Site investigations and some cleanup work at 10 polluted mining complexes in four states were suspended because of conditions similar to those that led to a massive wastewater blowout from an inactive Colorado gold mine, US Environmental Protection Agency officials said.

The sites include three in California, four in Colorado, two in Montana and one in Missouri, according to details obtained by the Associated Press following repeated requests for the information.

They have the potential for contaminated water to build up inside mine workings, EPA assistant administrator Mathy Stanislaus said. That would set the stage for a possible spill such as last month’s near Silverton, Colorado, where an EPA team triggered a 3m gallon blowout of toxic sludge while doing excavation work on the inactive Gold King Mine.

The accident fouled rivers in three states and attracted harsh criticism of the EPA for not being prepared despite prior warnings that such a spill could happen.

“We want to take extra caution before we initiate any work,” Stanislaus said of the work suspensions. Some the mines were abandoned decades ago and have grown more unstable over time, raising the risk of an accident.

The stop-work order was issued last month but officials for weeks refused to disclose specifics.

Cleanup efforts on some of the mines have been going on for years yet remain unfinished, underscoring the complexity of a long-running attempt to address an estimated 500,000 abandoned mines across the U.S. Work on others was in the early stages.

In a report to Congress delivered Friday, the Government Accountability Office said federal agencies identified thousands of contaminated mine sites in recent years – even as their attempts to assess what harm is being done to people and the environment have lagged.

Further investigations were needed to gauge the danger posed by the 10 mining complexes under the suspension before work could safely resume, according to internal EPA documents released by the agency.

That includes categorizing their level of hazard. For those deemed a “probable hazard”, the EPA plans to keep the work stoppage in place until emergency plans are drawn up to deal with any accident.

The agency also wants to get the results of an Interior Department investigation into the Colorado accident before proceeding on most of the other sites. That’s expected in late October, department officials said.

Prior to the 5 August Gold King spill, the EPA and its contractor, Environmental Restoration LLC of St Louis, appeared to have only a cursory emergency response plan in the event of a spill, according to documents released under public records requests.

There was no cellphone coverage at the remote site in the San Juan Mountains, and the workers did not have a satellite phone, according to EPA documents. As a result, they had no way to immediately communicate with the outside world when the rust-colored water loaded with heavy metals, including lead and arsenic, began rushing towards downstream communities.

One of the sites where cleanup work was subsequently halted was the Standard Mine in the mountains above Crested Butte, a ski town in west-central Colorado. Crested Butte mayor Aaron Huckstep said that after work was suspended, the EPA met with residents and officials and made sure cleanup workers could communicate directly with the town in an emergency.

“They understood that they needed to make sure that the communication channels and the communication protocols were in place and the folks knew who to call and when to call them,” Huckstep said.

EPA documents show wastewater at the site periodically spills over a crudely-built impoundment, raising concerns about a “potential catastrophic failure” and the possibility of tainting Crested Butte’s drinking water. But Huckstep said he didn’t believe the Standard Mine was a threat to blow out, based on EPA statements and differences in the land.

The EPA said the town’s water meets safety standards.

Colorado Department of Public Health and Environment spokesman Warren Smith said wastewater flowing from the mine was not considered an acute health threat. Work on the site resumed 4 September after officials determined appropriate safety measures were in place.

The 12 August stop-work order from EPA Administrator Gina McCarthy did not apply to sites where halting operations would pose a threat to people or increase the potential for harm to the environment, according to internal EPA documents.

Also exempted were portions of the 10 stopped projects where construction already was completed, such as treatment systems for contaminated water that pours continually from many abandoned mine shafts.

That’s the case for two sites listed in northern California: the Leviathan sulfur mine near the town of Markleeville and the Iron Mountain metals mine near Redding. Water continues to be collected at the sites, to be treated and then discharged.

“We have not received any direction from EPA to shut down our treatment. It’s been business as usual for us out there,” said Scott Ferguson, with the Lahonton regional water quality control board, which is involved with the Leviathan mine.

EPA spokeswoman Laura Allen said other work at the two mines has stopped, including plans to remove a beaver dam at Leviathan.

