WASHINGTON – The U.S. Geological Survey said in a report released Wednesday it found no evidence of groundwater contamination caused by gas drilling in north-central Arkansas.

USGS examined the water quality of 127 shallow domestic wells in Van Buren and Faulkner counties and found no indication of contamination associated with drilling in the Fayetteville Shale area.

"None of the data that we have looked at as part of this study suggests that any groundwater contamination is resulting from natural gas production activities," said USGS hydrologist Tim Kresse.

Kresse noted that the study focused on methane – the primary component of natural gas - and chloride. Salty water, or chloride, is a byproduct of the hydraulic fracturing process.

Chloride is a good indicator of whether chemicals used during hydraulic fracturing are reaching groundwater, Kress said. In this case, the chloride concentrations were not higher than samples taken from nearby areas from 1951 through 1983.

USGS found methane in some wells but it could not be attributed to "fracking," according to Kresse.

Gas production in the Fayetteville Shale area began in 2004. Exploration and production companies say they have invested more than $12.7 billion in Fayetteville Shale activities from 2008 to 2011. There are now more than 4,000 producing wells.

Rep. Tim Griffin, R-Little Rock, said the report offers "additional evidence that natural gas production is safe for our environment and communities."

"While we must always seek to ensure that energy developing is done responsibly, this report is an inconvenient truth for those who seek to ban fracking," Griffin said.

Duke University researchers reported similar results last summer for northeastern Pennsylvania where gas drilling is being done on the Marcellus Shale.

Environmentalists have raised concerns over the fracking process used to release gas from shale.

The Arkansas Department of Environmental Quality withheld comment on the report Wednesday, saying the agency is still reviewing the results.