Workers from the US Environmental Protection Agency (EPA) accidentally triggered a massive release of toxic waste from an old gold mine that sent about three million gallons of polluted water into a Colorado river, turning it orange.

The spill began August 5 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 one million gallons (3.8 million liters) of polluted water -- containing dissolved heavy metals -- spilled into the river.

But on Sunday, the agency revised that figure to three million gallons.

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 a mustardy-orange color and discolored waters stretched for more than 100 miles, all the way into the neighboring state of New Mexico.