The Yale research appears to confirm an earlier report from the US Environmental Protection Agency that widespread fraccing may not deserve its poor public image.

After analysing samples of groundwater collected from private residences in north-eastern Pennsylvania, host to the Marcellus and Utica shales, the researchers determined that groundwater contamination was more closely related to surface toxins seeping down into the water table than pollutants from fraccing operations seeping upwards.

“The dominant source of organic compounds to shallow aquifers was consistent with surface spills of disclosed chemical additives,” they wrote.

“[There is] no evidence of association with deeper brines or long-range migration of these compounds to the shallow aquifers.

“Organic compounds found in drinking water aquifers above the Marcellus Shale and other shale plays could reflect natural geologic transport processes or contamination from anthropogenic activities, including enhanced natural gas production,” the Yale researchers said.

They suggested that drinking water sources affected by disclosed surface spills could be targeted for treatment and monitoring to protect public health.

It is unclear whether chemicals injected into deep shale horizons reach shallow groundwater aquifers and affect local water quality, they found.

Their findings from the largest study of its kind, sampled a mere 64 private water wells near fraccing sites to determine if they could be contaminated by drilling fluids.

The Yale study seems to partially support a Cornell University study which suggested frac fluid causes the release of particles in soil that bind to pollutants, which have the potential to contaminate groundwater.

The flowback fluid causes associated pollutants, such as heavy metals, to leach out of the soil and into groundwater.

Scientists at the College of Agriculture and Life Sciences believe the frac fluid's chemical composition weakens the bind between tiny particles called colloids and the soil. This makes groundwater susceptible to contamination when these particles leach out of the soil, bringing bound pollutants and heavy metals with it.

The Yale results were recently published in the journal Proceedings of the National Academy of Science, and found those chemicals assessed likely not a threat to human health

They also found that there was a correlation between the concentration of toxins and the nearest gas well that has had an environmental health and safety violation reported.

Researchers also noted that shale underlying the Pennsylvania surface did not cause any organic chemicals to seep into groundwater aquifers.

However, these findings may not be applicable to all locations worldwide.

The researchers also said it was too early to declare that fraccing was completely safe without more investigation.

Earlier the US EPA found there was no evidence that fraccing has led to widespread, systemic impacts on drinking water resources in the United States.

The Yale report follows research from Duke University, published in the journal Environmental Science & Technology Letters which found that fraccing accounted for less than 1% of water used nationwide for industrial purposes.

This suggested that the natural gas extraction processes are far less water-intensive than previously thought.

Duke researchers evaluated the overall water footprint of hydraulic fracturing of unconventional shale gas and oil throughout the US, finding that between 2005 and 2014, unconventional shale gas and oil extraction used 708 billion litres and 232BL of water, respectively.

Integrated data from 6-10 years of operation yielded 803BL of combined flowback and produced water from unconventional shale gas and oil formations.

While the hydraulic fracturing revolution has increased water use and wastewater production in the United States, its water use and produced water intensity is lower than other energy extraction methods and represents only a fraction of total industrial water use nationwide, Duke found.