The federal government will spend $21 million to restore a contaminated lake to recreational use in the third and final phase of cleanup of the Lipari Landfill, the nation's No. 1 Superfund toxic waste site, officials said. The Environmental Protection Agency said contamination in Alcyon Lake, about 1,000 feet downstream from the landfill, should be reduced enough by the summer of 1991 to allow a resumption of fishing and swimming. The southern New Jersey lake has been closed to swimming since 1958 and limits on other recreational uses were imposed in the 1970s, EPA Regional Administrator Christopher Daggett said. Daggett approved a three-year plan that involves the dredging and removing of about 140,000 cubic yards of contaminated sediments and removing organic compounds with a rotary dryer unit.