*Correspondence author: Triantafyllou George, Hellenic Centre for Marine Research, Institute of Oceanography, 46.7 km Athens-Sounio Ave, PO Box 712 Anavyssos, Attica, GR-190 13, Greece.
How to cite this article: Kalaroni S, Hatzonikolakis Y, Tsiaras K, Gkanasos A, Triantafyllou G. Modelling the Marine Microplastic Distribution from Municipal Wastewater in Saronikos Gulf (E. Mediterranean). Oceanogr Fish Open Access J. 2019; 9(1): 555752. DOI:10.19080/OFOAJ.2019.09.555752
A three-dimensional hydrodynamic model is coupled with a Lagrangian-Individual Based Model to simulate the floating microplastics (<300μm) dispersal and transport in the Saronikos Gulf. Considering municipal wastewaters as their main source, simulations were carried out over 2011–2012. A comparison with hydrodynamic observational data has shown that the model qualitatively reproduces the main circulation structure and hydrodynamic features. To explore the fate and distribution of microplastics, model results were analyzed taking into account the seasonal variability of near-surface circulation. Simulation results gave a qualitative description of affected areas from microplastics pollution, suggesting that the most affected part of Saronikos Gulf is the coastal area that extends from Psitallia Waste water treatment plant to the east. Despite some limitations, this is a first model attempt to explore the dispersal and distribution of microplastics in the Saronikos Gulf.
Abbrevations: WWTP: Wastewater Treatment Plants; IBM: Individual Based Model; SI: Super-Individuals; GEBCO: General Bathymetric Chart of the Oceans; ΜDT: Mean Dynamic Topography; SLA: Sea Level Anomalies; SSH: Sea Surface Height
Most of the world’s marine pollution comes from the land according to the Joint Group of Experts on the Scientific Aspects of Marine Environmental Pollution , while the 60-80% of marine pollutants are made of plastics . Microplastics are plastic particles with diameter less than 5 mm, originating from the breakdown of larger plastic pieces and also from a variety of substances, such as personal care products and textiles, entering the ocean mainly from wastewater treatment facilities [3-5]. Many studies have shown that microplastics can be ingested by numerous marine organisms [6-10], due to their small sizes and large volume-to-surface area ratio. Furthermore, recently microplastics have been detected in human stools .
A significant population growth has occurred in Mediterranean coastal regions (from 95 million in 1979 to 143 million in 2000) , due to both touristic and industrial activities, contributing to an increased plastic pollution of the marine environment. Wastewater Treatment Plants (WWTP) are an effective solution towards the reduction of microplastics input to the marine environment, showing retention efficiencies that may reach ~99% . However, given the very large amount of wastewater
discharge from the major coastal cities and the significant number of smaller cities that still lack of wastewater treatment facilities, a considerable amount of microplastics is released into the marine environment, especially smaller particles (<3mm) for which the treatment is less efficient.
Saronikos Gulf is a semi enclosed area that receives an increased load of wastewater from the populous city of Athens (>4 million inhabitants), especially through the WWTP of Psitallia. To assess the current status of plastic pollution, identifying the main pathways and accumulation areas of marine microplastics in Saronikos Gulf, a modelling tool has been developed and applied to simulate the dispersion and pathways of microplastics, after their release from WWTPs.
A Lagrangian Individual Based Model (IBM) is on-line coupled with the three dimensional (3-D) hydrodynamic Princeton Ocean Model [14,15], which provides ocean currents and diffusion coefficients (horizontal and vertical) that are used for the transport of the Lagrangian particles.
The hydrodynamic model: POM is a primitive equation,
time-dependent, free surface and sigma-coordinate 3-D
circulation model that has been extensively described in the
literature and is accompanied by a comprehensive user’s guide
. It is a widely spread community model that has been used
both for coastal and open ocean studies [17-22]. POM prognostic
variables are temperature, salinity, velocity, sea surface height
and turbulent kinetic energy. The model uses a bottom-following
sigma coordinate system and an Arakawa-C staggered grid in the
horizontal. The vertical eddy viscosity and diffusivity coefficients
are computed using the Mellor-Yamada 2.5 turbulence closure
scheme , while horizontal diffusion is calculated along sigmalevels
following a Smagorinsky formulation . Time integration
is performed with a split (external/internal) time step.
