The EPA - which probably has more experience with the impacts of hydraulic fracturing than any other government agency in the world - was asked by the US Congress to develop a state-of-the-science assessment that provides a review and synthesis of available information concerning the potential impacts of hydraulic fracturing for oil and gas on drinking water resources in the US.

Over the past five years the EPA has investigated the issue, and has found that while there were some reports of contaminated drinking water in Texas and Pennsylvania, there was no evidence that the fraccing boom has led to systematic or widespread harm to US freshwater supplies.

While there have been more than 30,000 wells drilled and fracced since 2011, the incidents of contamination have been modest in comparison, although the EPA did point out there is a lack of long-term data on the quality of drinking water in many areas.

"We did not find evidence that these mechanisms have led to widespread, systemic impacts on drinking water resources in the US," the EPA found.

"Of the potential mechanisms identified in this report, we found specific instances where one or more mechanisms led to impacts on drinking water resources, including contamination of drinking water wells. The number of identified cases, however, was small compared to the number of hydraulically fractured wells."

The EPA found water withdrawals in situations of low water availability, frac fluid or produced water spills, below ground migration of liquids and gases or inadequate treatment and discharge of wastewater are all risk factors for contaminating underground drinking water resources.

The EPA admitted its findings could just as easily reflect a rarity of effects on drinking water resources or a simple lack of data.

It seems as if the EPA report won't solve the intractable issues between fraccing's proponents and opponents.

The oil industry has argued it supports the safety of fraccing, while green groups are emphasising the fact that the study found troubling instances when fraccing can go awry, saying the lack of data and reported intransigence from oil companies - wanting to keep propriety data secret - clouds the results.

Energy lobby groups claim the EPA's findings back up earlier studies by the Energy Department and US Geological Survey, with the American Petroleum Institute saying the study affirmed the sector's record of "continuous safety improvements."

However, the Natural Resources Defence Council claims the EPA report was the first time the body acknowledged the potential to poison drinking water, and the significant gaps in the scientific understanding of fraccing.

The Environmental Defence Fund says that even if fraccing were safe, it doesn't address all the issues, including the considerable amount of water used and the disposal of waste water, which must either be trucked away to surface pools or injected back underground.

In states such as Oklahoma, the latter practise has been tied to a jump in low-level earthquakes.

The EPA noted that accidents are relatively rare, but can be destructive. The EPA characterisation of hydraulic fracturing-related spills found that 8% of the 225 produced water events included in the study reached surface water or ground water.

These spills tended to be of greater volume than spills that did not reach a water body.

A well blowout in Bradford County, Pennsylvania, spilled an estimated 38,000 litres of produced water into a tributary of Towanda Creek, a state-designated trout fishery.

A recent paper in the US Proceedings of the National Academy of Sciences assessed 2-Butoxyethanol, a compound commonly found in Marcellus Shale drilling fluids, found in samples collected from three households in Pennsylvania in 2012, suggesting that more conservative well construction techniques should be used to avoid this in the future and that flowback should be better controlled.

The authors suggested a chain of events by which the drilling chemical ended up in a homeowner's water supply, albeit at levels which was within safety regulations and did not pose a health risk.

The contamination was likely from poor drilling well integrity from wells drilled by Chesapeake Energy in 2009 that were not cased below 300m.

The EPA said its assessment was not nation-wide, site specific, or a human health or risk assessment, and could not be used to evaluate best management practices or develop policy.

The EPA's draft assessment will now undergo a "rigorous peer review" by EPA's Scientific Advisory Board and will be open to public review and comment.

The EPA's report arrived in the same week that a new study from the University of Pittsburgh which found pregnant women who live close to multiple fracced wells in Pennsylvania were more likely to have babies with lower birth weights than were those who live further away.

While the study does not draw a conclusive link between fraccing and lower birth weights, the results have concerned the study's authors.

Pregnant women who lived within a few miles of six or more wells were 34% more likely to give birth to babies who were small for their age than were mothers who lived in lower well intensity areas.

Oil and gas industry advocacy groups criticised the study as being "full of deep and glaring flaws", saying there is no proof that benzene, toluene and xylene escaping from fracced wells could cause the issues.

But a study published last year found that pregnant women in rural Colorado who lived near multiple gas wells might be more likely to give birth to babies with congenital heart defects, while the Massachusetts Institute of Technology examined birth records from 2004 to 2011 and found that babies born to mothers who live near fraccing sites were more likely to have low birth weights and lower health scores than their counterparts who were born further away.

All want to conduct wider surveys.