Waltham, Mass. — Under an aluminum-gray sky, Ronald Gasper is making a small contribution to improving the nation's plumbing. The broom-mustached engineer lays a sensitive microphone on top of a buried water pipe and then places another on the same pipe about 150 feet up the street. Two wires are plugged into a microprocessor in a nearby van.
Within minutes, a Christmas tree of bubbles appears on a screen. ''This should show within a foot where the leak is,'' Mr. Gasper proclaims to a group of engineers, red-cheeked on a late-autumn day. ''Oh, wait a minute, we parked the van right over it.''
Mr. Gasper, a consultant, is part of a Massachusetts effort to tutor engineers on the latest methods of preventing leaks in city water lines from turning into small ponds in roadways and basements.
The program is symbolic of a new thrust across the country to pay more attention to that vast but decaying network of subterranean pipes and sewers so vital to the comfort of modern civilization.
A small but growing number of cities are tapping a new breed of engineer, armed with the latest tools and techniques, to find better ways of refurbishing the old and prolonging the life of the new.
New Yorkers were painfully reminded of this necessity in September, when bursting water mains shut down the bustling garment district and sent many Manhattanites squishing to work in rolled-up pants.
The trend to do something about aging pipes marks what may be an important shift in thinking: from the fix-it-after-it-breaks mentality to a more preventative approach. In some cases, thumbs are being put in city pipes as part of water-conservation programs - an ethic many believe must become more ingrained if the country is to have enough of this dwindling resource in the future.
''Most cities just deal with problems on a crisis response,'' says Albert Doyle, managing engineer with Brown & Caldwell, a California consulting firm. ''Now some are at least starting to recognize the problem.''
Spurring the movement is the rickety state of the nation's infrastructure, plumbing included. Water systems in many old Northeastern cities are in particularly bad shape. An Urban Institute study earlier this year, for instance , showed that Boston annually loses some 17 percent of the water entering its system because of leaky pipes. Philadelphia loses 19 percent; Buffalo 15 percent; Chicago 12 percent.
For many cities, the repairs are nothing a lot of money wouldn't help fix. But in these austere times, town officials are hard pressed to keep policemen on the beat, much less pour money into leak sleuthing. Even when city, state, or federal funds do come in, they often go for repair of more visible needs: roads and bridges, for instance. Underground utility lines have few boosters.
''If you were a politician, which would you rather have your picture taken next to - a new waste-water treatment plant or some old repaired pipe in the ground?'' asks Richard Noss, an assistant environmental-engineering professor at the University of Massachusetts at Amherst.
The price tag for repairs is huge. Studies estimate that $225 billion will be needed over the next 10 years to upgrade sewer and water systems. Some help may be on the way. Two bills now before Congress would boost federal support for water-main repairs.
States, too, are paying closer attention. The Massachusetts Metropolitan District Commission, for instance, is fashioning a $100 million leak-repair and water-conservation plan that, if implemented, would save 30,000 gallons a day over 40 years, it is said.
Even so, the continuing cash crunch and mounting problems of decay in the nation's basement are forcing cities to find more resourceful ways to cope. Some examples:
* In St. Louis, men armed with infrared cameras and ground-penetrating radar can be seen these days shuttling back and forth high above streets in hydraulic lift trucks (''cherry pickers''). The aim: to spot flaws that might lead to breaks in sewer lines.
The city has good reason to go to such lengths. Much of its 1,100-mile network of round-, egg-, and horseshoe-shaped sluiceways - made of timeworn brick, mortar, and wood - date back to the 1860s. Three major sewer collapses, including one that took a building down with it, have cost the city $5 million in repairs in three years.
For this reason, the Metropolitan Sewer District has gone beyond the usual inspection techniques. These include using TV cameras shuttled between manhole covers on dollies and visual surveys by oxygen-masked engineers. Instead, the St. Louis authorities hope infrared scanning, radar, and deflectometers (devices that put pressure on a roadway and then measure the dip) will point up the worst cracks in ancient archways as well as the cavities around them which often lead to cave-ins. Many cities are closely watching the St. Louis experiment.
''We may prove unsuccessful, but we can't afford to wait anymore for something to happen - and maybe someone to get hurt,'' says John Koeper, maintenance director for the sewer district.
* In suburban Washington, engineers are using unusual plastic liners to refurbish old sewers. Towns in Maryland's Montgomery and Prince Georges Counties admittedly don't have the most antiquated plumbing system in the US. But the Washington Suburban Sanitary Commission (WSSC) figures it can fix up old pipes for half the cost of putting in new ones.
One weapon: a felt and polyurethane tube saturated on the inside with resin. It can be slipped inside old sewer pipes. When filled with hot water, it turns inside out like a sock and cements to the duct walls, forming a plastic liner. A number of cities are now using similar systems. But probably no one has taken it as far as the WSSC, which lays 14 miles of the pipe sheathing a year. The process has long been used in repair-minded Britain, but only recently here.
* In Sacramento, engineers are lobbing numbers at woeful water mains and seaping sewers. They have been statistically mapping where the worst flaws are hiding. The reason: to determine where scarce funds can be best used. Nearly all towns, of course, keep inspection books. But some consultants consider Sacramento's dusty sewer logs and ''leak'' maps Michener-esque in thoroughness.
With the data, ''scenarios'' are worked out to help anticipate problems in the sewer system. Computers are used to create models of water flows and then to gauge where leaks may spurt.
None of this, however, will replace the field crews who visually scout for problems. Indeed, some old hands - using just their experience and a geophone (a water worker's stethoscope) - can pinpoint not only where leaks are but also their size from the sounds they make.
New tools, nevertheless, can cut down on the number of costly and irksome ''dry holes'' bored in streets in search of leaks. In a few cases, too, technology comes in handy for more than just repair work. When Sacramento water and sewer officials, for instance, meet with skepticism on the part of city politicians about the scope of municipal plumbing problems, they bring in ''dirty movies.'' These are TV tapes routinely taken of the city's buried sewer mains. What they show, often in vivid if not entertaining detail, are the piles of rubble and gaping holes in the lines - as well as an occasional rat.
Do the tapes produce the desired effect? The politicians ''at least realize what we're facing,'' Mr. Bitten says. ''They're convincing to us - and we see them all the time.''
Ultimately, though, observers agree that it will take a lot more footage, financing, and forward thinking to sort out the country's underground utility problems. But at least, they point out, a few fingers are beginning to be put in the dike. City water leaks
TThe following is a spot check of US cities and their estimated water losses
Leakage Percent of water City percentage not accounted for* Philadelphia 18.9 35.0 Boston 17.5 39.0 Buffalo, N.Y. 15.0 20.0 Houston 13.2 24.0 Chicago 12.0 (Not available) Portland, Ore. 8.5 17.0 New Orleans 7.6 8.0 Independence, Mo. 7.0 7.0 Dallas 6.6 6.6 Louisville 4.2 17.0 Tucson 3.0 5.0 Iowa City 1.5 2.0 Kansas City, Mo. 0 21.0 * Water not accounted for is due to faulty meters and/or unmetered municipal uses, such as in government buildings or for firefighting. Source: Urban Institute