PITTSBURGH (AP) -- Hydraulic fracturing for natural gas hasn't contaminated drinking water wells in Arkansas, according to a new study, but researchers said the geology there may be more of a natural barrier to pollution than in other areas where shale gas drilling takes place.
The most passionate critics and supporters of hydraulic fracturing, or fracking, often describe the process in extremes, suggesting it is either inherently dangerous for the environment or that it poses virtually no risk at all. But Avner Vengosh, a Duke University professor of geochemistry and water quality, said making generalizations about fracking in Arkansas, Pennsylvania and Colorado doesn't make scientific sense.
"Each basin will have its own dynamics and its own rules," he said of the possibility of contamination, adding that differences in well construction and regulations play a role, too.
Members of the U.S. Geological Survey were also part of the study, which examined 127 drinking water wells for evidence of pollution from methane gas or chemicals.
The study published Wednesday looked at an area of heavy drilling in north-central Arkansas known as the Fayetteville Shale. More than 4,000 new wells have been drilled there since 2004. The researchers did multiple tests to looks for the presence of contamination from drilling, or from naturally occurring gas or ultra-salty liquids that seep up through pre-existing faults.
Arkansas homeowners "typically had good water quality, regardless of whether they were near shale gas development," said Robert Jackson, a professor of environmental sciences at Duke.
Arkansas Department of Environmental Quality spokeswoman Katherine Benenati declined to comment on the study.
In the contentious debate over fracking the Duke team has shown a willingness to report both positive and negative studies. In 2011 Duke researchers found higher concentrations of methane gas near drilling sites in Pennsylvania, and state regulators denounced the findings even as environmental groups cheered them.
During fracking, large volumes of water, along with sand and hazardous chemicals, are injected into the ground to break rock apart and free the oil and gas trapped inside. In some areas fracking has been blamed for air pollution and gas leaks that have ruined well water, but the Obama administration and many state regulators say the practice is safe when done properly.
Deb Nardone, a spokeswoman for the Sierra Club, agreed that there "are going to be geological differences" in different areas. But she said what that means is that research should be done in each area before drilling begins, to identify possible problems. Nardone also questioned whether over time the Arkansas water wells might start to show pollution from drilling. "It would be interesting to come back at this after five years," she said of the study.
One representative from industry agreed with some aspects of the Arkansas study, but questioned others.
"It does clearly speak to the need to think about these things from a regional and local perspective," said Andrew Place, the interim director of the Center for Sustainable Shale Development, a Pittsburgh-based partnership between industry, environmental groups, and philanthropies that is promoting higher standards for drilling in the Marcellus Shale, which covers Pennsylvania, Ohio, West Virginia and New York.
Place questioned the suggestion that the Marcellus Shale is uniquely different from the Fayetteville Shale, noting that Marcellus wells are typically two or three times the depth of those in Arkansas. Place said there are 6,000 feet or more of different rock layers between Marcellus gas wells and surface aquifers, and suggested that makes a "pretty compelling" case that pollution doesn't migrate unless wells are improperly constructed.
"Well casing can be done right," Place said.
Vengosh said the Arkansas findings don't mean that contamination can't happen, since faulty well construction or areas with different geology could cause different results.
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