Soil geochemistry as a tool for the origin investigation and environmental evaluation of urban parks in Mashhad city, NE of Iran

Abstract

The total concentration of major oxides and trace elements, sequential extraction of heavy metals, and Pb isotope signatures of park soils were investigated in Mashhad, the largest city in northeastern Iran. The geochemical characteristics of park soils show two different trends: high silica soils (HSS) and low silica soils (LSS). The elements such as Mg, Fe, As, Co, Cr, Cu, Mn, Ni, Pb, Sn, and Zn were concentrated in LSS, while HSS samples were enriched of Si, Ga, Li, Nb, Ta, Th, U, Y, Z, and rare-earth elements (REE). The REE composition and chondrite—normalized patterns of LSS [low ∑REE (20–68 mg kg−1) and low LaN/YbN (1.9–7.4)]—are compatible with ultramafic-derived soils. The HSS samples display distinct REE composition with high ∑REE (102–336 mg kg−1) and LaN/YbN (13–51) which would be originated from granitoid, metamorphic, and sedimentary derived soils. The available percentage of heavy metals in the park soils is much higher than natural samples. The Pb characteristics of park soils are distinct from natural soils. These samples are less radiogenic than natural soils with lower 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, 206Pb/207Pb, and higher 208Pb/206Pb ratios. The calculated contributions of possible sources in Mashhad park soils release 6.6 and 93.4% for natural and anthropogenic (industrial and leaded petrol) sources, respectively. The central parks exhibit the highest pollution of heavy metals in Mashhad indicating high traffic intensity in this area. The concentration of potentially toxic elements in the surface soils of Mashhad parks is lower than the national maximum permissible concentration, but many of them such as Cd, Cu, Mo, Pb, Sn, Se, and Zn are highly enriched relative to non-urban soils.