We recently spoke to Gavin Lee, the Macraes Operation’s Environment and Community Manager, about how water is managed on site.
Water is essential to life and underpins the health of people and communities. It has significant social, ecological, cultural and economic values.
Water is also vital when mining for and producing gold, and at the Macraes Operation, water is used in a number of ways. From the trout hatchery, to the processing plant, to the way we capture stormwater and safely store it.
Each step in our processing plant requires water to separate the valuable minerals containing the gold from the host rock. We initially add water when grinding up gold-bearing ore turning it into slurry (a paste-like mixture of fine rock and water). At this consistency we’re able to move the mixture from one step to the next. Water is also used for mixing reagents like cyanide which dissolves the gold and allows it to be removed from all the other material.
After we remove the gold, the remaining material, which contains trace amounts of cyanide is passed through a cyanide destruction process (the INCO process) to reduce the environmental risk, then pumped as tailings to the tailings storage facility (TSF).
In the processing plant, about 90% of the water we use is recycled, much of it from the tailings storage facilities. About 5% of the water we use is taken from silt ponds, or other water storages which are not able to be discharged, and about 5% (between 1,500 – 1,800 cubic metres a day, or about two thirds of an Olympic Swimming Pool), comes from the Taieri River.
We have three permits with the Otago Regional Council that allow us to take water from the Taieri River. When the flow in the river is low, we work with a group of irrigators in the catchment, called the Strath Taieri Irrigation Group (STIG), to ensure a minimum flow is maintained in the river. STIG has a coordinator who sends out a text message when it is our time to switch on and off the water pumps.
Before the water gets used in the process plant, it is used to produce trout at the Macraes Trout Hatchery. Established in 2004 in collaboration with conservation organisation Fish & Game, the hatchery is a fantastic example of how we can create shared value with our stakeholders. From an initial capital expenditure of NZ$60,000 nearly 20 years ago, the trout hatchery has become enormously successful at continuously supplying trout for the benefit of anglers in Otago.
It is also critical we manage water around the surface mining areas. Erosion from rain leading to sediment run-off needs to be contained in appropriately designed drainage that feeds into silt ponds, where the sediment settles out before the water is discharged to local creeks. Water also needs to be pumped away from open pit floors and the underground to ensure safe and efficient mining operations.
Through our environmental monitoring program, we have found that water that passes through the waste rock stacks may collect sulphates and nitrates. In some cases, the concentrations of these compounds are such that this water cannot be discharged, and it is pumped back into the site water management system.
This process is possible in the short term, however once the mine closes it presents a challenge. Research is currently underway to examine two options. One option is to use this water for irrigation. As sulphates and nitrates are fertilisers, the use of waste rock seepage for irrigation offers the opportunity for a win-win for the mine and the farming community. We are currently working with the University of Otago and a number of lessees to understand the potential of this water.
Where irrigation may not be possible, our team is also working with a research company, Verum, on enhanced passive water treatment. This treatment uses natural processes to convert and remove the sulphates and nitrates from the waste rock water. We have completed a series of lab-based trials and the next step will be a field pilot program. We also worked with Verum on the passive water treatment plant for our Reefton Restoration Project.
With the expanding footprint of the mine, we are collecting additional water and a lot of this water cannot be discharged. At Macraes we have a negative water balance, meaning there is more evaporation than rainfall. So, we use a sprinkler system that utilises evaporation from the sun and wind, to help reduce the volume of water on site. This is particularly important for maintaining the correct freeboard (water levels) on the TSFs.
Water is becoming a scarcer and more valued commodity. Looking forward, we are likely to face changes in rainfall patterns with more intense rainfall and long periods without rain. What this means for mining is that we need to always be thinking about how we can reduce our water consumption by minimising use and increasing reuse.
In our Water Management Statement of Position we commit to reducing our water-related impacts through collaboration, partnerships, product stewardship, minimising our use and a strong focus on operational control and improving operational performance. Using water wisely is good for the environment, good for society and also good for business.