MILLIONS OF YEARS ago, seeping groundwater created cavities in the rock below the northern Arizona desert, which eventually collapsed under the weight of the stone above. The groundwater kept flowing, depositing minerals into the resulting formations, known as Breccia pipes. One of these minerals was uranium, the fuel that America’s nuclear power plants consume, making the area home to some of the highest-grade extractable uranium ore in America — and very attractive to mining companies. The Pew Environment Group reports that mining claims in a 1 million-acre zone jumped from 320 in 2004 to 3,200 in 2006, with 2,900 additional claims filed in 2007.

But that 1 million acres isn’t just any patch of desert; it’s within miles of the Grand Canyon. After pressure from environmentalists, Interior Secretary Ken Salazar put a temporary moratorium on new claims there in 2009, and on Monday he announced that he will recommend a 20-year ban this year. Existing claims might still turn into full-fledged mines, but the claims bonanza is over.

Mining groups blasted the decision, saying the government didn’t have sufficient scientific basis for it. A 2010 study from the U.S. Geological Survey indicated that past uranium mining near the Grand Canyon has resulted in limited contamination to surface water and soils.

Yet James K. Otton, one of the report’s authors, says that there is still much the government and industry do not know about the effects of uranium mining in the region, particularly on local and regional aquifers. How much water moves down the pipes, how much contamination does it pick up and how diluted does it become? The last question is critical: Mr. Otton points to one abandoned mine near a stream that carries relatively high levels of contaminants. When those contaminants enter the Colorado River, they get too diluted to pose much of a threat. But multiply that many times in an underground aquifer, and there might be a bigger problem. So there is reason to be cautious about allowing too many mines to open.

Mr. Salazar’s preferred option puts off limits a lot of uranium — the USGS estimates 102,744 tons, or about 7.8 percent of the total endowment in northern Arizona. It’s a resource that America should want to obtain if it can do so responsibly. Interior could have given more consideration to more limited restrictions. But since mining can continue on existing claims, the government has an opportunity to collect information on how dangerous the extraction is to the Grand Canyon and its environs. The monitoring of surface water, groundwater, aquifers and soils that wasn’t done when mines operated from the 1950s to the 1980s should be carried out this time — and regulators should make sure to require it in the mining plans they review. Otherwise, they won’t really know whether it makes sense to leave all that uranium trapped below the desert floor.