Traces of a chemical compound used in hydraulic fracturing, or fracking, have been found in drinking water at three homes in Pennsylvania, according to a study published this week. The findings of the study, published in the Proceedings of the National Academy of Sciences, add to growing concerns over the public health risks involved with fracking, which uses water, sand, and chemicals to release shale gas from rocks deep below Earth's surface.

"This is the first documented and published demonstration of toxic compounds escaping from uncased boreholes in shale gas wells and moving long distances" into drinking water, Susan Brantley, a geoscientist at Pennsylvania State University and one of the study's authors, told the Associated Press.

Contamination blamed on poorly built wells

Analysis of drinking water gathered from the homes revealed small amounts of 2-Butoxyethanol or 2-BE, a compound commonly found in cosmetics and household cleaners. 2-BE has been shown to cause tumors in rodents, though its effects on humans remain less clear, and researchers say the amounts discovered in Pennsylvania were within regulatory limits. The authors note that the contamination was likely not caused by fracking itself, but by poorly constructed drill wells.

The three homes are located in Bradford County, Pennsylvania, near gas wells built in 2009 over the Marcellus Shale formation. High levels of methane and sediment were found in their drinking water in 2010, prompting a lawsuit against the drilling company, Chesapeake Energy Corporation. State regulators levied a $1 million fine against the company in 2011, and enacted tougher regulations for drilling. New York and Vermont have banned fracking altogether amid concerns over public health and the environment. Last month, the Obama administration introduced new regulations that require companies to disclose the chemicals used when drilling for gas and oil.

The industry pushes back

The study's authors believe the contamination occurred in 2010, before Pennsylvania's drilling regulations went into effect, and acknowledge that the stricter rules could have prevented it. Experts not involved in the study said it demonstrates a clear (if rare) example of drilling's impact on drinking water, but representatives from the drilling industry called the findings into question. Shale gas advocates have long argued that drilling occurs too far below underground freshwater to pose any threat to human health.

"The entire case is based around the detection of an exceedingly small amount of a compound that’s commonly used in hundreds of household products," Katie Brown, a consultant for the petroleum advocacy group Energy in Depth, said in an email to The New York Times. "The researchers suggest the compound is also found in a specific drilling fluid, but then tell us they have no evidence that this fluid was used at the well site."

But lead author Garth T. Llewellyn, a hydrogeologist at Appalachia Hydrogeologic and Environmental Consulting, told the Times that the researchers found no traces of the compounds in homes located farther away from the wells. "When you include all of the lines of evidence," Llewellyn said, "it concludes that that’s the most probable source."