Defunct Pennsylvania oil and gas wells may leak methane and metals into water

In the dense forests of northwestern Pennsylvania, hundreds of thousands of retired oil and gas wells—some dating back to the mid-1800s, long before modern construction standards—dot the landscape, according to geochemists in Penn State's College of Earth and Mineral Sciences who recently led a study in the region. Left uncapped and exposed to air and erosion, they break down, leaching harmful chemicals into the atmosphere and, the researchers reported, into the groundwater.

Led by Susan L. Brantley, Atherton Professor of Geosciences and Evan Pugh University Professor Emerita in the Department of Geosciences and Earth and Environmental Systems Institute at Penn State, the researchers surveyed 18 abandoned wells in and near the Allegheny National Forest and found that they leak methane not only into the atmosphere but also into the adjoining groundwater. Some of the sites' groundwater also exhibited high concentrations of dissolved iron and arsenic.

Using a geochemical computer model, the team found that methane—a powerful greenhouse gas that traps more heat than carbon dioxide—interacted with the rock near wellbores to release metals into groundwater. The researchers published their findings this week (Nov. 1) in Geochimica et Cosmochimica Acta.

"As a greater portion of oil and gas wells worldwide are abandoned and their structural integrity declines, the issue of water quality will grow in importance," Brantley said. "This is because as gas pipes rust and break down, gases infiltrate nearby underground aquifers and can dissolve toxic elements like arsenic into the water."

After identifying the retired wells based on visual evidence of gas leakage, researchers—including a team of undergraduate research assistants from GeoPEERS, part of the Research Experiences for Undergraduates Program—collected 36 samples of water near wellbores and from streams and underground aquifers over the course of between one to seven visits to each site.

Researchers in the Laboratory for Isotopes and Metals in the Environment at Penn State and collaborators at the University of Wisconsin analyzed each sample and identified their unique chemical signatures.

The team found that some of the sampled sites had an abundance of methanotrophs, microorganisms that consume methane, while others had an abundance of methanogens, which generate methane.

Both methanogens and methanotrophs create issues for their surroundings, according to first and corresponding author Samuel Shaheen, an NSF postdoctoral fellow at the University of Minnesota, who completed his doctorate in geosciences in 2024 at Penn State under Brantley.

"Methanotrophs grow and feed off methane, which then dissolve the red iron oxide of the metal pipes or surrounding rock, contaminating the nearby water table with metals like arsenic," he explained. "Methanogens, on the other hand, produce more and more methane, which is also a problem for air pollution."

Shaheen said the team thought the methane produced via natural gas drilling would attract methanotrophs, but they had to reassess once they found more methanogens in some of the sampled areas. Upon further investigation, they found that sites with more methanotrophs had another similarity: they also had high amounts of dissolved metals in the groundwater. One-sixth of the samples were over the Environmental Protection Agency's (EPA) limit for arsenic in drinking water, and over half of the samples were over the EPA's limit for iron in drinking water.

"The truth in nature is that wherever you have microbiology and geochemistry, it is a puzzle—some wells grow methanotrophs and others grow methanogens," Brantley said. "Sam discovered that there was a 'switch' based on the rock in the aquifer and the speed at which the groundwater moves through the system that determined whether these microorganisms produced metal-rich or metal-poor groundwater."

To better understand their field results of different wells producing either methanogens or methanotrophs, the researchers created a geochemical model to simulate how methane migrates through abandoned wellbores into aquifers. The model helped clarify the role of iron and sulfur in interacting with methane to change groundwater chemistry, Shaheen explained.

"Pennsylvania is a powerhouse when it comes to production of hydrocarbon and fuel, but it comes at a toll: There are hundreds of thousands of wells around the state, and some of them leak," Brantley said. "Though the state has been working very hard to plug them, there is no way to get to all of them; there are just too many of them."

However, the researchers noted that though many studies have been conducted on atmospheric emissions, this is one of the first that studied how unplugged wells can pollute groundwater.

"There's a lot of discussion about how we prioritize which wells to plug," Shaheen said. "Until this study, we have had a much less comprehensive picture on the groundwater impacts, which could influence decision-making around well plugging."