A POPULAR party trick in areas rich in natural gas is to conjure fire from water. The host lights a match, brings it close to a running tap and steps back as the trickle bursts into flames. Guests look on in amazement, possibly tinged with terror. Although such pyrotechnics are decades old, many people fret that they are becoming more common with the rapid spread of hydraulic fracturing, or fracking, which involves injecting water, sand and chemicals into the hard shale rock under high pressure to break it up and extract the gas trapped within. Some of this gas, they warn, escapes as it rises to the surface, contaminating aquifers on the way. Fans of fracking retort that, done right, the technique is safe and clean—and that done right it for the most part is. In 2011 a report from the Massachusetts Institute of Technology concluded that only a handful of more than 20,000 wells drilled in the previous decade had led to groundwater contamination. All of these incidents resulted from breaches of existing regulations, which require, among other things, that the bore hole be encased in concrete to stop methane and other gases seeping out.

New research just published in the Proceedings of the National Academy of Sciences by Robert Jackson, from Duke University, and his colleagues will add fuel to the controversy. Dr Jackson studied water samples from 141 private drinking-water wells in an area home to 5,000 or so drilling sites sitting atop the Marcellus shale, a gas-rich geological formation stretching from northeastern Pennsylvania to southeastern New York. He found that four out of five wells contained methane. In some homes within 1km (0.6 miles) from the nearest drill site the gas had a chemical signature suggesting that it originated from the Marcellus, rather than being the product of biological processes closer to the surface.

Concentrations of the gas in such homes were also six times higher than for those farther away. Levels of ethane and propane, other ingredients of natural gas, were higher, too. All were well above the levels the Department of Interior considers safe. The relationship cannot be put down to gasmen's penchant for plonking their drills in spots where natural gas is most abundant in the first place. In the absence of drilling the gas, being trapped in the shale beds 1,500-2,500 metres beneath the countryside, would stay put; concentrations nearer to the surface would remain unaffected.

Nor, Dr Jackson insists, did his team cherry-pick homes whose occupants complained of high methane concentrations, as some critics of his research have suggested. Finally, the analysed drilling sites were dotted more or less randomly around the Marcellus so the contamination cannot be pinned on a clutch of rule-breaking wells. The existing rules, in other words, may not be tight enough.

America's recent natural-gas bonanza owes a lot to fracking, which has made tapping abundant shale reserves economical for the first time. As a result, the country now pumps 2 billion cubic metres of natural gas a day, up 30% since 2005. The benefits of the technique, in the form of improved energy security and lower electricity prices, may therefore still outweigh the drawbacks (though the majority of participants in our recent debate on the issue thought otherwise). The risks have always been evident; witness all the regulation already in place. Dr Jackson's research shows that they have not gone away.

Correction: An earlier version of this story suggest that all the water wells within 1km of drilling sites contained Marcellus methane. In fact, only some did. Sorry.