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A massive cyanide spill at a gold mine in Western Argentina contaminated five rivers after a leak of over 265,000 gallons of cyanide solution, according to the Argentina Independent.

The spill occurred in September, when the leak sprung in a defective valve, eventually leading a court to temporarily ban cyanide solution from being used at the site. (The solution is used during the “leaching process.”)

Officials had said in the days after the spill that there was no evidence of contamination in the rivers, but, on Tuesday, a government report said that cyanide and toxic heavy metals were found in the rivers near the mine, in Veladero.

The mine, operated by Canada-based Barrick Gold, is one of the largest gold mines in the world, yielding over 18 tons of gold in 2015 alone. The government has said that they will be investigating any potential health risks related to the spill.