Study reports fourfold increases in levels of zinc in the Upper Snake River
By Bob Berwyn
SUMMIT COUNTY — Even without significant impacts from abandoned mines or other human activities, concentrations of heavy metals in the upper Snake River have increased by as much 400 percent in recent decades, with potentially serious consequences in a watershed that, in certain reaches, is already deadly to aquatic bugs and trout.
The also study illustrates the potential for comparable increases of metals in similar Western watersheds, said USGS scientist Andrew Todd, lead researcher on the project.
The observed spike in metals sent researchers on a quest to try and figure why levels of zinc and other metals are increasing so dramatically in the small headwaters streams above Montezuma, Colorado. Concentrations of zinc, for example, have increased fourfold in the past 30 years.
By default, they think global warming may be a significant factor.
“This is a really undisturbed watershed as far as anything that might change the water chemistry,” said USGS scientist Andy 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 drainage in question is distinct from nearby Peru Creek, where abandoned mines and natural sources also contaminate the water. Downstream of Peru Creek’s confluence with the Upper Snake River, the water is so tainted that some species of trout can only survive a few days of direct exposure to the metals.
Given that there appear to be no significant anthropogenic causes for the increases, 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.
“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, and Manning said the researchers hope to follow-up studies to pinpoint the cause.
For one, 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.
That means more rocks and soil are exposed to air for longer periods of time, which could increase the weathering process that dissolves the minerals, which subsequently reach the stream.
“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.
There’s not a lot of detailed permafrost mapping in Colorado, but geologists assume that some of the highest slopes, especially in wind-scoured, north-facing terrain, includes some permanently frozen areas.
Manning said that, 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,” said U.S. Geological Survey director Marcia McNutt. “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.”
Metals concentrations spiked during September and other low-flow months, but were apparent during other times. Generally, high concentrations of dissolved metals in the upper Snake River watershed are the result of acid rock drainage, the USGS explained in a press release.
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.
Increased sulfate and calcium concentrations observed over the study period lend weight to the hypothesis that the increased zinc concentrations are due to acceleration of pyrite weathering.
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,” Todd said. “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.”
In cases where acid rock drainage is linked directly with past and present mining activities it is called acid mine drainage. Another Snake River tributary, Peru Creek, is largely devoid of life due to acid mine drainage generated from the abandoned Pennsylvania Mine and smaller mines upstream, and has become a target for potential remediation efforts.
Such cleanup efforts could become even more challenging if the background levels of pollution are a moving target, Todd said.
A previous study, led by CU grad student Caitlin Crouch, reached similar conclusions about 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 leaves more rocks exposted to weathering.
Crouch gave a presentation on the subject at the 2010 fall meeting of the American Geophysical Union in San Francisco. That study was co-authored by Professor Diane McKnight of CU-Boulder’s civil and environmental engineering department.
Collaborators on the most recent study include USGS, CU Boulder and the Institute of Arctic and Alpine Research (INSTAAR). The data analyzed for the study came from INSTAAR, the USGS and the U.S. Environmental Protection Agency.