Toxicity of Acid Water from Mt Morgan Mine Site, Central Queensland, to the Freshwater Shrimp Caradina Indistincta

A combination of toxicity testing using the freshwater shrimp Caradina indistincta and modelling of metal speciation has been used to assess water quality requirements for mine pit water releases from the Mt Morgan gold and copper mine site in Central Queensland to the Dee River. Analysis of the mine pit water indicated concentrations of some metals (730 mg/L Al, 35 mg/L Cu, 25 mg/L Zn) were several orders of magnitude above the Australian water quality guideline trigger values. Dilution of the mine pit water (pH 2.9) with Fletchers Creek (pH 8.3) water (a tributary of the Dee River) resulted in neutralisation to pH 5.3 and pH 6.4 at 1.5% and 0.8% dilutions, respectively, and precipitate formation. At a dilution of 0.8% mine water, dissolved concentrations of Al, Cu and Zn were calculated to be approximately 300, 2.5 and 5 times the guideline trigger values, respectively. The acute toxicity of the diluted pit water to C. indistincta (96-h exposures) was observed to be very pH dependent and was least toxic at pH 6 and most toxic at pH 5. No toxicity was observed at dilutions of 0.8% mine water at pH 6 or 7. For waters of pH 4, 5, 6 and 7, LC50 values of 1.8, 5.7, 20.5 and 12.9% pit water were determined, respectively. Calculated metals concentrations indicated that dissolved aluminium would contribute most to the observed toxicity. Speciation modelling calculations indicated that the lowest concentrations of bioavailable aluminium would occur at pH 6, consistent with waters of this pH being least toxic. The study indicated that neutralisation of pit waters to pH 6 prior for discharge would minimise toxicity to C. indistincta. Further testing using species such as algae, water fleas and fish is suggested to better assess the impact of the metal-rich waters on metal-sensitive biota.