By Nicola Jones of Nature magazine

When audiences saw dramatic scenes of people setting their tap water on fire in the Oscar-nominated documentary Gasland, hydraulic fracturing, or "fracking," was thrown into the spotlight. The technique, in which high-pressure fluids are pumped into shale formations to fracture the rock and force out natural gas, has been accused of releasing methane into well water (hence the flammable water), polluting groundwater with toxic chemicals and even causing earthquakes.

On May 5, U.S. Department of Energy head Steven Chu set up an expert panel to make recommendations on how to improve the safety and environmental performance of fracking. Several areas, including the state of New Jersey, the city of Buffalo, New York, and the Canadian province of Quebec, have recently banned the practice or issued moratoriums on new development. On May, 11 the lower house in the French parliament voted to ban the technique, with the upper house expected to follow suit next month--although the proposed ban does include an exemption for research on the technology and its environmental impact. Nature checks out how valid the accusations are and how the technique is being regulated.

Why frack?

Vast quantities of natural gas lurk in shale rock formations deep underground: shale reserves are thought to contain 187 trillion cubic metres of natural gas. This adds about 40 percent to previous estimates of global recoverable gas resources that don't include shale reserves. The largest known reserves are in China, the United States, Argentina and Mexico, in that order.

To get the gas out, companies drill down into the shale, then sideways through the deposit. This approach took off in the 1980s, with U.S. production increasing more than ten times from 2000 to 2010. The U.S. Energy Information Administration estimates that shale gas makes up 34 percent of the nation's natural-gas production, rising to 47 percent by 2035.

Does fracking release methane into tap water?

Many people living near fracking sites have complained of bubbly or flammable water. Methane can leak into wellwater totally naturally. A study in the May 9 issue of Proceedings of the National Academy of Sciences (PNAS) reported a link between methane in well water and fracking operations in northeastern Pennsylvania and nearby areas of New York state (see "Methane threat to drinking water"). The authors concluded the methane was probably coming from leaky well pipes, which would mean an easy fix. But it was also possible that the fracturing process created cracks that let methane seep upwards into groundwater.

Methane doesn't make water undrinkable--but the gas can cause asphyxiation or explosions in confined spaces. Some researchers have called for studies into the possible health effects of breathing in methane.

Does fracking pollute groundwater?

Fracking fluids contain many compounds, including sand, ground-up walnut shells, salt, citric acid, benzene--a known carcinogen--and lead. Some of these help to keep the fractures propped open; some reduce water surface tension and keep particles suspended in the fluid even at high pressures; and some are biocides that stop bacteria from clogging up the well.

A U.S. Congressional report released in April showed that the 14 most active hydraulic fracturing companies in the United States together used nearly 3 billion liters of fracking fluid, not including water. The products contained at least 29 chemicals that are known or possible human carcinogens.

The question is what concentrations these chemicals are used in, and whether the substances (or natural hazards such as radioactive radium present in the rock) escape the drilling area and contaminate water supplies--either by leaking out from the drill site, or through improper disposal. An investigation by The New York Times revealed worrying levels of radioactivity in fracking wastewater going to rivers in Pennsylvania. The PNAS study, however, found no evidence that fracking fluids were leaching into wells. The US Environmental Protection Agency launched a study of these questions in March last year--but the agency doesn't expect results to start coming in until the end of 2012, with a report scheduled for 2014.

Aren't there regulations in place to prevent pollution?

Fracking is exempt from the US Safe Drinking Water Act, although the U.S. House of Representatives did introduce a bill in March to close that loophole. The bill seems to have strong Democratic support, but it is unclear how popular it will prove overall.

In November last year, the US Department of the Interior said that it was considering requiring all US companies to reveal the chemicals used in their fracking fluids. Several states have already passed laws along these lines--Colorado has mandated fracking-chemical disclosures, for example, and Texas is now working on a similar rule.

Are efforts being made to reduce pollution?

Halliburton, a company based in Houston, Texas, that supplies products and services to energy companies, has developed CleanStim--a fracking fluid made with ingredients "sourced from the food industry" (though the company warns that the product should not be considered edible), along with water-treatment systems and more. And at least one company--Poseidon Sciences in New York--is investigating biocides that can't escape a well, such as selenium-coated sand particles.

Does fracking cause earthquakes?

There have been no cases in which hydraulic fracturing is suspected to have triggered a quake. But the way in which many companies get rid of their drilling water--by pumping it into disposal wells--definitely can. In Colorado in the 1960s, injection wells were linked to quakes as large as a magnitude-5 one that caused damage in Denver, says Scott Ausbrooks of the Arkansas Geological Survey in Little Rock. Today, injection wells in Arkansas have been shut down while geologists examine a possible link to a recent swarm of quakes, in an area that Ausbrooks says could potentially trigger a magnitude 6. There are thousands of injection wells across the country, says Ausbrooks, and only a couple have been linked to seismicity.

Fracking itself is a gentler activity. "You are generating micro seismic events, but the amount of energy is minute. We had a [seismic] station half a mile from where they were fracking and we couldn't detect it," Ausbrooks says.

When will the energy department release its results?

The panel will report on any immediate steps that would improve safety and environmental protection within 3 months; its full consensus advice will take 6 months. The panel is not providing media interviews.

This article is reproduced with permission from the magazine Nature. The article was first published on May 12, 2011.