It was mentioned here several times in recent years that many of Washington' bridges were becoming unsafe.

The city is chronically short of money, and PM (preventive maintenance) is an easy target. When city hall quietly cancels PM work, few voters know or care. Other budget cuts trigger protests, but an unrepaired bridge doesn't stage a demonstration outside the mayor's office.

When I wrote about bridges that weren't being maintained properly, I had no idea that our water and sewer systems were also being denied PM. Staff writer Thomas W. Lippman's report in yesterday's editions described the full scope of the problem.

Lippman obtained a copy of a study made by the Federal City Council, a private organization of local business and political leaders. He began his description of the study's conclusions with these words:

"The financial crisis that has forced the District of Columbia government to lay off workers and cut services is also taking its toll on and beneath the city's bumpy streets. Cutting corners on maintenance to save money, the District is setting the stage for above-ground and subterranean nightmares by allowing water pipes, sewer lines, roads and bridges to deteriorate faster than they can be repaired. Century-old pipes and bridges eroded by salt have been neglected instead of repaired, and the cost of catching up is rising faster than the available resources."

We have 1,436 miles of water pipe, and some of it is 160 years old. There are 27,500 valves in the water system, and some of them date back to 1825. One-third of the fire hydrants installed in the last century need to be replaced. Sewer pipes are deteriorating. Mayor Barry knew before he took office that half the catch basins in the city's storm sewer system were clogged with debris. Nearly 150 miles of streets that should have been resurfaced in the past 16 years have not been. Of our 215 bridges, the District's Department of Transportation concedes that 50 are in need of repairs, but only four are being worked on.

Why?

No money.

Bare bones budgets are chronic -- not just here, but in cities all over America. When gasoline was cheap, we continued to look for jobs in urban centers, but we moved our living quarters horizontally: out to the suburbs. Perhaps we'll now have to begin a vertical move: up into high-rise dwellings inside our cities. But meanwhile our cities face the more pressing problem of neglected maintenance work.

The Federal City Council estimates it will take 10 years to catch up with deferred PM work and bring our pipes and roads and bridges back up to acceptable standards. During that 10 years, we will have to allocate "$100 million a year in 1980 dollars."

"In 1980 dollars" means the $100 million annual cost will increase at a rate determined by inflation. If inflation runs at a rate of 10 percent a year, in the tenth year our PM expenditure will have to be about $260 million. If inflation runs at 12 percent a year, the $100 million annual price tag will rise to $310 million.

Don't blame Marion Barry or anybody in his administration for this horror story. The problem was with us long before Barry was elected.

We have been putting off absolutely essential maintenance work for decades with the excuse that we had to balance the budget by "saving money" on PM.

The truth is that failure to perform routine maintenance work costs more in the long run, not less.

What do you think it would cost to build new bridges today? Or a new water system?

As Arnold Palmer tells us in those television commericials, it's a lot cheaper to take care of machinery than to replace it.

But even if we're all in agreement about the value of preventive maintenance, we're between a rock and a hard place when we begin looking for the money to pay for it. We say this is a particularly bad year to be told that we're $100 million a year worse off than we thought, but when was there ever a good year for that kind of news? Is there anything left to be taxed, or taxed at a higher rate, before the law of diminishing returns comes into play? What should we do now? What can we do now?

I don't know the answers.

It is quite likely that nobody at city hall has any easy answers, either.

Perhaps we'll just have to do a lot of praying and belt-tightening and hope we find a way to muddle through.

One thing is certain: A city cannot live without a water system, a sewer system, a mass transit system, and a network of streets, bridges and traffic control signals that permit people to go about their business without undue hardship.

The position we're in now reminds me of the fellow who said he couldn't afford to go to a doctor.

When he got sick, he found he was even less able to afford that.