BOSTON (Reuters) - The contamination of water supplies near U.S. shale gas fields appears to be the result of leaky cement wells and casings and not the controversial production technique of hydraulic fracturing, according to a study released on Monday.
So-called "fracking" is a way of extracting natural gas from deep layers of rock using high-pressure fluid injections. The method has triggered a surge in U.S. gas production, but raised fears that breaking up rock formations underground could allow gas to seep into drinking water.
Scientists from several universities, including Duke, Ohio State, Stanford and Dartmouth, analyzed more than 130 drinking-water well samples overlying the Marcellus and Barnett shale gas formations and attempted to trace the source of any contamination, according to the study.
The researchers found eight clusters of drinking-water wells that were tainted by hydrocarbons and linked them, using "gas geochemistry data," to leaky cement from three production casings and one underground well used by energy companies to extract the gas.
The study said the research data "appear to rule out gas contamination by upward migration from depth through overlying geological strata triggered by horizontal drilling or hydraulic fracturing.
"Determining the mechanisms of contamination will improve the safety and economics of shale-gas extraction," according to the study, published on Monday in the Proceedings of the National Academy of Sciences.
The U.S. Environmental Protection Agency is in the midst of a years-long study into the impacts of fracking on drinking water, and is scheduled to release a draft this year, after reports of drinking water so badly contaminated that homeowners could light it on fire.
The Marcellus formation lies beneath a swath of New York, Pennsylvania and West Virginia. The Barnett formation is beneath a portion of northern Texas.
(Reporting by Richard Valdmanis. Editing by Andre Grenon)