Each year, some 3 million visitors are drawn to Lake Powell, a massive artificial body of water that straddles the Utah-Arizona border. Below gleaming red-rock cliffs, the reservoir is busy with Jet Skis, motorboats and luxurious houseboats that resemble floating Winnebagos. Out of sight but a crucial part of this iconic picture are Westerners who rely on the lake for more than recreation.

On the surface, Lake Powell seems like a dream, but below its dark-blue waters lurks an ominous legacy:

An estimated 26,000 tons of radioactive waste are covered by the silt of Lake Powell, which is part of a system that provides drinking water for 40 million people in the Southwest.

Huge piles of mining tailings rose along the banks of the Colorado River during the West’s uranium boom of the 1940s and 1950s. The White Canyon mill, built by the Vanadium Corporation of America in 1949, for example, crushed and treated 20 tons of ore per day with sulfuric acid, tributyl phosphate and other compounds, according to a Freedom of Information Act request. With 1 ton of ore yielding 5 or 6 pounds of uranium, some 39,900 pounds of tailings accumulated daily on the banks of the river, according to Jonathan Thompson, author of River of Lost Souls. Though the mill closed in 1953 and Vanadium Corp. went out of business, the tailings remained. After water began accumulating behind the Glen Canyon Dam about a decade later, the reservoir’s waters inundated the tailings.

The drought from 2000 to 2005 dropped the water level nearly 100 feet — about a fifth of its total depth.

“For the people enjoying Lake Powell, they’re not exposed to anything that’s going to hurt them,” says Phil Goble, uranium mill and radioactive materials section manager for the Utah Department of Environmental Quality. “The uranium mill tailings produce a sandy waste that contains heavy metals and radium, which is radioactive, but these tailings have been down there since around the 1950s, with several feet of sediment placed over top of them and the water used as a moderator, or a shield.”

The Environmental Protection Agency says two public water systems draw water directly from Lake Powell: the Navajo Generating Station and the city of Page, Arizona. “All community water systems are required to monitor and meet established drinking water standards for a suite of radionuclides to include gross alpha, combined radium 226/228 and uranium,” says Nahal Mogharabi, spokesperson for the EPA Region 9 (Pacific Southwest). “The city of Page water system is serving drinking water that meets the drinking water standards for radionuclides.”

Not everyone is so sanguine. “We have no reason to doubt that there is significant radioactive waste under Lake Powell,” says Sandy Bahr, chapter director for Sierra Club’s Grand Canyon (Arizona) Chapter. “What we have said repeatedly about the uranium mine and mills is that it is not worth the risk to our drinking water.” Goble spells out the risks if radioactive elements like uranium or radium get into the lining of a human’s lungs: anemia, fractured teeth, cataracts and cancer.

And it’s not just a power plant and a small Arizona city that could be affected. Jim Pokrandt, director of community affairs for the Colorado River District, says Powell is like a “savings account” whereby the four upper basin states — Colorado, Wyoming, Utah and New Mexico — can meet their annual water supply obligations under the Colorado River Compact of 1922 to the lower basin states and to Mexico. “In a sense, Powell serves all 40 million people by protecting upper basin uses, which occur before water gets to Powell, and then supplying resources to the lower basin, which stores water in Lake Mead to meet lower basin needs.”

While the situation is currently under control, there are scenarios that could upend the status quo. “The tailings could potentially become a problem if Lake Powell gets to a very, very low water level or if the lake is drained, and the tailings are exposed,” Goble says. “In this case, if someone were to dig down and expose those tailings, or the wind blows them, or people use the spot for recreational use of off-road vehicles, then there could be a health hazard.”

Bruce Rittmann, a regents’ professor in the School of Sustainable Engineering and the Built Environment at Arizona State University, agrees that the tailings currently pose no risk to people or aquatic life in the lake, but that could change if something disrupts the tailings, such as the draining of the lake or a dredging operation that stirs up sediment.

Draining the lake may sound ludicrous, but the reservoir hasn’t been full since the late 1990s, and ongoing overuse and the drought from 2000 to 2005 dropped the water level nearly 100 feet — about a fifth of its total depth. Some scientists predict that climate change will intensify and prolong the West’s drought cycles. If that happens and Powell becomes a red-rock canyon once again, then an old mess may finally have to be cleaned up.