Mine sites in Northern Australia struggle with managing water from tropical rain events in the wet season. Excess water can result in uncontrolled discharges from over topping of water containment structures allowing release of mine affected water. This type of discharge has the potential to result in downstream water quality exceeding water quality guidelines. To predict potential impacts from uncontrolled spills on downstream aquatic macroinvertebrate populations, an ecological risk assessment (ERA) methodology was developed. Aquatic macroinvertebrates were identified as receptors for this predictive ERA as, generally, macroinvertebrates in their early life stages are the most sensitive organisms to poor water quality. Since macroinvertebrates are typically monitored as part of routine Receiving Environmental Monitoring Programs (REMP), results of monitoring can be used to validate the results of the predictive ERA.
As part of the ERA, risk characterisation criteria (RCC) were developed for species relevant to the macroinvertebrate populations within the receiving river. RCC can be used to assess future water quality in the downstream receiving environment in the case of a spill when compared against exposure point concentrations (EPC) for that spill. The ERA follows the usual pathway according to the five phase NEPC (2013) process:
RCCs can be developed using EC10/NOEC chronic data. For a predictive ERA, maximum contaminant concentrations (measured during a similar event) can be applied as EPCs. Further investigations are conducted if the RCC<EPC and the difference between the RCC and EPC is >20% (to account for uncertainty of bioassays and application of conversion factor). Duration of exposure is also taken into account in further investigation as is bioaccumulation potential.