Weathering rates for mined lands exponentially higher than unmined sites

September 25, 2018 by Mary Guiden, Colorado State University
Weathering rates for mined lands exponentially higher than unmined sites
Credit: Matthew Ross, CSU

Mountaintop removal, a coal-mining technique used in much of Central Appalachia, is an extreme form of surface mining, that excavates ridges as deep as 600 feet—twice the length of a football field—and buries adjacent valleys and streams in bedrock and coal residue. This mining activity has long been known to have negative impacts on water quality downstream.

A new study led by watershed scientist Matthew Ross at Colorado State University found that many of these water quality impacts are caused by a dramatic increase in the chemical weathering rates of mined landscapes, which are melting away bedrock up to 45 times faster than unmined areas. In addition, the weathering has global consequences for the cycling of sulfur, which is a key nutrient for all life forms.

The findings show that when people move large quantities of bedrock and soil to build cities or to extract resources, they can completely alter and accelerate the natural weathering processes on land, which can impact water quality downstream.

Ross, an assistant professor in the Department of Ecosystem Science and Sustainability, described the chemical weathering rates as one of the highest rates ever observed, when compared to landscapes across the globe.

The study, "Pyrite oxidation drives exceptionally high weathering rates and geologic CO2 release in mountaintop-mined landscapes," was published in the journal Global Biogeochemical Cycles.

Carbon cycle disrupted

This increased weathering—like many mine-related impacts ­— starts when iron sulfide or pyrite, a mineral also known as fool's gold often found in coal, is exposed to air. This creates sulfuric , making water draining from the mine extremely acidic and caustic. To neutralize the acid, in much of Central Appalachia the pyrite-bearing rock is intentionally surrounded by and mixed with carbonate rocks.

While this limits acid-mine drainage problems, these acid-producing and -neutralizing reactions create ideal conditions for rapid chemical weathering of bedrock, with surprising implications for geologic carbon cycling of these landscapes.

In most areas that experience , carbon dioxide dissolves into carbonic acid, a weak weathering agent. When carbonic acid reacts with silicates or rock-forming minerals, carbon dioxide is permanently locked into the bedrock, balancing the carbon cycle over millions of years. In unmined landscapes, this process provides a slow but inevitable sink for , or CO2.

In mined landscapes with abundant sulfuric acid, the weathering reactions no longer rely on , and the potential for geologic carbon sequestration is eliminated. Instead, the sulfuric acid weathers out acid-neutralizing carbonates, which releases carbon dioxide into the atmosphere.

This means that long after mining in these areas has stopped, researchers estimate that between 20 percent and 90 percent of the carbon absorbed by plants on the surface will be cancelled out by the release of rock carbon to the atmosphere.

"Because this is happening so fast and it is powered by , it creates a that is a source for ," Ross said. "You're rapidly dissolving away the landscape and releasing a bunch of rock ."

This regional impact also has global consequences for the cycling of sulfur, an element that is important for all life forms. While mountaintop mining operations in Appalachia cover a small portion, .006 percent, of the land area on Earth, they may contribute as much as 7 percent of the total global delivery of sulfur from land to ocean.

This research, funded by the National Science Foundation, is part of an ongoing project led by Ross, who recently joined the faculty at the Warner College of Natural Resources.

Explore further: Thawing permafrost may release more CO2 than previously thought, study suggests

More information: Matthew R. V. Ross et al, Pyrite oxidation drives exceptionally high weathering rates and geologic CO2 release in mountaintop-mined landscapes., Global Biogeochemical Cycles (2018). DOI: 10.1029/2017GB005798

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3 comments

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carbon_unit
4 / 5 (4) Sep 25, 2018
These mineral companies are locusts. Profit by tearing up the landscape over a large region then move on, leaving a mess which future generations will have to deal with into the distant future. Aren't externalities great!
JamesG
1.8 / 5 (5) Sep 25, 2018
So land that's been torn all to pieces wears away at a higher rate than land that's still intact and has all it's natural pieces packed together. Wow. That's astounding.
rrwillsj
4.5 / 5 (2) Sep 29, 2018
Oh JG, what is amazing the is the sheer volume of toxic wastes flowing downstream from the coal fields during and after Hurricane Florence!

Let's face it. If you are incapable of learning from bitter experience? You are just a hopeless twit.

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