Risk assessment of heavy metal contaminated soil in the vicinity of a lead/zinc mine

Heavy metal contamination of soils through anthropogenic activities is a widespread and serious problem confronting scientists and regulators throughout the world. In this study we investigated the distribution, chemical species and availability of lead, zinc, cadmium and copper in nine surface（0 to 20 cm） soils from near an abandoned lead/zinc mine tailings located in Shaoxing, Zhejiang, China. Total heavy metal contents ranged from 5271 to 16369 mg/kg for Pb, 387 to 1221 mg/kg for Zn, 3.0 to 9.3 mg/kg for Cd and 65 to 206 mg/kg for Cu. In general, all heavy metals exceeded China National Standards for Soil Environmental Quality of Heavy Metals by a factor of 3—65 times. Comparison of the heavy metal concentrations（Pb, Zn, Cd and Cu） with clay content revealed a strongly significant relationship while significant relationship（P<0.001） was also obtained between Cd+Zn and Pb+Cu. Solid phase speciation of the soils using Tessier procedure showed that the heavy metals were distributed in the order: residual  organically complexed-Fe-Mn oxides occluded>carbonate bound>exchangeable > water soluble. In the organic matter fraction, the ratio of Pb（29.1%） to its total concentration in the soils was higher than those of Zn（4.70%）, Cd（3.16%） and Cu（9.50%）. The percentages of the water soluble and the exchangeable fractions of Pb（1.80%） and Cd（2.74%） were markedly greater than those of Zn（0.10%） and Cu（0.15%）, suggesting that Pb and Cd are relatively more mobile and hence more toxic in the contaminated soils. Strongly significant relationships between H2O-Pb, H2O-Zn and H2O-Cu, strong positive correlations between H2O-Pb, H2O-Zn, H2O-Cu and organic matter in soil were found. The content of H2O-Pb, H2O-Zn, H2O-Cu was negatively correlated with pH values. The similar negative relationships between pH values and exchangeable heavy metals were also recorded. It is suggested that increasing soil pH or liming the soil could decrease bioavailability of heavy metals in the soil.