This undated photo shows IDM’s Red Mountain project near Hyder, Alaska. (Photo courtesy of IDM Mining)British Columbia has green-lit another gold mine in the region’s prolific Golden Triangle near Hyder, Alaska.Listen nowIDM Mining CEO Rob McLeod said the 363-acre Red Mountain mine will be a relatively modest operation.“It’s a fairly short mine life at about five and half years,” McLeod said. “But in 2017, we added another 20 percent of resources and the deposit is wide open for expansion.”The environmental assessment approved by B.C. authorities envisions downstream monitoring for about 10 years after the mine closes down.Federal scientists in Juneau are concerned that’s not long enough and said so in comments filed with provincial authorities.NOAA hydrologist Sean Eagan said historic mines nearby have proven it can take decades for sulfides in mine waste to oxidize in a way that generates acid rock drainage.“They had piles of rock sitting around and it took 20 years for the acid to really develop, the acid leaching to really come out of those piles,” Eagan told CoastAlaska. “So the amount of time they are proposing to monitor does not seem sufficient.”Alaska regulators have also weighed in. They work with their B.C. counterparts and track transboundary mines because of potential downstream impacts in Alaska.Kyle Moselle is in charge of large mine projects for the Alaska Department of Natural Resources. His agency didn’t find any red flags with the project.Kyle Moselle is in charge of large mine projects for the Alaska Department of Natural Resources. His agency didn’t find any red flags with the project.“You know, I’m not going to second guess the regulatory decision by the B.C. regulatory agencies on setting that 10-year post-closure monitoring period,” Moselle said. “But it’s not unusual for a short-lived mine that you would have a proportionally short monitoring period.”The post-closure monitoring period isn’t necessarily fixed. If stream samples detect concerning levels of contaminants the monitoring period could be extended by B.C. regulators.“You monitor until monitoring is not needed,” Moselle said.But Alaska-based mine watchdogs also have concerns about the proposed tailing facility. That’s where mine waste is stored in reservoirs.“Tailings dams are the problem,” said Brian Lynch, a retired Alaska Department of Fish and Game biologist who now works for a group called Rivers Without Borders. “You know, not just in the gold mines of B.C. but tailings dams for coal mines in the U.S. — it’s just having those in the system that can be problematic.”Rivers Without Borders and other green groups urged IDM to use “dry-stacking” for its mine waste. That’s when water is filtered from the waste allowing it be stacked in dry heaps for long-term storage. It’s used at the Hecla Greens Creek Mine on Admiralty Island.But McLeod said in order to liberate the gold from the ore it’ll have to grind the material down to about 25 microns.“And with so with such a fine particle size that is not generally amenable to the dry-stack method,” McLeod said.IDM Mining estimates there is 524,000 oz. of measured gold available in the underground mine. (Photo courtesy IDM Mining)McLeod said his company understands the sensitivity around tailing ponds; IDM is still finalizing its storage plan.“We may mix the tailings in with concrete,” McLeod said. “But upon closure it would be sealed with another geomembrane and covered up and will be continued to be monitored at the headwaters.”The Canadian Environmental Assessment Agency is doing its own separate federal review and taking public comment through Oct. 18.IDM projects its Red Mountain mine will produce about a half-million ounces of gold – worth nearly $600 million at today’s gold prices.The company expects to begin construction in 2020.Editor’s Note: This article has been updated to reflect that B.C. regulators could order post-closure monitoring to be extended past the initial 10-year period.