Impacts of land use and geomorphology on some heavy metal concentrations in a part of Zayandehroud dam watershed

Introduction:

With the rapid development of industry and urbanization, soil pollution with heavy metals as a result ecosystem destruction has attracted global attention. Pollutants are considered environmental threats and among pollutants, heavy metals are known for their non-degradability and physiological effects on living organisms even at low concentrations. The close correlation of magnetic properties and heavy metals shows that magnetic measurement is an efficient and cheap tool to detect heavy metal contamination in soils affected by heavy industries and traffic pollution. Magnetic minerals in soil may be inherited from parent rocks (lithogenic origin), pedogenesis (pedogenic origin), or may result from human activities (secondary ferromagnetic materials). The concentration of metals can be influenced by geomorphology and various soil properties such as organic carbon, electrical conductivity. Land use directly or indirectly affects the geochemical behaviors of heavy metals through regulating soil properties. The main objectives of this study were to investigate the effect of land use change on magnetic receptivity and the concentration of some heavy metals including zinc, copper, iron, nickel, chromium, cobalt and manganese in the 20 cm soil surface layers, and to explore spatial distribution of magnetic receptivity and heavy metals under different types of land use and geomorphological units in the studied area.

Materials and Methods:

The present research was conducted in Isfahan province in the center of Iran with an area of 227 Km2. This area has an average temperature of 9.8 oC and an average annual rainfall of 324 mm and an altitude of 2380 meters a.s.l. Based on Kopen's classification, the climate was classified as semi-arid with cold winters. Geologically, it belongs to the Sanandaj-Sirjan zone, the dominant rocks of the area include limestone, shale limestone, slate and Quaternary sediments. The most important land uses in the region included pasture, rainfed and irrigated agriculture, and in terms of geomorphology, the region comprised river plains and pediments. Soil sampling was done by stratified random method. A total of 100 samples were collected from the surface layer (0-20 cm depth) in the summer of 2021. Magnetic susceptibility was measured at high and low frequencies using Bartington MS2 dual frequency sensor. The concentration of heavy metals including iron, zinc, manganese, nickel, copper, chromium and cobalt were measured by atomic absorption spectroscopy. pH, organic carbon, calcium carbonate, electrical conductivity were measured in all samples. Spearman's correlation coefficient was used to check the correlation between different parameters. Analysis of variance was applied to evaluate the effects of geomorphology and land use on heavy metals and magnetic susceptibility. Spatial analysis was performed for heavy metals and magnetic susceptibility, and the maps were prepared in ArcGIS v.10.7 software.

Results and Discussion:

The results showed that there was a negative and significant correlation between calcium carbonate, heavy metals and geomorphology. There was no significant correlation between organic carbon and heavy metals in land uses. But there was a negative correlation in the river plains and alluvium. There is a significant negative correlation between electrical conductivity, copper, manganese, and nickel. In the use of agricultural lands and river plains, there is a positive correlation between low-frequency magnetic susceptibility and high-frequency magnetic receptivity with electrical conductivity. Also, pH showed a significant negative correlation with magnetic susceptibility in pasture land and had no relationship in other land uses. There is a positive correlation between calcium carbonate and frequency-dependent magnetic susceptibility in agricultural land use and river plains. There is a significant positive correlation between heavy metals and magnetic susceptibility in pediments and some land uses, especially in rainfed lands. The results of analysis of variance showed significant difference (p*<0.05) in land use regarding heavy metal concentrations. In this analysis, there was a significant difference between cobalt, nickel and manganese elements according to land use, and the magnetic susceptibility among the studied geomorphic surfaces. According to the results of the test, there was a significant difference for heavy metals in various geomorphic surfaces. The content of iron, chromium, cobalt, nickel and manganese in river plains and pediment had significant differences with hills.

Conclusion:

The present study was conducted with the aim of clarifying the effect of land use and geomorphology on magnetic susceptibility and concentration of heavy metals in a part of the Zayandeh River watershed in Isfahan province. The average of nickel and manganese in the soils of the study area is higher than the normal range, due to parent materials effects and agricultural activities (plowing and irrigation) accelerate the soil formation processes and increase the amount of these elements in the soil. The highest concentration of cobalt, iron, zinc, copper, nickel and chromium elements were observed in dryland farming. In addition, investigating the spatial distribution of magnetic receptivity values and heavy metals in different places are significantly different. Higher values of magnetic susceptibility were seen in the center of the studied area. Spatial distribution of heavy metals iron and chromium are concentrated in the center of the region and other metals are concentrated in the west and northwest. Probably, parent materials such as shale, dolomite, limestone and sandstone and weathering and release of elements in the soil increase the concentration of these elements in the region.Keywords: Geomorphology, land use, LSD test, kriging