Management principles for heavy metal contaminated farmland based on ecological risk-A case study in the pilot area of Hunan province, China.

A pilot project for farmland soil remediation was carried out in the Changsha-Zhuzhou-Xiangtan (CZX) area of Hunan province, China. However, the pilot project focused mainly on the risk of exposure to heavy metals on grain safety, and little attention was paid to the risk to ecosystem quality. The study selected three areas in counties of in the CZX, and focused on five high toxicity heavy metals-Cd (cadmium), Cr (chromium), Pb (lead), As (arsenic), Hg (mercury) to explore the potential ecological risks of the soil. Probabilistic ecological risk assessment (new method) and traditional methods were introduced to quantitatively evaluate the ecological risk. Two target criteria levels (LC/EC/IC50 and NOEC/LOEC) were employed. Through constructing species sensitivity distribution (SSD) models and joint probability curves (JPC), the predicted no effect concentrations (PNECs) derived from the SSD models were 0.21, 1.57, 3.05, 0.86 and 0.16 mg/kg for Cd, Cr, Pb, As and Hg, respectively. Compared with the ecological risk assessment results of the traditional methods, the new method reached a different conclusion, Cr showed the highest risk, at 84.3%, which signified that there was an 84.3% probability that 5% of the species with their NOEC/LOECs exceeded in County C. Despite differences among the risk assessment approaches, all methods indicated that County C was the most contaminated. The case study signifies that traditional methods underestimated the soil ecological risk of exposure to heavy metals and there should be a strong focus on farmland ecosystem security. At the same time, this study provided a scientific basis for goal-setting in species protection and prioritizing ecosystem protection as a management principle for heavy metal contaminated farmland from the perspective of ecological risk.