Quaternary Journal of Iran

Geochemical Analysis of the Qarasu River Basin Rocks: Investigating Their Role in Heavy Metal Pollution of the Gorgan bedrock

Document Type : Original Article

Authors

Sara razavi 1
mohamad hashememamie 2
Homayoum khoshravan 3
Parvin Farshchic 4
Sayd Jamal sheikhzakariaee 5

1 PhD Student, Department of Earth Sciences, SR.C., Islamic Azad University, Tehran, Iran

2 Department of Earth Sciences, SR.C., Islamic Azad University, Tehran, Iran

3 Caspian Sea National Research Center, Water Research Institute, Ministry of Energy, Tehran,Iran

4 Assistant Professor, Faculty of Natural Resources and Environment, Islamic Azad University, Science and Research Branch, Tehran, Iran.

5 Faculty of Natural Resources and Environment, SR.C., Islamic Azad University, Tehran, Iran

10.22034/irqua.2025.2058584.1044
Abstract
Introduction:
The geochemical study of sediments in water basins such as rivers, estuaries, and seabeds can be an effective step in identifying the source and assessing the distribution model of environmental pollutants in the region. This research focuses on the geochemistry of rock units in the Qarasu River watershed and their contribution to the potential pollution of heavy metals in the sediments of Gorgan Bay. The schists in the area have a fine to medium grain size, and the original rock nature is preserved in most of them. The carbonate-clastic formations of Khush Yilagh and the carbonate of Lar are the most important sedimentary sequences in the studied area.
Method and Result:
 Thin section characteristics and field observations also indicate that most samples related to the Khush Yilagh formation consist of a carbonate matrix and clastic grains such as quartz and feldspar. The Lar formation is primarily composed of pure carbonate, including calcite and dolomite mineralogy. In the studied samples from this formation, rhombohedral dolomite crystals are well observed. 
Ultimately, the results of the geochemical data analysis showed that there is a significant similarity between sediment and rock samples, as well as between heavy metals in all three types of samples (rock, sediment, and water), indicating a common source for these metals. The concentrations of heavy metals in the studied samples are approximately: cobalt 1-27 (average 13), chromium 134-7 (average 88), copper 6-59 (average 32), nickel 1-76 (average 42), lead 1-49 (average 23), vanadium 8-185 (average 100), and zinc 9-157 (average 99) in ppm. 
Discussion:
The concentration of heavy metals gradually increases as one moves away from the river source towards the shores of Gorgan Bay. Overall, the main source of heavy metals is primarily the Gorgan schist and the Shamsak formation. Human involvement in the concentration of heavy metals in the Qarasu River and the shores of Gorgan Bay is very low and rare.
The geological units that are the most important study area with the greatest extent in the Qara Su watershed include the Gorgan schists, Khush Yalaq, Shamshek, and Lar. These units, due to their varying extents, are located at different distances from the riverbed. The Qara Su River and its various branches pass through these units and are often influenced by these substrates. This affects the sedimentary components of the riverbed and its geochemistry. In most cases, there is a positive correlation coefficient between the concentrations of heavy metals and the distance of the river from the geological units, with values above 0.5, indicating that a significant amount of heavy metals is controlled by the geological units of the region. Based on the data and evidence from this study, it can be stated that the main sources of cobalt and copper are from the Shamshek and Gorgan schist units, while the main sources of lead, chromium, nickel, vanadium, and zinc are supplied by the Shamshek, Lar, Gorgan schist units, and alluvial sediments. Although the primary cause of heavy metal accumulation in all samples is the geological units, for some heavy metals like chromium, vanadium, and zinc, anthropogenic factors are also significantly relevant.
Conclusion:
The chemical characteristics of water were examined, emphasizing elements and heavy metals, and the role of lithostratigraphic units in the pollution of elements and heavy metals in the Qara Su river (which subsequently leads to the pollution of the Gorgan Bay) was addressed. In Figure 7, a dendritic diagram has been drawn for rock samples. According to the diagram, except for calcium, elements can be divided into three groups. Magnesium and sodium each form a single-member group. However, in the most important and largest group, the heavy elements are observed together. This fact clearly indicates a common source for most of these elements (Olatunde et al., 2014; Zhang et al., 2016). In Figure 8, this diagram has also been drawn for the sediments of the Qara Su River. This diagram and the grouping relationships of the elements in it bear a strong resemblance to the clustering diagram of elements related to rock samples. As seen in the figure, in these samples, except for calcium, three distinct groups of elements can also be observed. Just like the rock samples, the two elements magnesium and sodium each form a group on their own. The largest and most important group in the clustering of elements in the rock samples, which is very similar to the rock sample, encompasses most elements, especially heavy elements."

Keywords

Pollutants
environment
geochemical heavy metals
Qarahsu
Iran

Subjects

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Quaternary Journal of Iran
Volume 10, 1 , 2
September 2025
Pages 239-266

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