Uranium Mill Tailings Dam Failures: Church Rock and Beyond

Reference article · Updated 2026-11-06

Key facts

On 16 July 1979 the tailings dam of United Nuclear Corporation's Church Rock mill breached near its south abutment.
EPA records describe about 94 million gallons of acidic effluent and about 1,100 tons of tailings solids; other sources quote 93 to 100 million gallons.
Investigators linked the breach to differential settlement of the foundation, with operating practices that departed from approved procedures contributing.
The Church Rock site is on the EPA National Priorities List, and groundwater at the tailings site has been listed as not under control.

Why uranium tailings need their own treatment

Uranium mills extract only a small part of the ore's radioactivity. Most of the long-lived decay products, including thorium-230 and radium-226, stay in the residue, together with the acid or alkali used in leaching and with metals such as molybdenum, vanadium or arsenic. A breach therefore carries both a chemical and a radiological hazard, and cleanup standards address radium and thorium concentrations in soil as well as water quality. The composition and long-term disposal of these wastes are described on uranium mill tailings: characteristics and management.

Most uranium mill tailings are stored in engineered impoundments, many built from earth or from the tailings themselves, so the engineering questions overlap with those for any other mine. For a comparison with failures at non-uranium mines, see the chronology of tailings dam failures.

Church Rock, New Mexico, 16 July 1979

The United Nuclear Corporation mill at Church Rock, on the edge of the Navajo Nation, operated a tailings pond behind an earth dam. On 16 July 1979 the dam near its south abutment breached. EPA documents describe the release as approximately 94 million gallons of acidified effluent and tailings slurry together with approximately 1,100 tons of solid tailings.

The liquid ran into Pipeline Arroyo, a tributary of the Rio Puerco, and travelled downstream; radioactivity in the river was observed at Gallup, New Mexico, roughly fifty miles away, by the morning. Other published estimates of the liquid volume range from 93 to about 100 million gallons, so the figure should always be quoted with its source.

A New Mexico state report in 1983 described the event as the largest single release of liquid radioactive waste in the United States. The company reached an out-of-court settlement with the Navajo Nation about a year later, reported at $525,000.

What caused the Church Rock breach

The company's own stability report and a report commissioned by the US Nuclear Regulatory Commission reached a similar conclusion: the probable cause was differential settlement of the foundation under the breached section, which stressed the embankment and opened cracks through which water eroded the dam once free liquid stood against its upstream face.

Cracks had been noted in the dam wall by consultants in December 1977 and were sealed with a bentonite and kerosene slurry in February 1978. The commissioned report also found that variations in pond operating practice from approved procedures contributed. A company executive offered a different explanation, pointing to the shape of the bedrock beneath the embankment.

The episode is a clear example of a general pattern: a defect that was known and patched, but whose cause (settlement in the foundation) was not resolved. The same pattern of uncorrected warnings appears in other cases, such as Stava.

Measurements, cleanup and long-term status

A technical survey carried out in 1979 sampled the Pipeline Arroyo and the Rio Puerco arroyo from the mill to the New Mexico-Arizona border at points about 500 feet apart between 24 September and 5 October. It measured lead-210, radium-226, thorium-230 and uranium-238 and compared them with action levels proposed by the NRC and adapted by the New Mexico Environmental Improvement Division. The results are in the published technical report (PNL-4122, NUREG/CR-2449).

The mill area was placed on the EPA National Priorities List in 1983. A later EPA statement notes that stabilisation work at the mill between 1988 and 1996 protected the Rio Puerco from repeats of the spill, and that the Navajo Nation asked EPA in the 2000s to take the lead on the broader cleanup. Current listings continue to record that groundwater migration at the tailings storage site is not under control. Readers needing the present status should consult EPA Region 6 and New Mexico Environment Department pages for the site.

Regulatory consequences in the United States

In the United States the Church Rock failure occurred in the same period as the passage of the Uranium Mill Tailings Radiation Control Act of 1978, which created a framework for the cleanup of inactive mill sites and for long-term management of tailings at licensed mills. Church Rock remains a standard reference case in discussions of dam engineering and licensing of active tailings impoundments. For the wider regulatory picture across countries, see the page on regulation of uranium mining and tailings.

Lessons drawn from the Church Rock case

Several practical lessons follow from the documented sequence. First, a crack in an embankment is a symptom and the cause has to be found: the patching of cracks in 1978 did not remove the settlement beneath the dam. Second, foundations need as much characterisation as the dam fill, because a transition zone with unexpected settlement behaved differently from the design assumption.

Third, operating practice matters: the commissioned report noted that variations from approved pond operation contributed, so surveillance has to cover how the facility is run as well as how it is built. Fourth, because the stored liquid was acidic and carried radionuclides, the downstream consequences extended beyond the immediate flood to sediments in an ephemeral channel that local residents used. These points are developed in general form on the page about tailings dam safety and the page on monitoring.

Large uranium tailings legacies that are managed, not failed

Not every large uranium tailings problem is a dam failure. Two documented examples of planned management illustrate the alternatives:

Wismut, Germany. Four large tailings facilities at Helmsdorf, Dänkritz 1, Culmitzsch and Trünzig hold more than 160 million m³ of tailings. The operator chose a dry in-situ approach: pond water is drained, tailings are covered, re-contoured and capped. Helmsdorf used subaqueous cover placement in 2002-2003 because very fine slimes had to be stabilised before the water was removed.
Moab, Utah. The US Department of Energy has been moving the tailings pile beside the Colorado River by rail to an engineered disposal cell at Crescent Junction, about 30 miles to the north. DOE has announced that over 16 million tons have been moved; the schedule for completion has been revised more than once and should be checked against DOE's current statement.

Other reports describe unstable Soviet-era uranium tailings in Central Asia, but the sources reviewed for this page did not allow figures to be verified, so no numbers are given here. Non-dam legacy problems are covered under incidents beyond dams.

Frequently asked questions

What was the Church Rock spill?

A breach of the United Nuclear Corporation mill's tailings dam on 16 July 1979 in New Mexico, releasing about 94 million gallons of acidic liquid and about 1,100 tons of solids according to EPA records.

What caused the Church Rock dam to fail?

Investigations found differential settlement of the foundation beneath part of the dam, with cracks and erosion, and operating practices that departed from approved procedures.

Why do volumes for the spill differ between sources?

Reports range from about 93 to 100 million gallons of liquid because of different methods and rounding. The EPA figure of about 94 million gallons is used here.

Is the Church Rock site cleaned up?

Cleanup and groundwater work continue under EPA oversight; current listings note that groundwater migration at the tailings site is not under control.

Are uranium tailings dams more likely to fail than others?

The sources reviewed do not support a comparison. Uranium tailings dams are subject to the same engineering failure modes as other tailings dams, and the radiological content raises the consequences rather than the probability.