Almost two months after it happened, work crews in New Mexico are still cleaning up what is being called the worst spill of radioactive wastes in U.S. history.

The spill occurred July 16 at a uranium mine and mill in Church Rock, N.M., when a muddy mixture of waste material stored behind an earthen dam poured through a 20-foot crack in the dam and gushed into a small stream.

Eleven hundred tons of mine tailings-- the radioactive solids that remain after urianium ore is processed-- and 100 million gallons of radioactive water escaped during the hour it took workmen to seal the crack. Traces of the spill were later found as far away as 75 miles -- across the Arizona border.

he spill occurred in a desert region that is sparsely populated for the most part, and health officials in both states believe it presents no immediate health hazard.

[Preliminary results of health studies of humans and livestock near the affected waterway have turned up no radiation above normal background level, Washington Post special correspondent Mark Acuff reported. Local, state and federal health officials are monitoring persons in the area, and Los Alamos Scientific Laboratory has done whole-body counts on five children and one adult. A whole-body count tests for excessive concentrations of uranium and thorium, according to Dr. George Voelz of the background radioactivity, Voelz said. The lab also took 24-hour urine samples. Results of those tests will not be available for another week or so, Voelz said.].

A federal official, Hubert Miller of the Nuclear Regulatory Commission, said: "This is something you need to be exposed to for many years before (health) effects would be seen."

But there is concern that some of the radioactive material might percolate into the earth and contaminate well water, or be absorbed by plants that livestock consume, or be carried by wind-borne dust particles into human lungs.

About a dozen Navajo farmers in the area have been told not to use their wells. Instead, they are being supplied with water for themselves and their livestock by the operators of the mine.

Authorities also have ordered that signs be posted-- in English, Spanish and Javajo-- warning people not to go near the contaminated stream, the Rio Puerco. Navajo livestock drink from the stream, but Miller, the federal nuclear official, said the radioactive water is so acidic that "I doubt cattle would drink it."

The largest city along the path of the Rio Puerco is Gallup, popultion 21,000. However, neither it nor any other municipality draws its water supply from the Rio Puerto "or anything reasonably close by," one state health official said.

Engineers are still trying to determine why the dam, owned by United Nuclear Corp. and built only two years ago, developed a crack. One theory is that the subsoil under the dam gave way, creating stress that the dam could not abosrb without fracturing.

When the dam failed, the waste material escaped with sufficient force to overrun the steep banks of the stream in some places-- leaving behind isolated pools of radioactive water the color of iced tea and patches of radioactive crystalline material the color and shape of yellow ant hills.

State health authorities have ordered United Nuclear to pump these stagnant pools dry and remove any underlying soil that is contaminated.

The patches of crystalline material, left behind when the radioactive water evaportates, also must be removed to prevent the material from being dissolved by rain and reentering the stream or percolating into the earth.

So far, 140 tons of radioactive waste have been recovered and returned to the storage pond behind the dam. But the work is slow. Mud conditions prevent the use of heavy machinery, and the cleanup so far has been accomplished by crews laboring with shovels and buckets. One estimate is that it will take until December to complete.

Federal officials have called the United Nuclear spill the worst of its kind in U.S. history because of the distance it covered. All other such spills have been contained at milling sites.

But even without that distinction, the episode probably would have drawn special attention. The Grants uranium belt of New Mexico, where the United Nuclear mine is situated, is said to be headed for a boom period, with up to 75 new mines and 20 new mills possibly opening in the next decade. Concern for the safe deposit of radioactive waste at these mines is likely to be heightened by the United Nuclear experience.