After winning the Democratic nomination for president, Walter Mondale went fishing. He retreated to Gunflint Lake in the Minnesota wilderness for six days, landing dozens of trout and resting for the arduous campaign ahead. A week with a rod and reel, he said, was the best relaxation he could think of.

Ronald Reagan has his ranch, but Mr. Mondale is doing his best to paint himself the candidate who cares most about the great outdoors.

In this effort he has been joined by environmental groups; the Sierra Club and Friends of the Earth have endorsed Mr. Mondale's presidential bid.

The League of Conservation Voters says Mondale has a ''fine'' environmental record, but adds that as a senator, he ''was not a leader on most of our issues.''

During his 13 years in the United States Senate, Mondale's main environmental interest was land conservation - specifically, protecting land in his home state , Minnesota.

He sponsored legislation that established the Boundary Waters Canoe Area along the Canadian border. He played an active role in establishing Voyageurs National Park and in designating parts of the St. Croix and other rivers as ''wild and scenic.''

In the early '70s, Senator Mondale also worked hard to delay the Alaskan oil pipeline and soften its effects on the environment.

As vice-president, Mondale annoyed environmental groups by opposing water-project reforms. But the Carter-Mondale administration backed amendments that made the clean-air and water acts more stringent, and helped set up the Superfund for cleaning up hazardous-waste sites.

If elected president, candidate Mondale proposes to increase the Environmental Protection Agency's budget at least enough to restore its purchasing power to pre-1980 levels.

He also says he will restore funds cut from the Land and Water Conservation Fund (used for purchase of national parkland and wetlands), although he gives no exact figure.

A Mondale administration would also immediately propose a program to fight acid rain, the candidate promises. Mondale has joined a number of scientists and environmentalists in calling for a 50 percent reduction in smokestack emissions of sulfur dioxide, a suspect in the acid-rain case.

He says he favors a ''national solution'' to the problem of paying for this reduction; the question of who pays for reducing acid rain has already sparked bitter congressional fights between regions.

Mondale also says he would clean up toxic-waste dumps faster than would a second Reagan administration. He has urged passage of a House bill that would pump $10 billion into Superfund for such cleanup over the next five years. (The Reagan administration says Superfund can't yet absorb such large sums.)

He also says the US government should in some way compensate victims of hazardous-waste dumping. (Reagan officials are concerned that such a provision would be very expensive.)

''I will end the lawlessness - and that's what it is, lawlessness - that's destroying our environment,'' Mondale said last month upon receiving the Sierra Club endorsement.

''I will take polluters to court and not to lunch.''

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