The head of the Environmental Protection Agency toured the sludge-coated banks of Colorado’s Animas River on Wednesday, as the Obama administration sought to limit the environmental and political damage from last week’s 3-million-gallon toxic waste spill — one caused in part by the agency’s own contractors.

EPA Administrator Gina McCarthy ordered a temporary halt to the agency’s cleanup at the Gold King mine and several similar sites after traveling to the region to pledge a thorough investigation into an accident she has called “tragic and unfortunate.”

“It is a heartbreaking situation,” McCarthy said at a news conference in Durango, Colo., about 48 miles downstream from the site of the Aug. 5 spill. “We are going to be transparent and collaborative in making sure people have the information they need.”

The regulatory agency has been criticized for its response to the spill, which began as crews were investigating leaks from a toxic waste pond in the inactive gold mine north of Durango. An earthen barrier gave way, sending toxic wastewater into the Animas, turning the river bright orange-yellow for miles.

A week after the incident, the contractor involved in the work at the site was identified as Environmental Restoration LLC, a St. Louis-based firm. EPA officials said the company’s crews worked under the direction of the EPA in consultation with Colorado’s Division of Reclamation, Mining and Safety.

McCarthy met with Colorado Gov. John Hickenlooper (D) and Sen. Michael F. Bennet (D-Colo.) to coordinate responses to the spill.

“The good news is, the river seems to be restoring itself,” she said. The EPA reported Thursday that water quality had returned to normal along the entire stretch of the Animas from the spill site to Durango. “EPA plans to continue to monitor, analyze and share data for downstream river segments as it becomes available,” the agency said in a statement.

The Environmental Protection Agency says the spill of toxic wastewater from a mine in Colorado is three times larger than previously thought. Residents are being advised not to drink or bathe in well water. (Reuters)

Photos of kayakers floating in mustard-hued river water last week embarrassed the agency at a time when it is battling Western states over new regulations for water and air pollution. While no injuries or serious damage to wildlife have been reported, the spill raised levels of arsenic, lead and other toxins in the river for dozens of miles through southwest Colorado and northern New Mexico. Wastewater from hard-rock mines often contains heavy metals that can be toxic at high concentrations. Residue from spills can linger on the bottom of a river for months or years, to be redispersed with new storms and floods.

Traces of orange residue from iron were still visible on some riverbanks Wednesday, as state officials met to consider possible lawsuits against the EPA. They have complained about what they describe as a slow EPA response to the spill and inadequate precautions to prevent the accident from occurring.

“I was just horror-stricken that this could happen in our state,” Colorado Attorney General Cynthia Coffman, a Republican, told Denver’s Fox 31 TV station after a Wednesday visit to the Animas River, near Durango.

Officials from Colorado, New Mexico and Utah have complained about what they describe as a slow EPA response to the spill and inadequate precautions to prevent the accident from occurring. Longtime foes of the EPA’s pollution controls for air and water seized the opportunity to bash the agency.

“This disaster emphasizes the need for the EPA to focus on fulfilling its existing responsibilities, instead of focusing its resources on imposing expensive new regulations that kill jobs and hurt family budgets,” Sen. Orrin G. Hatch (R-Utah) said in a statement.

At the time of the spill, the EPA was attempting to determine how to deal with a common pollution problem in many parts of the Rocky Mountain West: the steady leaching of toxic waste from hundreds of hard-rock mines scattered throughout the region. At the Gold King site, the crew was moving heavy equipment near the mine’s wastewater pond when the barrier gave way.

EPA officials accused critics of seeking to obscure a larger pollution problem affecting waterways through the West.

“EPA was assessing cleanup efforts in a mine that had been leaching toxic material for years,” said an EPA official familiar with the accident, speaking on the condition of anonymity because the accident investigation is still underway.

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