In a uranium mine, an effluent retention basin broke. The surrounding retention enclosure subsequently failed, releasing 370,000 m³ of solution and 1,100 tonnes of a radioactive powdery substance. The radioactivity of these released elements (primarily thorium 230 and radium 226) rose to 46 Curie (1,700 Gbq), a level comparable to what was released the same year during the Three Mile Island accident. The solid phase settled in the vicinity of the operator’s parcel as well as into the adjacent ARROYO River. The liquid component flowed until reaching the RIO PUERCO River. In all, 110 km of watercourses, including 45 located in the neighbouring state of Arizona were contaminated. The slurry mix clogged sewer systems and left in its wake contaminated puddles.

Despite implementation of the New Mexico emergency plan, which included extensive onsite sampling, the population was not immediately notified: 2 days after the accident, several individuals sustained burns to their feet after wading in the river as a result of the high level of acidity in the discharged solution. The press reported deaths to livestock. Aquifers also sustained long-term pollution due to high metal concentrations.

A total of 3,500 barrels of waste were recovered. Once the basin had been placed back into service, activity was suspended between 8 and 13 November upon order of the State of New Mexico as a result of noncompliance with minimum clearances (separation between the liquid and the dam core).

Supplies were brought by truck until 1981, especially for livestock feed and irrigation. River water was used once again despite the presence of residual contamination. The mine was closed in 1982 and included in 1983 on the EPA’s list of national priority sites, given the migration of radionuclides and chemical compounds found in groundwater.

In January 1980, the Nuclear Regulatory Commission, in a publication filed with the Federal Register, cited a design error (inaccurate assessment of settlement amplitude and heterogeneity among alluvial foundation materials) as the cause of the basin break. These inaccuracies resulted in differential settlements and dam cracking, which were then exacerbated by operating deficiencies: the operator failed to maintain a sufficient distance between the liquid phase contained inside the basin and the dam core, which led to dam weakening due to crack filling and material saturation.

An agreement reached in 1988 between the EPA and the operator imposed that rehabilitation work be conducted by the operator. The water from 3 shallow water tables, polluted by residue from acidic tailings, sulphates, thorium, radium and iron, were pumped via both existing and additional shafts and then routed to the evaporation basins. The compartments of the mining effluent basin were covered by a radon-proof membrane. This process, which enabled limiting pollutant migration in water yet did not sufficiently lower the observed content levels, was revised in 2008. Pollution cleanup processes relying on massive water injection and subsequent pumping are still under study in 2011.