Environmental contamination from operations to remove gas from deep within the Earth, known as hydraulic fracturing, often happens close to surface and not far below, said a US study released Thursday.

Spills at the drill site or problems with cement casing around upper well bores were examples of incidents that have led to shallow groundwater contamination in the United States, said the study by the University of Texas.

"Most of what we have seen happening related to shale gas development that impacts the environment was at or near the surface," said project leader Charles "Chip" Groat, presenting the findings at a major science conference in Vancouver.

"We found no direct evidence that hydraulic fracturing itself -- the practice of fracturing the rocks -- had contaminated shallow groundwater," he added.

"However that doesn't mean that there aren't other parts of the process of gas development that could get things you don't want in shallow groundwater."

Groat said the review of fracking operations in Texas, Louisiana, and Marcellus Shale area throughout the northeastern United States was funded by the university and that the team had turned down industry funds.

The report, called "Fact-based Regulation for Environmental Protection in Shale Gas Development," aimed to "separate fact from fiction" and give policy makers a tool going forward as the US experiences a major natural gas boom, Groat said.

However, he admitted that the amount of actual science out there to measure the reports of tap water that can be lit on fire, earthquakes resulting from fracking and potentially damaging methane emissions, is thin.

"We spent a lot of effort looking at the scientific evidence -- which isn't profuse unfortunately, although the scientists are getting more engaged -- and the regulatory evidence from those states to determine what violations had been identified and what the severity was," he said.

The study found that "surface spills of of fracturing fluids pose greater risks to groundwater sources than from hydraulic fracturing itself," and described those problems as common to other forms of oil and gas development as well.

It did not call for a strict new regulatory framework but said individual states could take steps to supplement the regulations already in place.

Questions about gas in drinking water and methane emissions will be examined "more intensely in the months ahead" as more research becomes available, Groat added.

Hydraulic fracturing, or "fracking," is a process by which high-pressure injections of water, sand and chemicals are used to blast through rock to release oil and gas trapped inside.

Advances in horizontal drilling have helped create a boom in the industry, and the US Energy Information Administration has said natural gas reserves could supply US needs for 110 years.

Estimates of US shale gas resources are about 862 trillion cubic feet, a figure which doubled from 2010 to 2011, and shale contributes to 23 percent of the US natural gas supply, expecting to reach 46 percent by 2035 according to the Texas study.

Over 3,000 gas wells have been drilled in Pennsylvania alone in the past six years, according to industry figures, and 15,000 in north Texas, helping drive down the price of natural gas.

Critics however say the industry has moved too fast with little regulation, and cite concerns about spills, leaks and contamination from chemicals used in the process. Similar debates are ongoing in Canada, France and other countries.