Shale gas drilling operations increase the risk of nearby drinking water becoming contaminated with methane, a study has suggested.
Researchers found, on average, methane concentrations 17 times above normal in samples taken near drilling sites.
Growing demand for energy has led to a sharp increase in shale gas extraction around the globe, prompting concerns about the impact of the technology.
The findings appear in the Proceedings of the National Academy of Sciences.
"We found surprising levels of methane in home-owners' wells that were close to natural gas wells, " co-author Rob Jackson, Nicholas Professor of Global Change at Duke University, North Carolina, explained.
"We found that within a kilometre of an active gas well, you were much more likely to have high methane concentrations," he told BBC News.
The team from Duke University collected samples from 68 private water wells in the north-eastern states of Pennsylvania and New York.