Internationally, acid and metalliferous drainage (AMD) is a common challenge for coal pit lake water quality. Coal mine pit lakes may form at mine closure when voids formed through mining extractions have extended below groundwater.

Without acidity, pit lake water quality may become degraded gradually through the dissolution of contaminants even without acid conditions such as heavy metals and from salts by evapoconcentration.

Contaminated coal pit lake waters can present significant risks to surrounding and regional communities and natural environments. Pit lake waters may discharge into surface and groundwater or directly present risks to wildlife, stock and human end users.

Poor pit lake water quality also may mean lost opportunities in the form of beneficial end uses such as recreation, fishing or wildlife habitat that cannot be realised as a result of poor water quality.

Flow-through of the pit void lake by diversion of regional streams and rivers is increasingly being proposed to mitigate pit lake water contamination. Chemical and biological processes such as dilution, absorption, flocculation and sedimentation reduce solute loads from river and lake.

Riverine lake flow-through is often a valid mine closure strategy for pit lakes with poor water quality. Although the maintenance of existing riverine system values must be maintained first and foremost, the river water quality outflowing from the pit lake can, in some circumstances, be improved.

This is especially so in examples of nutrient contaminated river waters typical of agriculture-modified catchments flowing through slightly acidic pit lakes.

Flushing with river water has proved to be a very useful strategy for management of some otherwise poor water quality pit lakes. A fundamental prerequisite for the use of river water and mine water for filling and management of pit lakes is water availability, proximity of a flowing source, and the volume of water able to be abstracted without impinging on existing end uses.

Water scarcity may be a limiting factor for flow-through solutions to pit lakes that currently function as terminal lakes due to regional water availability. The applicability of filling and flushing of pit lakes with river water and mine water strongly depends on the climate and the intensity of the use of water downstream of the pit lakes.

In the case of limited water availability, seasonal or occasional floods may be the only options for the filling of pit lakes under such arid conditions. This method of filling pit lakes can be evaluated against existing international examples.

However, the ecological and social needs of the river system downstream the pit lakes also have to be kept in mind, including the flow magnitude and variability of the flow rate under such conditions.

The water quality of the used river water also has to suit the requirements of the planned use of the pit lakes. It may be necessary to treat the river water, the mine water or the pit lake. Priority should be given to river water quality and end uses and the maintenance or improvement of existing water values.

River water quality should not be degraded by pit lake flow-through. This may limit the applicability of this strategy to rivers of relatively lower water quality. Ideally, hydrological and geochemical modelling will precede a trial period of flow-through which then validates the model expectations to stakeholders’ satisfaction.

Some pit lakes can also be used as biochemical reactors under certain conditions, for example, removing nutrients from river water and in turn precipitating metals from lake water. Nonetheless, hydro-chemical processes will vary between operations and sites based on the specific geological, hydrological and climate characteristic of each lake and its inflow/outflow characteristics.

Mitigating long-term acidification and salinisation by flushing with alkaline and fresher river water is typically the key driver to use flow-through as a closure strategy for pit lakes.

As a result, developing flow-through systems must be based upon reliable monitoring data and accurate predictions of water balance and water quality from deterministic models.

Flow-through closure proposals for coal pit lakes must be scientifically justifiable and follow a risk assessment of the options of no flow and through-flow. Due to the high levels of uncertainty, biological, physical and chemical attributes of both upper and lower river and lake must be well monitored.

Monitoring should directly feed into an adaptive management framework approved prior by key stakeholders with regular engagement during a trial program.

In summary, riverine lake flow-through can be a useful mine closure strategy, removing nutrients from river water and in turn precipitating metals from AMD contaminated lake water.

By following an informed risk-based approach and seeking appropriate professional advice for key steps, miners can ensure coal pit lakes are managed sustainably rather than leaving a lasting liability.

Dr Clint McCullough is principal environmental scientist and mine closure specialist at Golder Associates