The toxic wastewater that gushed from a shuttered gold mine in southern Colorado nearly a week ago — fouling the Animas and San Juan rivers with a mustard yellow tinge — has brought to the surface a more menacing concern.
With an estimated 55,000 abandoned hard-rock mines pockmarking a landscape across Colorado and other parts of the West, this could happen again — and it's only a matter of time for these "ticking time bombs" to go off, environmentalists and researchers warn.
"It could [happen] anywhere," Sinjin Eberle, a spokesman for the American Rivers conservation group, told NBC News on Wednesday. "Just in this little valley alone where the Gold King Mine lies are a pair of mines [named Red and Bonita] that have been leaking for decades that must be cleaned up. Any of these could be another disaster waiting in the wings."
The century-old Gold King Mine, near the town of Silverton, flushed more than 3 million gallons of sludge into the rivers, creating a chemical-laden stew of arsenic, copper, lead and other contaminants.
Related: A Week After Toxic River Spill in Colorado, Environmental Impact Remains Unclear
At the time of the spill, the Environmental Protection Agency was overseeing a tricky project: plugging the leaking Red and Bonita mines. To do that, crews went into the nearby Gold King Mine to ensure that it was stabilized as well.
During the process of excavating loose material, it fell — creating a chain reaction that led the Gold King Mine's tunnel to open and water sitting behind the collapsed material to surge into a tributary of the Animas River, the EPA said. The ribbon of sludge has since traveled into parts of Utah and New Mexico.
The federal government, meanwhile, has been under pressure to patch up these old mines to ensure the runoff isn't contaminating America's waterways.
But there's no guarantee that crews couldn't inadvertently set off another spill, said Jason Willis, Colorado mine restoration field coordinator with the conservation group Trout Unlimited.
"It could happen," Willis said. "All this stuff is very site-specific. You really have to know what you're dealing with before you go in there. And even then, there are years and years and years of pressure that has built up with a certain volume of mine water."
There are about 400 abandoned mines in the Silverton area alone, among an estimated 22,000 in Colorado. Most of them remain unexplored, and remediation for the most dangerous among them isn't guaranteed.
"Sometimes you can't do reclamation work because of liability concerns or if it's too expensive or you can't get access," Willis said. "There are a lot of hoops."
In the case of Silverton, residents resisted the EPA's designating local mines as a Superfund site. The recognition of hazardous waste, some said, would turn off tourism and hurt the economy.
But in the aftermath of this spill, whose long-term effects on wildlife remains unknown, a push to create more Superfund sites in the West is possible.
"The EPA has been forced to basically play Whack-a-Mole with these mine cleanups and [has] been hamstrung from executing a comprehensive, thoughtful plan to address this area as a whole," Eberle said.
The federal agency and state officials continued Wednesday discussing when the contaminated Animas waterways could open up again to the public.
The shocking yellowish color seen last week is gone, and Colorado says its tests show the concentration of metals in the water, including arsenic and lead, are decreasing.
Gov. John Hickenlooper wants the river reopened to boating and fishing, both important to the region's economy. He also pledged Tuesday to act on reassessing the abandoned mines "so we can make sure that those places where there might be a problem, that we'll be able to address it before the problem becomes serious."
