[Chemical speciations of heavy metals and their risk assessment in agricultural soils in a coal mining area from Xingren County, Guizhou Province, China.]

In this study, total concentrations and chemical speciations of heavy metals (As, Cr, Pb, Zn, Cd, Hg, Cu and Ni) in 64 agricultural soil samples were determined to evaluate the level of contamination from a coal mining area in Xingren County, Guizhou Province. The single factor index, the Hkanson potential ecological risk index and the risk assessment code (RAC) were used for the evaluation of potential ecological and environmental risks in four main utilized types of soils (paddy soil, Coix lacryma-jobi soil, tobacco-growing soil and vegetable soil). The results demonstrated that the concentrations of heavy metals were significantly higher than their soil background values in Guizhou Province, with the exception of Zn. According to the evaluation results of the single factor index method, the soils were severely contaminated with As, Pb, Hg and Cu. And the chemical speciations of heavy metals significantly varied among the different utilized types of soils. As and Cd mainly existed in acid soluble fraction. Cr, Zn, Cu and Ni mainly existed in residual fraction. The existence of Pb was mainly in reducible fraction and residual fraction. And the content of Hg was distributed mainly in acid soluble fraction, reducible fraction and oxidizable fraction which accounted for about 55% of the total concentration. The bioavailability of heavy metals was characterized by the order of As(63.6%) > Hg(57.3%) > Cd(56.4%) > Pb(52.5%) > Cu (45.7%) > Zn (32.8%) > Ni (26.2%) > Cr (13.2%). The Hkanson potential ecological risk index (RI) suggested that the heavy metals were at considerable ecological risk and the ranking order was vegetable soil (505.19) > C. lacryma-jobi soil (486.06) > tobacco-growing soil〖JP2〗(475.33) > paddy soil (446.86). The risk assessment code (RAC) indicated that As was at high risk in the paddy soils, C. lacryma-jobi soils and tobacco-growing soils, while As was at medium risk in the vegetable soils. Cd and Hg were at medium risk, Cr, Pb, Zn, Cu and Ni were at low risk. Therefore, management measures must be taken to control contamination of As, Cd and Hg in this area.