EPA officials are anxious that work on each site could lead to a repeat of last month’s massive spill at an inactive Gold King Mine in Silverton, Colo. The fallout from the August 5 disaster has left Congress calling for answers and a more complete picture of the risk of abandoned mines throughout the nation.

A stop-work order affecting the sites was issued last month but officials did not disclose specifics.

In recent years, the EPA has been trying to clean up roughly 500,000 abandoned mines across the United States. 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.

As a result, the EPA said it is being exceedingly careful about its work on similar sites.

The report says abandoned mines make up the majority of un-inventoried hazardous waste sites on land managed by the departments of Agriculture, which oversees the U.S. Forest Service, and Interior, which oversees the Bureau of Land Management.

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

U.S. Rep. Lamar Smith (R-TX), chairman of the Science, Space and Technology Committee, led the first congressional hearing on the Gold King Mine spill, which was triggered by Environmental Protection Agency (EPA) inspection activity on August 5.

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.