Abandoned Coal Mines Are Secretly Releasing Massive Amounts of Carbon Into the Air

For over 250 years, Pennsylvania has been a major hub for coal mining. By the 1830s, Pittsburgh alone was burning more than 400 tons of coal per day, fueling the industrial age and, unfortunately, the rise of climate change. But the real story doesn’t end when the mining stops. Recent research reveals that abandoned coal mines are quietly continuing their damage—this time by leaking carbon dioxide (CO₂) into the atmosphere through contaminated water known as mine drainage.

At the GSA Connects 2025 conference in San Antonio, geochemist Dr. Dorothy Vesper from West Virginia University presented findings that shed light on a hidden climate threat lurking beneath our feet. Her research suggests that mine drainage from old, unsealed coal mines can emit significant levels of CO₂ for decades or even centuries after mining operations have ended.

The Hidden Carbon Source Beneath Abandoned Mines

Coal mining, especially before 1977 when environmental regulations were introduced under the Surface Mining Control and Reclamation Act, left behind a network of unmonitored tunnels and pits. These abandoned mines continue to fill with groundwater, which reacts with the minerals in the surrounding rock. Over time, this creates acidic water rich in dissolved minerals—a phenomenon known as acid mine drainage (AMD).

But what Dr. Vesper’s research uncovered goes beyond the typical pollution concerns of AMD. Her studies show that these waters carry enormous amounts of dissolved carbon dioxide. In fact, the CO₂ levels are sometimes up to 1,000 times higher than what’s typically found in normal surface water.

In a 2016 study, Vesper’s team estimated that drainage from just 140 abandoned coal mines in Pennsylvania emitted as much CO₂ annually as a small coal-fired power plant. Since Pennsylvania has thousands of undocumented or forgotten mine sites, the total emissions could be far higher than anyone has realized.

How Acidic Water Produces Carbon Emissions

The process begins when sulfuric acid, formed by the oxidation of sulfide minerals like pyrite, seeps through the mine’s geological layers. This acid reacts with carbonate rocks, such as limestone or dolomite, that are often found alongside coal seams.

When the acid dissolves these rocks, it releases carbonate ions (CO₃²⁻) and dissolved inorganic carbon (DIC). As the water exits the mine and reaches the surface, it encounters the open air. The carbon in the water then de-gasses, meaning it escapes as CO₂.

What’s particularly striking is that this CO₂ isn’t newly created—it comes from ancient carbon locked away millions of years ago when the rocks first formed. Mining effectively reopens that geological vault, setting that carbon free once more.

Measuring the Unmeasurable

One of the biggest challenges for scientists has been accurately measuring CO₂ in mine drainage. Most field instruments can’t handle such high concentrations, and traditional sampling methods often underestimate the true amount of dissolved gases.

To overcome this, Vesper’s team got creative. They turned to an unexpected tool: a carbonation meter used in the soda and brewing industry. This portable device, originally designed to test CO₂ levels in beverages, could withstand extreme concentrations and acidic conditions.

Armed with this unusual gadget, Vesper and her students trekked through forests and old mining regions across Pennsylvania and West Virginia, often searching for discharges that had been forgotten for decades. Some mine outflows showed CO₂ concentrations comparable to hydrothermal springs, far exceeding normal natural water systems.

Their fieldwork also revealed that CO₂ emissions fluctuate with seasonal hydrology—for instance, water levels and rainfall patterns influence how much CO₂ escapes at different times of the year.

Estimating the Scale of the Problem

Quantifying how much carbon is being released is complex. In one recent assessment, Vesper’s team analyzed 25 drainage sites and found that collectively, they released around 6 tons of carbon per year directly into the atmosphere and exported another 8 tons per year in dissolved bicarbonate form through flowing water.

When these findings were scaled up to the entire bituminous coal region of Pennsylvania, the total emissions were estimated at around 14,000 tons of carbon per year. That’s equivalent to tens of thousands of gallons of gasoline being burned annually—just from water seeping out of the ground.

And remember, this is likely an underestimate. Thousands of discharges remain unmonitored, and many old mines are undocumented, especially those dating back to the 19th century.

Can This Be Fixed?

Dr. Vesper believes remediation could make a meaningful difference. She suggests relatively simple design changes could reduce emissions. For example, keeping the discharge underground in pipes or introducing it directly into treatment wetlands below the surface can limit its exposure to air—preventing much of the CO₂ from degassing.

She’s also exploring the role of other gases like methane (CH₄) in mine drainage, which could make the total greenhouse gas output even more significant. Future research will focus on long-term monitoring across different seasons and mine types, aiming to refine estimates and identify the best mitigation strategies.

Why This Matters

While we often associate mining’s environmental impact with the burning of fossil fuels, this discovery highlights a non-combustion source of emissions—a geochemical process triggered by mining that continues long after the last shovel of coal was removed.

Globally, the implications are big. Similar mine drainage issues exist in Europe, China, Australia, and South Africa, where centuries of coal mining have left extensive underground networks. If these sites are emitting CO₂ in a similar way, then abandoned mines could represent a global blind spot in the carbon cycle.

Climate models and national carbon inventories typically don’t include emissions from mine drainage, meaning our total human-caused CO₂ emissions might be slightly underestimated. Recognizing and quantifying these “legacy emissions” could be crucial for understanding our full climate impact and designing better strategies for carbon neutrality.

Beyond Coal: Acid Mine Drainage Around the World

Mine drainage isn’t unique to coal—it also affects metal mines that extract copper, gold, or zinc. The same oxidation processes create acidic runoff that leaches metals and minerals into rivers and groundwater. These discharges can devastate ecosystems, lower pH in streams, and harm aquatic life.

In the U.S. alone, the Environmental Protection Agency (EPA) estimates that more than 20,000 kilometers of rivers and streams are affected by acid mine drainage. Globally, it’s one of the most persistent forms of industrial pollution, often lasting centuries after mining ends.

But what’s new about Vesper’s research is her focus on the carbon side of the equation. While acid mine drainage has long been recognized as an environmental hazard, its role as a carbon emitter has largely been ignored. Now, it’s becoming clear that these seemingly quiet trickles of orange water may be contributing more to climate change than anyone expected.

The Road Ahead

Vesper’s research emphasizes the importance of mapping and cataloguing old mine sites to identify where these emissions are happening. Without knowing where the discharges are, it’s nearly impossible to estimate their collective effect on the climate.

Her team’s work represents an early but essential step toward quantifying this hidden source of greenhouse gases. As she continues expanding her monitoring efforts, adding methane analysis, and experimenting with remediation designs, scientists are beginning to see just how complex and interconnected our legacy with fossil fuels truly is.

Coal powered the industrial revolution—but even its leftovers are still heating the planet.

Research Reference:
CO₂ Releases from Abandoned Coal Mines in Pennsylvania and West Virginia