PITTSBURGH (AP) — New research on Marcellus Shale gas drilling in Pennsylvania may only add fuel to the debate over whether the industry poses long-term threats to drinking water.

A paper published on Monday by Duke University researchers found that gas drilling in northeastern Pennsylvania did not contaminate nearby drinking water wells with salty water, which is a byproduct of the drilling.

"These results reinforce our earlier work showing no evidence of brine contamination from shale gas exploration," said Robert Jackson, director of Duke's Center on Global Change and a co-author of the paper, which appeared online in the Proceedings of the National Academy of Sciences. The team evaluated 426 samples from groundwater aquifers in six counties.

The findings are noteworthy because last year the same Duke team found evidence that methane from gas wells had contaminated drinking water in Pennsylvania. That prompted harsh criticism from the top official at the state Department of Environmental Protection, who accused the researchers of bias and shoddy science.

Department of Environmental Protection spokesman Kevin Sunday said the agency was still reviewing the new study, but an industry group welcomed the results.

"This research demonstrates that freshwater aquifers in northeastern Pennsylvania have not been impacted by natural gas development activities," said Kathryn Klaber, president of the Marcellus Shale Coalition.

The Marcellus Shale is a gas-rich rock formation thousands of feet under large parts of Pennsylvania, New York, Ohio and West Virginia. Over the past five years, advances in drilling technology made the gas accessible, leading to a boom in production, jobs, and profits — and concerns about pollution.

The gas is pulled from the ground through a process called hydraulic fracturing, or fracking, in which large volumes of water, plus sand and chemicals, are injected deep underground to break shale apart and free the gas.

Environmentalists have claimed the brine water that comes up with the gas, the gas itself or the chemicals could pollute drinking water aquifers. The industry and many state and federal officials say the practice is safe when done properly, but there have also been cases where faulty wells did cause pollution.

Avner Vengosh, the new paper's lead author, said researchers are still in the early stages of understanding the links among reservoirs of deep brine, surface aquifers and gas drilling. The industry has long claimed that the final drilling takes place so far underground that fluids could never reach the surface.

But Vengosh said the research found that naturally-occurring pathways can bring the brine up into shallow aquifers, especially at the bottom of valleys. That could mean some areas are naturally more at risk of groundwater contamination from drilling, he said.

One groundwater expert said some, but not all, of the Duke findings seem to match existing research.

"They're basically supporting a lot of the things I found," said David Wunsch, Delaware's state geologist and the director of science and technology for the National Groundwater Association. He wrote a 1993 paper that looked at brine in Kentucky valley bottoms.

But Wunsch said the Duke team's claim that the shallow brine is coming from deep underground may be too simplistic.

"There's a lot of work already out here, they just haven't looked at it all," he said of research into Marcellus Shale gas drilling, adding that the Duke team may be "re-examining something that might have already been explained."

Wunsch believes it is more likely that the reservoirs of brine in valleys have been there for a long time, and he questioned the theory that much of that fluid comes from deep underground.

The Duke team still plans to analyze the recent water samples for evidence of fracking chemicals, Vengosh said. Their paper last year didn't find any evidence of those chemicals in water wells.

John Detwiler, an activist with Marcellus Protest, a group critical of fracking, declined to comment on the Duke study.

George Jugovic, president of PennFuture, an environmental group, said the potential for natural pathways to bring deep brine to surface areas raises concerns about whether fracking in some areas could increase the risks of such contamination.

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  • In this file photo from Oct. 14, 2011, a drilling rig is seen in Springville, Pa. State regulators blamed faulty gas wells drilled for leaking methane into the groundwater in nearby Dimock, Pa. It was the first serious case of methane migration said to be related to the Pennsylvania Marcellus Shale gas field drilling boom. (AP Photo/Alex Brandon, FILE)

  • British police secure the area where demonstrators erected a mock fracking rig with a banner reading 'No fracking in the UK' in a protest against hydraulic fracturing for shale gas outside the Houses of Parliament in London on December 1, 2012. AFP PHOTO / JUSTIN TALLIS

  • SPRINGVILLE, PA - JANUARY 18: A truck with the natural gas industry, one of thousands that pass through the area daily, drives through the countryside to a hydraulic fracturing site on January 18, 2012 in Springville, Pennsylvania. (Photo by Spencer Platt/Getty Images)

  • SAN FRANCISCO, CA - MAY 30: Protestors stage a demonstration against fracking in California outside of the Hiram W. Johnson State Office Building on May 30, 2013 in San Francisco, California. Dozens of protesters with the group Californians Against Fracking staged a protest outside of California Gov. Jerry Brown's San Francisco offices demanding that Gov. Brown ban fracking in the state. (Photo by Justin Sullivan/Getty Images)

  • People demonstrate on August 3, 2013 in La Petite Brosse, near Jouarre, outside Paris, to protest against an exploratory oil shale drilling, considering that it opens the door to the exploration of shale gas in the Parisian Basin. Banner reads 'Stop gas and oil shale'. AFP PHOTO / PIERRE ANDRIEU

