[9] The spill contaminated groundwater and rendered the Puerco unusable to local residents, mostly Navajo peoples who used the river's water for drinking, irrigation, and livestock.

[2] The Governor of New Mexico Bruce King refused the Navajo Nation's request that the site be declared a federal disaster area, limiting aid to affected residents.

"[13] At around 5:30 am on July 16, 1979, a previously identified crack opened into a 20-foot-breach (6.1 m) in the south cell of United Nuclear Corporation's Church Rock temporary uranium mill tailings disposal pond, and 1,100 short tons (1,000 t) of solid radioactive mill waste and about 93 million US gallons (350,000 m3) of acidic, radioactive tailings solution flowed into Pipeline Arroyo, a tributary of the Puerco River.

[17] In addition to radioactive uranium, thorium, radium and polonium, it also contained various other metals, including cadmium, aluminium, magnesium, manganese, molybdenum, nickel, selenium, sodium, vanadium, zinc, iron, and lead, and also high concentrations of sulfates.

[17] The contaminated water from the Church Rock spill traveled 80 miles (130 km) downstream, through Gallup, New Mexico, and reached as far as Navajo County, Arizona.

The EPA reported no long-term effects of the spill, but noted that contaminant levels from uranium mine effluents and natural sources were "environmentally significant".

[4] Several days after the spill, the Indian Health Service and the Environmental Improvement Division of New Mexico warned local residents over the radio and with signs written only in English not to drink from, water livestock at, or enter the Puerco River.

[21][22] United Nuclear Corporation employees were dispatched to warn Navajo-speaking residents downstream in accordance with a state contingency plan, but not until a few days after the spill.

[4] Although the New Mexico Environmental Improvement Division said the spill's "short-term and long-term impacts on people and the environment were quite limited",[24] ponds of uranium-contaminated water lined the Puerco River and seeped into wells.

"[24] Navajo Tribal Council's vice president Frank Paul said of the worst spill in US history, "Somehow, United Nuclear Corporation was permitted to locate a tailings pond and a dam on an unstable geologic formation.

[2][3][4] The spill has been called "the largest radioactive accident in U.S. history," but the Nuclear Regulatory Commission has said that this is "an overstatement," and that "there have been a number of other events that have been more significant in terms of radiological impact.

[36][37] At the same Congressional hearing, the United States Army Corps of Engineers testified that had the dam been built according to legal specifications, the failure would not have occurred.

[38] United Nuclear initially claimed that only one curie of radioactivity had been released in the spill, but that figure was later revised upward by the New Mexico Environmental Improvement Division.

[45] An epidemiological study conducted by the NMEID in 1989 concluded that "the health risk to the public from eating exposed cattle is minimal, unless large amounts of this tissue, especially liver and kidney, are ingested.

"[46] An Indian Health Service study found significantly higher levels of radionuclides in Church Rock cattle compared to livestock from non-mining areas.

[47] A few Navajo children were sent to Los Alamos to be checked for radiation exposure, but no long-term monitoring was undertaken, prompting a local writer to comment that the IHS spent more effort studying livestock than the people affected.

[4][48] Studies have shown since the 1950s that the Navajo have had significantly higher rates for some cancers than the national average, associated with contamination from the uranium mines and the exposure of workers to radiation.

[49][50] United Nuclear dispatched small crews with shovels and 55-US-gallon (210 L; 46 imp gal) drums to begin cleanup, but expanded the workforce after complaints from local residents and pressure from the state.

[35] The pools contained high levels of sulfuric acid and remained for more than a month after the spill, despite cleanup efforts by the New Mexico Environmental Improvement Division.

[52] The Navajo Nation appealed to the governor to request that the president declare the site a federal disaster area, but he refused, reducing the aid available to local residents.

[54] In 1983, the site was entered on the National Priorities List of the Environmental Protection Agency's Superfund investigations and cleanup efforts, as radionuclides and chemical constituents were found to be contaminating local groundwater.