Poorly constructed shale gas wells, not hydraulic fracturing, are to blame for contaminated water in Pennsylvania and Texas, says a study by scientists at Duke University and four other schools.
The study was published Monday in the Proceedings of the National Academy of Sciences.
Researchers analyzed the gas content of more than 130 drinking water wells over the Marcellus shale formation in Pennsylvania and Texas' Barnett shale. They found contamination in eight clusters of wells.
Hydraulic fracturing extracts gas from deep underground. The suspicion has been that methane -- the main component of natural gas -- from fracking or horizontal drilling had migrated up into drinking water aquifers.
The new study appears to rule that out. Instead it found, in four contaminated clusters, that the methane leaked at shallow depths from faulty rings of cement around gas-well shafts. Three more clusters suggested methane leaked through bad well casings.
While drilling has contaminated water, the scientists say, most of the causes can be prevented.
The researchers used noble gases such as helium to trace methane emissions because they mix with natural gas but aren't affected by microbes or oxidation. Measuring the noble gases determined the source of the methane and how it reaches drinking water aquifers.
Scientists from Duke, Ohio State University, Stanford University, Dartmouth College and the University of Rochester took part in the peer-reviewed study. Funding came from the National Science Foundation and Duke's Nicholas School of the Environment.