Old Oil Wells Damage Stream Ecosystems More Than Fracking in Pennsylvania's Waterways

Syracuse University

The public debate over oil and gas drilling and water quality has focused heavily on hydraulic fracturing. Fracking is newer, more visible, and more controversial. But in Pennsylvania, where conventional oil wells have been operating for over a century alongside the modern shale gas boom, the bigger threat to stream biodiversity may be the infrastructure nobody talks about.

Pennsylvania as a natural experiment

Pennsylvania is uniquely positioned for this kind of analysis. It has one of the longest histories of conventional oil and gas drilling in the world, with some wells dating back more than 100 years. It simultaneously sits at the center of the Marcellus Shale gas boom that began in the mid-2000s. And it maintains a strong, long-running stream biological monitoring program that provides decades of high-quality ecological data.

"What makes this moment special is that we now have decades of high-quality biological monitoring data available," said Tao Wen, an assistant professor in the Department of Earth and Environmental Sciences at Syracuse University and a co-leader of the study, published in ACS ES&T Water. "That gave us a rare opportunity to step back and ask, what has all of this development meant for stream life at a statewide scale?"

Reading stream health through bottom-dwellers

The research team, led by Ryan Olivier-Meehan at Syracuse with collaborators at UCLA, Carnegie Institution for Science, and the University of Colorado Boulder, analyzed more than 6,800 benthic macroinvertebrate samples collected from streams across Pennsylvania. Benthic macroinvertebrates, the insect larvae, small crustaceans, and worms that live on stream bottoms, serve as sensitive biological indicators of water quality. They live in the water year-round, continuously exposed to local conditions. When conditions deteriorate, pollution-sensitive species disappear and are replaced by hardier, more tolerant ones.

"By looking at the community as a whole, we get a long-term picture of stream condition, not just a snapshot of water chemistry on a single day," said Olivier-Meehan. These organisms also form the base of the aquatic food web, so disruptions to their communities can cascade upward through the entire ecosystem.

Conventional wells leave the deeper mark

The statewide patterns were clear. Conventional oil and gas development was linked with fewer species, less diversity among them, and an overall decline in ecosystem health. Communities of aquatic organisms near conventional development shifted toward hardy, pollution-tolerant species, a classic sign of ecological degradation and reduced ecosystem resilience.

Shale (unconventional) development showed detectable but more limited effects on the same measures.

"Public debate often centers on shale gas because it's newer and more visible. Our results show the story is more nuanced," Olivier-Meehan said. "In Pennsylvania, conventional drilling, much more widespread and often decades old, was more strongly associated with declines in stream biodiversity."

Why old infrastructure does more damage

Several factors likely explain the disparity. Conventional wells in Pennsylvania include many that were drilled decades ago under older, less stringent environmental regulations. Abandoned and orphaned wells, those with no responsible owner, number in the hundreds of thousands across the state. These aging wells can leak brine, heavy metals, and hydrocarbons into surrounding waterways for decades after active production ceases.

The sheer density of conventional wells also matters. Pennsylvania has far more conventional wells than unconventional ones, and their cumulative footprint on the landscape, including access roads, well pads, and associated infrastructure, is correspondingly larger.

Modern shale development, while not impact-free, operates under stricter regulations, uses more contained waste management practices, and benefits from decades of environmental oversight improvements that older conventional operations predate.

Caveats and complexity

The researchers emphasize that the study does not give shale development a clean bill of ecological health. "Environmental risk reflects the age and density of infrastructure along with regulatory oversight and landscape factors," they note. A newer, well-regulated conventional well might have minimal impact, while a poorly managed shale operation in a sensitive watershed could cause significant damage.

The study's observational design also means it identifies associations rather than proving causation. Streams near conventional wells may differ from streams near shale wells in geology, land use, agricultural activity, and other factors that independently affect water quality. The researchers used modeling and network analysis to disentangle these influences, but residual confounding cannot be entirely eliminated.

The biological monitoring data, while extensive, was not originally collected for this purpose. Sampling locations were not chosen to optimally compare conventional and unconventional drilling impacts, which introduces potential spatial biases.

Beyond Pennsylvania

The implications extend beyond one state. "While our study focuses on Pennsylvania, many other states and countries have similar histories of conventional oil and gas development," Wen said. "The broader message, that legacy infrastructure can have lasting ecological effects, likely applies elsewhere."

The team's next steps include examining how outcomes vary based on the density of inactive, abandoned, and orphaned wells, their proximity to streams, and local geology. They also plan to expand the analysis to other regions with similar histories of layered energy development.

"Our goal is to help communities make informed decisions that balance energy needs with environmental protection," Wen said. "Good long-term monitoring lets us move beyond assumptions to evidence-based conversations about sustainability."

For policymakers, the practical takeaway is that the environmental legacy of past energy development deserves as much attention as the footprint of current operations. The wells that nobody is watching may be doing more damage than the ones that dominate headlines.