* EPA says needs information to conduct fracking study
* Of nine companies, EPA only subpoenaed Halliburton
* EPA says Halliburton submitted publicly available info (Recasts, updates with comment from environmental group)
By Timothy Gardner
WASHINGTON, Nov 9 (Reuters) - The U.S. Environmental Protection Agency said on Tuesday it has issued a subpoena to Halliburton (HAL.N), demanding information about chemicals it uses in a natural gas drilling technique called "fracking."
In September, the EPA had asked nine companies that practice hydraulic fracturing, or fracking, to reveal the mix of chemicals they use in the practice which is opposed by environmental groups worried about its effect on drinking water.
All but Halliburton provided the necessary information, the EPA said.
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The EPA says it needs the data on fracking fluids to complete its comprehensive study of the technique. During fracking, companies inject millions of gallons of water, sand and chemicals as far as two miles (3.2 km) underground to break open fissures in the gas-bearing shale.
The EPA is slated to release preliminary results of the study, commissioned by Congress, by the end of 2012.
Fracking has been around for decades, but companies have expanded its use in recent years to extract abundant but hard-to-access reserves of shale gas.
Homeowners near some shale field developments say fracking has made their tap water toxic, and that in some cases flammable gases have escaped through their spigots.
Halliburton said it has been working with the EPA on its request and had provided some 5,000 pages of documents as of last week. The company said it was meeting with the EPA "in order to help narrow the focus of their unreasonable demands" that could require them to prepare about 50,000 spreadsheets.
An EPA official said Halliburton had submitted publicly available forms that describe the general hazards associated with various chemical mixes.
"While these are helpful, they do not identify each and every chemical in a mixture, which is precisely the sort of information EPA needs to understand the risks these ... mixtures may pose to drinking water supplies," the official said.
The EPA had also asked all of the companies for standard operating procedures that govern the choice and amount of chemicals and water used in fracking jobs, along with locations of every fracking job undertaken in the previous year or planned for the next. Halliburton has not given that information.
In 2005, the administration of George W. Bush passed the so-called "Halliburton Loophole" which exempts fracking companies from regulation under the Safe Drinking Water Act.
Environmentalists said Halliburton should be required to reveal the chemicals.
"As if being exempt from the Safe Drinking Water Act wasn't enough, Halliburton is now refusing to comply with EPA requests designed to protect public health," said Megan Klein, an associate lawyer with Earthjustice.
The companies that had provided EPA with the information are Schlumberger (SLB.N), BJ Services BJS.MX, Complete Production Services CPX.N, Key Energy Services (KEG.N), Patterson-UTI (PTEN.O), RPC Inc (RES.N), Superior Well Services SWSIP.PK and Weatherford (WFT.N). (Reporting by Timothy Gardner; Editing by Marguerita Choy)