Summertime, and all the familiar elements are with us: Driving down 16th Street to the office, the White House nestled below in the distance, the sun bearing down, the streets deserted, the president and the Congress away, the car radio playing, the announcer giving the latest air quality index and warning against the pollution by saying: "Do not breathe too deeply."

He delivered that message with all seriousness and solemnity, unaware, it seemed, of the wonderful irony in the words. Don't breathe too deeply . It was a classic restating of other, also familiar, dilemmas:

Don't eat red meat: don't charcoal-grill your steak, or hamburger; don't eat eggs, or fried foods, or saccharine, or refined sugar, either; don't smoke cigarettes, or cigars or pipes: don't spend much time in smoke-filled rooms unless you want to suffer an angina attack; don't drink colas with cyclomates: don't eat certain sea food from certain wasters. And, always, watch out for kepone, mercury, red dye and other deadly substances that continually creep into the food chain.

Don't do anything, almost. And don't, in this summer season, dare do anything as rash as go swiminf in many of our public waters.

Therein, contented citizens, lies another familiar element in this tale of American progress in the 1970s. For today we bring you an update on what's happened to a notable political promise.

It was 14 years ago that Lyndon Johnson set another national goal: to make the Photomac River "a conservation model for our metropolitan areas." That wasn't a happenstance remark of LBJ's, a president with a well-known penchant for hyperbole. In his State of the Union message in January of 1965, he promised to turn the Potomac into a "model of scenic and recreation values for the entire country." He followed up with a message on national beauty one month later. Again he proclaimed the goal of cleaning up the Potomac. Then, in a special ecological essage to Congress that fall, he employed even more dramatic language - and a more sweeping kind of promise.

Johnson's campaign to reverse the tide of filth in tha nation's streams would begin has a right to use America's rivers and America's waterways, that belong to all the tomac, there had been a time well into the 20th century when presidents and ordinary citizens swam there regularly. "But today, the Potomac is a river of decaying sewage and rotten algae. Today all the swimmers are gone. They have been driven from the banks."

Then came the great promise: "I pledge you are going to reopen the Potomac for swimming by 1975." That is akin to LBJ's promise to win the war on poverty for all time - and also win the war in Vietnam we were then entering so boldly.

It's said that when Capt. John Smith sailed up the Potomac in 1608 to what is now Georgetown he was entranced by what he had seen. The river water had been so pure you could see every pebble at the bottom. And all around was an abundance of fish and game.

"The fish were lying so thicke with their heads above water, as for want of netw, we attempted to catch them with a frying pan, which we found a bad instrument to catch fish with." As for the river itself, it was "fed with many small rivers and springs, frequented by otters, beavers, martins and sables. Neither better fish, more plenty, nor more variety for small fish had any of us ever seen in a place."

Even Lyndon Johnson, with all his monumental self-assurance and conviction that nothing was impossible, particularly if he himself put his imprimatur on the effort, didn't expect to see the Potomac returned to the pristine quality of colonial days. And his main promise was, like so many others from that time of excessive political expectations, almost surely unrealistic. You don't see anyone swimming in the Potomac this weekend.

The record of bureaucratic squabbling, petty inter-jurisdictional disputes, political footdragging since LBJ's pledge is one of the less elevating of the past decade. Consider just the following lead sentences from hundreds of Washington Post articles in that period:

About 600 tons of raw sewage sludge is being pumped daily into the Potomac River - desipite available remoral techniques - because Maryland has jailed to live up to a 1971 sewrage agreement. Maryland state officials confirmed yesterday - Jan 1, 1974.

District of Columbia officials yesterday threatened to start dumping raw, untreated sewage - 290 tons of it daily - into the Potomac River on Friday unless Prince George County lifts its objections to a state-ordered plan to transport treated sludge from the Blue Plains plant in the District to Andrews Air Force Base for fill dirt - Feb. 6, 1974.

The Fairfax County Board of Supervisors decided yesterday against giving top priority to a sewer project to help clean up the Potamac River, even though the clean-up was ordered by a county court six years ago. - Sept. 21, 1976.

And the federal government itself clearly has not performed as promised. As James B. Coulter commented last spring at a governmental seminar sponsored by the Interstate Commission on the Potomac River Basin: "In the interest of truth, one has to observe that the ever-changing, upredictable, disjointed efforts of a parade of [Environmental Protection Agency] administrators have made a mess of things."

Happily, that isn't the entire story. As Coulter also reports, "a major improvement" has been made in the Potomac becuase of work already accomplished and vast sums expended. Indeed, one of the most intriguing aspects about the present condition of the Potomac is the difference between the public perception of it and the reality as seen by those who watch it most closely.

If you spend several hours talking to the people at the interstate commission, for instance, you'll come away not nearly as gloomy as you might have expected. The river, by all accounts, is far better than just a few years ago.

When a group of British water pollution experts conferred with their Washington counterparts a few months ago, the Americans were struck by their different ways of looking at the issue. "They have recreational facilities, including swimming, in areas that we would say are terribly polluted," one of the Americans remarked. "The British perception is that it's remarkably clean."

The same sort of outlook, he said, can be applied to the Potomac today.

"What it gets down to is that we'll have to lower the level of public consciousness on environmental hazards," the American said. "Our level of expectation has been so raised that people believe we can have a risk-free society. We havent been able to get across the idea that there is no such thing as a risk-free society. Our expectations are too high not only in the field of environmental concerns, but in many other areas as well. There's a limit to what government and technology, can deliver. I think that's what we're beginning to learn now."

There's another lesson, and another "don't" to add to the long list: Don't believe in political promises. But then you don't have to be told that.