By Deborah Zabarenko, Environment Correspondent

WASHINGTON, July 15 (Reuters) - The United States unveiled plans on Tuesday to bury climate-warming carbon dioxide emissions deep underground to keep the greenhouse gas from further heating up the atmosphere.

The burial process, known as carbon capture and storage or geologic sequestration, has long been part of the Bush administration's strategy to combat climate change without imposing any economy-wide limit on carbon emissions.

But this is the first time the U.S. government has proposed requirements on how to do it. No federal rule is expected until late 2010 at the earliest, according to Benjamin Grumbles of the Environmental Protection Agency.

A carbon storage operation has been in place since 1996 in porous rocks under the seabed off Norway, where operator StatoilHydro has stashed some 10 million tonnes of carbon dioxide. And last week, the oil-rich Canadian province of Alberta said it would set aside C$2 billion ($2.04 billion) for carbon capture and storage programs.

Carbon dioxide is already being injected into the ground in the United States to help energy companies recover more oil and natural gas, a process known as enhanced oil recovery, Grumbles said in a telephone briefing.

The proposed rule would create a new kind of well designed specifically for long-term storage of carbon dioxide, he said.

"We think that geologic sequestration is a promising, yet unproven, technology," Grumbles said.

The rule would be issued under the Safe Drinking Water Act and aims to safeguard underground water supplies from possible contamination.

"We want to make sure that there are environmental safeguards to prevent the migration of CO2 or any other type of substance into underground sources of drinking water," he said.

He noted that carbon dioxide is "not toxic or radioactive" -- it occurs naturally in addition to being emitted by vehicles, factories and coal-fired power plants -- but added that the carbon sequestration plan would put large amounts of it underground for long periods under high pressure.

The rule provides "extensive testing and monitoring so that the carbon dioxide does not migrate into an underground source of drinking water." If that happened, Grumbles said, the carbon dioxide could push other underground substances, like salts, into the water source.

Grumbles has been called to testify on July 24 before members of the U.S. House of Representatives Committee on Energy and Commerce on the environmental effects of carbon sequestration. ($1 = C$0.9981) (For more Reuters information on the environment, see http://blogs.reuters.com/environment/) (Editing by Eric Walsh)

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