On August 7, 2014 ten million gallons of a mining wastewater solution broke through containment at the Buenavista copper mine just outside of Cananea, Sonora. The sulfuric acid solution was released into the Rio Bacanuchi, a tributary of Rio Sonora, one of the largest watersheds in Sonora.
Rio Sonora is a key water source for many communities including Arizpe, Sinoquipe, Banámichi, Aconchi, Baviácora, Mazocahui, and Ures, along with substantial agriculture along its lower reaches. Rio Sonora headwaters are near the spill area just south of the US border. It then flows south over 200 km before turning west towards Hermosillo, which until recently used a reservoir on the Rio Sonora as its primary municipal water source. The Indepencia Pipeline connecting to the Rio Yaqui started delivering water to Hermosillo and its 800,000 people last year, freeing the city of its reliance on water from the Rio Sonora.
The contamination has colored the river a burnt orange and killed livestock, fish, and likely innumerable other aquatic animals. Grupo Mexico owns the Buenavista copper mine, which contains one of the larger known veins of copper on Earth. Effective cleanup of such a spill is not feasible. Likely many of the contaminants will be flushed by monsoon rains and flooding to downstream reservoirs, where it will remain in water and sediment.