By Alex Dobuzinskis
(Reuters) - A U.S. Environmental Protection Agency team working to address seepage at an abandoned Colorado gold mine underestimated water pressure at the site before triggering a blowout that sent toxic waste into rivers, according to an internal report released by the agency on Wednesday.
The Aug. 5 blowout at the Gold King Mine located on a creek outside Silverton in southwest Colorado sent more than 3 million gallons (11,360 cubic meters) of acid mine sludge into the nearby Animas River, with the plume later flowing from the Animas into the San Juan River in New Mexico.
The spill of bright orange wastewater, which carried high concentrations of heavy metals such as arsenic, lead and mercury, was an embarrassment for the U.S. Environmental Protection Agency.
It led officials to temporarily shut off intake of river water in Durango, Colorado, and the New Mexico towns of Farmington and Aztec, while creating problems for farmers needing to irrigate their crops on the Navajo Indian reservation and other areas along affected waterways.
The internal EPA report said a team was at the Gold King Mine on Aug. 5 as part of a project to reopen a passage into the mine and assess wastewater seepage. During an excavation of a blockage, something appears to have been knocked loose that sent wastewater spurting out, the report said.
The team was experienced and had followed a solid work plan, but the "underestimation of the water pressure in the Gold King Mine workings is believed to be the most significant factor relating to the blowout," the report said.
The EPA could have been tipped off that water pressure was building up by the fact that flows from mine passageways had decreased in previous years, the report said.
The U.S. Department of the Interior is working on a separate report on the factors that led to the blowout.
On Aug. 14, a stretch of the Animas River in Colorado was reopened to kayaking and rafting, and officials said waterways affected by the spill were returning to normal through dilution.
The EPA has measured for contamination as far away as Lake Powell on the border between Arizona and Utah, but has said it expects no significant effects from the spill at the lake.
Analysts say contaminants have settled into river sediments, where they can be churned up and unleash a new wave of pollution when the rivers flood.
(Reporting by Alex Dobuzinskis in Los Angeles; Editing by Daniel Wallis and Peter Cooney)