Underground drilling that threatens to pollute half the nation's drinking water will come under some federal control with new rules announced yesterday by the Environmental Protection Agency.

The oil industry has threatened to take the rules to court on the grounds that they are unnecessary and could cost more than $5 billion to implement. But EPA officials estimated the cost at $1 billion or less and said that, considering the danger, the rules are cheap at the price.

"About half the U.S. population now depends on underground water for drinking," said EPA Administrator Douglas Costle in a statement. "Once polluted an underground source is difficult, if not impossible, to clean up."

Groundwater may get contaminated by seepage from abandoned waste dumps, unsafe landfills or other disposal sites and by polluted streams, all of which are being tackled under other EPA programs.

Yesterday's rules are the first federal attempt to deal with contamination from direct injection, which may occur in disposing of sewage sludge or hazardous chemicals, in oil and gas drilling techniques for secondary recovery that use saltwater or chemical injection to boost production, or in various mining methods. Some injection wells that penetrate far below groundwater levels also leak pollution higher up.

Groundwater is generally pumped directly into homes for drinking, unlike river water, which is treated at central locations for distribution to the public. Polluted groundwater therefore poses a potentially serious public health problem, Costle said.

There are an estimated 650,000 wells nationwide that will have to be surveyed under the program announced yesterday. The plan outlines minimum state and local control rules for five kinds of wells.

An estimated 7,500 wells are of the Class IV type, the "bad actors," in the words of Victor Kimm, EPA's deputy assistant administrator for drinking water. These wells inject known hazardous wastes into or above underground sources of drinking water.

EPA wants to stop this practice. Under the new rules, states and localities have 18 months to device plans to meet the federal minimum, and an additional six months to begin implementation.

But Class IV wells will be subject on an interim basis to rules promulgated under the Resources Conservation and Recovery Act (RCRA) that go into effect in October, and to EPA rules on hazardous wastes expected to be issued that month.

In effect, Kimm said, hazardous waste wells are being covered from two directions, although not until October. "Eventually they will get picked up under the state program," he said, which is part of the Safe Drinking Water Act.

Most states now regulate underground injection but "the quality of their programs is uneven," the EPA statement said. Even under the new federal guidelines, most nontoxic-injection well owners will have four years after the state program is enacted to apply for a permit to continue.

Roy Carlson, director of production for the American Petroleum Institute, said most state laws are sufficient now. "There's been little or no evidence of contamination of groundwater operations from injection operations" by oil and gas drillers, he said.