The Environmental Protection Agency has changed a rule requiring cleanup of ponds holding coal mining waste to give states more flexibility and postpone the deadline to close facilities that have contaminated the surrounding groundwater.

The decision changes a 2015 Obama administration rule about ponds and landfills that store coal ash, a byproduct of burning coal that contains heavy metals like arsenic, mercury, and cadmium. Coal ash is stored in ponds at sites around the country and testing at hundreds of those sites have shown that those toxic substances have spread from the holding pond into the groundwater.

There are hundreds of active coal ash sites across the country that handle more than 100 million tons of ash every year, according to the EPA. As much as 95 percent of the sites have contaminated groundwater with levels of the metals higher than is considered safe, according to the Environmental Integrity Project.

The EPA says the changes will let states address coal ash contamination on a case-by-case basis, but environmental groups say the change will weaken protections.