A rapid infiltration basin keeps non-contact water away from the Cortez mine.
In Nevada, where dry, sandy vistas peppered with sagebrush are commonplace, you might be inclined to think there is a severe water shortage—and you'd be right. Nevada has a desert climate, with just seven inches of annual rainfall a year, but that is more than enough to replenish large underground aquifers in the northern part of the state.
These aquifers, which contain trillions of gallons of water, are the reason why mining companies in Nevada often encounter large volumes of ground water when excavating an open pit or an underground mine. This water must be safely managed, and ultimately returned to the aquifer that it came from. One example of this is Barrick's Cortez mine in northern Nevada.
"Nevada's desert status is relatively recent in geologic times," says George Fennemore, Cortez's Manager of Growth, who oversees tasks such as mine dewatering. "The state was largely lakefilled 5,000 to 10,000 years ago, and while surface water is relatively scarce today, the ancestral lakes have left really large groundwater bodies just beneath the surface."
Barrick operates dewatering systems at a number of its mines around the world. While these systems are not identical at every operation, they function similarly, in that they move clean water away from active mining areas, and return it to aquifers or other receiving bodies of water away from the mining operations.
In Cortez's case, water is removed—in accordance with strict permit requirements—to allow the mine to continue its underground and open pit operations, and to start a planned underground expansion. To do this, a complex pump, pipe, and basin system is used to remove water from operating areas of the mine, in a process called "dewatering."
To dewater the mine, nearly 40 different wells have been strategically located around Cortez to pump water away from the mine. Pumping water this way ensures that it maintains its natural quality. Each well is equipped with a large pump that moves the water thousands of feet from the mine into plastic pipes that lead to rapid infiltration basins several miles away from the mine property. These basins are essentially mile-long trenches, typically 20 feet deep and 200 feet wide, made up of soils that are easily penetrated by water, such as sand or gravel deposits. Once in the basins, the water percolates through the soil to the underground aquifer.
Barrick operates dewatering systems at a number of its mines around the world. While these systems are not identical at every operation, they function similarly, in that they move clean water away from active mining areas, and return it to aquifers or other bodies of water away from the mining operations.
"All of this helps to ensure that the water we intercept is kept clean," says Fennemore. "This also helps us to control our water treatment costs."
Like all mines, Cortez requires some water for milling, mining, and processing. The operation is allowed to use only about 10 percent of the water that flows through the property for these purposes, Fennemore says. "We're required to put the remainder of that water back into the groundwater body, and we do that through the rapid infiltration basins," he says.
To dewater the mine, nearly 40 different wells have been strategically located around Cortez, to pump water away from the mine.
Water used for mining purposes is collected in sumps located throughout the underground mine and in the milling area. It is then routed to the mine's processing areas to aid in the extraction of gold. This water is known as "contact water" because, unlike the water that flows through the Cortez dewatering system, it has made physical contact with the ore contained in the mine as well mine equipment and may have been modified as a result. This water is recycled and reused at the mine. At some of Barrick's other mines, such as Pueblo Viejo in the Dominican Republic, this contact water may be reused or treated to meet regulatory water standards before being safely discharged back into the environment.
A mine operator cannot arbitrarily engage in dewatering. They must first ensure that the water quality, the quantity of water to be pumped, and the destination of this water meet state regulatory requirements.
Hence, before any water management infrastructure is built, and before any water is pumped or used in the mining process, a mine must obtain all of the necessary permits from regulatory agencies. In Nevada, the following permits or approvals are required:
Consumptive water rights
Consumptive water rights are necessary for the mine to use water for mining and milling ore. They also allow mines to pump water. Barrick often pumps clean groundwater from its Nevada mines to the nearby ranches it owns to grow alfalfa and corn for livestock. In Nevada, these water rights are legal property, but still require approval from the Nevada State Engineer, who works for the Nevada Department of Conservation and Natural Resources.
Water pollution control permit
This permit is focused largely on water quality and requires the mine to monitor all of its water use and its water quality. In Nevada, this data is submitted quarterly to the Nevada Division of Environmental Protection and used to demonstrate that the quality of the water being returned to the environment meets state waterquality criteria. Once data is submitted to the state regulator it becomes part of the public record.
National Environmental Policy Act
In Nevada, mining activities are allowed on public land only after an extensive environmental review and permitting process. Bureau of Land Management approval is necessary to build water management infrastructure, such as rapid infiltration basins, on public land. The Bureau will issue a record of decision after a rigorous review of the proposed water infrastructure. In other parts of the world, mine operators require similar permits to build this type of infrastructure.