Natural gas has likely seeped into Northeast Pennsylvania water supplies from both deep gas drilling and natural processes, researchers at Duke University reported in a paper released Monday.
The article published in the journal Proceedings of the National Academy of Sciences found that methane, ethane and propane concentrations were higher in drinking water wells located within a kilometer of a gas well than in more distant water supplies.
The researchers detected methane in 82 percent of 141 water wells and ethane in 30 percent of 133 sampled wells, but methane levels were six times higher and ethane levels were 23 times higher on average in the water supplies close to gas wells.
The study is an expansion of a paper the researchers published in 2011 using 60 water wells, which was criticized for lacking information on methane concentrations in wells before drilling started, among other issues.
This time the scientists used multiple tools, including studying carbon, hydrogen and helium isotopes, propane and ethane concentrations and the ratios of gases, to help distinguish the source of gas in water wells in Bradford, Lackawanna, Sullivan, Susquehanna, Wayne and Wyoming counties. They found evidence that some water wells may have been contaminated by gas from the Marcellus Shale while others showed signs of gas from shallower rock layers or a mixture of gas types that might come from either gas drilling or natural pathways.
"Our research strongly suggests contamination with stray gas for a subset of homeowners," said co-author Robert Jackson, a professor of environmental sciences at Duke.
"Not everyone has a problem," he said. "The emphasis should be on how to keep the problems that we've seen from happening in other places."
The study is the newest entry in unfolding and sometimes contradictory research into the prevalence and cause of natural gas in Northeast Pennsylvania water supplies.
Regulators have determined that poorly constructed natural gas wells caused high levels of methane to seep into drinking water wells in parts of Susquehanna and Bradford counties and the state revised its drilling laws and regulations to try to prevent future problems. In other cases, including a high-profile investigation in Franklin Twp., the Department of Environmental Protection determined that high methane levels in water wells could not be attributed to natural gas drilling activity.
Researchers have also detailed the widespread natural occurrence of methane in the region's water wells in places where drilling had not yet begun.
Last week, the U.S. Geological Survey released a study of 20 randomly selected water wells in an area of Sullivan County without shale drilling and found that seven contained some naturally occurring dissolved methane with one containing enough of the gas - 51.1 milligrams per liter - to create a potential explosion risk. The methane in two wells was thermogenic - gas that comes from deep underground, not from the breakdown of biological matter near the surface.
A recently updated study by Cabot Oil and Gas Corp. contractors and employees found detectable levels of methane in 78 percent of 1,700 water wells it sampled before drilling nearby in Susquehanna County, but only 3.4 percent of those samples exceeding the state's action level of 7 milligrams per liter for dissolved methane. It also found that high methane concentrations in water wells generally correspond to how close they are to valleys, not their proximity to gas wells.
The Duke study released Monday reaches a different conclusion: The researchers found that distance to gas wells was "highly significant" in influencing the amount of methane in water wells and was "the only statistically significant factor" influencing ethane, while distance from a valley bottom or a geological deformation called the Appalachian Structural Front were less or insignificant.
Thermogenic gas does occur naturally in areas without shale gas development, the researchers found. In fact, "most of the natural methane" in the sampled wells "is thermogenic," Dr. Jackson said. But the combination of multiple tools helped the researchers distinguish between natural seeps and man-made problems.
Dr. Jackson said cases of contamination by Marcellus Shale-like gas are likely caused by structural problems with the steel casings used to keep gas in a well from seeping into aquifers, while contamination by shallower thermogenic gas is likely tied to failures in the cement that is meant to keep gases from migrating between casings or up the outside of wells.
The research did not find evidence of contamination from the gas extraction process of hydraulic fracturing, or fracking.
Fred Baldassare, the state's former stray gas inspector who now consults for industry and other clients with his firm ECHELON Applied Geoscience Consulting, said the most notable aspect of the Duke paper is the identification of "post-mature" thermogenic gas - deeply buried gases similar to those produced from the Marcellus Shale - in some of the sampled water wells. Most cases of drilling-related methane contamination have traced the thermogenic gas in water wells to shallower rock layers, not the target shale.
But he said that the Duke researchers picked water wells to sample in areas with known drilling-related gas contamination so it would be wrong to conclude from their data that water wells in close proximity to gas wells are likely to be tainted by gas.
"That's not taking into consideration the thousands of gas wells that have been drilled in that area," he said. "The percentage that don't have gas well migration incidents is far higher than those that do. If you read their paper you wouldn't know that."
Industry groups criticized the study for an apparent lack of random sampling and pointed out that measurable concentrations of methane were found in more than 50 sampled water wells that were not close to gas wells.
"Not exactly a smoking gun," Energy in Depth spokesman Steve Everley wrote in a blog post on the industry group's website.
Contact the writer: llegere@timesshamrock.com