THE COUNCIL of Governments' two-year, $3 million effort to define the region's future water-pollution control policies looks as though it's going to be a bust. COG might have tried to produce a detailed 20-year plan, since that's what federal law requires as a condition of future cleanup aid. Or it might at least have brought some imagination to the dead-serious problems of protecting the area's water supplies and coping with the pollution generated by two more decades of growth. Instead, the draft being circulated for review amounts to a two-inch-thick status report.

In some ways that status is good. Pollution control here is far better than it was decade ago, and far ahead of that in many metropolitan areas now. High standards of sewage treatment are firmly established. The most complex problems now invorlve "nonpoint" pollution - the dirty runoff from construction sites, paved areas, rooftops, fields and the like. COG estimates that that threat to water quality could increase by 25 percent in the next 20 years. Area governments are gradually devising controls.

What are the biggest defects in the draft report? First, there is Dickerson. The Environmental Protection Agency disapproved that proposed upper Montgemery County treatment platnt two years ago, but County Executive James Gleason and Maryland state officials refuse to give up. They have taken EPA to court, enlisted the District's aid, and pushed COG to endorse the project in the curent draft. But that plant may never be built - and quite possibly shouldn't be. It's soaring costs and energy demands make alternatives such as land treatment or expansion of other plants look wiser all the time. The COG plan does mention such options, but leaves the whole subject just hanging there.

Then there's the matter of water supplies. Should Dickerson or any other treatment be allowed t discharge upstream from water intakes? COG leaves that to be fought out case by case. And there's the converse question: How much water should suppliers leave in the river to protect its quality? That is a major issue in debates over the new low-flow agreement and Fairfax County's proposed Potomac intake. A serious regional plan should surely resolve such a basic question of water quality.The COG draft ducks.

All this is disappointing. It suggests that area officials are still ready to get together on some of the main pollution-control questions of the day. Worse, it suggest more years of wrestling out such issues one by one through tedious negotiations, short-term alliances, more lawsuits and, no doubt, more intervention by EPA. An important opportunity to avoid this gloomy prospect will have been lost unless area leaders do a lot of rethinking and redrafting in the next few months.