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Management of Salinity for Closure of Open Cut Coal Mines

Open Cut » Environment

Published: May 04Project Number: C11050

Get ReportAuthor: K David, S A Prathapar, Bob Creelman, GR Hancock, Tony Voller | Department of Infrastructure, Planning and Natural Resources

This project is a scoping study that examines salinity issues facing the mining industry in the Hunter Region that is challenged with mining in a saline environment associated with Permian geological sediments and with the mining process increasing the potential mobilisation of salt.

Salinity in the Hunter Valley of New South Wales occurs naturally due to its geological features. Mining activities in the Hunter study area are part of the human disturbance of the landscape with an impact on the dynamic hydrogeochemical equilibrium of the natural state. The mining process increases the potential for the dissolution and mobilisation of salts in the mined landscape. There is also increase in the potential for salt export to the Hunter River, particularly in the medium to long term.

During the operational phase of mining this salt is contained on-site with options for subsequent transfer saline water between minesites during approved water sharing arrangements or for controlled release it under the Hunter River Salinity Trading Scheme (HRSTS). The minesite contribution of salt load to the Hunter River in controlled discharges is estimated at 3% under the HRSTS between 1995 and 2001.

The study also estimates that groundwater inflows from the study area contribute 16% of the salt loading in the Hunter River at Singleton. Constructed potentiometric maps of the regional groundwater table highlight a short-term benefit of mining on salinity. The effect of depressurisation of the groundwater table in the vicinity of an active mine pit reduces the risk of the local acquifer system discharging saline groundwaters into the Hunter River that occurs as a natural feature of many Hunter sites.

However at mine closure there is a repressurisation of the groundwater tables over time. The options for actively addressing salinity issues are less available following mining. Post mining there is the potential for continued mobilisation of salts within the mined landscape and the progressive accumulation of salts in sink areas like the final void. This study highlights the challenge for the mining industry to minimise the adverse impact of salinity issues post closure through salinity minimisation designs that are incorporated during the planning and mining phases into the mined area.

The study examines the task of addressing salinity issues, particularly after mining ceases. It examines salt mobilisation processes on minesites and identifies potential medium to long term problems. It promotes a risk management approach to salinity and the implementation of appropriate guiding principles during the planning and operational phases that will assist with the minimisation of adverse impacts in the post-mining phase.

The report includes a proposed groundwater monitoring guideline to standardise the approach to monitoring and data presentation. The use of monitoring to verify EIS predictions will promote the on-going refinement of predictive models that accurately quantify salinity processes on minesites. This will increase community confidence that the mining industry is addressing salinity issues both in the short and the long term.

The minimisation of salinity impacts from mining is essential to ensure the continued sustainable development of the industry in the region. There are many gaps remaining in the knowledge of salinity issues on minesites. This study highlights areas where further work is required.

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