It’s now possible to manage evaporation on mine pit lakes using an accurate, autonomous system. It takes daily water measurements and combines them with local weather data to predict how evaporation rates will change in the future. LOUIS WHITE reports

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Lake with sandy banks reflected in water

One of the industry’s pressing tasks is to manage discontinued mining sites to minimise environmental impacts with water management a top priority.

To tackle this problem, CSIRO has developed a monitoring system that improves evaporation estimates through a combination of measurements and improved computer models.

“The main purpose of the system is to understand evaporation losses from mine pit lakes, water storages, tailings dams or any other open water surface,” CSIRO principal research scientist, David McJannet, says.

A disused open-cut mine that has been excavated beneath the water table is likely to fill with water from rising groundwater and rainfall, forming a pit lake with a unique chemical composition.

Effective water management relies on accurate information

Effective environmental management of such a site depends on information about the geology and any possible contaminants remaining in the mine tailings, as well as the lake’s water balance – the ebbs and flows of water and its evaporation from the lake.

The evaporation data collected is then coupled with information from surrounding weather stations to generate computer models to help predict how evaporation rates will change as pit lake levels rise in the future.

Tracking evaporation rates

The system is designed to float upon the surface of open-cut mine lakes and take daily evaporation measurements, with its durability allowing it to remain on the water for long periods of time. Because it’s autonomous, it requires minimal labour once deployed.

“The main data we are aiming to provide is daily evaporation rates,” Dr McJannet says. “We do this through an intensive monitoring period where we make measurements from two technologies.

“We deploy a floating evaporation pan to directly measure evaporation and we also deploy a buoy that measures meteorological conditions such as humidity, wind speed, temperature and water temperature.”

Trialed in the Pilbara and Hunter Valley

The system has been applied in the Pilbara in three locations for BHP iron ore and in the Hunter Valley for Anglo American coal operations. Most recently, CSIRO installed the monitoring system at Mogalakwena mine in South Africa, and Las Tortolas tailings facility in Chile.

“We typically run the system for six to 12 months, and during that time, we develop a model that relates lake evaporation to the meteorological variables measured by the buoy,” Dr McJannet says.

“We then develop code for the buoy that enables it to directly estimate evaporation. That buoy is then left at the water body to provide ongoing daily evaporation numbers. We then remove the pan for work at other sites.

“The evaporation pan uses state-of-the-art instruments, ultrasonic algae control systems, and fully automated filling and emptying routines.”

The combination of accurate data and evaporation modelling will enable the team managing the mine to predict pit lake levels and possible negative impacts from changing water levels on the surrounding environment. Anticipating the evolution and impact pit lakes pose for adjacent water resources is crucial for effectively managing the long-term legacy of mining.

Data for accurate water monitoring and reporting

In the end, the CSIRO system will not only save companies money but also provide for far more accurate water reporting.

“In water balance reporting for many of these mine pits, evaporation is the most uncertain component,” Dr McJannet says.

“By improving accuracy of these estimates mining companies can better predict available water resources.

“Evaporation numbers can be used to help understand evolution of pit lakes, which form after extraction activities have ceased and can potentially save rehabilitation costs particularly around earth works costs, as rehabilitation occurs to the predicted mature lake level.

“Better evaporation numbers also help in other mining water bodies such as dams to help understand potential flood retention capacity and ability to store and evaporate excess water.

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