) winds of 10 m/s (results are not presented here). Furthermore, it is noteworthy that a rapid dispersion of particles away from the mouth is observed under the influence of SW winds. More specifically, the dispersion of floating microplastics one day after their release is shown in Figure 20, followed by their eventual accumulation along the coasts of Chalkidiki, which is opposite the Pinios River mouth, three days after the discharge event. This pattern is attributed to the transport of floating (buoyant) material by the surface currents generated by southwesterly (SW) winds of 10 m/s (Figure 21).

Figure 20. Dispersion of particles 1 day after the release. The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 20. Dispersion of particles 1 day after the release. The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 21. Dispersion of particles 3 days after the release. The grey scaling of the bar on the right indicates the number of microplastic particles.

Figure 21. Dispersion of particles 3 days after the release. The grey scaling of the bar on the right indicates the number of microplastic particles.

4. Discussion

The prevalence of plastics has particularly increased in the sampling areas and close to river/stream banks (Figure 22). Photodegradable, oxodegradable, and biodegradable plastics may fragment into microplastics under environmental conditions such as high temperature and high levels of sunlight and microbial activity [45]. These microplastics can adversely affect soil microbiology and nutrient cycling, which are crucial for agricultural productivity. Recent studies have highlighted the presence of microplastics in food and water, posing direct health risks. Their lightweight and buoyant nature of some microplastics facilitates transport by wind and water [45]. Given the connectivity of the Litheos and Portaikos rivers to the Pinios River, which flows into the Aegean Sea, there is a pathway for microplastics to enter marine ecosystems.

The present research constitutes a preliminary study; however, it provides valuable insights into the dispersion of microplastics in the broader area of the Thermaikos Gulf and NW Aegean Sea following their discharge at the Pinios River mouth.

The weathered plastic materials used in agricultural practices tend to accumulate—either through natural transport mechanisms or due to uncontrolled dumping—along various stream banks and riverbanks, ultimately converging in the Pinios River. The plastics commonly found in marine environments are often fibers, with PET being particularly prevalent. This pattern is typically associated with sources such as fishing, tourism, and harbor activities, as well as domestic wastewater and industrial discharge. However, a study conducted by Zeri et al. in 2021 [48] on microplastic concentrations in the Pinios River found that polyethylene (PE) was the most abundant polymer, which aligns closely with the findings of the present study.

As outlined in the introduction, this study focuses on the wind-driven circulation and associated dispersion of microplastics (MPs), employing an idealized process-based modeling approach. Accordingly, the numerical simulations included only the wind forcing, and the hydrodynamic model was implemented in a barotropic configuration. Thermohaline circulation and vertical stratification in the Thermaikos Gulf were not incorporated in the current simulations. While these processes may influence circulation patterns, particularly during periods of intense river discharge (e.g., the Daniel storm), they will be addressed in future work using a two-layer or multilayer hydrodynamic model. Additionally, the release of microplastics was modeled as an instantaneous event, although, in reality, discharge occurred over several days during and after the storm. This simplification was deemed appropriate for the preliminary, idealized nature of the present process study.

Soil properties significantly influence the degradation and transport of agricultural plastics to aquatic environments. Factors such as pH, organic matter, microbial activity, texture, moisture, and temperature affect the breakdown of plastics in soil. While biodegradable plastics may degrade under favorable microbial and chemical conditions, conventional plastics like PE and PP primarily undergo physical fragmentation. Soil texture also affects microplastic mobility, with clayey soils—dominant in this study—facilitating transport mainly through rainfall and irrigation. High irrigation demand and flood events further enhance microplastic runoff into water bodies, particularly in erodible soils with steep slopes and sparse vegetation. Tillage and soil erosion contribute to the fragmentation and mobilization of plastics toward streams and rivers, two facts that can be interconnected, along with the increased irrigation during spring/summer that aligns with the results of the present study.

The settling velocities of the microplastics were determined from laboratory experiments (which were in agreement with reported findings of published research), and there was no need for parameters such as density, shape, and geometrical information. These parameters are used for the computation of settling velocity values if such information (as mentioned above) is available and can be included in the simulations during the process of the model’s improvement.

The model simulations demonstrated that the majority of microplastics accumulate in areas near the Pinios River mouth. Additionally, for a period exceeding 10 days after the initial discharge of microplastics into the marine environment, a portion of the material appears to reach the coasts of Pieria (west coast of the Thermaikos Gulf, north of Thessaly Region) under both prevailing wind conditions modeled, namely southwesterly (SW) and northeasterly (NE) winds of 10 m/s. Moreover, under SW wind conditions, minor quantities of microplastics seem to reach the western coasts of Chalkidiki (east coast of the Thermaikos Gulf) approximately 15 and 20 days after the discharge event. It should be noted that these circulation patterns were identified under relatively strong winds (10 m/s) for particles characterized by a settling velocity of 0.1 m/s that are transported primarily by depth-averaged currents. Furthermore, it is of interest that particles with a settling velocity of 0.05 m/s exhibited similar dispersion patterns which are not presented in this paper.

