Well water near shale-gas drilling sites in the eastern United States carried high levels of leaked methane, researchers said Tuesday, placing a further question mark over this fast-growing energy source. Scientists tested water samples taken from 68 private wells in five counties in Pennsylvania and New York to explore accusations that \"hydro-fracking\" a contested technique to extract shale gas contaminated groundwater. Methane was found in 85 percent of the samples, and at sites within a kilometer (0.6 mile) of active hydro-fracturing operations, levels were 17 times higher than in wells far from such operations, said the study by researchers at Duke University in North Carolina. \"In these rural areas, almost everybody has a well that they are using the groundwater for some purpose they are using it for drinking, for their livestock, for agriculture,\" lead author Stephen Osborn told AFP. However, little is known about the health impacts of consuming methane in drinking water. \"We were surprised, and we have spoken with many health officials,\" he said. \"There is really no literature that address that particular issue the physiological response is methane really non-reactive in the body? What are the effects of consuming high concentrations of methane?\" The paper found no evidence of contamination from the chemicals used to fracture the rock or from \"produced\" water the wastewater that emerges from the wells after the shale has been fractured. Shale gas is found in dense sedimentary rock which is fractured by large volumes of water, sand and chemicals that are piped in horizontally at very high pressure. After the fracturing, large amounts of water return to the surface within a few days, along with significant amounts of methane, which comprises the bulk of the shale gas. The gas is enjoying a boom in North America and Europe, buoyed by high prices and fears over the political risks of imported fossil fuels. Opponents say the technique is environmentally destructive because the methane can contaminate groundwater and, if leaked to the air, add to the greenhouse effect. The study, published on Monday in the peer-reviewed journal Proceedings of the National Academy of Sciences (PNAS), said the methane sampled near the fracking sites had an isotopic fingerprint that pointed to its source. Water from wells farther from the gas sites had lower levels of methane and a different isotopic signature. As a gas, methane is flammable and causes suffocation. In April, scientists from New York\'s Cornell University found that current extraction techniques meant that shale gas carried a greater carbon footprint than oil, coal and conventional gas over at least a 20-year period. According to the US Department of Energy, total domestic production of natural gas will grow by 20 percent by 2035. Shale gas alone will increase its share of production from 16 percent in 2009 to 45 percent in 2035.