At Savage River mine (Tasmania), historic acid mine drainage contributes important inputs of acidity, high sulphate concentrations and high dissolved metal concentrations (aluminium, cobalt, copper, iron, manganese, nickel and zinc). The high seasonality observed in contaminant concentrations along the river is associated with rainfall, driving concentrations of dissolved organic carbon (DOC) in the water as well as alkalinity and hardness. Seasonal variability of the contaminant mixture represents a challenge for management.
The combination of metal speciation model and biotic ligand models (BLMs) was used to provide a more in-depth understanding of the mechanisms responsible for bioavailability in the system to inform management. The speciation modelling provided estimates of the fraction of the element in the form of either free metal ion or charged inorganic complexes in solution which, together, form a “potentially bioavailable” fraction. BLMs use speciation data to further assess bioavailability by adding a “biotic ligand” representing typical aquatic organisms in the modelled system. This approach provided a more accurate estimate of bioavailability as it considered competitive binding of ions at the biotic ligand.
The key advantage of using a modelling approach in the case of Savage River was that it provided a long-term monitoring solution to address the large temporal variability of the metal mixture. This assessment showed that only a fraction of the measured dissolved concentrations of contaminants in Savage River were bioavailable, therefore new site-specific criteria could be developed. Appropriate water management strategies will need to be established to achieve these new water quality objectives.