A team from the EPA was investigating contamination at the abandoned mine when loose material that had collapsed into the cave entry unexpectedly gave way, opening the mine tunnel and spilling the water into Cement Creek, a tributary of the Animas.
There were several workers at the site at the time of the breach, but no one was harmed.
As of Aug. 8, the flow from the mine was measured at 548 gallons per minute, and the discolored water from the spill had stretched more than 100 miles from where it originated, according to a report in the Denver Post.
Water collected at sampling stations along Cement Creek and the upper Animas on Aug. 9 found higher-than-normal levels of arsenic and other heavy metals. The mine water is being diverted to settling ponds constructed near the portal and treated with lime and sodium hydroxide solution to facilitate sedimentation of the metals in the ponds. Flocculant is being added to increase the amount of sedimentation.
The metal levels are dropping as the plume drifts farther down the river and is diluted; however, heavy metals will settle on the riverbed as the tail of the plume travels downstream, so a long-term monitoring plan will be needed.
La Plata County and Durango both declared a state of local emergency as a result of the spill.
It is unclear at this time if the spill will have negative impacts on drinking water, public health, agriculture, fish and wildlife. The city of Durango uses drinking water from the Animas, but an intake valve was turned off before contaminated water reached it, city officials said. Fish are especially sensitive to changes in water quality. Colorado Parks and Wildlife officials have been monitoring the effects of the spill on terrestrial and aquatic wildlife since the incident began.