Keywords: 
• heavy metal /
• earthworm /
• antioxidant enzymes /
• 8-OHDG /
• MT /
• ecological risk assessment

Abstract: Typical soils contaminated by multiple heavy metals were collected from a mining area in Guizhou Province for 28 d chronic toxicity assay. Activity of antioxidant enzymes, i.e. catalase (CAT) and superoxide dismutase (SOD), 8-hydroxy deoxyguanosine (8-OHDG) and metallothionein (MT) in the earthworm (Eisenia foetida) were determined to evaluate correlations between these indicators and soil heavy metal contents. Feasibility of using these indicators to assess ecological risk of heavy metal contaminated mining soils was investigated. Results showed that the activity of CAT and SOD in earthworms increased firstly, and were inhibited thereafter during the exposure duration. This might suggest that activity of CAT and SOD in earthworm was raised to remove free radicals caused by stress of heavy metal contamination so as to adapt to changes of the environment. The accumulation of metabolites in earthworms after alleviating of metal toxicity inhibited activity of antioxidant enzymes, which led to decrease in CAT and SOD activity. When exposed to sub-lethal heavy metal concentrations, content of 8-OHDG in earthworms showed significant decrease with exposure time. With increase in exposure concentrations, more serious damages on 8-OHDG were observed. Within the exposure duration of 28 days, MT in earthworm played a detoxification role under the stress of heavy metal contamination. MT contents increased after 7 days of exposure and decrease thereafter. This study performs a comprehensive evaluation on ecological risk of typical heavy metal contaminated soils of a mining area based on toxicity assay. Results of this study will be helpful to guide remediation of heavy metal contaminated soils as well as its reuse after treatment.