NEW HAVEN, Conn. — Oct. 12, 2015 — Trace contamination of organic compounds in Pennsylvania drinking water wells comes from hydraulic fracturing surface operations, rather than gas wells.

That’s the key finding of a Yale University-led study which analyzed 64 samples from drinking water wells of residential properties near the Marcellus Shale in northeastern Pennsylvania over a three-year period.

Chemical analyses conducted by the team showed that some of the groundwater samples contained low levels of organic compounds in areas close to natural gas wells. But there was no evidence that this contamination came from deep fracking shale horizons, underground storage tanks, well casing failures or surface waste containment ponds.

In fact, the analyses indicated that the compounds most likely entered the groundwater supply from gas extraction operations above the ground surface, for example as a result of surface spills of chemical additives, Yale reported.

Publishing their findings in the journal Proceedings of the National Academy of Sciences, the researchers said the results are encouraging because drinking water sources affected by disclosed surface spills could be targeted for treatment and monitoring to protect public health.

However, the study results should not be used to draw conclusions about the effects of hydraulic fracturing in other regions, said Desiree L. Plata, assistant professor of chemical and environmental engineering at Yale.

“Subsurface geology is wildly different across the United States,” she noted in the release. “While there are shale deposits across the country, the geology above those shale deposits is highly variable, so in some places you might have much different communication between the deep formation fluids and the shallow groundwater.”