Exposure of earthworm (Eisenia fetida) to bauxite residue: implications for future rehabilitation programmes

uxite residue is typically alkaline, has high sodium content and elevated concentrations of trace elements. Ef fective rehabilitation strategies are needed to mitigate potential environmental risks from its disposal and stor age. Increasingly, the importance of viable soil faunal populations as well as establishment of vegetation covers is recognized as key components of successful rehabilitation. Inoculation with earthworms is a strategy for accel erating mine site rehabilitation, but little is known on the effects of bauxite residue properties on earthworm sur vival and viability. In the current study, earthworms (Eisenia fetida) were exposed for 28 days to a series of bauxite residue/soil treatments (0, 10, 25, 35, 50, 75 and 100% residue) to evaluate possible toxic effects on earth worms, investigate the bioavailability of relevant elements (e.g. As, Cr, V), and assess the risk of element transfer. Results showed that soil containing ≥25% residue (pH ≥ 9.8; ESP ≥ 18.5%; extractable Na ≥ 1122 mg/kg) signifi cantly impacted survival (mortality ≥28%) and reproduction (cocoon production inhibition ≥76%) of the exposed earthworms. Alkalinity, sodicity and bioavailable Na were identified as major factors causing toxicity and some earthworms were observed to adopt compensative response (i.e. swollen body) to cope with osmotic stress. Con versely, soil containing 10% residue (pH = 9.1; ESP = 9.2%; extractable Na = 472 mg/kg) did not elicit significant toxicity at the organism level, but biomarker analysis (i.e. superoxide dismutase and catalase) in earthworm coelomocytes showed an oxidative stress. Furthermore, earthworms exposed to soil containing ≥10% residue took up and accumulated elevated concentrations of Al, As, Cr and V in comparison to the control earthworms.