The fear is that in a few hundred years, the plutonium could reach an underground area called the saturated zone, where water flows, and from there enter the Columbia River. Because the area is now arid, contaminants move extremely slowly, but over the millennia the climate is expected to change, experts say.

The finding on the extent of plutonium waste signals that the cleanup, still in its early stages, will be more complex, perhaps requiring technologies that do not yet exist. But more than 20 years after the Energy Department vowed to embark on a cleanup, it still has not “characterized,” or determined the exact nature of, the contaminated soil.

The department has been weighing whether to try to clean up 90 percent, 99 percent or 99.9 percent of the waste, but because the extent of contamination is unclear, so is the relative cost of the options. For now, the preferred option is 99 percent.

Government officials recognize that they still have a weak grasp of how much plutonium is contaminating the environment. “The numbers are changing,” said Ron Skinnarland, a radiation expert at the Washington State Department of Ecology, which is trying to enforce an agreement it reached with the Energy Department in 1989 for the federal government to clean up Hanford.

So far, the cleanup, which began in the 1990s, has involved moving some contaminated material near the banks of the Columbia to drier locations. (In fact, the Energy Department’s cleanup office is called the Office of River Protection.) The office has begun building a factory that would take the most highly radioactive liquids and sludges from decaying storage tanks and solidify them in glass.