New York: Living near oil or natural gas fields may not necessarily affect your drinking water unless the well is poorly constructed or there are problems associated with hydraulic fracturing or fracking.

‘We have found a number of homes near active wells with very high levels of natural gas in the tap water,’ said Rob Jackson, professor of earth system science at Stanford University.

‘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,’ he explained.

Most documented cases of groundwater contamination were 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,’ he explained. ‘The gap allowed gases to move up and down freely like a chimney and contaminate the drinking water supply.’

Besides structural issues, Jackson and his colleagues have identified problems associated with hydraulic fracturing, or fracking.

The technology uses pressurised sand, water and chemicals to crack open rocks and release trapped reservoirs of oil and gas.

Fracking wells are often installed a mile or more below the surface, far from underground sources of drinking water.

But in a recent study, Jackson found that at least 2,600 wells in the US have been fracked at depths shallower than 3,000 feet, some just hundreds of feet below the surface.

‘We found a surprising number of places where companies are fracking directly into shallow freshwater aquifers,’ he said. ‘In no other industry would you be allowed to inject chemicals into a source of drinking quality water.’

Jackson cited a high profile case in Pavillion, Wyo., where the U.S. Environmental Protection Agency (EPA) found that shallow fracking operations had released natural gas and other toxic compounds into freshwater aquifers.

In California, Jackson has identified hundreds of fracking wells drilled into aquifers located less than 2,000 feet below the surface. In the U.S., hydraulic fracturing is typically regulated by individual states.

An expert on the health impacts of fossil fuels, Jackson presented the findings at the annual meeting of the American Association for the Advancement of Science (AAAS) in Washington, D.C. Last weekend.