Rare earth elements and old mines spell trouble for Western U.S. water supplies

Nate Rock and his dog Nita collect aquatic insects from Snake River on a field trip to CU Boulder in 2015. This was part of a course by Diane McKnight on stream ecology. These are called "bioassessments" which are based on the diversity and abundance of insects to determine habitat quality and pollution tolerance. Later, they are analyzed in a laboratory to detect metals in their biomass. Credit: Stephen Cardinale
A new study in Environmental Science & Technology has found that rare earth elements are making their way into Colorado's water supplies. This is due to changes in climate.

Many high-tech devices such as cell phones and hard drives rely on rare earth elements for their components. However, it is becoming more apparent that even low concentrations can pose a danger to the environment.

"This is concerning because their concentrations have not been monitored and there aren't water quality standards for them," said Diane McKnight (study author, INSTAAR Fellow, and engineering professor at University of Colorado Boulder).

This study is the first to examine how rare earth elements travel within a watershed rich in minerals. It also examines how climate change is altering stream flow and natural weathering processes to release more rare earth elements into streams.

Since the 1990s, Diane McKnight has been leading her students in research into water quality in Colorado's Snake River watershed. Their primary focus was to observe and measure acid rock drainage. Rocks containing sulfide-based minerals like pyrite oxidize in this process when they are exposed to water and air. This chemical reaction results in sulfuric acid, dissolved metals such as iron, and eventually the water runs off into streams. Additional acidic water can dissolve heavy metals like lead, zinc, and cadmium. It can also carry rare earth elements.

"The pH is what controls the mobility and distribution of rare earth elements. "Acid literally leaches it from the rocks," said Garrett Rue, first author. Rue earned a masters degree in limnology under McKnight and a Ph.D. at CU Boulder.

With its pyrite rich geology, acid rock drainage is a natural phenomenon in the west United States. However, historic mines that have disturbed large quantities of soil and rocks can accelerate the process and lead to downstream water pollution.

The Snake River watershed has seen many towns affected by acid mine drainage forced to adapt to low water quality. Silverton, a former mining boomtown, now imports water from faraway sources. Some rely on costly water treatment plants. Because the water contains too much zinc to support any native species of fish, all fish in the Snake River are stock. Rue says that the problem is not limited to the west. "About forty percent of the headwaters for major rivers in the West have been contaminated with acid mines or rock drainage."

Since the Snake River is a natural laboratory for both, the Peru Creek portion of the watershed was heavily dredged, but the Upper Snake River wasn't, the Snake River has been a great place to investigate both. McKnight and Rue found that both the upper and lower parts of the watershed now contribute significant amounts to the downstream metals, due to climate change which has seen longer summers and less snow during the winters. The stream flows are longer and more concentrated, making it easier for metals in the watershed to leach.

These same processes that lead to more heavy metals in streams also affect rare earth elements. Researchers found rare earth elements in the Snake River. Rue says, "We found a concentration range from one to hundreds of micrograms/liters. This is a significant increase in surface water concentrations. The highest concentrations were near the headwaters and the areas receiving drainage from abandoned mine works."

They also found that the Snake River had an increase in rare earth elements. This was due to warmer summer temperatures. Insects living in streams are also accumulating rare earth elements at levels comparable to toxic metals like lead and cadmium.

Rue says, "We are beginning to understand that once rare Earth elements get into the water they tend to stay there." "They don't get removed by traditional treatment processes either," Rue says. This has implications for reuse, and led some European cities designate REEs an emerging contaminant in drinking water supplies. This could pose a problem for the future, considering the Snake River flows into Dillion Reservoir which is Denver's largest stored water source.

Researchers suggest that investing in technology to recover rare earth elements naturally from water could lead to valuable commodities. This would also help to address problems such as acid rock and mine drainage. These issues are likely to get worse with climate change.

Rare earth elements can be used to make many products. The majority of the supply comes mainly from China. Rue says that while our government has been searching for sources, mining has also left an indelible impression on the waterways of the West. It might be worth treating the water to recover some of the materials already found in our environment and harvesting them.

McKnight adds that "this problem is getting worse" and urges us to address it. McKnight says that if we solve the problem holistically, it will be a valuable resource. He also suggests climate adaptation.

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More information: Garrett P. Rue et.al, Enhanced Rare Earth Element Mobility in a Mountain Watershed in the Colorado Mineral Belt with Consecutive Detection In Aquatic Biota, Increasing Climate Change Driven Degradation of Water Quality, Environmental Science & Technology (2021). Information from Environmental Science & Technology Garrett P. Rue et.al, Enhanced Rare Earth Element Mobility in a Mountain Watershed in the Colorado Mineral Belt with Consecutive Detection In Aquatic Biota. Increasing Climate Change Driven Degradation of Water Quality. (2021). DOI: 10.1021/acs.est.1c02958