People living within one kilometer of shale gas wells appear to be at higher risk of having their drinking water contaminated by stray gases, a new U.S. study has found.

Shale gas accounts for an increasing fraction of the U.S. natural gas supply, but the environmental implications of shale gas extraction, a process that includes hydraulic fracturing, remain uncertain.

Researchers from the Duke University analyzed 141 drinking water samples from private water wells in the U.S. state of Pennsylvania. Their study found not only higher methane concentrations in drinking water within a kilometer of shale gas drilling, which past studies have shown, but higher ethane and propane concentrations as well.

On average, methane concentrations were six times higher and ethane concentrations were 23 times higher at homes within a kilometer of a shale gas well. Propane was detected in 10 samples, all of them from homes within a kilometer of drilling, they reported in the journal Proceedings of the National Academy of Sciences on Monday.

"The methane, ethane and propane data, and new evidence from hydrocarbon and helium isotopes, all suggest that drilling has affected some homeowners' water," Robert Jackson, a professor of environmental sciences at Duke's Nicholas School of the Environment, said in a statement.

The team examined which factors might explain their results, including topography, distance to gas wells and distance to geologic features. "Distance to gas wells was, by far, the most significant factor influencing gases in the drinking water we sampled," said Jackson.

Hydraulic fracturing, also called hydrofracking or fracking, involves pumping water, sand and chemicals deep underground into horizontal gas wells at high pressure to crack open hydrocarbon- rich shale and extract natural gas. Accelerated shale gas drilling and hydrofracking in recent years has fueled concerns about contamination in nearby drinking water supplies.

Two previous peer-reviewed studies by Duke Researchers found direct evidence of methane contamination in water wells near shale- gas drilling sites in northeastern Pennsylvania. A third study conducted with U.S. Geological Survey scientists found no evidence of drinking water contamination from shale gas production in the state of Arkansas. None of the studies have found evidence of contamination by fracking fluids.

"Our studies demonstrate that distances from drilling sites, as well as variations in local and regional geology, play major roles in determining the possible risk of groundwater impacts from shale gas development," said Avner Vengosh, professor of geochemistry and water quality at Duke's Nicholas School. "As such, they must be taken into consideration before drilling begins." Endit

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