In a high school half a mile from a radioactive-waste dump, in a community with abnormally high rates of leukemia, Walter F. Mondale said today that the Reagan administration's efforts to clean up toxic wastes are a "super fraud."

"It's time to declare war on toxic waste," the Democratic presidential nominee said on what he called "national toxic waste" day on his campaign. "We've lost four precious years. Let's get on with our work."

Mondale accused the administration of playing politics with environmental cleanup, noting that "just by magic, three days before I arrived, they announced they were going to do something" about the Weldon Spring site.

He said the administration had done the same thing when he visited a site in California. "If I only had 761 days to go in this campaign, I'd go to a dump every day and clean them all up," he said.

Mondale's appearance at the Weldon Spring site was part of a coordinated effort by Democrats and environmentalists to draw attention to the issue today in many parts of the country.

Democratic vice-presidential nominee Geraldine A. Ferraro, in her only public campaign appearance of the day, charged in Allentown, Pa., that "across the board, this administration has the worst environmental record since Sherman marched through Georgia."

"For every chicken coup, they found a fox to guard it," Ferraro told 2,000 students at Moravian College. "Remember, this is the administration that gave you James Watt once and Anne Burford twice."

Referring to the coordinated Democratic emphasis on environmental issues today, Ferraro charged that Democrats are visiting more toxic dump sites in one day than the Reagan administration has cleaned up in four years.

The campus crowd included about 100 vocal pro-Reagan students who countered the chants of "Ger-ry! Ger-ry!" with their own cries of "Rea-gan!" and "Four more years!" Although boisterous before and after Ferraro's speech, they generally remained quiet when she spoke.

Sen. Gary Hart (D-Colo.) was in San Francisco announcing cleanup plans with Sierra Club officials, while former representative John B. Anderson, the Illinois Republican who ran for president as an independent in 1980 and is supporting Mondale this fall, was in Connecticut. Half a dozen governors and senators participated in local events.

The toxic-waste speech, and a luncheon fund-raiser that followed in nearby St. Louis, were the only public events of a day that opens an abnormally light week of campaigning for Mondale, who has decided to concentrate on preparing for Sunday's televised debate on foreign policy.

Mondale will appear at a rally and a fund-raiser in California on Tuesday, then return on a late night flight to Washington. He will keep a low profile the rest of the week.

"It's his call," said traveling aide Thomas Donilon. "He wants the time to restudy the issues, to rest up, to make sure he looks good. We think a sort of freeze phenomenon has set in this week, and that there isn't much room for movement until after the debate."

Peter Hart, Mondale's pollster, predicted today that the gap between Mondale and Reagan will narrow to about 10 points by Sunday's debate. Hart said Reagan now leads Mondale by about 12 to 13 points in his polls.

The Mondale campaign has scrubbed the politicians' traditional appearance at the Al Smith Dinner in New York Thursday night. The public explanation is that Mondale needs the time for debate preparation. Privately, aides acknowledge that their decision also was prompted by the presence of the dinner host, Archbishop John O'Connor of New York, who has locked horns with Ferraro this fall over Mondale's position on abortion.

Coming off what were easily his most boisterous receptions of the campaign last week -- Mondale today called it "the most remarkable week of my life" -- today's receptions were subdued and Mondale's performances flat. Half an hour after appearing at Weldon Spring, he had trouble recalling the name of the dump site. In his remarks at the high school, Mondale played on his familiar old Reagan-new Reagan theme.

"For four years on the issue of the environment, the president sounded like Ronald Reagan. Now in the last three weeks, he's trying to sound like Walter Mondale," he said. "The new Reagan announces the cleanup of Weldon Spring; the old Reagan cleaned up only six dumps in four years.

"The new Reagan admires Teddy Roosevelt and Ansel Adams; the old Reagan turned the Superfund into a super fraud.

"The new Reagan poses for pictures on Chesapeake Bay; the old Reagan appoints James Watt, Rita Lavelle and Anne Gorsuch."

The Weldon Spring toxic-waste site consists of four sludge pits filled with untreated radioactive waste from an old Army uranium processing plant.

Late last week, after Mondale made it known he would visit the site, the administration announced it was seeking a $250 million appropriation for 1987 to clean it up. The announcement drew a skeptical response from an editorial in the St. Louis Post Dispatch.

Mondale's toxic waste proposals include pledges to strengthen the Superfund program and move for passage of the Safe Drinking Water Act, both of which were tied up this year by the Republican-controlled Senate.