Can fracking contaminate drinking water?

The salt of the Earth may hint at trouble for the fracking industry's safety claims, according to a new geological study ? although other researchers disagree.

Hydraulic fracturing uses pressurised fluid to crack open deep shale rocks to release the methane trapped within them. Geologists say this potentially harmful fluid is unlikely to percolate up through a few kilometres of rock to reach the shallow aquifers that supply drinking water ? but Avner Vengosh of Duke University in Durham, North Carolina, thinks the methane itself could do so. The gas would be an explosion risk.

Last year his team claimed drinking wells close to fracking sites in Pennsylvania were contaminated with methane ? perhaps from fracking ? a finding that was met with a storm of criticism. Now Vengosh claims more evidence of possible contamination.

Some 40 of the 158 Pennsylvania aquifers his team studied were unusually salty, contaminated with brine from salt aquifers that occur at the same depth as fracking operations. Cracks in the rock must have allowed the brine to migrate hundreds of metres upwards. Gas from deep fracking operations could travel in the same way.

Fast-moving gas

The process is likely to take millions of years, making it far too slow to pose a serious problem, says Mike Stephenson of the British Geological Survey. Vengosh disagrees, pointing out that the salt water must be moving upwards fairly fast otherwise Pennsylvania's heavy rainfall would wash it out of the shallow aquifers. He says gas would probably move even faster.

There are much more plausible ways for fracking to cause gas leaks, says Richard Davies of Durham University in the UK. If boreholes aren't properly sealed, gas can leak out of them. That may be what happened in Dimock, Pennsylvania; residents there are suing Cabot Oil & Gas Corporation for contaminating their wells. Cabot claims that its tests of the water in the area reveal no signs of illegal levels of contamination.

What's more, around 184,000 wells were drilled in Pennsylvania before records were kept, says Davies, and we don't know where they all are. Most of these probably aren't sealed, so if someone fracks near one the gas could escape up it.

Last December an industry-funded study claimed that the methane in the Pennsylvanian aquifers was chemically different to that liberated from the shale during fracking (Oil & Gas Journal, vol 109, p 54).

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1121181109

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