As expected, floating/buoyant particles (zero settling velocity) were transported and dispersed more intensively and rapidly, as their movement was predominantly governed by surface currents. Notably, under NE wind conditions, floating/buoyant microplastics were rapidly advected and accumulated around neighboring coastal areas near the river mouth, while under SW winds, dispersion was more extensive: within just three days, particles were transported across the Gulf and reached the opposite coast of Chalkidiki.

The model results for NE winds, one day after microplastic discharge at the Pinios River mouth, are presented in Figure 15 and Figure 23b (modified from Figure 15 and adjusted in order to enhance the comparison) and are generally consistent with the satellite image presented in Figure 23a, thus supporting the validity of the model’s performance. The meteorological conditions prevailing in the area prior to the September flooding event (as well as until the end of the month) were characterized by NE winds. The satellite image displays the dispersion of sand and silt material under the influence of NE winds, closely matching the microplastic dispersion patterns predicted by the model. Specifically, the significant accumulation of sediment particles depicted in Figure 23a appears consistent with the modeled accumulation of microplastics with settling velocities similar to those of fine sand and silt particles. Minor quantities of material visible farther from the Pinios mouth in Figure 23a likely correspond to clay particles, which possess significantly lower settling velocities.

It is well-known that the transport of suspended particulate matter in aquatic environments consists of two simultaneous processes, namely advection and diffusion [50]. The same principles clearly apply to microplastics. Polyethylene (PE) and expanded polystyrene (EPS) are major plastic contaminants in agricultural soils and are directly introduced primarily through the use of plastic mulch films, greenhouse materials, and packaging, as well as indirectly via wind-blown litter and construction debris. Once in the soil, these polymers undergo degradation through a combination of photodegradation, thermal oxidation, mechanical fragmentation, and chemical reactions, gradually forming persistent microplastics. EPS is particularly prone to physical disruption due to its porous structure. Although microbial degradation is limited, biofilm formation can enhance localized breakdown. These microplastics pose risks to soil health, structure, and microbial communities and may facilitate agrochemical transport. Additionally, PE and polypropylene (PP), while similar in density and marine buoyancy, show different degradation and sinking behaviors due to variations in biofouling susceptibility, with implications for modeling the fate of these plastics in terrestrial and aquatic systems.

Therefore, monitoring in order to quantify and identify the polymer species that the microplastics are composed of may facilitate an accurate representation and implementation of the dispersion process. As the dispersion coefficients significantly influence the transport of microplastics, in the present study, the Smagorinsky formula, widely applied in mathematical simulations globally [51], was employed to calculate the horizontal turbulent diffusion coefficients for both momentum and mass transport. Values ranging between 10 and 100 m²/s were computed in the model simulations.

Regarding the transport model, the vertical mass diffusion coefficient was assumed to be constant and equal to 0.01 m²/s. The application of these diffusion coefficient values produced patterns of particle dispersion considered realistic and suitable for comparison, evaluation, and validation against available satellite imagery, such as that discussed above.

Furthermore, the dispersion of microplastics appears to be largely confined within the Thermaikos Gulf, with no significant transport of the material observed toward the southern or eastern regions of Chalkidiki.

5. Conclusions

This preliminary study highlights the significant role of extreme flooding events in facilitating the transport of microplastics from agricultural soils into marine environments. The extensive damage to agricultural areas and rural infrastructure led to the release of a variety of plastic materials, which subsequently degraded into microplastics under environmental conditions. These microplastics, originating from post-flooded soils, were carried by interconnected river networks into the NW Aegean Sea and pose a significant threat not only to the ecological quality of the receiving ecosystems but also to human health through the agrifood food chain and consumption of marine species.

Model simulations demonstrated that microplastics predominantly accumulate near the river mouth, with a portion dispersing along adjacent coastal zones under the influence of prevailing winds and sea currents. The dispersion patterns revealed limited transport beyond the confines of the Thermaikos Gulf, suggesting that microplastic contamination remains largely localized within this marine area. However, if the particles present zero settling velocity, i.e., move as a floating/buoyant material, they may be transported rapidly by the surface currents to distances far away from where they initially were discharged.

In the next stage, the use of more complex and analytical models is planned in order to complete the preliminary study presented here and to produce more detailed results.

The presence of microplastics poses a dual threat: degradation of soil quality and disruption of key biological processes in agricultural environments and long-term ecological impacts on marine ecosystems, including potential effects on food webs and nutrient cycles. These findings underline the urgent need for comprehensive monitoring and management strategies to address microplastic pollution arising from extreme weather events, especially in regions where agriculture and riverine systems are closely linked to coastal marine environments.

Future research should focus on predictive models and long-term monitoring of microplastic dispersion in soils and marine environments, enhanced modeling that couples terrestrial and aquatic systems, and assessment of ecological impacts at multiple trophic levels. Additionally, exploring mitigation strategies to prevent microplastic generation and transport following extreme weather events will be crucial for protecting both agricultural productivity and marine ecosystem health.