Warming escalates river zinc levels

U. COLORADO (US) — Climate change affecting the timing of the annual snowmelt may be responsible for rising concentrations of zinc in the Snake River on Colorado’s Western Slope.

The watershed just west of the Continental Divide near Keystone, Colo., has experienced  a four-fold increase in dissolved zinc over the last 30 years during the lowest water flow months, says Caitlin Crouch, a master’s degree student in environmental studies at the University of Colorado-Boulder.

The high levels of zinc affect stream ecology, including deleterious effects on microbes, algae, invertebrates and fish.

The increased zinc concentrations may be tied to changes in groundwater conditions and stream flow patterns caused by climate change and the associated snowmelt that has been peaking two to three weeks earlier than normal in recent years, largely because of warming air temperatures, Crouch says.

The result is lowered stream flows and drier soils along the stream in September and October, which increases metal concentrations.

“While most of the talk about climate change in western waterways is about decreasing water quantities, we are evaluating potential climate influences on water quality, which is a whole different ball game,” she says.

The zinc in the Snake River watershed is primarily a result of acid rock drainage, or ARD, which can come from abandoned mine sites along rivers or through the natural weathering of pyrite in the local rock, says Crouch.

Sometimes enhanced by mining activity, weathering pyrite forms sulfuric acid through a series of chemical reactions, which dissolves metals like zinc and carries them into the groundwater.

There are nearly 2,000 miles of waterways in Colorado affected by ARD, says Diane McKnight, professor of civil and environmental engineering.

One of the most noticeable impacts of ARD in the Upper Snake River drainage is on the fishery downstream. Rainbow trout populations in much of the river are not self-sustaining because of ecologically harsh stream conditions, and the waterway requires stocking several times a year.

The elevated zinc in the Snake River comes from several ARD sources. Crouch’s study site—where an increasing trend in zinc concentrations is sustained by groundwater discharge—is above the Peru Creek tributary to the Snake River, where natural pyrite weathering is thought to be the main source of ARD.

Peru Creek is largely devoid of life due to ARD from the abandoned Pennsylvania Mine and other smaller mines upstream and has been a target for potential remediation efforts.

Another factor involved in rising zinc levels in the Snake River watershed—which runs from the top of the Continental Divide to Dillon Reservoir—could be the result of the severe 2002 drought in Colorado that significantly lowered waterways, allowing more pyrite to be weathered in dry soils of the watershed and in wetlands adjacent to the stream.

As part of the study, Crouch measured zinc concentrations in an alpine tributary of the Upper Snake River and found that zinc concentrations there were 10 times higher than in the main stem of the waterway and correlated with increased sulfate, so-called “hard water” containing calcium and magnesium, and a variety of metals.

“This supports our contention that the increasing zinc concentrations we are seeing in the watershed are driven by the acceleration of ARD,” Crouch says.

“One of the things I still am trying to parse out is whether metals like zinc are coming from one discrete source or are being diffused into the watershed from the groundwater beneath.”

Cleaning up abandoned, polluted mines like the Pennsylvania Mine remains a problem largely because of liability issues since the mine owners who normally would be responsible for the mine cleanup are long gone.

The Environmental Protection Agency has begun an agency-wide effort to reduce barriers to the cleanup of abandoned mine sites by local environmental groups and volunteers.

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