are low and clean water is discharged. "The tailings storage and man- agement plan for Pebble includes an investment of approximately $500 million in modern water treat- ment facilities to allow the water to be treated and safely released into the environment, meeting all required standards, instead of accumulat- ed," 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 En- vironmental Impact Statement said that they could not conceive how a failure could occur. The fact is that Alaska has a very rigorous dam per- mitting process which will ensure that the facility will be safe before it is built." More than 30 alternative tailings sites were evaluated before the fi - nal 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. "We invested a further $150 mil-
lion in studies of the climate and po- tential for earthquakes in the area, conducted by a team of industry ex- perts to ensure the TSF (tailings stor- age 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. The earthquake caused signifi - cantly 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 mod- eled to test the stability of proposed tailings embankments and other mine-site facilities at Pebble include: n A repeat of the 9.2-magni- tude megathrust event that occurred in Alaska's Prince William Sound
in 1964 (the second largest seismic event ever recorded). n 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. n A 7.5-magnitude event along the entire length of the Lake Clark Fault, including those portions clos- est to the Pebble site, for which no evidence of movement for more than 10,000 years has been detected. n A 6.5-magnitude event that occurred immediately below the pro- posed Pebble mine site, where no ev- idence of a fault exists. "Still our approach to seismici- ty (ground movement factor) was to ensure the facilities can withstand the likely earthquake events that might impact Southwest Alaska, in- cluding a repeat of the 1964 Valdez 9.2-magnitude earthquake," Thies- sen continued. "This was the largest event ever recorded in North America and the second largest ever."
— A.J. Roan North of 60 Mining News
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Fall 2022
The Alaska Miner
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