* Propane and ethane also detected in water
* Scientists suggest leaky natural gas wells are the cause
* Industry group disputes study's findings
Elevated levels of methane, ethane and propane gases were found in drinking water wells in Pennsylvania, close to operations that shake natural gas loose from underground shale formations in a process known as fracking, scientists reported on Monday.
Detection of contaminated drinking water suggests the gas wells are leaking, according to Robert Jackson of Duke University, lead author of a study published in the Proceedings of the National Academy of Sciences. An industry group disputed these findings, saying that methane occurs naturally in water in this area.
The United States has the world's biggest natural gas reserves, followed by China, Argentina and Algeria. With the advent of horizontal drilling and fracking, formally called hydraulic fracturing, U.S. oil and natural gas production has soared in the last decade.
The Obama administration supports fracking-derived natural gas as an alternative to coal, which emits more climate-warming carbon dioxide than natural gas. President Barack Obama is expected to call for cuts in carbon dioxide emissions from coal-fired power plants in a speech on Tuesday.
However, environmental advocates have questioned whether fracking, in which water and chemicals are injected at high pressure into rock deep underground to blast out the natural gas, interferes with water quality at the surface.
Jackson's study suggests that it does, in some cases.
Based on analysis of 141 drinking water wells in northern Pennsylvania that sit atop a natural gas-rich underground formation called the Marcellus shale, Johnson and his colleagues found 82 percent of drinking water samples contained methane, with concentrations six times higher for homes within .62 miles (1 km) of natural gas wells than for homes farther away.
Ethane concentrations were 23 times higher for homes close to natural gas wells; propane was detected in 10 drinking water wells, also within .62 miles of a natural gas well.
"We found much higher concentrations of methane, ethane and propane in people's drinking water within one kilometer of the shale gas wells," Jackson said by telephone. "What that means to me is that those gases are leaking out of the wells and into the shallow aquifers."
He noted that no fracking chemicals or radioactivity were detected in drinking water wells.
"The researchers found methane in virtually every water well they sampled, irrespective of its proximity to gas drilling. They suggest a link to Marcellus gas wells, but pre-drill testing in the same part of the state directly contradicts them," Steve Everley of Energy in Depth said in an email response to questions about the study.
Energy in Depth is a program of the Independent Petroleum Association of America that focuses on shale and fracking.
Fracking operations in the Marcellus shale - which takes in Pennsylvania, New York, Ohio and West Virginia - are often more than a mile deep, while aquifers are only a few hundred feet underground, Everley said.
This indicates, he said, that there is no direct leak from where the gas is being extracted up through the rock. Instead, it is more likely that in some cases, the wells are leaking closer to the shallower drinking water sources.
Jackson said little is known about long-term health effects from methane and other gases in drinking water, and the Environmental Protection Agency does not regulate methane in drinking water.