Proposed mine shares watershed with world's largest sockeye salmon spawning ground.

The Mount Polley mine spill, in which a tailings pond failure dumped more than 8 million gallons of water, silt and sand into waterways, is likely to provide further ammunition for opponents of the Pebble Mine, an even larger copper and gold project planned in Alaska.

Located about 200 miles southwest of Anchorage, the Pebble Mine would be an open-pit mine owned by Northern Dynasty Minerals Ltd., Vancouver, British Columbia.

Northern has not filed specific plans with the U.S. Environmental Protection Agency, but the agency’s analysis and a recent determination are based on plans Northern filed with the U.S. Securities and Exchange Commission. Those plans are based on mines producing between 2 billion and 6.5 billion tons per year. The EPA analyzed both sizes as well as a smaller 250-million-ton mine.

“The extraction, storage, treatment and transportation activities associated with building, operating and maintaining one of the largest mines ever built would pose significant risks to the unparalleled ecosystem that produces one of the greatest wild salmon fisheries in the world,” the EPA said in a report issued July 18.

“In simple terms, the infrastructure necessary to mine the Pebble deposit jeopardizes the long-term health and sustainability of the Bristol Bay ecosystem.”

The EPA would drastically restrict discharge of dredged or fill material from the mine. It is taking public comment until Sept. 19 on the report.

The Pebble Mine “is likely to involve excavation of the largest open pit ever constructed in North America, covering up to 6.9 sq miles and reaching a depth at much as .77 miles” or almost as deep as the Grand Canyon, the EPA report says.

Tailings ponds are another issue.

“Disposal of resulting waste material would require construction of up to three mine tailings impoundments” covering 18.8 sq miles and waste rock piles covering up to 8.7 sq miles, it says.

The volume of mine tailings and waste rock produced from a 2-billion-ton mine would be enough to fill a professional football stadium more than 880 times, volume that would grow to more than 3,900 stadiums for a 6.5-billion-ton mine, the EPA says.

The mine pit, tailings ponds and rock piles “would cover an area larger than Manhattan,” it says.