Pennsylvania’s coal story didn’t end when the last shifts clocked out. After 250 years of industrial mining – Pittsburgh alone was burning more than 400 tons a day by 1830 – the region is dotted with abandoned workings that still leak acidic water.

New research presented at GSA Connects 2025 in San Antonio adds a twist. Drainage from abandoned coal mines can carry massive loads of carbon dioxide, quietly venting to the air decades or even centuries after mining stops.

Geochemist Dorothy Vesper from West Virginia University (WVU) has been chasing these flows for years.

In a 2016 study, she and colleagues estimated that drainage from just 140 abandoned Pennsylvania mines releases as much CO2 annually as a small coal-fired power plant.

The truth is, no one even knows how many abandoned mines exist in the state, let alone nationwide. That makes the climate tally incomplete.

“We would like to have a much better handle on how big these carbon emissions are,” Vesper said. “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.”

Most public discussion of mine drainage focuses on orange streams, poisoned trout, and corroded infrastructure.

Vesper’s work shows there’s also an invisible plume: CO2 degassing from water as it leaves mine portals and seeps into creeks. The leaks are continuous, not episodic. That makes them a quiet, persistent climate source.

Acidic mine water turns rock into gas

The chemistry starts underground. Many coal seams are accompanied by sulfide-bearing minerals. When mining exposes those minerals to oxygen and water, they generate sulfuric acid.

The resulting acidic water moves through limestone and other carbonate rocks that were laid down millions of years ago, back when the carbon they contained was part of the atmosphere.

Acid attacks carbonate. As the water dissolves limestone, it picks up carbonate ions that are transformed into dissolved inorganic carbon, including CO2. Inside the mine, water can hold a surprising amount of that gas.

Once the discharge hits daylight and mixes with air, the excess CO2 escapes. The process unlocks ancient, geologically stored carbon and sends it skyward – long after boilers went cold.

Before Vesper’s studies, few had tried to quantify this degassing. That’s partly because the targets are slippery.

Abandoned mines aren’t well cataloged, and reported portals often turn out to be dry or lost to time. Fieldwork can mean hours of bushwhacking to a dot on an old map, only to find nothing.

Measuring carbon from coal mines

Another hurdle is hardware. Off-the-shelf environmental probes choke at the extreme CO2 levels found in mine water.

Some sites carry concentrations up to a thousand times what you’d expect in normal springs or streams. Vesper’s fix came from an unlikely place.

“It’s basically out of the soda industry. Bottling plants and breweries have them,” Vesper said. 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.”

Armed with that meter, Vesper’s teams have logged mine discharges that mimic hydrothermal vents in their CO2 load, far exceeding values seen in ordinary limestone cave waters.

The signal isn’t static. Concentrations rise and fall with rainfall, groundwater levels, and seasonal changes that alter how much water moves through mine workings and surrounding rock. Each site becomes a living gauge of local hydrology and geochemistry.

From field notes to climate math

Scaling up remains the central challenge. A handful of instrumented drains can’t represent tens of thousands of unknown sites scattered across old coal country and beyond.

Yet the physics is consistent: more acidity, more carbonate rock, more contact time, more dissolved carbon. That gives modelers a path forward if they can map where the ingredients co-occur.

Policy also matters. The U.S. tightened mine reclamation in 1977, but many older mines were never sealed or treated. Those legacy discharges are the worst offenders.

Adding CO2 to the list of harms strengthens the case for finding and fixing them, even when the water looks clear.

Locking carbon in old coal mines

Vesper’s group is now testing ways to keep CO2 from reaching the air at all. Some solutions are surprisingly simple: move the discharge out of the open.

“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,” Vesper said. “Then you’re fine. It’s not going to degas in the environment as easily.”

Other ideas include promoting in-water mineralization that locks carbon into stable solids before it sees daylight, or routing flows through alkalinity-generating systems that change the carbon chemistry.

The team also plans to add methane to its sampling suite, in case old workings are burping another potent greenhouse gas.

The climate legacy of coal

Coal’s climate legacy was never just at the smokestack. Vesper’s measurements suggest abandoned mines are still part of the problem, quietly exhaling carbon long after the last cart rolled out.

Counting those emissions will take better maps, longer monitoring, and some ingenuity in the field. Cutting them will take modest engineering and steady funding.

In a warming world, that’s an opportunity hiding in plain sight. Treat the water, keep the gas from escaping, and you turn a lingering liability into a manageable fix – one that cleans streams and shaves a little off the atmospheric tab at the same time.

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