THE CLEAN water act has been on the books for a quarter-century. Most of the time has been spent reducing pollution from large individual sources, mainly factories and sewage treatment plants. The effort has been remarkably successful, yet a lot of the water in the country is still dirty -- too dirty to fish or swim in, for example -- and the remaining pollution, much of it diffuse urban and agricultural runoff, is harder to get at.

The Clinton administration last week announced a strategy for doing so. The announcement -- a single paragraph in the president's Saturday radio address, plus a short press release from the Environmental Protection Agency and an invitation to read more on its Web page -- was almost casual, given the transformations the strategy would entail. The extensive pollution that remains is harder to deal with not so much for technical as for political reasons. It requires changes in behavior, not on the part of a relatively limited number of easily identifiable companies and municipalities but on the part of the populations of entire watersheds, often spread across multiple states. Who's going to tell them what to do, and how?

The EPA proposes to require the not always willing states to enforce a long-neglected part of the act involving what are known as TMDLs -- the total maximum daily loads of given pollutants that a body of water can absorb and still meet the definition of clean. States will have to inventory their lakes and streams, identify -- worst cases first -- those that, despite the industrial and municipal cleanup that has occurred thus far, are still not clean enough, and figure out how much more each pollutant needs to be reduced to get them clean. Then comes the hard part. The states would have to allocate the further reductions among classes of polluters -- so much to come from agriculture, so much from urban areas, so much more from factories and municipal facilities already regulated. Then they would have to come up with plans providing "reasonable assurance" that the reductions would actually occur. Among much else, the regulation envisions a market in pollution reduction, in which those able to reduce pollution more than the law required could sell their excess ability to others having to struggle to comply.

The government was driven to act in part by a series of mostly successful lawsuits by environmental and other private plaintiffs, insisting, in more than half the states, that the TMDL provision be enforced. There is almost sure to be resistance in both courts and Congress. The argument in the courts will be that the EPA lacks the power under the act to regulate generalized urban and agricultural pollution. The effort in Congress will meanwhile be to strip it of such power in the future. The House passed legislation in 1995 -- it ultimately died in the Senate -- that would have weakened the EPA's enforcement power, and that was well before the administration took the action it now has.

The TMDL proposal provides the framework for accomplishing a necessary task. But enforcement is the key, and ultimately that becomes a political question, or a long string of them. The regulation is unlikely to take formal effect until about the time the Clinton administration is leaving town. The meaning of last Saturday's radio address will be determined by the next administration.