Chemical analysis links methane in drinking wells to shale-gas extraction. As shale-gas operations expand across the United States, industry officials and environmentalists are at loggerheads over whether or not shale-gas extraction can contaminate groundwater. Now researchers have traced low levels of methane and other contaminants to a source of shale gas: the sprawling Marcellus Formation, which lies beneath much of New York state, Pennsylvania, West Virginia and Ohio (see ‘On tap’) . The study, led by researchers at Duke University in Durham, North Carolina, expands on an earlier analysis of drinking water in northeastern Pennsylvania, where energy companies have used...