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Warming temperatures are causing a steady rise in copper, zinc and sulfate in the waters of 蜜糖直播 mountain streams affected by acid rock drainage. Concentrations of these metals have roughly doubled in these alpine streams over the past 30 years, , presenting a concern for ecosystems, downstream water quality and mining remediation.

Iron oxides stain the bed of Upper East Mancos River in southwestern 蜜糖直播, an alpine stream affected by acid rock drainage. Concentrations of zinc, copper and sulfate have doubled in 蜜糖直播鈥檚 high-altitude streams, like the Upper East Mancos, where groundwater flows through bedrock rich in metal sulfides, according to a study that links the trend to climate change. Photo by Andrew Manning.

Natural chemical weathering of bedrock is the source of the rising acidity and metals, but the ultimate driver of the trend is climate change, the report found.

鈥淗eavy metals are a real challenge for ecosystems,鈥� said lead author Andrew Manning, a geologist with the U.S. Geological Survey in Denver. 鈥淪ome are quite toxic. We are seeing regional, statistically significant trends in copper and zinc, two key metals that are commonly a problem in 蜜糖直播. It鈥檚 not ambiguous and it鈥檚 not small.鈥�

The study was published in Water Resources Research, AGU鈥檚 peer reviewed journal for original research on the movement and management of Earth鈥檚 water.

Although the mechanism coupling warming temperatures to increased sulfide weathering is still an open research question, the new results point to exposure of rock once sealed away by ice as a top suspect, Manning said. The sudden appearance of 鈥渞usting Arctic rivers鈥� flowing out of regions of thawing permafrost in the last couple of years is likely the same process, magnified.

蜜糖直播 is riddled with patches of bedrock rich in metal sulfides. Shiny iron sulfide, familiar to many Coloradans as fool鈥檚 gold, or pyrite, is the most common of these sulfide minerals, but copper, zinc and other metal sulfides are also common.

Exposure to air oxidizes the metal sulfides in bedrock, releasing the metals into groundwater, which flows into surface streams. Rusty red deposits in streambeds are distinctive signs of iron sulfide oxidation. Sulfides also acidify the water, which can accelerate weathering. Some alpine streams sampled were found have a pH as low as 3 or 4.

The study drew on 40 years of water chemistry data, taking final samples from all sites in 2021, from 22 headwater streams in 17 watersheds that are naturally acidic and metal-rich enough to limit aquatic plants and animals. Sampling sites were above 3,000 meters (10,000 feet) elevation and included a mix of pristine, untouched areas and places that had been mined historically, but left alone for 50 to 100 years.

鈥淭he key point is no recent mining or remediation work has been done,鈥� Manning said. 鈥淭hese watersheds have just been sitting there responding to nothing other than the climate.鈥�

Warming, drying mountains

I have been studying these increases in zinc and decreases in pH in the upper Snake River near Dillon for more than three decades and have suspected that similar changes have been occurring across the 蜜糖直播 Mineral Belt. It鈥檚 sobering to have found out that indeed this is the case. 
- Diane McKnight

Mountain streams were sampled from mid-July to November, spanning the late summer and fall low-flow period. Long-term records of flow volume from nearby stream gauges show streamflows have been dwindling with warming temperatures and smaller snowpacks, suggesting smaller water volumes could explain the higher metal concentrations.

But Manning and his colleagues found less water could only account for half the effect they observed. To reach the concentrations they were seeing, the mountains had to be putting metals and sulfate into streams at a faster rate.

As these metal-rich mountain streams flow down into larger rivers, the effect of the extra metal load is diluted, the researchers noted.

鈥淚 don鈥檛 think this is a big red flag for major metropolitan or agriculture users way downstream at lower elevations,鈥� Manning said, 鈥渂ut some of our mountain communities get their water only a short distance down from these mineralized streams.鈥� To help mitigate the water quality risk, managers could benefit from advance knowledge of what metals are entering the stream, and where and how fast they are increasing, Manning said.

More metals and acidity in these mountain streams could also impact decisions about where to invest limited funds for remediation of those that have been altered by historical mining, and where to stock fish to benefit tourism.

