Government scientists have found that it may cost hundreds of millions of dollars and take as long as 35 years for Canada to clean up rivers contaminated with mercury from industrial waste disposal.

The scientists are searching for solutions to an environmental problem that, following widely publicized revelations in the 1970s, came to symbolize the hazards of industrial waste for Canadians in much the same way that Love Canal has for Americans.

Their recently released report seems to bear out the worst fears of environmentalists about the difficulties of restoring damaged ecological systems.

Even expensive and long-term measures cannot ensure that fish in mercury-polluted rivers will be made fit for human consumption, the team of scientists concluded after one year of study funded by the Federal Environment Department and the Ontario provincial government.

Their laboratory has been a system of interlocking rivers and lakes in central Canada that, to quote from the report, "has the dubious distinction of being one of the more severely polluted waterways" in the country. But their findings are pertinent to other, similarly affected Canadian rivers in northwestern Quebec.

The ecological state of these rivers has been of national concern since the early 1970s. At that time it was discovered that fish caught in these waterways contained high levels of mercury, posing a threat to the health of thousands of Indians who relied on locally caught fish for protein.

Scientists traced the pollution to pulp and paper mill operations, whose unrestricted dumping of effluents had allowed mercury to build up in the riverbeds. There, it was transformed into methyl mercury, an organic compound of the metal easily absorbed by plants and fish.

Beginning in 1970, the federal government hastily took steps to avert a tragedy similar to that in Minamata, the Japanese village where mercury poisoning caused widespread nervous system disorders among residents.

Under government pressure, paper mills introduced waste treatment procedures to reduce the outflow of mercury. Commercial fishing was banned in the affected areas, and Indians were urged not to eat the contaminated fish.

Despite these measures, medical surveys conducted until 1978 continued to show that some individuals from Indian reservations in both Ontario and Quebec had blood mercury levels exceeding 100 parts per billion, the level at which an adult is considered to be "at risk" of mercury poisoning. t

The federal and Ontario governments in 1978 began a two-year, $300,000 study of the English-Wabigoon river system in northwestern Ontario near the border of that province and Manitoba.

Some of the methods considered for cleaning up the waterway, the scientists said in their interim report, were dredging, flushing the mercury-laden sediment into a chain of man-made ponds and then removing the contaminant or treating the waterway with chemicals.

The report is far from optimistic about the possibilities of success.

"Even the more promising methods appear to have limitations which are not altogether predictable," including the possibility that the cleanup could itself cause further environmental damage, the report said.

The team of scientists concluded that it appears "there is no simple, rapid, fully effective, inexpensive, trouble-free course of action for overcoming the mercury problem."

Dredging, offered as probably the most promising solution, could cost up to $800 million, the scientists estimated.

Even then, they said the project "would not be guaranteed to remove all of the contaminated sediment and might temporarily aggravate the mercury problem by causing resuspension of fine-granted contaminated particles, which would then be swept downstream." Also, "there remains the important question of finding a suitable way to dispose of the dredge soil," the scientists said.

The report pointed out that, in the early years after the discharge of mercury into the river system became regulated in 1970, levels of the pollutant declined rapidly in the waterway.

"However," it continued, "the decline has been leveling off and is now so gradual that fish in this lake are expected to continue to exceed the guidelines for human consumption for a long time -- probably for many decades -- unless remedial measures can be taken."

Although commercial fishing on the English-Wabigoon system was banned, the report said, sport fishing continues to be permitted. As a result, the scientists warned, Indians hired as guides by fishing lodges may be particularly vulnerable to chronic mercury poisoning because of their practice of having "shore lunches" of freshly caught fish.