THE CLEAN Water Act as amended over the years has had important success. Part of the cleanup was achieved through billions of dollars in federal grants to help polluting population centers build adequate sewage treatment plants.

The Reagan administration came to office determined for fiscal and philosophical reasons to cancel the grants program. It argued that the construction projects had too often turned to pork, that the program had largely accomplished the objectives for which it was created and that it was time to return the function of waste-water treatment, like so many costly other projects, to the state and local level from which it had come and where it belonged.

In 1987 a reluctant Congress, which had already reduced the federal share of construction costs, appeared to yield. Legislation was passed to phase out the construction grants in favor of helping the states create revolving loan funds. Once those were filled, the feds would retire from the field.

Now, however, in what may prove to be the most important environmental legislation in this Congress, the Clean Water Act is back up for tinkering, the ideological climate has changed and Congress appears disposed to reconsider its 1987 decision. The idea is not necessarily to revert to the old grants program but to preserve the current spending level of about $2 billion a year on clean water, one of the few large environmental expenditures in the budget.

The basic argument is that worrisome levels and sources of pollution persist and that this is the time to reduce neither federal investment in this form of infrastructure -- favorite buzzword -- nor federal aid to ailing state and local governments. The mere maintenance of current spending will not increase the projected deficit, those opposed to a phase-out say. Meanwhile, many small communities least able to help themselves still need help with sewage treatment, and other problems beg to be addressed -- the problem of separating older storm from sanitary sewers for example, and the problems of generalized or "non-point" pollution that falls or seeps into a body of water instead of entering from a particular pipe.

Non-point sources now account for more than half the nation's water pollution, but are particularly difficult for the federal government to reach. Often the only way to control the pollution is to control land use, a jealously guarded state and local government preserve. Congress needs to induce state and local governments to act, which means it must offer them funds.

Congress ought to resist just reviving the often porky public works program it rightly voted to terminate in 1987. But if ways can be found that the same money can be more broadly and imaginatively spent, $2 billion a year is hardly too much for the government to spend on clean water.