WASHINGTON — The Interior Department is poised to issue a final rule that will make it easier for mountaintop mining companies to dump their waste near rivers and streams, the agency announced Friday.

The environmental impact statement released Friday afternoon by Interior's Office of Surface Mining overhauls a 1983 regulation protecting water quality that has been regularly flouted by mining companies. It marks the next-to-last step in a 4 1/2-year battle over how companies should dispose of the rubble and slurry created when they blow the tops off of mountains to get to the coal buried below.

The revised rule will take effect after a 30-day review by the Environmental Protection Agency, making it one of the last significant changes to environmental regulations by the Bush administration.

For a quarter of a century, the government has prohibited mining operators from dumping "valley fills," massive piles of debris created by mountaintop removal, within 100 feet of any intermittent or permanent stream if the material harms the stream's water quality or reduces its flow.

Mining companies have frequently disregarded the law: by the administration's estimate, roughly 1,600 miles of streams in Appalachia have been wiped out by such activities since the mid-1980s.

The revised rule calls on companies to avoid the 100-foot stream buffer zone "or show why avoidance is not possible," said a statement from the Office of Surface Mining. If they do dump the waste in the buffer zone, they must try to minimize or avoid harming streams "to the extent practicable."

The agency said the change in regulation would have a "slightly positive" effect on the environment "because it requires coal mining operations to minimize certain impacts," but Joan Mulhern, senior legislative counsel for the environmental law firm Earthjustice, called the environmental impact statement "totally inadequate."

"It didn't even include the alternative of actually enforcing the rule on the books," she said in an interview. "The implications of this ruling are devastating, they're widespread and they're irreversible."

Mountaintop mining removal — which is both less expensive and somewhat safer than traditional underground mining — is used widely in West Virginia and Kentucky and, to a lesser extent, in Virginia and Tennessee. The practice provides access to valuable low-sulfur coal seams but generates large amounts of waste. While companies are required to restore the land they have bulldozed and dynamited to reach the coal, they are left with large amounts of rubble and sludge that is usually trucked away to other valleys.

Environmentalists have been feuding with state and federal agencies over how to interpret the 1983 law for more than a decade. A federal court ruled in the environmentalists' favor in 1999, but that judgment was overturned two years later. Former President Clinton pushed to restrict dumping of mining waste but left office before enacting any changes. The Bush administration has been seeking to rewrite the law since it took office.

Before the rule takes effect, Environmental Protection Agency administrator Stephen L. Johnson must certify in writing that the new regulation complies with the Clean Water and Clean Air Acts and determine that the environmental impact statement was adequate.

Mulhern said that she hoped Johnson would block the rule, but if he does not, environmental groups will fight the regulation in court: "I can't imagine a circumstance in which this is not going to be challenged by environmental groups."