A study of 68 water wells in active gas-extraction areas of Pennsylvania and New York States found that fracking within a kilometre of the well increased the wells' methane levels to the point of being an explosion hazard.

However, the study found no evidence of well contamination by saline groundwater or fracking fluids.

"We conclude that greater stewardship, data, and — possibly — regulation are needed to ensure the sustainable future of shale-gas extraction and to improve public confidence in its use," wrote the researchers from the Nicholas School of the Environment and Duke University who undertook the study.

Natural gas is the rising star of the fossil fuel industry. The study noted that in one Pennsylvania county alone, gas well approvals rose 27-fold between 2007 and 2009.

Gas extraction is also powering significant investment in greenfield regions like Queensland's Surat Basin.

Fracking, a technique that involves forcing liquids at high pressure into underground formations to fracture them and release natural gas trapped in fissures, has become one of the favoured methods of cost-effectively recovering gas.

Fracking has come under considerable pressure from those who fear contamination of underground water reservoirs. Critics of the methods say that fracturing formations could lead to mixing of freshwater with saline aquifers, pollution of water by fracking liquids, or pollution of water reserves by gas.

According to the US research, this latter concern has been realised in the intensively-mined and closely settled areas of the Appalachian Basin.

The researchers reported that there are up to a million private water wells in Pennsylvania alone, most of them unregulated and untested.

The wells tested were 36-190 metres deep. Methane levels in shallow wells near active drilling sites had methane levels on average 17 times higher than wells where there was no drilling.