Langrangian-IBM plastic dispersion model: The
Lagrangian-IBM plastic dispersion model is based on Pollani
et al.  and follows the concept of Super-Individuals (SI) for
computational efficiency. Each SI represents a group of particles,
sharing the same attributes (position, weight, origin, type of plastic
etc.). The position of every SI is described by its coordinates (x, y,
z) in a Cartesian system, which are updated every time-step using
the 3-D displacement produced by currents and waves, obtained
by bi-linear interpolation at the SI location (x, y, z)
where uc, vc and wc are the ocean current velocity field
components, obtained (on-line) from the hydrodynamic model.
The terms uw and vw represent the wave stokes drift, obtained (offline)
from POSEIDON operational wave model (www.poseidon.
This is assumed to decrease exponentially with depth as
where uw0 and vw0 represent the stokes drift at surface, k is
the wave number and Z is the depth of the water column. The
stochastic horizontal displacement (Rx, Ry) of particles depends on
the horizontal diffusion as follows:
where Kh is the horizontal diffusion, obtained from the
hydrodynamic model and the terms rx and ry represent random
numbers between [0, 1]. The stochastic vertical displacement (Rz)
is derived as:
where kz is the vertical turbulent diffusion, obtained from the
hydrodynamic model, rz represents a random number between
[0, 1] and Κw is the vertical displacement from wave that decays
exponentially with depth as follows:
where Wh is the significant wave height, while Wt and k are the
wave period and number, respectively.
Sources: Microplastics inputs in the Saronikos Gulf through
wastewater treatment facilities were estimated, considering the
wastewater discharge from all coastal cities with more than 2000
inhabitants. Inputs data were obtained from UNEP/MEDPOL
2011 report. The amount of microplastics in the wastewater,
depending on the type of treatment, was estimated using available
data from the literature. Untreated wastewater was assumed to
contain ~450.000 particles/m3  with size varying between
20μm and 300μm. The microplastics concentration was assumed
to decrease by 25% and 75% respectively, when primary and
secondary treatment is applied, based on existing studies [25,26].
A slightly lower (10%) decrease was applied for pre-treatment
and a slightly higher (85%) for tertiary treatment. Recent studies
report efficiencies up to 98%, although these are often related to
additional filtering, such as reverse osmosis.
The 3-D coupled hydrodynamic-Lagrangian-IBM model
domain covers the entire Saronikos Gulf (2.9792ο Ε to 24.3844ο Ε
and 37.5250ο Ν to 38.2292ο Ν) Figure 1, with a resolution of 1/240o
×1/240o (~500 ×500m) in the horizontal and 24 sigma-levels in
the vertical, following a logarithmic distribution approaching the
surface and bottom layers. The General Bathymetric Chart of the
Oceans (GEBCO; www.gebco.net) was used to build the model
bathymetry using bi-linear interpolation into the model grid. The
atmospheric forcing (10m wind speed, 2m air temperature, 2m
relative humidity, precipitation, incoming long-wave radiation
and net short-wave radiation) was obtained from the POSEIDON
atmospheric dataset . The initial conditions for temperature
and salinity were obtained from simulations of the 3D operational POSEIDON hydrodynamic model for the Aegean Sea [28,29].
The Saronikos Gulf plastic dispersion model has a south-eastern
open boundary, where boundary conditions for temperature,
salinity and ocean current velocity were also obtained from the
operational POSEIDON Aegean model. The Lagrangian-IBM model
was initialized with a homogeneous distribution of microplastics
of 0.5 *106 particles/km2. After a 3-year model spin-up, a
simulation with the 3-D on-line coupled model was performed
over 2011-2012 period.
