Variation in pollution status, sources, and risks of soil heavy metals in regions with different levels of urbanization

Soil heavy metal (HM) pollution is an increasing threat to ecosystem integrity and human health with rapid urbanization. Nevertheless, how soil HMs vary with the process of urbanization remains unclear. Here we used index evaluation, spatial analysis, and a positive matrix factorization (PMF) model to determine the pollution characteristics and sources of eight soil HMs (Mn, Cr, Cu, Zn, As, Cd, Pb, and Ni) among regions with different urbanization levels (urban area, suburb, and ecoregion) in Baoding City, Northern China. We also assessed the risks posed to the ecosystem and human health using risk assessment models. The results indicated that the mean levels of Cu, Zn, As, Cd, and Pb in the study area exceeded the soil environmental quality standards by 10.7 %, 10.7 %, 12.5 %, 23.2 %, and 3.57 %, respectively. A pronounced regional spatial distribution was discovered with high levels in suburban areas. Both the geo-accumulation index and potential ecological risk index revealed significantly higher HM contamination in suburban areas than in urban or ecoregion areas. Source apportionment based on the PMF model and correlation analysis showed that soil HMs in suburban areas primarily originated from agricultural activity, industrial sources, and natural sources. Those in urban soils originated from industrial sources, urban traffic, and natural sources, whereas those in ecoregions derived from natural sources and agricultural activity. The complex sources of soil HMs in suburban areas resulted in the highest carcinogenic risks to children health, followed by the ecoregion, but not in urban areas. This study identified the differences in pollution levels, sources, and risks of soil HMs among regions with different urbanization levels and can guide future efforts to mitigate and manage soil HM pollution during urbanization.