Stray gases have been detected in drinking water wells near fracking drill sites in northeastern Pennsylvania, according to a study published today in PNAS.

Hyrdraulic fracturing — also known as 'hydrofracking' or 'fracking' — pumps water, sand, and chemicals deep underground to crack shale rock that is encasing deposits of natural gas.

Grave concerns have amounted against the use of fracking near communties.

Some studies have noted changes in air quality and human health near drill sites, but little to no conclusive evidence of groundwater contamination by fracking chemicals has been uncovered. In 2011, the U.S. Environmental Protection Agency announced that fracking had caused groundwater pollution in Wyoming, but opted to drop its follow-up investigation last week.

In this report, scientists from Duke show that stray natural gas has made its way into drinking water wells near fracking sites in Pennsylvania's Marcellus shale.

"The methane, ethane and propane data, and new evidence from hydrocarbon and helium isotopes, all suggest that drilling has affected some homeowners' water," said author Robert Jackson, a professor of environmental sciences at Duke's Nicholas School of the Environment. "In a minority of cases, the gas even looks Marcellus-like, probably caused by faulty well construction."

Although fracking is now common across the U.S., it has spawned countless political debates, especially in Pennsylvania. The state's Marcellus shale formation, which it shares with New York and West Virginia, is suspected to house one of the largest shale-natural gas deposits in the nation and could yield billions of dollars in industry and tax revenue.

Duke scientists sampled water from 141 drinking wells across six counties in northeastern Pennsylvania — Bradford, Lackawanna, Sullivan, Susquehanna, Wayne, and Wyoming — along with parts of New York.

Methane was found in 82 percent of the wells, with concentrations of the gas being six times higher within two miles of drilling sites. Propane and ethane were also detected. Ethane levels were 23-times higher near fracking locations.

"Distance to gas wells was, by far, the most significant factor influencing gases in the drinking water we sampled," said Jackson.

Little research has been done on the potental health effects of natural gas contamination in drinking water, but these gases are highly flammable and pose a risk of explosion. High levels are also linked to asphyxiation.

Although gas levels were high near drinking wells, neither this study nor prior work from this group has found contamination from fracking fluids. Salt, metals, and radioactivity levels around the wells were also normal.

Source: Jackson RB, Vengosha A, Darraha TH. Increased stray gas abundance in a subset of drinking water wells near Marcellus shale gas extraction. PNAS. 2013.