Wastewater injection wells that accompany natural gas “fracking” sties are linked to an increase in the probability of earthquakes, according to a University of Texas study published this week in the scientific journal Proceedings of the National Academy of Sciences.

Cliff Frohlich, the University of Texas researcher who conducted the study, used seismic data from the Barnett Shale region of Texas taken between November 2009 and September 2011. He found that while injection wells were typically near the site of small quakes, they did not always cause tremors, leading Frohlich to speculate that injection wells near existing faults were more likely to cause seismic activity. Most of the quakes registered for the study were so light that they could not be felt on the surface.

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The study is just one of several that have warned about the possibility of inducing seismic activity by injecting hundreds of thousands of gallons of water underground in a short period of time. The National Research Council said earlier this year that while the risks posed by fracking itself are rather low, wastewater injection wells, along with the carbon capture and storage techniques typically used with “clean coal,” do in fact pose a heightened risk of earthquakes.

The U.S. Geological Survey also said as much in April, when it confirmed that America’s oil and natural gas boom has led to a dramatic increase in seismic activity in midwestern states where earthquakes used to be a bizarre occurrence. Even the British energy firm Cuadrilla Resources admitted in a 2011 study of its own operations that the combination of “unusual combination of geology” and underground pressure resulting from the injection wells caused “a number of minor seismic events.”

The University of Texas study did not look at other reported hazards of fracking like drinking water contamination. Testing last year on 68 private wells in Pennsylvania and New York, all near fracking sites, found that most had been contaminated by methane leaks. The oil and gas industry maintains that fracking and injection wells are safe.

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