A comprehensive investigation of hazardous elements contamination in mining and smelting-impacted soils and sediments.

Extensive mining and smelting activities in the Baiyin district have resulted in a serious hazardous elements (HEs) contamination in the soils and overbank sediments. In this study, the concentrations and chemical fractions of HEs were analyzed to evaluate the environmental risks of these HEs in the focus areas. In soils, Zn, Cu, Pb, and Cd exhibited an obvious decline compared to the results in 2012, which confirmed that the treatments of the contaminated soils by the government have played a very important role in the remediation of the soils. However, Zn, Cu, and Pb still exceeded the background values, and the study areas were still extremely contaminated with Cd. The spatial distribution of HEs showed that the contaminated areas were mainly focused around the mining and smelting regions and the sewage irrigation regions. Sequential extraction showed that Zn was mainly present as a residue fraction, while the percentages of unstable fractions increased in the sewage irrigation region samples. As for Cd, the bioavailable fractions were extremely high (over 90%) and the ecological risk was much higher than Zn. In the case of the sediments, the concentrations of HEs were extremely high. However, few researches have investigated HEs contamination in the sediments from the study area. With changes in climate and environmental conditions, HEs in sediments will easily release and influence the groundwater and the irrigation water. Furthermore, the available fractions of Zn and Cd were over 80%, which suggests high bioavailability and mobility in sediments. HEs pollution in sediments should receive more attention compared to that in soils.