Spatial Distribution, Contamination Levels, and Health Risks of Trace Elements in Topsoil along an Urbanization Gradient in the City of Urumqi, China

For this study, we collected a total of 77 topsoil samples from urban, suburban, and rural gradients in the Urumqi city area in northwest China and analyzed their concentrations of seven trace elements: Arsenic (As), cadmium (Cd), nickel (Ni), lead (Pb), mercury (Hg), copper (Cu), and Zinc (Zn). To assess and compare the contamination levels and the potential health risk of trace elements in topsoil along an urbanization gradient, we employed the pollution index and the health risk assessment model introduced by the US Environmental Protection Agency; we also used geostatistical analysis to analyze the spatial distribution patterns. The results we obtained indicate that the contamination levels of trace elements in soil decreased in the order of urban > rural > suburban gradients. We found a similar spatial distribution pattern of contamination levels of Ni, Zn, Cd, and Pb elements, with a zonal distribution pattern, while the spatial distribution patterns of As, Cu, and Hg elements show high concentration patches in many of the areas investigated in this study. Furthermore, based on the identified concentrations, the topsoil is heavily contaminated by Hg and slightly contaminated by Pb, As, and Zn in the urban gradient, whereas it is heavily contaminated by Hg in suburban and rural gradients and slightly contaminated by As in the suburban gradient. The total non-carcinogenic risk index of the analyzed trace elements for adults and children decrease in the order of urban > suburban > rural gradients, whereas the total carcinogenic risk index of the analyzed elements for adults and children decreased in the order of suburban > urban > rural gradients. However, the contamination of topsoil in all gradients is more harmful to children’s health than to that of adults. Overall, urbanization has had obvious effects on the accumulation of trace elements in soil, and As is the main carcinogenic and non-carcinogenic risk factor among the investigated elements in topsoil in all urbanization gradients.