Most documented cases of groundwater contamination are caused by poorly constructed oil and gas wells, an expert has claimed.
Rob Jackson from Stanford University in US said that living near an oil or natural gas well usually does not affect drinking water, but there are exceptions.
"We have found a number of homes near active wells with very high levels of natural gas in the tap water," he said.
"Where the chemistry suggests contamination, the problem usually lies with the integrity of the well, either the cementing used to isolate it from the surrounding rock and water or the steel casing that allows gas and oil to flow upwards," Jackson said.
Most documented cases of groundwater contamination are caused by poorly constructed wells, he said, pointing to a widely publicised case in Parker County, Texas.
"At that site, the company cemented very near the surface and deep underground, but they put no cement for 4,000 feet in between," said Jackson.
"The gap allowed gases to move up and down freely like a chimney and contaminate the drinking-water supply," he said.
Besides structural issues, he has identified problems associated with hydraulic fracturing, or fracking. This technology uses pressurised sand, water and chemicals to crack open rocks and release trapped reservoirs of oil and gas.
"We found a surprising number of places where companies are fracking directly into shallow freshwater aquifers. In no other industry would you be allowed to inject chemicals into a source of drinking-quality water," Jackson said.
Jackson cited a high-profile case in Pavillion, Wyoming where the US Environmental Protection Agency (EPA) found that shallow fracking operations had released natural gas and other toxic compounds into freshwater aquifers.
"At Pavillion, they were fracking less than 1,000 feet deep, while people were getting drinking water at 750 feet," said Jackson.
"Contamination is more likely to occur when there isn't enough separation between the hydraulic fracturing activity and the drinking-water sources," said Jackson.
The 2010 documentary, Gasland, sparked controversy with footage of a kitchen tap catching fire at a home located near a natural gas well.
Natural gas consists primarily of methane, a greenhouse gas that is more than 30 times more potent than carbon dioxide over a 100-year period.
"Concerns about global warming and the potential impacts if natural gas on drinking water have led several European countries to take a much more cautious approach to hydraulic fracturing," Jackson said.
