Workers from the US Environmental Protection Agency (EPA) have accidentally triggered a massive release of toxic waste from an old gold mine that sent about 11 million litres of polluted water into a Colorado river, turning it orange. The spill began last Wednesday when EPA workers were trying to drain water from the mine but accidentally sent the toxic water flowing into a tributary of the Animas River. The EPA originally said about 3.8 million litres of polluted water - containing dissolved heavy metals - spilled into the river. But on Sunday, the agency revised that figure to 11 million litres. Affected water contained elevated levels of arsenic and other metals, the Denver Post reported, noting that it was unclear if the spill posed health risks to humans and aquatic life. The normally clear waters turned mustard-orange and discoloured waters stretched for more than 160km, all the way into the neighbouring state of New Mexico.