As part of our client’s Mine Closure Strategy, options surrounding the creation of a ~20 GL pit lake that has a valuable after-use were investigated. Left to fill naturally, the pit lake would take over half a century to fill with the resulting water quality having a low pH (<3; reflective of the background catchment conditions). Therefore, the preliminary Mine Closure and Rehabilitation Plan sets the pathway to achieve their long-term Water Quality Objective of establishing a self (alkalinity) sustaining water body requiring minimal ongoing management with the potential to provide long-term improvement in downstream surface water ecosystems.
Due to the elevated dissolved metals and acidity within the surface water, active and passive treatment methods have been developed to be incorporated in the Mine Closure and Rehabilitation Plan to increase alkalinity and reduce dissolved metal concentrations. These methods included:
- Identifying water sources to accelerate filling;
- Constructing a discharge over flow channel to allow connectivity to surface water features;
- Spreading lime and Sodium Hydroxide on the pit spoil and into the recovering pit water to raise pH during filling; and
- Adding organic carbon (and associated nutrients) to the pit water body to facilitate stratification of the water body and establishment of biological processes.
Regulatory Requirements
All work approvals associated with the hydrogeological aspects of the Mine Closure Strategy have been undertaken in consideration of the following regulations and guidelines:
- Water Act 1989.
- Environment Protection Act (1970).
- Land Acquisition and Compensation Act (1986)
- Water Industry Regulations (2006)
- Water Industry Act (1994)
- SEPP Waters (2018).
- NHMRC (2008).
- ANZECC (2000).
- Flora and Fauna Guarantee Act (1988).
- Environment Effects Act (1978).
- Planning and Environment Act (1987).
- Conservation, Forests and Lands Act (1987).
- EPBC Act (1999).
Project Results
Integral to the success of this project the following assessments have been undertaken:
- Risk assessment in accordance with AS/NZ ISO 31000:2009
- Drilling, completing and sampling investigation bores within and surrounding the coal mine to aid the development of a conceptual site model.
- Sampling and hydrograph analysis of surface water data (including in-Pit and first flush acidic events).
- Sub catchment scale geochemical modelling and analysis including spoil and pit wall acid generating potential calculations;
- Civil works to provide for the long-term stability of the pit walls;
- Undertaking numerical and analytical modelling to assist informing potential future management actions such as:
- Spreading lime and Sodium Hydroxide on the pit spoil and into the recovering pit water to raise pH and improve water quality during filling.
- Covering the remaining exposed coal faces and other high acidity sources to limit some of the fluxes of acidity into the water body.
- Adding organic carbon (and associated nutrients) to the pit water body to facilitate stratification of the water body and establishment of biological processes.