Groundwater contamination near North Texas shale gas wells could be caused by a faulty casing or poor cement construction surrounding the casing, according to a new study published online Monday in the Proceedings of the National Academy of Sciences of the United States of America.

Researchers from Dartmouth College, Duke University, Ohio State University, Stanford University and University of Rochester conducted the study, sampling 113 domestic groundwater wells overlying the Marcellus Shale in Pennsylvania and 20 groundwater wells overlying the Barnett Shale in North Texas.

Researchers identified eight “discrete clusters” of gas contamination, seven in Pennsylvania and one in North Texas, that showed groundwater contamination increasing over time. They linked four of the clusters to gas leakage from failures of cementing or faulty production casings in wells.

“Optimizing well integrity is a critical, feasible and cost-effective way to reduce problems with drinking water contamination to alleviate public concerns accompanying shale gas extraction,” the authors wrote, adding that future research should evaluate whether the large volumes of water used in hydraulic fracturing affect well integrity.

This study constitutes yet another chapter in the ongoing debate over the gas well drilling process called fracking. Voters in the city of Denton will go to the polls on Nov. 4 and either approve or shoot down a ballot proposition that calls for a ban on fracking within city limits.

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