Local case, global pattern

蜜糖直播鈥檚 watersheds are a dramatic case because of the unusual abundance of bedrock metal sulfides, Manning said, but scientists are observing more subtle rising sulfate concentrations in mountain streams around the world. The new study is the first to statistically connect accelerated sulfide weathering to rising temperatures on a large scale across an entire region.

鈥淚 have been studying these increases in zinc and decreases in pH in the upper Snake River near Dillon for more than three decades and have suspected that similar changes have been occurring across the 蜜糖直播 Mineral Belt," says study coauthor Diane McKnight, an INSTAAR fellow and Distinguished Professor in Engineering. "It鈥檚 sobering to have found out that indeed this is the case. But it is also important for understanding how to move forward with planning for remediation of abandoned mine sites and for protecting downstream ecosystems.鈥�

The study found the biggest gains in metal loads in the highest, coldest mountain streams. Manning said this pattern points to thawing underground ice. 蜜糖直播鈥檚 highest elevations have annual average temperatures close to zero degrees Celsius (32 degrees Fahrenheit), putting them right at the boundary conditions for permafrost. Some peaks have warmed past the freezing threshold since 1980.

鈥淚ce is like armor. Melt it and you create windows for groundwater to get into rock that has not seen water and oxygen for millennia, and it will begin to oxidize quite quickly,鈥� Manning said.

Other possible mechanisms are falling water tables exposing fresh rock to air and melting rock glaciers releasing pockets of concentrated metals stored in the ice. Wetlands accumulate metals and may release a burst when water returns after dry periods. The study did not find a correlation between rates of rising metal concentrations and the presence of wetlands, rock glaciers or factors linked to falling water tables, although these could be playing a role in other regions. But all these possible mechanisms are consequences of climate change.

鈥淭here鈥檚 just no other logical explanation than this is a changing climate signal,鈥� Manning said. 鈥淣othing else would reach all these watersheds universally.鈥�

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A researcher brought her love of birds to a watershed in Summit County, 蜜糖直播, affected by acid mine drainage. Recently published in the journal Diversity, connects some dots between water quality, life in streams, and life on land in the granitic landscape of the West.

Former 蜜糖直播 graduate student Kelly Watson is lead author of the study with Professor and INSTAAR Fellow Diane McKnight. Based on her master鈥檚 degree research, the study aimed to see if breeding birds were avoiding streams with high metal concentrations.

The study found that, despite an order of magnitude increase in metal concentrations in some streams, birds did not seem to be avoiding them.

A stream in the Snake River watershed flows over rocks with a red coating, the result of high metal concentrations in the stream.

Diane McKnight has led investigations of water quality and stream ecology in 蜜糖直播鈥檚 Snake River watershed for decades. The watershed has a section with pristine streams, and a section with streams impacted by acid mine drainage.

Acidic drainage can happen wherever geology puts sulfide-based minerals together with air and water. But this natural weathering process is greatly intensified by mining activities that expose more rock. As a result, sulfuric acid and dissolved metals, like iron, drain into streams, where the more acidic water can further dissolve heavy metals and rare earth elements. Warmer, earlier summers are causing longer, lower stream flows, making it easier for metals to leach into streams.

鈥淎cid mine drainage is a source of water pollution in the Rocky Mountains and around the world,鈥� said Watson. 鈥淲e know it has a lot of impacts on organisms living in the stream.鈥�

McKnight has conducted research on many aspects of water quality in affected streams, as well as how it affects life within streams. The acidity of the water controls which species can live in it. For instance, the nymphs and larvae of mayflies populate the pristine streams of the Snake River watershed, but only the acid-tolerant larvae of stoneflies live in streams affected by acid mine drainage. Streams with particularly high metal content may develop a coating on their beds that inhibits insect and other life.

Relatively little is known, however, about how elevated metals and rare earth elements may affect terrestrial species living near polluted streams.

Then Watson joined her research program. Watson 鈥渉ad studied birds as an undergraduate at the College of William & Mary,鈥� said McKnight. 鈥淪he wanted to work with birds for her master鈥檚 thesis.鈥� It was an opportunity to investigate links between stream chemistry and terrestrial life in the Snake River watershed.

