THE AREA'S SEWER pipes are leaking, and that's bad news.But it could be worse - the leaks aren't letting sewage out. Instead, they're letting water in, from other pipes and from the ground. Even this sort of seepage, however, is serious enough when it occurs on the scale reported by staff writer Thomas Grubisich this week. District officials think that infiltration along the city's 3,500 miles of aging pipes increases sewage flows to Blue Plains by at least 30 million gallons per day (mgd) - and much more in wet weather. The Washington Suburban Santary Commission estimates that leaks feed 52 mgd into its 3,300 miles of pipes. In sum, at least 30 percent of the roughly 280 mgd treated at Blue Plains seems to be water that need not be there.

Repairing the pipes would obviously let Blue Plains accomodate more development. The city and Montgomery County had hoped to avoid growing pains by building the controversial regional plant at Dickerson. Since that expensive project has been shelved, officials are focusing more on leak-plugging as a way to gain nearly as much capacity at far less public cost. Right now the District and WSSC can justify spending about $22 million, three-fourths of which could be federal aid, to stop about 30 mgd of infiltration. As the price of new plants keeps rising, better maintenance will become an even better deal.

Besides being good policy, this attention to maintenance is a very good sign. Like other recent efforts, it points toward a new regional approach that emphasizes conservation and wiser use of existing water and sewer facilities. There are some drawbacks; patching pipes will, involve digging up some streets, and hardly anyone likes that. The alternatives, though, could be a building moratorium - or some project even more immense than Dickerson. Given those choices, we think most taxpayers would agree: The District and WSSC should plug those leaks.