Heavy metals concentrations are increasing in the Snake River, near Keystone, Colorado, and some scientists think global warming may be a factor.
We may not yet know exactly how global warming will affect all the complex parts of Rocky Mountain ecosystems, but it’s not for lack of trying. Scientists are prodding the soil, counting wildflowers, measuring winds and gauging snowfall nearly every day to unravel the mysteries of the anthropocene, this present-day geological epoch in which humans are having a big impact on the environment.
Take, for example, the Snake River, a medium-size mountain stream near Keystone Ski Area, perhaps one of the most-studied streams in the West. EPA wetlands specialists have poked around in the stream’s muddy wetland headwaters, USGS hydrologists several years ago used tracer chemicals to assess how fast certain substances move downstream and aquatic biologists electroshock parts of the river each year to count fish.
All this because, 100 years ago, mountain miners found a big vein of silver in the ore-rich mountains above the rivers. The miners are long since gone, but their toxic legacy lives on. Oozing, acid drainage from the abandoned mine taints the river far downstream. Even several miles below the old mine, caged fish survive for only a few days in the water before the heavy metals, primarily zinc, kill them.
In 2010, USGS researchers said there’s a trend of increasing metals concentrations in recent years, especially during low-flow seasons. In some sections of the Snake River, concentrations of heavy metals have increased by as much 400 percent in recent decades, and those measurements come from a section of the drainage that’s not even affected by the Pennsylvania Mine.
A few years later, another study suggested that global warming may be factor. After ruling out other causes, the scientists said melting permafrost, tucked in among the highest crags in the drainage, could be part of the reason for the rise in pollution, increasing at a rate that threatens to outpace a cleanup effort under way at the mine.
USGS researcher Andy Manning emphasizes that the findings are only a hypothesis at this point — it’s hard to know for sure because there’s not a lot baseline permafrost information for the Rocky Mountains, only predictive models that show, based on elevation and aspect, where permafrost should be.
When frozen soils thaw (by definition, permafrost is ground that’s frozen year-round), the process creates more passageways for water trickling down the steep slopes and more places where the water comes into contact with the naturally mineralized rocks in the basin, as well as the waste rock around the mine diggings.
If melting permafrost is a factor, it could be a big deal across the Rockies and the West, where federal land managers have counted almost 40,000 abandoned mines, many of them in high mountain drainages. Already, more than 5,000 miles of streams and rivers are affected by acid mine drainage. With a surge in metals, the impacts could spread farther downstream.
The long-studied Snake River is a great lab because of the controlled conditions and wealth of water quality baseline data.
“This is a really undisturbed watershed as far as anything that might change the water chemistry,” said Manning. “It’s really important to be clear about the conclusions. What we found is that the concentrations are going up. Then we laid out some hypotheses, and the cause appears to be climate warming, in general,” Manning said.
The researchers started with what they knew. During the study period, local mean annual and mean summer air temperatures increased at a rate of 0.2-1.2 degrees Celsius per decade (about 0.3 to 1.8 degrees Fahrenheit).
“Temperatures are clearly warming. The statistical evidence is solid on that. So then you can ask, what is it about warming that could increase metals?” Manning said.
“If you warm the system the reaction rates go up, but probably, the warming in Upper Snake is not enough to be the main cause directly,” he said.
That leaves a couple of other possibilities.
“It’s possible that, when you warm the mountains, then the amount of water that’s being used by plants goes up. That means there’s less for infiltration and water recharge,” he said, offering one explanation for how warming could affect stream dynamics and lead to higher levels of water pollution.
“The other possibility is this melting of frozen ground,” Manning said.
In permafrost soils, the spaces between the particles of earth and rock are filled with ice, resulting in a relatively impermeable mass.
Manning said if that layer were to thaw, it would create more of a “Swiss cheese” texture, with new spaces for water and air, once again introducing more oxygen that promotes the weathering process.
“This study provides another fascinating, and troubling, example of a cascading impact from climate warming as the rate of temperature-dependent chemical reactions accelerate in the environment, leaching metals into streams,” U.S. Geological Survey Director Marcia McNutt said as the findings were announced. “The same concentration of metals in the mountains that drew prospectors to the Rockies more than a century ago are now the source of toxic trace elements that are harming the environment as the planet warms.”
Chemically, the acid rock drainage is formed by the weathering of pyrite and other metal-rich sulfide minerals in the bedrock. The weathering forms sulfuric acid through a series of chemical reactions, and mobilizes metals like zinc from minerals in the rock and carries these metals into streams.
Numerous studies have also reported that increasing concentrations of sulfates and sulfuric acid in other mountain lakes and streams over the past three decades may be linked to climate warming. The buildup was observed even in lakes from basins that aren’t highly mineralized, suggesting that weathering of minerals like gypsum, rather sulfide minerals, are to blame.
“Acid rock drainage is a significant water quality problem facing much of the Western United States,” said USGS scientist Andrew Todd, who has been studying the Snake River Basin for years. “It is now clear that we need to better understand the relationship between climate and acid rock drainage as we consider the management of these watersheds moving forward.”
A previous study, led by CU grad student Caitlin Crouch, reached similar conclusions about increases in acid rock drainage in the Snake River Basin, with a focus on seasonal impacts, including a snowpack that’s been documented as melting up to three weeks earlier. The result is lowered stream flows and drier soils along the stream in September and October, which also leaves more rocks exposed to weathering.
Rocky Mountain Climate Watch is part of a two-month crowdfunded journalism project investigating how global warming is changing the Rocky Mountains, from glaciers and Alpine tundra down, to forests, fields and streams. In a series of reporting treks, Bob and Dylan Berwyn will visit with scientists who are monitoring the changes, and talk to ranchers, skiers and mountain town residents who are experiencing the changes. The series appears as the “Rocky Mountain Climate Watch” blog in the right-hand column of the Colorado Independent homepage.
We’re encouraging readers to ask specific questions about global warming in the Rockies, and we’ll make every effort to have the right person answer the question. We’ve also been fostering a social media dialogue via Twitter at the #ClimateRangers hashtag and the @bberwyn feed, and we’d love to see your comments and questions on the Colorado Independent Facebook page.
[Photos by Bob Berwyn]