Abstract
The way of pollution spreads one of the most important issues in the marine environment. In this study, the spread of mercury pollution on the surface and bed areas of the northern and southern region of Kish Island was investigated by using the 3D MITgcm model in the winter season. Primary data (temperature, salinity, wind, net heat flux, evaporation and precipitation) were interred to the model and modeling was carried out by considering the three factors of wind, density gradient and tide for 10 years. After the stability of the model, comparing the results of the hydrodynamic model with the measured data showed a good agreement among them. Moreover, the results of using the passive mercury detector showed that in June and February, the mercury pollution spread to the Strait of Hormuz because of the influence of the prevailing currents around the Kish Island, but over the time, on March, it can be seen that the pollution spread towards the northern shores of the Persian Gulf. In the lower layers, due to the reduction of wind stress, mercury pollution spreads slowly in the bed. At the beginning of the modeling, mercury contamination is observed up to a depth of 70 meters, while at the end of the modeling, its concentration decreases due to mixing the pollution with nearby waters.
Introduction
Mercury is one of the most dangerous environmental pollutants that can enter the living organism through various digestive, respiratory and skin routes. Mercury pollution can cause irreparable damage to ecosystem, including coral reefs on the coast of Kish Island. In general, 27 species of Waterstone-forming corals have been identified in the Persian Gulf, of which 21 species are around Kish Island. The coral ecosystem is the richest and most energetic marine ecosystem in the Persian Gulf. Coral reefs are the second most productive biomes in the world and their area is 17% of all marine climates. A basic and fundamental thing in identifying and determining the pollution occurred in any aquatic environment is to determine its transmission and distribution under the influence of ocean currents, which is usually done by numerical models. So far, there has been no study on the spread of mercury pollution in this area. Therefore, the purpose of this study is 1- Modeling the circulation pattern in the Persian Gulf. 2- Investigating the spread of mercury pollution around Kish Island.
Methodology
In this study, the MITgcm model has been used according to its special modules to track mercury pollution in the Persian Gulf. The bathymetry range used in this research in the geographic range of 30-24 N and 56-48 E was obtained from the GEBCO website with an accuracy of 30 seconds. Then in the ARCGIS software with an accuracy of 2 minutes (3706 meters, 0.033 degree) has been converted and intered to the model as a 312x600 grid in binary format (Figure 1).
Separability along the orbit and hemisphere is 3706 meters and the maximum depth of the basin is 93 meters. In order to accurately solve the equations in the pycnocline area, the model is divided into 8 layers along the Z axis with spatial resolution varying from 5 to 25 meters (110 meters deep) and sliding conditions have been applied to the bed and lateral boundaries. The model was implemented hydrostatically. The finite element method is used to solve the equations.
In this modeling, the time step discretization of the cluttered denser is used. The advantage of this methode is stratification phenomena and internal gravity waves that may have limiting processes for a stable time step. The linear state equation is used and the coefficients of linear thermal expansion in this equation were 2ˣ10-4 1/℃ and linear salinity contraction coefficient of 2ˣ10-6 1/psu is considered in this equation. Coriolis constant changes have been calculated according to the latitude of the modeling domain. Input data to the model include: sea surface temperature (SST), sea surface salinity (SSS) collect from the WOA website, meteorological data including precipitation, evaporation, latent heat, net flux of long waves, net flux of short waves, sensible heat flux and data and the wind stress data in both orbital and hemispherical directions have been received from the NOAA and ECMWF websites. TOPEX8_atlas amplitude and inertial phase data were obtained with an accuracy of 1/12°. Then by using the TMD toolbox in MATLAB, the amplitude of the radiative velocity in m/s and the phase in degrees, with 8 inertial components (M2, S2, N2, K2, K1, O1, P1 and Q1) have been extracted and intered to the model in the open border cells. Modeling was carried out in the target basin for 10 years without considering the tracer until the model reaches stability (temperature and salinity changes periodic with time) (Figure 2). The modeled temperature and salinity results were compared with the measurement data of HYCOME2009 and NCODA at station A and B. The results show a good match between the model results and real data (Figure 3).
In this study, mercury pollution was considered as a concentrated inactive chemical tracer in the MITgcm model. The permissible limit of mercury pollution is 0.85 (μgr/lit) in the sea, which was taken from the World Health Organization (WHO) website. Then mercury pollution with a concentration of 6 (μgr/lit) was released in the northern and southern regions of Kish Island.
The result of model shows that the surface currents in winter are affected by the increase in the intensity of northwest winds. The difference in density between the waters of the Oman Sea and the Persian Gulf leads to the creation of surface inflows from the Strait of Hormuz. Due to baroclinic instabilities, the incoming water mass forms medium-scale eddies near the coast of Iran. The thermohaline current entering the Gulf moves along the coast of Iran and travels a counter-clockwise path in this basin. Another factor affecting the flow in the Persian Gulf is the tides, which will have a noticeable effect on the currents on the coasts of Iran. The western currents in the Persian Gulf are influenced by the incoming plume from Arvandrud.
A northwesterly current near the coast of Iran with a speed between 30 and 40 cm/s between the Strait of Hormuz and the north of Qatar causes the formation of the northern slope of this cell. Between July and August, the coastal currents of Iran are unstable. Its cause is the pressurization mechanism as a result of potential energy stored in the transverse layer of the density gradient. As a result, the twists and turns in the coastal currents of Iran appear in a series of micro-scale eddies, which are known as coastal eddies of Iran. it is in good agreement with the studies done by Hogan and Prasad.