Introduction
Mandalay Resources owns and operates the Costerfield antimony and gold underground mine in central Victoria. Mine dewatering is an essential part of safe underground mining practice, and all groundwater pumped from the mine must be managed on site. Expansion of the underground workings has resulted in an increase in dewatering rates and has lead to investigation of alternative groundwater management options.
Extracted mine groundwater is currently used in mine operations and excess is managed through an evaporation facility and a reverse osmosis (RO) plant. The community was consulted to gauge support of the water management options available.
Discussion
Construction and operation of additional evaporation and storage facilities are expensive and long-term projects that may create negative impacts to the environment. The community determined this option to be the least preferred.
The RO plant is a high-cost and energy intensive process producing permeate and a brine by-product. The permeate is used onsite for dust suppression and excess may be discharged into a local waterway under EPA licence conditions, or provided to local community for agricultural use. The community is supportive of the process but permeate production is limited by brine storage capability, which must be stored on surface, in lined dams.
Alternative water management options were investigated to supplement current methods to meet the expected increase in dewatering from mine expansion.
Aquifer recharge (AR) was investigated to return groundwater from mine dewatering back to its original source in the regional basement aquifer (RBA). The high salinity of the RBA limits the potential for current and future beneficial uses. Due to lack of porosity within the fractured rock, large geologic fault structures were targeted to increase permeability. The confined nature of the aquifer prevents contamination of any shallow alluvial aquifer (SAA).
A 90-day AR trial was completed, discharging a total of 76ML at a rate of 7-20 L/s over two injection bores. No seepage to surface was observed and surface water quality was not impacted. The trial confirmed the movement of recharge water towards the aquifer depression created by mine dewatering. Transport modelling indicates that feed water quality would be contained within a localised area surrounding each injection well.
Conclusion
The community are supportive of the AR scheme to return groundwater back to the aquifer previously depleted by mine dewatering activity and that any risk to the beneficial uses would be actively managed by the mine.