Assessing the potential ecological risk of heavy metal in the soils around Garmsar salt mines

Introduction
In the past decades, industrial activities, application of fertilizer, sewage sluge, landfilling, mining and etc., have caused heavy metals to enter the soil. These processes increase the initial amount of these metals, accumulation or leaching of heavy metals from the soil, which has the potential to contaminate surface and groundwater. Due to the harmful effects of heavy metals on the human and the environment, the assessment of heavy metal pollution is important.
Heavy metal pollution assessment is determined using various factors including enrichment factor (EF), geoaccumulation index (Igeo), contamination factor (CF), contamination degree (Cd) and pollution laod index (PLI).
There are 27 salt mines in Garmsar city. Of these, 16 mines are active. Salt extracted from these mines are used as one of the food spices. On the other hand, due to mining activities, the soils of this region may be contaminated with heavy metals. Therefore, this study was conducted to evaluation of heavy metal in the soil around some Garmsar salt mines using pollution indices.

Methodology
The study area is Garmsar Sar Darreh region. This area is part of Garmsar plain, which is located in the south of Alborz mountaions. This plain is located between 35 º 5ʹ to 35 º 17ʹ north latitude and 52 º 15ʹ to52 º 35ʹ east longitude. There are 27 salt mines in this area and salt purity of these mines is over 98% , of which 16 mines are active. Seven soil samples were collected from the surface 0-20 cm of soils around some of the active salt mines in the area. Soil samples were air-dried and put through a 2 mm sieve. Acid digestion method was used to determine the concentration of total heavy metals in the soils. For each sample 2 gr of soil was placed in a centrifuge tube and then were extracted using 4 M HNO3 for 12h. Using a Whatman 42 paper the supernatant was filtered and the Atomic absorption Spectrometry (AAS)(Varian, Spectra 220) measured the concentration of Arsenic (As), Cadmium (Cd), Copper (Cu), Iron (Fe), Lead (Pb), Nickel (Ni) and Zinc (Zn).
Personʹs correlation coefficient analysis, cluster analysis, pollution indices and potential ecological risk were used to evaluate the source and degree of soil pollution. The resultes of this study were analyzed using Microsoft Excel software. In this study, Minitab 16.0 Software was used to calculated the person correlation coefficient between variables and cluster analysis was used to investigate the relationship between metals in soil samples.
Conclusion
The concentration of heavy metals at the study points is presented in Table 5. The maximum and minimum values for As were 27.14 and 9.54, for Cd were 1.82 and 0.79, for Cu were 54.11 and 18.35, for Ni were 24.47 and 9.8, for Pb is equal to 30.48 and 19.61, for Zn is equal to 4.92 and 1.47 and for Fe is equal to 25045 and 17024. Comparison of the average concentration of heavy metals in soil samples with the average concentration of heavy metals in the earth's crust (Taylor, 1964) shows that the concentrations of As, Cd, Pb and Fe were higher than the values of crust. Also, the average concentration of heavy metals in soil samples was compared with the standard introduced by the Environmental Protection Organization of Iran and the results showed that the concentration ofmetals such as As and Cd in the studied samples exceeded the values provided for soil.
The results show that based on the values of EF, the studied soil samples with respect to Cd and Pb in all soils and As in some soil samples were in the range of highly enriched while, Cu, Ni and Zn is classified in range of unenriched. High enrichment indicates anthropogenic origin (mining activity in the area). Also based of Igeo and Muller classification, soil samples with respect to heavy metals were in the range of non-polluted (Igeo<0). The calculated degree of contamination in all soils (except soil sample S1) showed low degree of contamination.The PLI obtained for soil samples is less than one, which indicates the normal and un-contaminated concentration of the studied elements. Based on the values of RI, the studied samples have low to medium environmental risk.
To better understand the relationships between the metals, methods can be used to separate homogeneous and similar groups. Cluster analysis is one of these methods. In this analysis, the variables are classified into homogeneous groups so that the variables of each group have the most similarity and the least similarity with the variables of other groups. Variables located in a cluster have the same geochemical behavior or similar origin. According to the obtained cluster diagram, three main groups of metals can be identified (Figure 3). The first group includes the metals Ni, Pb and Cu. In this group, the elements Ni and Pb have the most similarity to form the first cluster, and then Cu is attached to the first cluster. In the second group are the elements As and Zn and the element Cd is in the third group. These results indicate the existence of different sources for the studied elements in soil samples. In other words, the elements that are in a group have the same origin. Correlation coefficients are used to investigate the relationship between metals, variability of geochemical behavior and to determine the probable origin of metals in soil. In this study, a positive and significant correlation between Pb and Ni elements (r = 0.87) was obtained at a statistical level of 5%, which indicates a possible common origin between these elements in the study area. The existence of a high correlation coefficient between the elements indicates the same source of emission of the elements.
In general, the result of this study showed that As, Cd and Pb are more important, environmentally. Despite the studies conducted in the last decade, which show that the concentration of heavy metals in the salt samples of active mines in Garmsar region are in range of national standards but it is recommended in addition to the ecological indicators, the heavy metal concentration are periodically examinate in the salt rock of this area.