عنوان مقاله [English]
Heavy metals have become a serious environmental problem for ecosystems due to their high bioaccumulation capacity and durability (Tam & Wong 2000; Clark et al. 1998). Compared to organic pollutants, heavy metals cannot they are destroyed by chemical or biological processes and, therefore, accumulate locally over long distances (Merian et al. 2004). In natural environments, the accumulation and distribution of heavy metals depends on various factors such as oxidation and reduction conditions, pH (Langmuir 1997) and the amount of organic matter (Berkowitz et al 2008). In addition, these metals can be adsorbed by the surface of minerals such as clay minerals and iron, magnesium or aluminum hydroxides (Eby 2005). As a result, high concentrations of heavy metals can accumulate in sediments, especially fine-grained particles with high contact surface (Berkowitz et al. 2008). Because marine sediments are potential settlements and sources of heavy metals in the water column, high levels of heavy metals in coastal sediments have many negative effects on terrestrial and aquatic organisms (Defew 2005). In addition, polluted coastal sediments reduce tourism on these beaches (Muzuka 2007). These effects reveal the importance of assessing and monitoring changes in heavy metal concentrations in coastal environments. Makran Basin is located in southeastern Iran and southwestern Pakistan. The Makran Trench is the physiographic expression of a subduction zone along the northeastern margin of the Oman Sea adjacent to the southwestern coast of Baluchestan of Pakistan and the southeastern coast of Iran. Chabahar Bay, with a length of 17 km and a width of 20 km, is the largest bay on the southeastern coast of Iran, which is connected to the Oman Sea by a span of 13.5 km. According to the structural geological divisions, the study area is located in the inner Makran geological zone. Since the pollution of water and sediments by heavy metals and their impact on the ecosystem is very important. In this study, an attempt has been made to investigate the level of heavy metal pollution in Chabahar Bay sediments.
To study the sediments of Chabahar Bay, studies were performed in several different stages, which are as follows:
• Library studies and reviews
• Field surveys and sampling
• Laboratory tests
• Data processing
First, all research related to Chabahar Bay was collected and studied. Surveys and sampling were performed by sampling tools according to the sampling environment. Sampling was performed in Chabahar Bay (and the deeper part of the sea for comparison with coastal samples and some samples were selected from the coastal strip land formations). To sample the bay, first the location of the sampling points was determined using a GPS and with a number of Manual samplers (Grap) sampling of soft surface sediments was performed and the boat was used to move and transfer to the sampling site. A depth finder was also used to determine the exact depth of the sampling points.
In this study, at first, library and documentary studies (include collected of maps, articles, books and researches of related to the research topic) were performed. Then, in the field studies and field visits, for this purpose, 13 sediment samples were collected from the shallow bed of Chabahar Bay and in the laboratory, they were subjected to sediment analysis and geochemistry. Therefore, the fine-grained sediments (less than 200 mesh) to determine the major elements, trace and heavy metals were analyzed ICP. By comparing the grain size analysis and Geochemistry of sediments, pollution-prone areas were identified. Finally, by combining the obtained data, the necessary interpretation and conclusions were made.
Chabahar Bay is a dual sedimentary environment because although in most parts of the Gulf the sedimentary environment is destructive, but in some parts carbonate sedimentary environment is seen which is accompanied by the release of biochemical sediments. Detrital sediments in the bay include quartz, clay minerals, feldspar, calcite and destructive dolomite. The origin of these detrital sediments is rock units (conglomerate, sandstone and marl) around the bay. In the sediments of the Gulf bed, 9 sediment types including gravel, sand, silt, mud, sandy silt, sandy clay, sandy mud, silty sand and mud sand were identified, which are the predominant species of sandy silt. Of course, coastal sediments are mainly sandy. Scattering of fine-grained particles is often seen in the center of the bay and offshore, while coarse-grained sediments are often more widespread along coasts and coastal areas. The amount of worm element in Chabahar Bay is higher than the average in the crust and it is almost unusual. Therefore, areas prone to environmental pollution in the study area are divided into four parts: the first area with the most pollution in the west and southwest of the bay (around Konarak jetty), the second area includes areas prone to pollution but with less pollution in the middle South to the sea and north of the bay, the third region includes areas prone to more pollution and finally the area with the least pollution but prone to the east of the bay and the coast of Tis (Shahid Kalantari wharf and Tis coastal beach). One of the most important sources of pollution is human pollution such as dumping engine oil or fuel into ponds or repairing the hulls of fishing vessels, which causes heavy elements to enter the waters and marine sedimentary basins of the region. Also, erosion of upstream altitudes and the entry of detrital sediments as a geogenic origin (terrestrial and extra-basin) increase the concentration of heavy elements. The washing of vessels, which contain large amounts of fish waste, as well as the direct dumping of human wastewater into the pond water, has caused the contamination of organic matter in the environment of the docks. Due to the performance of waves, especially in summer and the monsoon rule, many metal elements that have high specific gravity (such as chromium, nickel and iron) are driven from contaminated seabed sediments to sandy beaches, increasing the concentration of heavy metals. They roam in these areas. Based on the Folk classification (1984), 9 sedimentary types (Gravle, Sand, Silt, Mud, Sandy silt, Sandy clay, Sandy mud, Silty Sand & Muddy sand) were identified in the sediments of Chabahar Bay. Some heavy elements such as As, Bi, Cr, Sr, Zn, W and Cu are highly concentrated in sediments located in Chabahar Bay, especially along in the Konarak and Shahid Beheshti Jetty’s. The cause of this pollution seems to be human origin (Anthropogenic) such as pollution caused by the maintenance of launches and fishing boats.