The Bullfrog mine in early 2017 before backfill work began.
The Bullfrog mine was discovered in 1904 by Frank “Shorty” Harris and Eddie Cross. Its name, according to David Myrick’s Railroads of Nevada and Eastern California, was chosen either because of Cross’s fondness for singing ‘Oh, the bulldog and the bank and bullfrog in the pool…’, or because the first gold-bearing ore discovered there looked like a bullfrog.
Barrick took possession of the mine when it acquired Lac Minerals in 1994. The Company ran Bullfrog until 1999, terminating operations because costs exceeded the price of gold, which was trading below $300 an ounce.
But the cessation of mining didn’t put an end to Barrick’s environmental obligations. At Bullfrog, this meant managing groundwater in the mine’s open pit. While Bullfrog is located in the southwestern Nevada desert, the mine’s open pit descends below the water table, causing groundwater to pool in the lower levels of the pit. When the mine was operating, Barrick would pump this water out of the pit in accordance with strict permitting conditions. However, the Company’s water experts knew that when Bullfrog closed the water would return and form a pit lake. The quality of this water could deteriorate over time and potentially damage the local environment. Recognizing this, Barrick began planning for the closure of Bullfrog soon after acquiring the property.
Historically, where there was a risk to water quality after a mine closed, the mining industry’s usual method of addressing the issue was to build a water treatment plant and treat water in perpetuity. This is still common practice. But as strict mine closure laws come into effect around the world, large companies like Barrick, which manage a growing number of closure properties, are seeking sustainable, environmentally sound, and cost-effective ways to meet their mine-closure obligations.
The best solutions, says John McCartney, Vice President of Water Management at Barrick, come when a multi-disciplinary team of experts work together to resolve challenges.
“On a mine site, water touches just about everything, and it is such an important consideration throughout the mine life cycle,” he says. “So when it comes to closure planning and water management, no one person or group can do the job in isolation. We need to start planning early, and we need to bring all the right people together to develop an effective plan. The results of such collaboration are always better than the sum of the parts.”
Given the high cost of building, operating, and maintaining water treatment plants, Barrick, increasingly, is looking to obviate the need for such plants by preventing water-quality deterioration from occurring in the first place, says Patrick Malone, Vice President, Permitting and Mine Closure.
“It’s good for business and good for the environment if we invest in preventing problems rather than treating problems,” he says.
Bullfrog in late August 2017 after the completion of backfilling. The fine layer of soil at the top of the backfill helps facilitate the evaporation process, which offsets the inflow of groundwater.
Bullfrog was a good early example of this strategy. Recognizing that a pit lake would form when mining stopped, the Company developed a plan to use backfill, rock stockpiled at the mine site, to cover the water that had begun to accumulate in the open pit, preventing a full-on pit lake from forming and protecting the natural groundwater system.
“Backfilling as a closure measure works in this instance because it is well-suited to local hydrological conditions, and it is totally sustainable,” McCartney says, adding that using rock stockpiled at the mine as backfill helped reduce costs and retain the region’s natural look and feel.
Malone notes that the Company consulted closely with regulators and local communities in developing the closure plan.
“It’s not enough for a plan to be economically viable, our community and government partners must feel comfortable that it will work and that it’s the right thing to do,” he says.
Barrick began backfilling the open pit in late 1999 and completed the work to a height of 906 meters by the spring of 2000. In the ensuing years, Barrick’s water expertise increased, and the Company began running sophisticated water modelling exercises to determine the status of groundwater beneath the open pit. Dr. Johnny Zhan, Senior Manager of Hydrology at Barrick, carried out this work. By 2009, Zhan’s models were predicting that groundwater would begin reentering the open pit by August 2015, which is exactly what happened.
“Johnny’s models were incredibly accurate,” McCartney says.
Johnny Zhan (right), Senior Manager of Hydrology at Barrick, travels to Barrick sites around the globe to provide expertise on water-related issues. Here, Zhan visits the Donlin Gold project in Alaska. With him are Mike Rieser (left), Senior Environmental Engineer at Donlin, and Danny Twitchell (center), Community Relationship Coordinate at Donlin.
Barrick closely monitored conditions in the open pit, providing regular updates to local and federal authorities and community leaders. In 2017, the Company did additional remedial work at Bullfrog, backfilling the open pit to a depth of 927 meters. One key difference from the work done in 1999 was the inclusion of a one-meter thick, fine layer of soil at the top of the newly-backfilled material. This fine-grade soil contains tiny pore spaces that facilitate the evaporation process, which occurs at a rate that offsets the inflow of groundwater below the backfill surface. Zhan’s water models indicate that, at the current backfill height of 927 meters and with the help of evaporation, no further backfill work will ever be needed.
“The backfilled pit is now an evaporative sink with no outflow of mine-impacted water or adverse effects on the regional groundwater system,” Zhan says.
Managing water after a mine closes is one of the bigger challenges for the mining industry in the 21st century, Malone says. It requires time, planning, and a wide range of expertise.
“It really pulls on all aspects of modern mining: engineering, geochemistry, construction, earth moving, hydrology, permitting, and community relations,” he says. “Our experience at Bullfrog shows that, when we bring these people into the room and plan ahead, mine closure is a challenge that can be overcome.”