The Largest Accidental Release Of Radioactive Material In US History: What Happened At Church Rock?


Francesca Benson


Francesca Benson

Copy Editor and Staff Writer

Francesca Benson is a Copy Editor and Staff Writer with a MSci in Biochemistry from the University of Birmingham.

Copy Editor and Staff Writer

rio puerco

Image Credit: Barbara Ash/

The Puerco River should be pretty dry in the summer. However, in the early hours of July 16, 1979, the river raged with a radioactive flood – the largest accidental release of radioactive material in United States history.

Despite the 94 million gallons of radioactive liquid cascading into the environment and the long-lasting effect on the local population, the situation attracted relatively little attention, outrage, or aid compared to other nuclear disasters like Chornobyl or Fukushima. What follows is a frustrating tale of incompetence and indifference – but how did this come to be?


The Mill

The spill originated from a uranium mill in Church Rock, New Mexico, operated by the United Nuclear Corporation (UNC). The mill processed uranium ore extracted from the Northeast Church Rock Mine, also operated by UNC from 1967.

Even before the Church Rock spill, the mill and mine were releasing radioactivity and pollution into the environment. Untreated water would be discharged into the Pipeline Arroyo, a tributary of the Puerco River – with some remembering the banks being coated with a “yellowish slime.”

Accessed through two main underground shafts, around 3.5 million tons of ore were extracted in the years the mine was active. The Church Rock mill aimed to process 4,000 tons of this ore a day, grinding it down and soaking it in sulfuric acid to extract the uranium oxide, leaving behind a slurry of radioactive waste called tailings.


These tailings were stored in big pools surrounded by huge earthen dams. However, UNC failed to insulate the walls of the dams to prevent the acidic tailings from eating away at them, and the ground they were built on was unstable, raising the risk of the dams cracking – and crack they did.

“Fist-sized cracks” in the south side of a dam were reported by surveyor Larry King shortly before the fateful spill in 1979. UNC was aware of cracks forming from at least 1977.

The Spill

It is important to note that most of the local population was Indigenous, mostly Navajo. Due to the dry climate, residents relied on the Puerco River to water their livestock and crops, and groundwater for drinking. Some even called the river Tó Nizhóní, translating to “beautiful water”.


However, that summer morning, it was far from beautiful.

At 5:30 am on July 16, 1979, disaster struck, and the dam failed. A worker flagged up the breach at 6:00 am and a temporary barrier was constructed, stopping the avalanche of solid tailings by 8:00 am. But it was too late – 94 million gallons of radioactive liquid waste was cascading down the river, alongside some of the 1,100 tons of solid tailings that also escaped.

"I remember the terrible odor and the yellowish color of the water," Larry King told VICE.

"I didn't know what was going on but it was an ugly feeling. I went to work and found out the dam broke," Church Rock Chapter Vice President Robinson Kelly told the Navajo Times.


This flood of waste was acidic, with a pH less than 2. It also held around 1.36 tons of uranium, plus radium and thorium, and had a gross alpha activity of 46 trillion picocuries. For context, the maximum level of alpha particles allowed in drinking water today is 15 picocuries per liter.

Alpha radiation can be detrimental to health over time when ingested – for example via eating or drinking contaminated material – increasing the risk of developing cancer. Exposure to elevated levels of uranium can cause kidney damage.

People who tried to cross the river developed sores, blisters, and burns on their feet and legs. The gushing waste backed up sewers, and radioactivity spiked in the water, soil, and air. Just after the dam broke, the river below it measured 6,000 times the allowable standard of radioactivity.

The Aftermath


Following the spill, Navajo UNC employees that spoke the local Indigenous language were sent out to warn residents, and the harrowing news was broadcast on the radio. However, many residents had no idea that their wells, springs, pools of water were contaminated, with children even playing in the water.

church rock spill
A warning sign placed by the Puerco river by the New Mexico Environmental Improvement Division after the Church Rock uranium mill spill on July 16, 1979. Image credit: EPA Public Domain

High levels of radiation were found over 161 kilometers (100 miles) down the river, as far away as the Petrified Forest National park.

The chairman of the Navajo Tribal Council’s Emergency Services Coordinating Committee requested in August 1979 that a state of emergency and a federal disaster area be declared, but was denied by the Governor of New Mexico, Bruce King.

A report from the New Mexico Environmental Improvement Division describes the Centers for Disease Control and Prevention (CDC) testing six Navajo individuals who were likely exposed to the spill, finding “amounts of radioactive material normally found in the human body.” However, it has been pointed out that this initial study only accounted for exposure pathways like inhalation, not others such as drinking groundwater or consuming contaminated vegetables, which can accumulate over time.


The same report recommends that “The Puerco River should not be used as a primary source of water for human consumption/ livestock watering or irrigation,” citing radioactivity and toxic metals in the water. However, due to the scarcity of water in the area, locals had little choice but to use it anyway.

Cattle, goats, and sheep from near the river had higher concentrations of radioactivity in their bones and organs, but as older animals had higher levels, the CDC put this down to the radioactivity already present in the river. Larry King told VICE that in a nearby community, "Their sheep were born hairless… like baby rats," with butchered animals having lurid yellow stomach fat.

The initial cleanup effort could be described as half-hearted at best, with only 1 percent of solid waste being removed. Contaminated sediment was loaded into oil drums by workers equipped with only shovels and buckets. The cleanup only focused on the top ~7.6 centimeters (3 inches) of the riverbed, ignoring the danger seeping further into the riverbed and contamination of groundwater.

However, UNC remained unrepentant. As reported by the Washington Post in 1983, UNC spokesman Juan Valesquez said that "The situation with the spill is that it has all been cleaned up," saying that it would be "simply erroneous to assume" that UNC’s mining activities caused elevated radiation in the river. The LA Times reported that in 1992, UNC was ordered to set aside over $16 million for the cleanup effort, but the money was instead given to UNC’s parent company.


The Current Situation

In 1983, the site of the spill was declared a Superfund site by the United States Environmental Protection Agency (EPA). The EPA states that this program "allows EPA to clean up hazardous waste sites and to force responsible parties to perform cleanups or reimburse the government for cleanups led by EPA."

The EPA states that the Navajo Nation requested the EPA to take the lead in the cleanup in the 2000s. Since then, around 200,000 tons of contamination were removed from the local residential area by UNC, and the EPA has conducted a number of five-year reviews that the protective measures put in place are effective. An annual corrective action report including groundwater analyses have been submitted by UNC since 1989.

UNC’s mining at Church Rock stopped in 1982 after it was no longer profitable. However, it left a lasting imprint on the local population. Although there were no deaths recorded as a direct result of the spill, many locals cite it as the source of ailments affecting them such as cancer, kidney disease, and diabetes.


A 2014 paper quotes a local Navajo resident Jackie Bell-Jefferson saying: “This area used to be my playground, now it’s just a huge wound.”


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