Historical mining practices at the inactive Hercules mine (Tasmania, operated 1894-1999) have left acid and metalliferous drainage (AMD) source hazards, long-term AMD discharge via natural drainages and degradation of downstream aquatic ecosystems.
Default guideline values (DGVs) for freshwater ecosystem protection are potentially inapplicable because the naturally acidic (pH ≥4.5), organic-rich quality of west coast Tasmanian waterways can strongly influence metal toxicity, and local species are adapted to these conditions. To support closure and relinquishment planning for the Hercules site, site-specific water quality criteria (SSWQC) are being developed for minewater discharges.
To supplement limited ecotoxicological data for locally relevant species and water characteristics, we harnessed long-term (25 years) water quality and biological monitoring datasets generated by MMG Australia Limited (MMG) for environmental license compliance. Time-series data from water quality and biological monitoring location pairs, were used to correlate contaminant concentration (Al, Cd, Cu, Fe, Pb, Zn and salinity) and biological metrics (benthic macroinvertebrate abundance/diversity, fish counts and algal coverage) on a seasonal and interannual basis. Monitoring locations were selected to reflect the spectrum of mine water impacts along stretches of local rivers. Minimally-impacted reference sites informed biological reference conditions, against which impacted sites were assessed, to extrapolate threshold contaminant concentrations for biological health.
Multiple lines of evidence (laboratory toxicity data, field-derived threshold concentrations, reference site data, DGVs, international guidelines) ultimately informed the SSWQC, consistent with best-practise guidance. For some contaminants, the proposed SSWQC are up to 2 orders of magnitude higher than applicable DGVs.
This case study draws attention to the substantial but often unrealised value of compliance-driven environmental monitoring datasets. Further, we demonstrate the power of catchment-specific studies when protecting local species that have adapted to natural and anthropogenic stressors related to AMD.
