Pebble is conscientious of its impacts

In spite of the current uncertainty regarding a final decision by the Environmental Protection Agency, Northern Dynasty Minerals Ltd. and Pebble Limited Partnership has outlined the potential locations and solutions of a tailings pond for the proposed world-class copper-gold-silver-rhenium mine in Southwest Alaska.

To alleviate concerns over the environmental impacts a mine like Pebble would have on the surrounding waters, especially the Bristol Bay watershed that supports the largest sockeye salmon fishery in the world, Northern Dynasty has opted to take the initiative and address these concerns.

At a large hardrock mine such as being proposed for Pebble, once the sought-after minerals are extracted, the leftover tailings are stored in a large containment pond.

The most sustainable tailing and rock waste management method presently is to stably contain the material, minimize the water and energy inputs, as well as the surface footprint of wastes, and seek alternative solutions.

Currently, the greatest risk of tailings ponds is dam failure, with one of the major causes being due to water overflow or overtopping, according to the paper "Regional Distribution and Causes of Global Mine Tailings and Dam Failures."

"The failure of a tailings storage facility around the world is rare," said Northern Dynasty President and CEO Ron Thiessen. "When one does occur, however, failure is often caused by the accumulation of too much water."

For the proposed mine at Pebble, this concern has been addressed with an overabundance of water treatment capacity to ensure that water levels are low and clean water is discharged.

"The tailings storage and management plan for Pebble includes an investment of approximately $500 million in modern water treatment facilities to allow the water to be treated and safely released into the environment, meeting all required standards, instead of accumulated," continued Thiessen. "Because of this and other design features, such as constructing embankments on bedrock with flatter slopes than is typical in the industry, the Army Corps of Engineers in the 2020 Environmental Impact Statement said that they could not conceive how a failure could occur. The fact is that Alaska has a very rigorous dam permitting process which will ensure that the facility will be safe before it is built."

More than 30 alternative tailings sites were evaluated before the final site was selected to minimize the tailings footprint on wetlands. Northern Dynasty says the Pebble project would impact less than one mile of stream for every 1,000 miles of streams feeding into Bristol Bay.

Alaska's earthquakes taken into account

"We invested a further $150 million in studies of the climate and potential for earthquakes in the area, conducted by a team of industry experts to ensure the TSF (tailings storage facility) would be safe," Thiessen added. "Our studies and the research and reports out of the United States Geological Survey demonstrate that Pebble isn't a high seismicity area, as most earthquakes in this region of Alaska are offshore and are almost abated by the time they reach the north shore of Lake Iliamna/Pebble."

This was shown to be the case when an enormous 8.1 magnitude earthquake hit roughly 300 miles from the Pebble site.

You can read about the earthquake at Pebble stands firm after 8.1M earthquake in the August 13, 2021, edition of North of 60 Mining News.

The earthquake caused significantly less ground movement than was initially factored in the design of the proposed tailings pond site – essentially 20 to 30 times less ground movement than engineers designed for the proposed tailings facility.

The extreme seismic events modeled to test the stability of proposed tailings embankments and other mine-site facilities at Pebble include:

• A repeat of the 9.2 magnitude megathrust event that occurred in Alaska's Prince William Sound in 1964 (the second largest seismic event ever recorded).

• An 8.0 magnitude earthquake that occurred approximately 50 miles east of the Pebble Project site, similar to but of larger magnitude than the 7.1 magnitude event that occurred north of Anchorage in 2018.

• A 7.5 magnitude event along the entire length of the Lake Clark Fault, including those portions closest to the Pebble site, for which no evidence of movement for more than 10,000 years has been detected.

• A 6.5 magnitude event that occurred immediately below the proposed Pebble mine site, where no evidence of a fault exists.

"Still our approach to seismicity (ground movement factor) was to ensure the facilities can withstand the likely earthquake events that might impact Southwest Alaska, including a repeat of the 1964 Valdez 9.2 magnitude earthquake," Thiessen continued. "This was the largest event ever recorded in North America and the second largest ever."