Shale gas production in Arkansas has not contaminated groundwater, a study by Duke University and the U.S. Geological Survey indicates.

The scientists analyzed 127 shallow drinking water wells in areas overlying Fayetteville Shale gas production in north-central Arkansas for major and trace elements and hydrocarbons, using isotopic tracers to identify the sources of possible contaminants, a Duke release reported Wednesday.

"Our results show no discernible impairment of groundwater quality in areas associated with natural gas drilling and hydraulic fracturing in this region," Duke geochemistry Professor Avner Vengosh said.

The researchers compared the chemical composition of the contaminants in the drinking water to those found in samples from nearby shale gas drilling sites using so-called fracking techniques.

Fracking, or hydraulic fracturing, 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.

Fracking in recent years has fueled concerns about water contamination by methane, fracking fluids and wastewater from the operations.

"Only a fraction of the groundwater samples we collected contained dissolved methane, mostly in low concentrations, and the isotopic fingerprint of the carbon in the methane in our samples was different from the carbon in deep shale gas in all but two cases," Vengosh said.

This indicates the methane was produced primarily by biological activity in the region's shallow aquifers and was not the result of shale gas contamination, he said.

"These findings demonstrate that shale gas development, at least in this area, has been done without negatively impacting drinking water resources," Duke doctoral student Nathaniel R. Warner, the study's lead author, said.