Contamination and risk assessment of heavy metals in soils irrigated with biogas slurry: a case study of Taihu basin.

The accumulation of heavy metals in different soils resulting from irrigation with biogas slurry obtained from Taihu basin may create a potential public health risk. We quantified the concentration of heavy metals (Zn, Pb, Cu, Cr, As, and Cd.) in soils. Results indicated that the concentrations of Ni, Zn, Cd, and Pb in soil exceeded the maximum permitted levels set by Chinese Soil Environmental Quality Standard (GB15618-2008). The highest mean level in the soil was noted for Zn, followed by Zn, Pb, Cu, Cr, As, and Cd, while maximum geoaccumulation index (Igeo) was found for Cd in all soil samples which ranged from strongly polluted to extremely polluted. Pollution levels varied with metals and soil types. Moreover, the concentrations of Mn, Co, Ni, Cu, Zn, As, and Pb in soils showed significant correlations with OM; pH showed positive correlations with Cd, Cu, As, and Cr; pH and OM were the most important factors controlling the uptake of heavy metals by soils. Multivariate principal component analysis showed anthropogenic contributions of Zn, Pb, Cu, As, and Cd in the different kinds of soils. The target hazard quotient (HQ) values of six metals in soils were less than 1, which suggested that non-carcinogenic risks of metal exposure to soils were generally assumed to be safe. The assessment results of carcinogenic risks in soils showed higher risks than an acceptable range of 1E-06 to 1E-04 that would pose potential cancer risks to the farmers due to the work of leafy and root vegetables grown locally.