THREE people, bundled against the Siberian cold, begin a long walk across the frozen surface of Lake Baikal. On their way, they glance toward the edge of the ice, where the headwaters of the Angara River well up.
They may not realize it, but they are seeing a natural phenomenon that was some three to four centuries in the making.
When it started, Rembrandt was still at his canvas, the Manchu Dynasty was still holding sway over China, and Galileo was still gazing at the stars.
It took from then until now for the water from the 336 rivers that feed into Lake Baikal to flow down its mile-deep 250-mile length, and surge out here, at the beginning of the Angara.
Yet that's not so long in the life of this lake. Scientists say it is 20 million to 25 million years old, making it among the oldest on earth. It is indisputably the biggest, holding fully one-fifth of the world's fresh water. The fact that it has stayed relatively fresh is one of the Soviet Union's environmental success stories.
Unfortunately, however, the same cannot be said for many of this country's other great lakes, rivers, and seas. The Soviet Union is facing serious water pollution problems across much of its breadth. The list of affected bodies of water includes the Volga, Ural, Tom, and Dnieper Rivers, and the Black, Azov, Baltic, and Caspian seas. And one incident on the Dniester River last year was of truly disastrous proportions.
Lake Baikal has also suffered from pollution, but it appears to have been checked before serious damage occurred.
One reason is the intense scientific interest in the lake, which is a unique environmental laboratory, containing more than 2,750 forms of life - including a species of seals that somehow came to this landlocked body of crystal-clear water.
The Soviet government rerouted its showcase construction project, the Baikal-Amur Mainline railway (BAM), away from the shoreline of the lake, running up substantial extra costs in the process.
Further, by government decree a cellulose plant on the shores of the lake has cleaned up its water discharges. By next year, the plant is expected to be recycling 90 percent of the water it uses, cutting down the pollution of Lake Baikal even further.
Such steps, along with constant environmental monitoring, have ensured that ''the lake is in a very sound and safe state today,'' according to Valentina Galkin, a researcher at the Baikal Institute of Limnology here on the lake's southern tip.
But it remains to be seen whether the Soviet Union will devote the same attention to a number of other serious cases of water pollution that have recently come to light.
The mere fact that they have come to light is notable in itself. Western analysts believe the Soviet Union has some of the most severe environmental problems of any industrialized nation. For years, these were virtually ignored by the government-controlled press, apparently reflecting an official preference for silence on the matter.
However, former Soviet leader Yuri Andropov, in last December's message to the Communist Party's Central Committee, stressed the need to find solutions to serious environmental pollution that had resulted from industrial development in various parts of the country.
Subsequently, a number of news stories spotlighted specific cases of environmental pollution. The most serious ones, however, concerned the fouling of the country's rivers, lakes, and seas.
It is unlikely, however, that this was the result of a newfound environmental zeal on the part of the Communist Party hierarchy. Instead, some analysts suggest that pollution has become such a severe problem here that it is virtually impossible for the country's leaders to avoid acknowledging it.
Whatever the reasons, with the lid on public discussion raised a bit, the Soviet public - and the world - has in recent months had a glimpse of the scope of the pollution of this country's water. And the scope is, it appears, large indeed:
* This month, Water Conservation Minister Nikolai Vasilyev admitted in the pages of Izvestia, the official government newspaper, that water pollution ''accidents'' are widespread across the Soviet Union. Some industrial plants guilty of pollution have been closed down, he added, and more may need to be.
* Pravda, the official Communist Party newspaper, published a biting article in February on the state of the Dnieper. The report charged that a shipbuilding plant was releasing wastes that were causing the river to glisten ''with all the colors of the rainbow.''
* Various articles have revealed that Lake Saki, in the Crimea, is threatened by pollution from chemicals and herbicides. The area is a health resort, and the mud and water in the area are reputed to have therapeutic properties. But chemicals from a nearby herbicide factory are already killing off organisms in the mud. A single dike, reportedly in need of nearly $2 million in repairs, is all that prevents chemical sediments from escaping into the lake. Pravda has called for the closing of the factory.
* Government authorities, according to the official Soviet news agency Tass, are undertaking a major campaign aimed at halting pollution of the Baltic Sea. The effort is centered in Tallinn, the capital of Estonia, according to Tass. But Tass indicated that sources of the pollution were widespread, involving factories in Leningrad and the Baltic republics.
* Probably the most serious water pollution problem occurred last September, when a dam burst at a fertilizer plant in the Ukraine, releasing millions of tons of potassium-laced waste water. A poisonous wave of the solution washed across 16 miles of the countryside and entered the Dniester, wiping out virtually all plant and animal life as it swept downstream.
Much of the wastes came to rest in a reservoir that provides drinking water for the cities of Odessa and Kishinev, forcing disruption of water supplies and a massive treatment effort which continues.
Three-quarters of a million tons of the wastes were eventually washed into the Black Sea. There are reports that life has returned to many parts of the rivers, although some foreign experts have their doubts.
Water Conservation Minister Vasilyev says the USSR has constructed 20,000 large water treatment plants during the past decade, and during the five-year period ending next year will have spent over $9.3 billion on water conservation and ''rational utilization of water resources.''
But other experts point out that the state still doesn't pay enough attention to environmental concerns.
One economist, writing in Pravda, noted that although some 40,000 environmental specialists graduate each year, ''only a small percentage of them are assigned to jobs in the field for which they have been trained.''
The economist, Prof. Yuri Tunytsa, stressed the need to include the costs of pollution control and environmental cleanup in the state's overall economic plans.