This story was updated at 1:12 p.m. EDT.
As Rob Jackson and his fellow researchers delve into concerns about stray natural gas contaminating Pennsylvania drinking water wells, they're finding that not all of it comes from the Marcellus Shale.
Some of it comes from layers of rock between the productive Marcellus gas formation and the surface, formations that companies aren't trying to produce, said Jackson, a biology professor at Duke University.
That gas likely works its way up into aquifers from those middle layers through gaps and cracks in the cement seal that is supposed to encase the steel wellbore near the surface, he said.
Marcellus gas can escape through weak points in the steel pipe between the bottom of the wellbore and the cement seal, he said.
"We have at least two sources of contamination," Jackson said in a phone interview yesterday.
The study, done by Jackson with fellow researchers from Duke University along with the University of Rochester and California Polytechnic State University, was published yesterday in the Proceedings of the National Academy of Sciences.
The peer-reviewed study is intended to build on the team's 2011 report that found a correlation between Marcellus Shale drilling in Pennsylvania and contamination of drinking water wells (Greenwire, May 9, 2011).
It contrasts with a report done on groundwater in the Fayetteville Shale formation in Arkansas for the U.S. Geological Survey by a team that included Jackson and three other researchers from the original study. They found no groundwater contamination associated with gas production there.
For the study released yesterday, the researchers took additional samples and were able to analyze samples for ethane and propane, two gas elements that are not found in shallow, "biogenic" gas.
"They really rule out a biological source," Jackson said.
The study is also a response to an industry study that discounted the Duke findings by deeming methane in the area's drinking water "ubiquitous" (EnergyWire, May 31).
Jackson agrees that methane is commonplace in water from wells in the area of northeastern Pennsylvania where his team sampled. But that doesn't mean all methane in drinking water is natural.
"There's no doubt there are historic cases of people being able to light their water on fire in Pennsylvania before fracking," Jackson said.
But within 1 kilometer, about 3,300 feet, of drilling wells, the study found that the concentration of methane was six times higher than for homes farther away.
The team sampled 81 new drinking water wells in six counties in northeastern Pennsylvania.
It combined the data with results from 60 previously sampled wells in Pennsylvania and included a few wells in New York's Otsego County.
Of 12 houses where the concentration of methane were greater than the federal threshold for immediate remediation, 11 homes were within the 3,300-foot radius. The only exception was a house 1.4 kilometers (4,600 feet) from a well.
The study also found that ethane and propane were more common closer to wells. They are both components of natural gas but are found in the deep "thermogenic" shale gas that drillers want. They are not found in the shallow, "biogenic" gas that is commonly found in well water supplies.
But thermogenic gas can get into water wells even in areas without drilling. A recent U.S. Geological Survey study of 20 water wells done in Pennsylvania's Sullivan County (west of Scranton) in advance of drilling found thermogenic gas in two of the wells. Ethane was also found, but in much smaller concentrations than what the Duke team found near drilling wells.
The team's 2011 study was criticized by industry and state regulators. And the industry group Energy in Depth quickly jumped to criticize the new study yesterday.
EID, a campaign of the Independent Petroleum Association of America, said the USGS study contradicts the new study, questions whether the team intentionally sought out wells where there were allegations of contamination and said the study was funded by the anti-drilling Park Foundation.
"So, in the end, the researchers found methane in virtually every water well they sampled, irrespective of proximity to a natural gas well," EID's Steve Everley wrote in a blog post.
Jackson said the study was not funded by the Park Foundation. The foundation did provide money for other work Jackson did taking baseline groundwater samples in New York.
"None of that is in here," Jackson said.
Energy In Depth has since updated its post.
Jackson said the team didn't seek out contaminated wells. He said it sought out areas where drilling is expected but has not yet occurred.
Stray gas from weak or faulty well construction is a well-known problem in the drilling world that companies have contended with for years.
The area covered by the Duke study includes Dimock, Pa., where state officials determined that shoddy well construction by Cabot Oil & Gas Corp. caused methane contamination in the area. The state required Cabot to deliver water to residents in the area for two years, fined the company and negotiated a $4.1 million settlement.
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