Investigation of heavy metals pollution in agricultural soils irrigated by Khiavchay River water- Meshginshahr, Ardebil Province

Pollution sources are basically divided into two groups: geogenic and anthropogenic. Geogenic sources include pollution from geological formations, mineral indices, mineral springs, brackish and saline water sources, and industrial activities, transportation, using agricultural fertilizers, mining activities, and wastes and effluents can be mentioned as anthropogenic pollution sources. Essential nutrients and other metals may enter the food web through the soil. The natural concentration of potentially toxic elements in the soil is usually low, except in the case of soils composed of ore minerals. The amount of trace elements in soils can also increase sharply due to human activities. In recent years, industrial and agricultural development in Iran has grown exponentially and various researches have been focused on assessment of environmental pollution. Therefore, it is necessary to use a set of different quantitative and qualitative methods of soil pollution assessment to gain a better understanding of the extent of pollution. Regarding the pollution of Khiavachay River in Sabalan geothermal region by natural springs and Meshginshahr iron mine, and due to irrigation of arable lands by this river, the aim of this study was to evaluate the pollution of arsenic, cobalt, lead, zinc, copper, iron, manganese and chromium using geochemical indices and statistical methods in the surface soil of Meshginshahr city.

A total of 14 soil samples (0-20 cm) were collected using composite sampling method from agricultural lands irrigated by the water of Khiavachay River. The samples were dried at room temperature, then a part of them passed through a 230 mesh sieve for digestion by four acids and elemental analysis by induction coupled plasma mass spectrometer (ICP-MS) in Zarazma laboratory. In this study, Geo-accumulation index (Igeo), contamination factor (CF) and enrichment factor (EF) were used to determine the severity of soil contamination with heavy metals. Also, in order to investigate how the elements relate to each other and also to determine their origin, the results were analyzed using statistical analysis. Descriptive statistics, correlation coefficient and principal component analysis were performed using SPSS 20 software.
 

Comparison of the concentration of elements in the region with their world average in soils and Earth crust shows that the average concentrations of arsenic, zinc and phosphorus in the agricultural soil of Meshginshahr are higher than their values in the world soil and Earth crust. Among the studied elements, As, P and Fe with values of 68.61, 34.27 and 30.76% respectively show the highest coefficients of variation in the region. The high variation coefficient, especially for As, indicates high concentration differences in different parts of the study area and possibly differences in the source of the contaminant. The results show that the amount of arsenic in the soil of station S1 (Moil village, adjacent to the iron mine) is in the range of severe to extremely polluted (Igeo level is 4.62). It is also present in 78% of the collected samples (11 samples) in the range of moderate to severe contamination (Igeo level between 2.37 to 2.95) and in two stations in the range of moderate contamination (Igeo level between 1.79 to 1.87). Zinc is in the moderate contamination range at all sampling points (CF between 1.04 and 2.27). Iron, lead and copper are in both the medium and low pollution ranges, and cobalt, chromium and manganese are only in the low pollution range. Therefore, the region shows significant enrichment to arsenic. These results are in good agreement with the previous study on the Khiavachay River and are reasonable considering the type of formations and irrigation of agricultural lands with arsenic-contaminated water. The results obtained from the pollution indices indicate that the maximum arsenic pollution is present in station number one and close to the iron mine of Moil village. High correlation of arsenic with iron (r = 0.911) and lead (r = 0.637) and its negative correlation with phosphorus (r = -0.65) was observed. The negative correlation of arsenic with phosphorus can be due to the competition of these two elements in the form of anions (arsenate and phosphate) in adsorption by soil colloids. Among the iron ores of the region, the highest concentration of arsenic has been measured in limonite. In the process of limonite formation, insoluble free hydride is formed by the oxidation of divalent iron, and during this reaction, arsenic, which originates from hot springs and hydrothermal waters of the region, is co-precipitated with iron. Therefore, As in the Moil iron mine has secondarily deposited in limonite under the influence of geothermal resources and hot springs. The results of correlation coefficient have been confirmed by principal component analysis.

Existence of hydrothermal springs with high concentration of arsenic, which are considered as causes of water pollution in Khiavachay River, and irrigation of agricultural lands with river water are important factors of arsenic enrichment in the soil of the region. Considering the amount of geo-accumulation index, the soil of the study area has moderate to severe As contamination. The results of the enrichment factor indicate significant enrichment for As and moderate enrichment for P, which is influenced by geological formations and natural factors for arsenic and anthropogenic factors for phosphorus. The results of statistical analyzes showed the important role of Moil iron mine and hydrothermal springs in the region in increasing the concentration of As, iron and lead in the soil of the region. Considering the negative effects of soil pollutants on the environment and due to the high concentration of arsenic in the study area, it seems necessary to study the concentration of this element in crops in the region and assess the health risk. For this purpose, evaluation of mobility and bioavailability of elements by sequential extraction methods, study of the role of soil physicochemical parameters in elemental mobility and investigation of the role of plant species in the uptake of pollutants from soil can be of great help in better analysis of the results.