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Experts debate benefits, drawbacks of post-mining pit lakes

While not always pretty, mining pit lakes may offer a bounty of benefits … at a hefty cost.

Achieving a productive post mining land use for a mining pit lake can be complex, expensive and risky in the long term for mining companies and for taxpayers if the former mining pit eventually becomes a Superfund site.

But with proper planning and financing, a mine pit lake can become a beneficial source of wildlife habitat, recreation opportunities, storing precious water resources, and even a laboratory for organisms that benefit medical research.

Pit lakes result when a mining pit is located at least partly below the natural level of groundwater or the water table and is regularly dewatered during mining operations. However, when mining ends, the company stops dewatering and the pit begins to fill with water as the water table tries to restore itself.

More recently, the quality of water left behind in pit lakes and the decrease in groundwater tables have become socioeconomic or social-license-to-operate concerns, ranging from the gold mines of Nevada to the copper mines of Montana to South American farmers.

Pit lakes are closely regulated in the mining areas of Nevada, before, during and after mining. Nevertheless, not every pit lake is a good candidate to become a fishing hole, sustain wildlife, or provide safe access to the water.

In less developed nations, old mine pit lakes are now surrounded by residential development and sometimes used as a source of drinking water for a populace that lacks resources to determine if the water is indeed safe for human consumption.

Future pit lakes may be attractive nuisances

University of Nevada Professor Dr. Glenn Miller is concerned about pit lake chemistry and the potential attractive nuisance factors of post-mining pit lakes. In a talk to the Nevada Water Resources Alliance symposium on pit lakes Monday, Miller said water constituents of particular concern to environmentalists, regulators, recreationists and other mining stakeholders are selenium, arsenic and antimony, mercury, nickel and zinc.

Meanwhile, wildlife and avian species are bound to ingest pit water at some time in arid Nevada, which can impact the food chain at some point.

Nevertheless, Miller stressed, “We have gone from concerns that pit lakes will be toxic and simply an evaporation surface to having options of serving as a resource for recreation and wildlife habitat.” Among potential post-mining pit lake uses are boating, fishing swimming, and wildlife habitat.

For instance, endangered species can be nurtured by pit lake habitats. Economic diversification can also be a realistic post-mining land use.

But there are still remaining issues including safe access to the pit lake, pit wall stability, water quality, regulatory oversight, and liability, he said.

Miller, a longtime environmental advocate, declared, “Pit lakes now can be considered a valuable public asset” that can become a focal point of post-mining uses such as recreation and wildlife habitat that, if not properly planned and regulated may present “an attractive nuisance”.

In arid Nevada, mine pit lakes can cause a loss of precious water resources through evaporation and/or possible contamination of some water resources.

“The requirement of a ‘productive post-mining land use’ should be taken seriously, since the pit lakes will, at the very least, release valuable water resources to the atmosphere,” Miller observed.

Reno-based SRK Consulting’s Corporate Consultant Jeff Parshley–an award-winning specialist with experience in environmental geology and geochemistry, feasibility and due diligence studies, mining permitting, closure and remediation, and environmental audits and liability assessments—told the mining and drilling professionals, scientists, consultants, and regulators attending the symposium that mining companies now consider pit lakes to be an important component of mining’s social license to operate.

However, Parshley noted, location, size, land ownership, and chemistry are all factors which must be considered regarding the viability and cost of post-mining pit lakes.

For instance, Vale’s Mina de Aguas Claras now has residential development near its pit lake shores, along with a booming tourist trade and “a lot of opportunities for commercial development”, said Parshley.

In less developed nations, however, post-mining land uses are left up to the landowner, not regulators, he said.

Parshley observed that former mines are particularly attractive to developing nations because they provide access to infrastructure such as roads and electricity. Mining pit lake water can become a source of water for feeding livestock or even drinking water. In poorer nations, people do not necessarily test the water before they drink it.

Meanwhile, regulators in these jurisdictions may lack the capacity to monitor pit lake water quality or may be bureaucrats, not technicians, for whom talking about pit lake water issues “becomes very, very difficult,” he stressed.

Post-mining pit lake cost concerns

The biggest cost concern for mine operators is long-term pit lake water treatment, which Parshley referred to as “an expensive solution of last resort” to create and maintain a potable water supply. Another formidable obstacle is that some jurisdictions don’t require financial assurance or guarantees for long-term pit water management costs.

An example of a massive never-ending money pit cited by several conference attendees and speakers is the Berkley Pit in Butte, Montana, one of the United States’ largest Superfund sites.

The pit and its water present an environmental problem because the water allows pyrite and sulfide minerals in the ore and wall rocks to decay, causing acid drainage. When the pit water level eventually reaches the natural water table, estimated to occur around 2020, the pit water may reverse flow back into surrounding groundwater, polluting the Silver Bow Creek, which is the headwaters of the Clark Fork River.

The Berkley Pit water carries a heavy load of dissolved heavy metals. The water is believed to carry so much dissolved metal that some material is mined directly from the water. Ironically new life forms have been discovered in the pit water and are being utilized in treatments for cancer and in other medicines.

The German government had turned reclamation and post-mining uses of Soviet-era East German coal mining pits into desirable recreation areas, thanks to a US$16 billion program to close 190 of 214 pit lakes, said Parshley.

The government decided to stabilize the land and the pits walls and then create massive pit lakes, which were linked together and connected to the nation’s river network. The lakes also function as flood mitigation systems, saving the government millions of dollars in flood damage.

Vacation homes are now built on the shores of pit lakes, which attract a steady stream of tourists. New crops, such as grapes, are being grown on the slopes of former mine sites.

Parshley also stressed pit lakes can be transformed into water reservoirs or habitats for medical research.

Is modern pit lake treatment science reliable?

While treatment of pit water is technically possible, Mark Logsdon of Geochimica Inc. in Aptos, California, observed that implementing long-term water treatment programs “may be expensive to super expensive” and wondered “who is going to execute perpetual water treatment over time?”

Nowadays sophisticated methods of applied mathematics exists that can be applied to pit lake modeling. Nevertheless, he asked, “How will we obtain the type, quantity and quality of input data needed to produce meaningful model estimates?”

While Logsdon said pit lake science is “getting where we need to go”, the data gathered requires more precision to provide potential better post-mining utilization of pit lakes.

Meanwhile, the challenge for mining proponents and regulators is bringing other stakeholders into the conversation concerning the future of pit lakes, a point also stressed by Parshley.

Logsdon observed that mining companies and regulators have to be able to explain pit lake water treatment options in language clear enough to gain the understanding of aboriginal peoples in Papua New Guinea as well as a more mining-educated audience in Nevada.

Perhaps, mining companies and regulators should consider the option of providing independent technical assistance for stakeholders concerned about the legacy of mining pit lakes, he suggested.

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