Sonora River spill (Buenavista del Cobre mine)

Copper has been mined at Cananea without interruption since at least 1899; the open pit, now Buenavista del Cobre, belongs to Grupo México and is one of the world's largest copper deposits (Wikipedia). On 6 August 2014 the Tinajas 1 leaching system released about 40,000 m³ of acid copper sulphate solution carrying aluminium, arsenic, copper, iron, manganese, lead and zinc. According to Wikipedia the mine told the authorities only after 24 hours, when residents had already seen the river turn orange.

The company built a retaining dam, added lime and collected the precipitated solids, taking them back to the mine. In 2014 a trust, the Fideicomiso Río Sonora, was set up to cover the damage; according to SEMARNAT's diagnosis it covered less than 6% of the real damage (Mongabay 2023).

The scientific assessments do not agree. A study of 187 sediment samples finds no statistically significant difference between the most affected reaches and the local baseline, and puts this down to natural geological enrichment, historical mining and attenuation after the spill (Gutiérrez-Ruiz et al. 2022). Another, using the 2014–2017 government surveys and sampling in 2018, concludes that the spill worsened concentrations in surface water and that other point sources keep contaminating the river (Archundia et al. 2021).

The environmental diagnosis presented by SEMARNAT in September 2023 attributes the spill to the deficient hydrological design of the Tinajas 1 system and puts total damage at more than 20,508 million pesos. Of 37 wells analysed, more than half exceeded the limits for arsenic, manganese and sulphates.