  • In this Nov. 26, 2012 photo, Steve Lipsky demonstrates how his well water ignites when he puts a flame to the flowing well spigot outside his family's home in rural Parker County near Weatherford, Texas. The U.S. Environmental Protection Agency had evidence a gas company's drilling operation contaminated Lipsky's drinking water with explosive methane, and possibly cancer-causing chemicals, but withdrew its enforcement action, leaving the family with no useable water supply, according to a report obtained by The Associated Press. The EPA's decision to roll back its initial claim that hydraulic fracturing, or “fracking,” operations had contaminated the water is the latest case in which the federal agency initially linked drilling to water contamination and then softened its position, drawing criticism from Republicans and industry officials who insisted they proved the agency was inefficient and too quick to draw conclusions. (AP Photo/LM Otero)

  • In this file photo of Jan. 17, 2013, Yoko Ono, left, and her son Sean Lennon visit a fracking site in Franklin Forks, Pa., during a bus tour of natural-gas drilling sites in northeastern Pennsylvania. Ono and Lennon have formed a group called “Artists Against Fracking,” which has become the main celebrity driven anti-fracking organization. (AP Photo/Richard Drew, File)

  • In this March 29, 2013 file photo, a worker checks a dipstick to check water levels and temperatures in a series of tanks at a hydraulic fracturing operation at a gas drilling site outside Rifle, Colorado. (AP Photo/Brennan Linsley)

  • In this March 29, 2013 file photo, a worker switches well heads during a short pause in the water pumping phase, at the site of a natural gas hydraulic fracturing and extraction operation outside Rifle, in western Colorado. (AP Photo/Brennan Linsley)

  • In this March 29, 2013 file photo, workers tend to a well head during a hydraulic fracturing operation at a gas well outside Rifle, in western Colorado. (AP Photo/Brennan Linsley)

  • Josh Fox, director of the anti-fracking, Oscar-nominated documentary “Gasland” testifies during a House Committee hearing on oil drilling, "fracking" legislation at the Illinois State Capitol Tuesday, May 21, 2013, in Springfield, Ill. (AP Photo/Seth Perlman)

  • This is a Thursday Aug. 15, 2013 image of the Cuadrilla exploration drilling site in Balcombe, southeast England. (AP Photo/Gareth Fuller/PA)

  • A child plays near a farmers' protest in an area where oil company Chevron plans to put a drilling rig exploring for shale gas in the south-eastern Polish village of Zurawlow on June 11, 2013. AFP PHOTO / JANEK SKARZYNSKI

  • Protesters hold a banner during a protest outside of the Momentive resin plant, Monday, July 8, 2013, in Morganton, N.C. Dozens of environmental activists blocked a chemical plant Monday to protest against the company's sale of products used in the natural gas drilling process called hydraulic fracturing, or fracking. (AP Photo/The News Herald, Mary Elizabeth Robertson)

  • A fracking rig exploring for shale gas of oil company Chevron on June 11, 2013 in a village of Ksiezomierz in south-eastern Poland. AFP PHOTO / JANEK SKARZYNSKI

  • People demonstrate on August 3, 2013 in La Petite Brosse, near Jouarre, outside Paris, to protest against an exploratory oil shale drilling, considering that it opens the door to the exploration of shale gas in the Parisian Basin. AFP PHOTO / PIERRE ANDRIEU

  • Opponents of hydraulic fracturing in New York state attend a news conference and rally against hydraulic fracturing, also known as fracking, on January 11, 2012 in New York City. (Photo by Spencer Platt/Getty Images)

  • Eric Weltman of Food & Water Watch attends a news conference and rally against hydraulic fracturing, also known as fracking, in New York State on January 11, 2012 in New York City. (Photo by Spencer Platt/Getty Images)

  • Opponents and supporters of gas-drilling, or fracking, walk into the last of four public hearings on proposed fracking regulations in upstate New York on November 30, 2011 in New York City. (Photo by Spencer Platt/Getty Images)

  • Engineers on the drilling platform of the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • Engineers at work on the drilling platform of the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • General views of the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • Engineers look at the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • A lump of shale rock on display at the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • Engineers on the drilling platform of the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • Engineers at work on the drilling platform of the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • Drill heads on display at the entrance to the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • An engineer displays a lump of shale rock at the Cuadrilla shale fracking facility on October 7, 2012 in Preston, Lancashire. (Photo by Matthew Lloyd/Getty Images)

  • Actor/director Mark Ruffalo (C) speaks at the Hydraulic Fracturing prevention press conference urging the protection of the drinking water source of 15 million Americans at Foley Square on April 25, 2011 in New York City. (Photo by D Dipasupil/Getty Images)

  • (L-R) Actor/director Mark Ruffalo, Denise Katzman, Wenonah Hauter, and Water Defense co-founder/campaign director Claire Sandberg attend the Hydraulic Fracturing prevention press conference urging the protection of the drinking water source of 15 million Americans at Foley Square on April 25, 2011 in New York City. (Photo by D Dipasupil/Getty Images)