The Clinton administration announced yesterday that it is preparing to enforce a largely ignored part of the federal Clean Water Act, requiring states to take aggressive additional steps to lessen pollution in 20,000 of the nation's rivers, lakes and bays.

Under new rules proposed by the Environmental Protection Agency, each state would have to write an individual game plan to clean every body of water that is too dirty for fishing and swimming--about two-fifths of the waterways in the United States.

For the first time, the federal government would compel states to determine the cause of the pollution and figure out how to reduce contaminants that wash off farm fields and city streets and toxins that spew from such identifiable sources as factories and sewage treatment pipes, which have been regulated for more than two decades.

Some states already have taken steps in that direction, including Maryland, which in 1997 became the first state to regulate farm runoff as part of its efforts to improve the Chesapeake Bay. But the EPA requirement would have a profound impact on other states--such as Virginia, which has not, for example, set limits on the discharge of nitrogen, a major source of water pollution.

"These steps will chart a course to clean up 20,000 waterways and ensure that they remain safe for generations to come," President Clinton said yesterday in his weekly radio address.

A quarter-century after Congress adopted the Clean Water Act, the proposed regulation represents the deferred second phase of what lawmakers originally had envisioned as a two-pronged attack on pollution of the nation's waters. It includes measures that environmentalists have sought and agriculture and business interests have resisted.

The proposed regulation was signed yesterday by EPA Administrator Carol M. Browner, who predicted that it would become final by the end of the year. In recent years, environmentalists in 31 states have lodged lawsuits against the EPA, seeking to force the agency to impose the kind of additional pollution controls the administration is embracing. More than half of those suits have led to settlements dictating when the EPA must step in if states do not act. Environmentalists interpreted the administration's action as capitulating to that wave of litigation.

"The bringing of more than 30 cases . . . has brought considerable pressure on EPA to revive this long-dormant part of the Clean Water Act," said Mark Izeman, a senior attorney in New York for the Natural Resources Defense Council. Izeman hailed the administration's action as "a major environmental proposal" but said its effectiveness would hinge on whether states insist on stringent efforts to clean up individual bodies of water--and whether the EPA intervenes if those efforts fall short.

The administration began work on this part of the Clean Water Act three years ago, when it assembled a broad-based group of advisers to help the agency forge an approach.

The proposal differs from current water regulation in two main ways. For one, it broadens the focus from the specific quality of discharges from individual polluters--such as factories and sewage treatment plants--to the overall quality of a body of water, taking into account its ability to handle contaminants and all the sources of pollution that enter it.

This focus on overall quality would require states to set a limit, known technically as a "total maximum daily load," for each body of water. Until now, states have been responsible mainly for granting permits that cover the kind of technology that must be used by steel mills, wastewater plants and other specific sites of "point-source" pollution to make sure their discharges do not exceed government standards.

The second key change is that states would be forced for the first time to reduce "non-source" pollution from more diffuse sources, including agricultural and urban runoff. That kind of pollution is more difficult to measure and control, but it has become a major threat to the nation's water, accounting for about 60 percent of current pollution.

For each body of dirty water, the state would have to negotiate with all the parties responsible for the pollution to determine which ones had to take--and pay for--the cleanup steps. States would have to submit their cleanup plans within two to three years, which in some cases could involve toughening restrictions already in place for identified polluters. States would have several years after that to make sure they are carried out. If a state shirked its job, the federal government would take over.

"This is the last chapter in how we get to fishable, swimmable waters for the people of this country," Browner said at a White House briefing.

EPA officials have assembled the first national inventory of the bodies of water that need such work. The list of 20,000 includes 300,000 miles of river and shoreline and nearly 5 million acres of lakes, Browner said.

Clinton also used his radio address to renew criticism of Republican plans to cut taxes by $792 billion over 10 years. He said the plan would damage the government's environmental programs, hindering its ability to crack down on toxic waste sites and forcing some national parks to close.

Staff writer Peter S. Goodman contributed to this report.

CAPTION: A recent fish kill hit the lower Chesapeake Bay in Maryland, which already regulates farm runoff. The EPA's plan requires other states to follow suit.