The observational data that were used for the model
validation of the Saronikos Gulf hydrodynamics are Mean
Dynamic Topography (ΜDT, spatial resolution: 0.0625ο X 0.0625ο)
for the 1993-2012 period, obtained from the European AVISO+
altimetry data base , and daily Sea Level Anomalies (SLA,
spatial resolution: 0.125ο X 0.125ο) that were obtained from the
European Copernicus data base (http://marine.copernicus.
eu/). Additionally, satellite remote sensing daily Sea Surface
Temperature data (SST, spatial resolution: 0.04ο X 0.04ο) were used,
obtained also from the Copernicus data base [31,32]. To compare
with the model outputs, all satellite data were interpolated at the
In Figures 2 and 3, the model simulated mean February and
July, SST and sea surface height (SSH), for the 2011-2012 period,
are compared against satellite data over the same period. The
near-surface circulation of Saronikos Gulf is also overlapped.
A general west-to-east flow is observed during both winter
(February) and summer (July), which is consistent with in situ
measurements of Kontoyiannis . The eastward flow, following
a broad anticyclonic loop in the north-western sub-basin, forms a
cyclonic meander in the north-eastern area of Saronikos Gulf. The
main flow exits to the south forming a closed anticyclonic loop in
the south-eastern sub-basin.
A more quantitative validation of the simulated near-surface
circulation can be obtained comparing the model simulated SSH
pattern with satellite altimetry data, given that low/high SSH
areas represent cyclonic/anticyclonic flows. Our results show that
the model captures quite well the observed SSH pattern Figure
2, although a model deviation from the observed values was
produced. Given the Mediterranean basin scale SSH variability,
differences between simulated and observed absolute values
are expected. Nevertheless, the model simulates the main SSH
horizontal variability. More specifically, the simulated southern
anticyclone (indicated by high SSH) is consistent with the
observed one, with a slight westward displacement. Although
satellite observations are limited in the north-eastern sub-basin,
the model captures the decrease of SSH. Finally, in the western
sub-basin, an increase of SSH is simulated, which is associated
with the anticyclonic loop.
The model successfully reproduces the observed spatial
variability of the Saronikos SST characterized by an eastward
decreasing gradient Figure 3, associated with the inflowing colder
Aegean waters. The north-western area is occupied by relatively
warmer waters due to the anticyclonic flow, while inside the
north-eastern cyclonic meander the surface temperature is colder.
In the southern anticycloclic loop, the colder blob indicates that
it contains cold water quantities of Aegean Sea origin. Simulated
SST is quite close to the satellite observations during both winter
(February) and summer (July) period. However, the model tends
to underestimate the southern SST during wintertime, which can
be attributed to the underestimated simulated inflowing Aegean
waters. A slight overestimation is also simulated in the northwestern
anticyclonic loop, which is probably related to the coastal
upwelling that is not simulated by the model [33,34].
The simulated distributions appear to be significantly
affected by both the source mapping and the mean nearsurface
circulation. The input of microplastics from municipal
wastewaters in the Saronikos Gulf is shown in Figure 4, with the
most important source being the Psitallia WWTP. Moreover, there is a contribution of microplastics from the open boundary with
the Aegean Sea, mainly during February and March. The seasonal
microplastics concentration, overlapped with the near-surface
(0 – 5m) currents, are shown in Figure 5. For 2011 winter, the
strong south -westward current with inflowing Aegean waters,
turned into a strong westward current inside the basin that ended
to an anticyclone in the western sub-basin, while in the central
sub-basin, a relatively weaker current was simulated but with the
same anticyclonic pattern . This resulted to an entrapment
of the pollutants to the north-eastern and southern parts of
the Saronikos Gulf. During spring, the anticyclonic circulation
in the western and central area of the Gulf became stronger,
strengthening the microplastics dispersion from Psitallia WWTP
to the west. However, microplastics concentration in the northeastern
sub-basin remained high.
The summer microplastics distribution pattern resembled
that of spring, though some difference could be found due to the
weakening of the cyclonic circulation east of the Psitallia WWTP.
