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The Pinedale Anticline in Wyoming is one of the highest-producing gas fields in the U.S.

A definitive study published by the National Academy of Sciences has concluded that horizontal drilling and hydraulic fracturing to extract shale gas and oil do not, in themselves, cause contamination of drinking-water sources. However, faulty cementation and construction techniques, such as improperly cementing well casings and linings, can potentially trigger migration of methane into groundwater wells and aquifers, according to the report, published in the Sept. 12 issue of the Proceedings of the National Academy of Sciences.

The study's conclusions are significant because fracking remains controversial, polarizing opinions on whether the practice causes contamination of drinking-water wells and aquifers near drilling sites. A handful of states and cities, including New York state and Fort Collins, Colo., have put in place temporary moratoria on fracking.

The team of researchers, led by Thomas Darrah, an Ohio State University assistant professor of earth sciences, analyzed 113 samples from drinking-water wells in the Marcellus shale play in Pennsylvania and 20 from the Barnett play in Texas. They found eight separate clusters of fugitive gas contamination, seven in Pennsylvania and one in Texas.

Four of the clusters seemed to be linked to poor cementing along the wells' annuluses, three to faulty production casings and the other to an underground well failure. An analysis of noble gases associated with methane seems to have ruled out any upward migration of methane triggered by horizontal drilling or hydraulic fracturing.

Noting that public and political support for unconventional means of energy extraction is sometimes "tempered by environmental concerns," the study authors conclude that "determining the mechanisms of contamination will improve the safety and economics of shale-gas extraction."

Katie Brown, a spokeswoman for Energy in Depth, a pro-industry group, says the study debunks claims by environmental groups that hydraulic fracturing causes environmental harm. "It's important to keep in context" that the amount of cases of actual contamination is less than 1% of all wells drilled in the U.S., she says. Some of the contamination that occurs in groundwater could result from natural causes, she says.

But environmental groups, including the Sierra Club and Food and Water Watch, say states do not have adequate regulatory protections in place to ensure that wells are drilled and cemented properly.

In a report released on Sept. 16, Food and Water Watch argues that, in addition to concerns over water contamination, the methane emissions associated with fracking contribute to climate change and that the drilling process itself can lead to earthquakes.

The advocacy group says that even if more-stringent regulations were put in place and "adequately enforced," fracking would still not be safe.

A separate technical report from the Dept. of Energy's National Energy Technology Laboratory, released the same week, looked at a single hydraulic-fracturing operation in Greene County, Pa., for upward fracture growth out of the target zone and upward gas and fluid migration. The site-specific study showed fracture growth ceased more than 5,000 ft below drinking-water aquifers, and there was no detectable upward migration of gas or fluids from the hydraulically fractured Marcellus play.