Abandoned coal mine drainage significant source of carbon emissions

Currently unaccounted for, unremediated discharge from old mines can transport CO2 from bedrock to the atmosphere

News Release 31-Oct-2025

Geological Society of America

Contributed by Rudy Molinek, GSA Science Communication Fellow

Boulder, Colo., USA: For the past 250 years, people have mined coal industrially in Pennsylvania, USA. By 1830, the city of Pittsburgh was using more than 400 tons of the fossil fuel every day. Burning all that coal has contributed to climate change. Additionally, unremediated mines—especially those that operated before Congress passed regulations in 1977—have leaked environmentally harmful mine drainage. But that might not be the end of their legacy.

In research presented last week at GSA Connects 2025 in San Antonio, Texas, USA, Dr. Dorothy Vesper, a geochemist at West Virginia University, found that those abandoned mines pose another risk: continuous CO2 emissions from water that leaks out even decades or centuries after mining stops. 

In a 2016 study, Vesper and her collaborators found that the water draining from just 140 of these mines in Pennsylvania add as much CO2 to the atmosphere each year as a small coal-fired power plant. Considering officials and scientists don’t know the number of abandoned mines in Pennsylvania, let alone elsewhere, the impact of these mines is an unresolved and important part of understanding the sources of human-caused climate change.

“We would like to have a much better handle on how big” these carbon emissions are, says Vesper. “A huge part of it is just not even knowing where the discharges are. And it’s not just Appalachia. It's all over the country. It's all over the world, really, these mine waters.”

The mine waters, loaded with sulfuric acid as a byproduct of the coal’s geology, break down carbonate rocks like limestone that is associated with the coal seams. Part of that rock is ancient CO2 that was locked away when the rock formed millions of years ago. The acidic water dissolves the carbonate rocks, releasing carbonate ions (CO3) that then convert to CO2 or other forms of carbon in the water. Once the discharge leaves the mine and is exposed to air, any CO2 present in the water can “degas” and be released into the atmosphere.

Prior to Vesper’s work, CO2 emissions from degassing of mine drainage hadn’t been extensively quantified. Part of the problem is the sheer number of abandoned mines and the fact that they aren’t well catalogued. Often, Vesper and her students would tromp through the woods to measure a reported mine and find no trace of the opening or that the discharge no longer flowed. 

Another problem is that standard field instruments can’t measure extremely high concentrations of CO2, and Vesper has found that some mine drainage contains up to 1,000 times more CO2 than would be expected in normal water. To make her measurements, then, Vesper had to turn to an unexpected source for a measuring device.

“It’s basically out of the soda industry. Bottling plants and breweries have them,” says Vesper. The beverage instrument is “designed to be carried around the brewery floor and connect to these giant vats. So, it's really portable, and it can handle really high CO2.”

Specialized instrument in hand, Vesper, along with her students and collaborators, track down old mines to measure the CO2 being carried out through water drainage. The results from some mines were comparable to the CO2 given off by hydrothermal springs, and vastly higher than water draining from typical natural limestone caves. Further, the amount of CO2 at each site changed through time dependent on hydrologic conditions around the mine.

In the future, Vesper hopes to measure more mines for longer periods and in different conditions, add methane to her suite of analyses, and explore how different remediation techniques might prevent the CO2 from being released to the atmosphere and contributing to climate change.

“I think that even just small things in remediation design could make a difference, like keeping the discharge underground in pipes and introducing it to treatment wetlands from the subsurface,” says Vesper. “Then you're fine. It's not going to degas in the environment as easily.”

CO2 Releases from Abandoned Coal Mines in Pennsylvania and West Virginia