Finally, during autumn microplastics concentration was increased
in the north-eastern and the western sub-basin. This is mainly
attributed to the strong anticyclonic structure of the near-surface
circulation that tends to accumulate microplastics in the western
sub-basin. In the second year of the simulation (2012), the seasonal
microplastics distribution is quite similar to that of the first. The
main differences can be related to the increasing microplastics
concentrations due to the previous year’s contributions. This is
apparent in all seasons except for spring, which can be attributed
to the 2012 May, where the western anticyclone that spreads
microplastics from the source to the western sub-basin, is
weakened in comparison to that of the same period of 2011.
In the present study, a 3-D coupled hydrodynamic-Lagrangian-
IBM model was developed and applied to simulate the microplastics
(< 300 μm) distribution of municipal wastewaters origin
in the Saronikos Gulf. Hydrodynamic features were validated
against satellite data and historic in situ observations. Due to the
lack of available in-situ data, microplastics dispersion was qualitatively
described, based on the near-surface circulation. We should
note that microplastics with diameter less than 300 μm have been
proved difficult to be measured in the field, since the sampling
nets that are commonly used, have a net-mesh size larger than
The analysis of a 2-year simulation output of the 3-D coupled
model (2011-2012) qualitatively reproduces the main hydrodynamics
and gives an overall description of microplastics
distribution in the Saronikos Gulf. On average, the model simulated
hydrodynamics are in line with the observed structure of
the near-surface circulation. Bearing in mind that microplastics
dispersion is mainly depended on the near-surface circulation,
dispersion model results can be considered as a preliminary qualitative
description of affected areas from microplastics pollution.
Additionally, model results have shown that the most affected area
of the Gulf is the coastal area that extends from Psitallia WWTP to
the east. This part of the Saronikos Gulf exhibits a microplastics
accumulation during the entire year, mainly due to its proximity to
the WWTP and the lack of strong permanent currents that would
transport pollutants off-shore.
However, there are certain limitations in the model, which
have been identified. The microplastics input from municipal
wastewaters is just an estimation based on literature, and thus
may suffer from significant uncertainties. Therefore, the initial
concentration of microplastics that were dispersed in the marine
environment, could be overestimated. Additionally, inputs from
other sources (rivers load, coastal and marine human activities,
fragmentation of larger particles etc.) have not been taken into
account. Moreover, the biofouling effect on microplastics is missing
from the model, while this seems to have a countable contribution
to their sinking. Recently, Kaiser et al.  have proved that
biofouling changes the buoyancy of microplastics, causing their
sinking to lower levels of the water column. Furthermore, the
permanent anticyclonic flow in the southern open boundary of the
basin would normally result into an accumulation of microplastics.
However, this had the opposite effect on microplastics distribution,
which is attributed to the computational domain that not includes
the whole area where the southern-east anticyclonic loop is
formed. Finally, future comparisons with observations are clearly
necessary to confirm the model results, which emphasizes the
need for a more systematic in situ sampling of microplastics in the
marine environment. This will help to a better calibration of the
model and a further examination of the pollutant’s distribution in
the Saronikos Gulf.
The 3-D coupled hydrodynamic-langragian-IBM model presented
here is a first attempt to simulate the dispersal and distribution
of microplastics in the Saronikos Gulf. The model shows
real promise for the future and will form the basis of several theoretical
and modelling studies on marine plastic pollution. Preliminary
simulation results suggest that the proposed 3-D model
approach can be considered as a valuable tool to describe the
current status of microplastic pollution in the Saronikos Gulf and
contribute to risk assessment solutions.
This work was funded under the EU project CLAIM (Cleaning
Litter by developing and Applying Innovative Methods in
european seas, H2020 Grant agreement ID: 774586) and the
national project GLAFKI - KRIPIS (Blue growth with innovation
and application in Greek Seas, MIS: 5002438). We would like to
thank Kontoyannis Harilaos for his valuable contribution in the
hydrodynamic model results analysis.