鈥淪omething to consider is, what should we be looking at when we remediate mine drainage sites?鈥� said Watson. 鈥淲e鈥檝e known for a long time that benthic macroinvertebrates are indicators of stream health. What about species that live on land around the streams?鈥�

McKnight and Watson designed a research project to look at water quality, insect and other life in the streams, and birds breeding in the watershed. Were birds avoiding sites next to streams affected by acid mine drainage?

Because the insects and other invertebrates living in acidic streams accumulate metals in their bodies, there is concern that birds or fish that use them as a food source might also start to accumulate metals. There was some evidence of concern from a previous study in the area, which found elevated metal concentrations in tree swallow nestlings. But little was known about potential effects on migratory birds that breed in the area.

鈥淎s an undergraduate, I was part of a bird monitoring program in Virginia,鈥� said Watson, who is now the Principal Floodplain Planner for Boulder County. 鈥淚 learned how to do point counts and run the occupancy models we used in this study.鈥�

The takeaway is that that birds that live by the stream are probably fine even though metals in the water have gone up by an order of magnitude.
- Diane McKnight

Watson worked together with Stella Koliavas, a student in the Undergraduate Research Opportunities Program (UROP), to conduct bird counts at twenty study sites along the main streams of the Snake River watershed during breeding season. They observed robins, dark-eyed juncos, Wilson鈥檚 warblers, Lincoln鈥檚 sparrow, mountain chickadees, ruby-crowned kinglets, white-crowned sparrows, and yellow-rumped warblers at many of the sites.

She also took samples of the water and macroinvertebrates living in the stream and analyzed them for concentrations of trace metals and rare earth elements. She then used a computer model to relate the presence of birds to various types of habitat and conditions, and to the potential for trace metal and rare earth element exposure.

The most acidified stream was about the acidity of soda or grapefruit juice, and showed more dissolved metals and very little resident life. Stonefly larvae dominated the acidified streams, which were about as acid as a cup of black coffee; while mayflies were the most abundant macroinvertebrate in the pristine stream, with a neutral pH. Overall, the water quality was similar to that found in many prior studies of the same watershed, though quantities of metals and rare earth elements in the streams are generally increasing.

Watson found that birds were found throughout the watershed area. The only pattern in their residency seemed to be habitat鈥攁 few birds are 鈥渉abitat specialists鈥� who nest in shrubby environments rather than forest. Habitat, but not water quality or invertebrate prey quality, is an important factor for these species during the breeding season.

The study also showed that the relationship between the amount of metals in a stream and the amount of metals accumulated in the bodies of insects living in the stream is not straightforward. This may affect how many metals birds ingest if they prey on the insects. While the species present in the streams are controlled by the acidity of the water, insects鈥� metal accumulations seem more related to what they eat and whether those food sources originate in the stream or out of it.

Watson and McKnight recently adapted Watson鈥檚 master鈥檚 thesis into the journal article. 鈥淚 have a full-time day job, so on nights and weekends, Diane and I would collaborate and work on revisions,鈥� said Watson.

鈥淭he takeaway is that that birds that live by the stream are probably fine,鈥� said McKnight, 鈥渆ven though metals in the water have gone up by an order of magnitude.鈥�

鈥淭he reason that鈥檚 interesting is because the stream conditions are getting worse,鈥� she said. While the study was not designed to show any possible negative effects of birds breeding near acidic streams, the birds may be an example of resilience in a changing climate.

Another aspect of the study was finding that the concentrations of some rare earth elements in invertebrates were high and aligned with concentrations of lead. There is little information about rare earth elements in aquatic organisms, and this finding might point the way to a need for further research on how metals and rare earth elements are transferred through ecosystems.

鈥淭his study indicates further inquiry on how metals may or may not be moving into terrestrial ecosystems is needed,鈥� said Watson.

McKnight will give a free public talk on the study at the Mountain Research Station, near Nederland, 蜜糖直播, on Wednesday June 28. All are invited.

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