FIFTEEN YEARS AGO, President Johnson pledged that "we are going to reopen the Potomac for swimming by 1975." Seven years later, Congress, in the Clean Water Act, directed that the Potomac (along with all other waters in this country) should meet this goal by 1983. Last week a new study suggested that swimmable, fishable water in the Potomac lies as far off as ever.

Excesses of nitrogen and phosphorus that support the growth of algae and of viruses and other disease-causing organisms constitute the Potomac's biggest problem. These lead to the slimy, smelly, floating mats that Washingtonians have come to know and hate, and to other kinds of algae suspended in the water, giving it a clarity about equal to pea soup. Algal growth and decomposition remove so much oxygen from the water beneath that fish can no longer survive.Controlling the algae, then, is the key to cleaning up the water.

In most lakes and rivers, the algae run out of phosphorus before they use up the available nitrogen, so the usual clean-up method is simply to remove the phosphorus. But the new study, performed by the Environmental Defense Fund at its own expense, concludes that where there is a lot of mixing in the water, such as in the Potomac basin, the reverse is true, and nitrogen is the growth-limiting factor. Therefore, money and effort spent on removing phosphorus may be wasted. Nearly $5 million is now being spent each year to remove phosphorus at the Potomac's Blue Plains treatment plant, and there are plans to double that amount. Doing this, the study's authors believe, wastes the taxpayers' money. The river is getting marginally less polluted, but the phosphorus approach will never control the algae enough to make the water clean.

The point is this: if what's being done is not working, local and federal authorities should say so and tackle and admittedly more difficult job of removing nitrogen. Conventional methods for doing that are too expensive, so a measure of technical creativity and political guts will be needed. Whatever it takes, we'd like to see it happen. Like everyone else we have long been determined to take a swim in the Potomac before we die. But not just a few minutes before.