THURSDAY, May 12 (HealthDay News) -- A new study finds that water wells near shale gas drilling and hydraulic fracturing sites have high levels of potentially explosive methane gas.

Shale gas accounts for about 15 percent of the natural gas produced in the United States, according to a Duke University news release.

Hydraulic fracturing -- also called hydrofracking or fracking -- involves pumping water, sand and chemicals deep into the ground in order to extract natural gas.

Researchers analyzed water samples from 68 private groundwater wells in five counties in northeastern Pennsylvania and New York.

"We found measurable amounts of methane in 85 percent of the samples, but levels were 17 times higher on average in wells located within a kilometer of active hydrofracking sites," Stephen Osborn, a postdoctoral research assistant at Duke's Nicholas School of the Environment, said in a news release.

Methane is flammable, and in very high concentrations it can cause asphyxiation. The health effects of drinking methane-contaminated water are not well-known, according to the study.

There was no evidence of well water contamination from the chemicals used in hydrofracking.

The study was published online May 9 in the Proceedings of the National Academy of Sciences.

"At least some of the homeowners who claim that their wells were contaminated by shale-gas extraction appear to be right," Robert B. Jackson, a professor of global environmental change and director of Duke's Center on Global Change, said in the news release.

More information

The U.S. Centers for Disease Control and Prevention has more about private groundwater wells.