THIS REGION'S impressive efforts to curb water pollution and cope with growth have been fueled by an equally impressive $690 million or so in federal grants for sewage-treatment projects. Now that federal tap is being turned off. Instead of aiding more projects that mainly serve the new development, the Environmental Protection Agency has decided to give priority to areas such as tidewater Virginia and Baltimore where pollution control has lagged. In national perspective, that policy is hard to quarrel with. But for this region, it's a jolt. Area leaders must now find different ways to maintain the cleanup progress without stopping growth, imposing huge new costs on local taxpayers or getting mired in more dreary squabbling.

Some promising methods exist. The trick is seeing them. Consider the problem that Montgomery County and the District have been trying to solve in the old way, by expanding their waste-treatment capacities. Lame-duck Mayor Walter Washington has touched off another row by asserting a claim to a larger share of the Blue Plains regional treatment plant. Meanwhile, retiring Montgomery County Executive James P. Gleason has capped his career by persuading the outgoing county council to build a 20-million-gallon-per-day plant in, of all Places, Potomac. Even if the project gets all necessary clearances, it will take several years to complete - and the county will have to bear the whole cost, $52 million or more.

Is all this necessary? Maybe not - if the newly elected leaders in Maryland and the District can find a way to do things differently. To start with, the District's own studies show that fixing leaky pipes, to keep groundwater from coming in, could cut flows to Blue Plains by far more than the city stands to gain by upsetting the regional pact. Repairs at the McMillan Reservoir alone could reportedly save up to 4.5 million gallons per day. Patching suburban pipes could greatly ease Montgomery County's strains. And federal aid for stopping leaks is still available, making it an even better bargain from the local point of view.

Beyond tending to maintenance, the incoming District and suburban officials should rebuild regional political accords. Prince George's County Executive-elect Lawrence J. Hogan is the crucial player in this field. Prince Georg's officials have snarled progress for years by refusing to expand their Piscataway treatment plant to accommodate Montgomery County. If sewer policy were controlled by economics, engineering, water-supply protection or the force of gravity - anything but politics - that would have been done long ago. In the process, pressure on Blue Plains could also have been reduced. If Mr. Hogan uses his considerable political charms to advance more far-sighted policies, the whole regional atmosphere could be vastly improved.

Finally, where existing systems are inadequate or out of reach, EPA is encouraging local governments to try new technologies that may well consume less money and energy than conventional treatment plants. In outlying areas, land treatment or innovative septic systems may work well. A growing array of self-contained systems offer alternatives for sub-divisions and even for urban office complexes. In this respect, too, local leaders should see EPA's refusal to finance more of the same as an encouragement - not a threat - to regional growth.