Uranium Mill Tailings in the Colorado River Basin

by Tom Dansie

From Hidden Passage, the Journal of Glen Canyon Institute
Volume V

Visitors to hite marina on the far east end of Lake Powell are, for the most part, unaware of the unique pre-dam history of the area. Few realize that not far from the boat launch at Hite and buried under hundreds of feet of silt and water lies a potentially hazardous pile of uranium ore mill tailings.

These tailings, a pile of almost 26,000 tons of mill waste, were deposited in the post World War Two uranium production boom that swept through the intermountain west. The pile lies near the mouth of White Canyon, the site of a former uranium ore mill. More than sixteen similar tailings piles, many much larger than the pile at Hite, are strewn throughout the Colorado River Basin; many are in proximal location to Lake Powell tributaries.

Uranium ore was mined and milled in the Colorado River Basin beginning in the late 1940’s and continued through the 1950’s at an ever-increasing rate. When production finally reached its peak in 1958 nearly 8,960 tons of uranium ore were being milled each day in the Colorado Plateau.

Uranium ore in the Colorado Plateau is found in Mesozoic shale dominated formations with sporadic interbeds and lenses of sandstone. The Morrison and Chinle formations are the two richest uranium ore bearing formations in the area; both have been heavily prospected. Ore, after it has been mined is transported to a mill where uranium is isolated and the remainder of the ore discarded. Through a process of crushing, grinding, leaching, filtration and centrifuge the uranium is isolated and processed into "yellow cake," a moniker for the final product of the milling process.

Uranium ore typically contains 0.25% recoverable uranium. Thus it is necessary to process a full ton of ore in order to produce five pounds of yellow cake. The residual 1,995 pounds of ore become mill tailings, deposited as slurry with acids used in the milling process. These tailings form a pile of sand-like material that poses significant health risks.

Concentrated in mill tailings piles are a number of heavy metals including arsenic, barium, cadmium, lead, vanadium and selenium. In addition to these contaminants mill tailings piles contain radioactive materials not removed in the production process. In fact, 85% of the radioactive material in ore remains after the milling process. Radionuclides concentrated in tailings piles include Thorium-230, Radium-226 and Radon-222.

These contaminants in tailings piles are introduced to human contact through a number of pathways. Continued radioactive decay through alpha and gamma particle emissions, inhalation of windblown particles, and inhalation of radon gas, a daughter product of Radon-222, are all potential contaminant exposure pathways. These exposure pathways can be effectively mitigated and eradicated by capping the piles with a layer of impermeable material.

The most threatening exposure pathway is contamination of ground and surface water with heavy metals and radionuclides. Preventing contamination of ground and surface water is a more complicated problem than mitigating the other exposure pathways. Mitigation of this pathway usually involves relocating the tailings to an offsite disposal cell. Due to the large volume of most tailings piles this procedure is both complicated and costly.

The submerged tailings at White Canyon near Hite present a particularly complicated groundwater contamination scenario. Shortly before inundation of the site by Lake Powell the federal Nuclear Regulatory Commission ordered the operators of the White Canyon mill to remove a small amount of high-grade waste from the pile. The remainder of the tailings is now covered by hundreds of feet of silt and water.

Due to the silt cover over the tailings at White Canyon there is little potential for mechanical transportation of the sediments away from the site and into the reservoir. Also due to the silt cover there is little danger of radioactive decay emissions or radon gas emissions from the pile.

However, as effective as the silt cover is in trapping the release of these contaminants it is ineffective in isolating the tailings from groundwater. Silt sediment deposited by the Colorado River on the tailings pile is water saturated and thus water from the reservoir easily passes through the sediments. Thus Lake Powell water is free to associate with the tailings covered by the permeable silt cover.

As water flows through the sediments, into the tailings, and then back into the reservoir there is a high potential for transportation of heavy metals and radionuclides into the reservoir proper. Once introduced to reservoir water these contaminants can rapidly spread. Heavy metals, in particular selenium, are readily transported in the food chain from microscopic organisms to fish to humans.

Two independent studies of large mouth bass from Lake Powell report that selenium concentrations in the fish greatly exceed national averages. This abnormally high selenium concentration in fish reflects the high concentration in the reservoir. Selenium transported from the submerged tailings piles is a likely contributor to the high concentration.

Mitigating contamination from the pile presents a complex problem. Currently there is a likely water contamination problem, but the tailings are fairly stabilized. Any attempt to mitigate the pollution with the site submerged will disturb the tailings and cause extensive spreading of the contaminants. As the reservoir is drained, however, the tailings can be isolated and stabilized. The tailings could then be transported to an adequate disposal site.

Thus the 26,000-ton mill tailings pile at White Canyon presents certain present pollution and contamination issues in the reservoir. This contamination will continue as long as the site is submerged and covered by water-saturated sediment. More studies are necessary in order to fully assess the hazards presented by the site and to formulate a feasible remediation plan. Ideally, remediation plans will be accessory to the full restoration of Glen Canyon through decommissioning of Glen Canyon Dam.