Mashhad in northeastern Iran, is a fast-growing metropolitan city which has been faced with a high rate of urbanization and industrial activities during recent decades. Despite the high geological diversity of Mashhad plain, the effect of anthropogenic and lithogenic factors on concentrations of potentially toxic elements (PTEs) in the soil have not yet been documented. The objectives of this study were to investigate spatial distribution, anthropogenic and lithogenic origins, and soil pollution intensity of Ni, Zn and Pb in Mashhad plain. A total of 178 soil samples (0–10 cm) were collected across an area of about 1500 km 2 . The aqua-regia extractable concentrations of selected PTEs were determined. Results showed concentration for Ni, Zn, and Pb ranged from 33.3–415.8, 35.4–149, and 19.6–69.1 mg kg −1 with mean values of 61.4, 74.1, and 31.6 mg kg −1 , respectively. Calculated background (median + 2MAD) concentrations of Ni, Zn and Pb were 60.1, 78 and 28.6 mg kg −1 ,respectively. Pb concentration in 70% of total soil samples was greater than background value; as for Ni and Zn, this figure was 16% and 35%, respectively. The highest concentration of Ni was found in soils formed on ultramafic rocks, whereas the highest concentrations of Zn and Pb occurred in the urban cities of Mashhad and Chenaran. Maximum pollution indices of Ni, Zn and Pb were 6.39, 1.91, and 2.42 with mean values of 0.94, 0.95, and 1.11, respectively. Spatial distribution maps indicated high concentrations of Pb and Zn in Mashhad and Chenaran. The concentration of these elements measured highest in the center of the city. Ni concentration decreased as the distance from ultramafic rocks increased. Results indicated that Ni concentration in the studied soils was controlled by parent material whereas increasing concentrations of Pb and Zn were under the influence of anthropogenic sources. The spread of urbanization and industrial activities in recent decades seem to be responsible for the contamination of soils by Pb and Zn. Although PI values revealed that the soil was not highly polluted, certain strategies must be considered to prevent further pollution in the future.

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