Assessment of trace element pollution in northern and western Iranian agricultural soils: a review

The pollution of Iranian agricultural soils with trace elements (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) was assessed and compared with other agricultural soils around the world. Experimental data on trace element content in Iranian agricultural areas in the west and north were collected from the literature: 39 studies covered a total of 46 agricultural sites of 17 provinces in Iran, in order to characterize their patterns of accumulation of trace elements. Two pollution indices, namely, the pollution index (PI) and the integrated pollution index (IPI), were used to evaluate trace element accumulation. The data revealed a remarkable variation in trace element content among soils in different areas. Exploratory data analyses (EDAs) showed that a number of trace elements (Pb, Cu, and Zn) are asymmetrically distributed and scattered. Surveys indicated that 45.5% of the studied samples had elevated PI values for Cd, 13.0% for Cu, and 16.7% for Pb, clearly indicating an anthropogenic contribution of these three elements. The IPI of the agricultural soils also indicated that most areas are classified as having moderate and high pollution. Higher contents of trace elements (except for Mn) were found in some cities of the Isfahan, Hamadan, and Tehran provinces. Excessive application of conventional and organic fertilizers, pesticides, animal manure, and sewage sludge for enhancing crop production is responsible for high trace element content in Iran’s agricultural soils. This in turn, through the food chain, is a threat to human health. Analysis of the correlation between trace elements exhibited that Cu, Pb, and Zn (Cd, Pb and Zn) were very closely associated with each other, showing that their prevalent sources are common and the efforts to regulate them linked in common actions. We consider this evaluation as a viable approach to other similar areas in the Middle East and beyond, which could be used by environmental scientists for risk assessment and decision making.