For the first time ever, the EPA is regulating the ash waste left over after power plants burn coal. But environmentalists have criticized the rule, saying the rules are too weak to safeguard against future spills. Every year, US coal plants produce 110 million tons of coal ash, which can contain toxic elements and is either recycled or stored in landfills and ponds. Those ponds have spilled before: In 2008, a big storage unit in Tennessee ruptured, releasing 1.1 billion gallons of coal ash slurry into the surrounding area. North Carolina had another big ash spill in early 2014. Under the new rule, the EPA will regulate coal ash as a "solid waste" — akin to household trash. That means existing storage sites will require regular inspections. Ponds that are currently polluting groundwater will have to close. New ponds will need to be built away from wetlands. But enforcement of these standards will be left to the states — and the rules won't deal with old, abandoned sites (such as North Carolina's Dan River site, the one that leaked in 2014). Environmentalists had pushed the EPA to go further and regulate coal ash as a "hazardous waste" — which would have forced utilities to phase out existing storage ponds and build new facilities with stricter standards. But utilities had opposed this option, saying it would have cost billions per year.

What is coal ash?

When pulverized coal is burned to generate electricity, it leaves behind inorganic matter, including fly ash, bottom ash, and boiler slag. This residue is referred to as "coal ash." Nowadays, most of that ash gets trapped by filters and scrubbers rather than being released into the air.

There are more than 2,000 coal ash storage sites in the US

There's a staggering amount of coal ash produced in the United States each year: In 2012, the nation's coal plants generated some 110 million tons of it. About 40 percent of that ash gets recycled to help make concrete, pavement, and other materials.

The rest is stored in landfills, quarries, and ponds as either dry ash or as a wet slurry. Currently, there are more than 2,000 storage sites around the country, including over 400 dry landfills and 676 wet ash ponds.

Why is coal ash so controversial?

The biggest concern with these storage ponds is that they could leak — leading to a catastrophic spill. Coal ash often contains a variety of toxic elements like selenium, mercury, and lead (although the exact amounts vary). In some cases, those heavy metals can pose health risks to humans and wildlife

The EPA began working on new rules after a major 2008 spill in Tennessee

In 2008, a giant storage unit at the Tennessee Valley Authority's Kingston Fossil Plant ruptured, releasing 1.1 billion gallons of coal ash slurry that damaged three homes and poured into the Emory River. After that disaster, the EPA began working on rules to regulate the ash.

More recently, in February 2014, a major spill occurred at Duke Energy's retired Dan River coal plant in North Carolina — a water pipe under the plant's coal-ash basin failed, leaking some 39,000 tons of ash into the river. Municipal officials filtered contaminants from drinking water, but the ash turned the river into a gray sludge, threatening wildlife.

In theory, major spills could happen again. The EPA has identified 45 wet ash ponds around the country that are "high hazard" — that is, if the encasing broke, it could lead to a loss of human life (the effect would be similar to the breaking of a massive dam). Two of those high-hazard ponds are located at Duke Energy's Dan River site in North Carolina:

There's also the risk of smaller-scale contamination, as the EPA noted in 2010. If the ash was deposited in an unlined landfill or sand pit or quarry, toxic elements could leach into the groundwater or migrate off-site. Or liquid waste could leak into surface water during a flood. Or dust from dry ash could become airborne.

The environmental group Earthjustice has found 207 sites in 37 states where coal ash has already contaminated the water or air in violation of federal health standards. For example: In Prince George's County Maryland, millions of tons of coal ash from a landfill leaked into a nearby creek after two recent hurricanes. In Nevada, the Moapa River Reservation alleged that dry coal ash was blowing into their communities from uncovered dumps, leading to a rash of illnesses.

What is the EPA proposing?

In its rule today, the EPA said it will regulate coal ash as "solid waste" — akin to household trash. Among other things, that will mean:

All storage landfills and ponds that don't meet certain engineering and structural standards will no longer be able to receive coal ash.

Storage ponds will need regular inspections to check for structural integrity.

New coal-ash storage sites can't be built in sensitive areas — such as wetlands or earthquake zones. New storage ponds will also require liners so as to prevent leakage.

Any storage sites that are currently polluting groundwater will need to be monitored, cleaned up immediately, and eventually closed.

Dry landfill sites will have to impose controls on coal ash dust.

One catch here: There wouldn't be any federal enforcement of these standards — this would all be left to either the states or citizen lawsuits.

Another twist: The EPA's regulations would not apply to storage sites that had already closed. That means these rules would not have applied to Duke Energy's Dan River site between the time that it closed in 2012 and then had a massive spill in 2014.

Back in 2010, the EPA estimated that complying with these rules would cost utilities some $587 million per year, or $8 billion total.

Could the EPA have gone further than this?

Yes. Many environmental groups had pushed the EPA to coal ash as "hazardous waste" — an alternative option that the agency had also proposed back in 2010. Doing this would have involved an additional set of rules on top of what's being put out today.

The 'hazardous waste' option would have required companies to phase out their existing ash ponds

For starters, the federal government would have direct say over enforcement and monitoring, not the states. What's more, under this option, utilities couldn't build any new wet ash ponds and would have to phase out their existing ponds — moving everything over to specially designed landfills.

The EPA would also be required to oversee every aspect of coal ash handling, from storage to transportation to disposal. And companies would have to show that they have the financial resources necessary to clean up after disasters. Back in 2010, the EPA estimated that the "hazardous waste" option would cost companies $1.4 billion per year, or $20 billion in all. (Industry groups said the cost could be triple that.)

Mining and utility companies, for their part, argued that the "hazardous waste" rule would be far too costly. They'd have to move coal ash off-site or set up expensive hazardous-waste landfills. Opponents of this option also tended to note that there's little evidence to suggest that states are doing a poor job of regulating coal ash themselves. Finally, they argued, recycling companies could be more reluctant to purchase coal ash if it was deemed "hazardous."

Utilities won this fight, and environmentalists are unhappy: "The EPA is bowing to coal-fired utilities' interests and putting the public at great risk by treating toxic coal ash as simple garbage instead of the hazardous waste that it is," said Scott Slesinger, legislative director of the Natural Resources Defense Council, in a statement. "Too much of the agency's new rule is left to the discretion of states, which all too often have favored powerful utility companies instead of the public."

Why is the EPA regulating ash, anyway?

For decades, there were no federal standards on how coal ash could disposed or where and how it can be recycled. It was all handled by the states, and the rules often varied from state to state. But after the big 2008 spill in Tennessee, the EPA decided to look at whether coal ash should be regulated under existing federal waste rules.

In 2010, the agency proposed the first-ever rules to regulate the stuff — putting forward both the "solid waste" and "hazardous waste" options. At the time, coal mining and utility companies argued that overly strict rules (the "hazardous" option) would impose unreasonable costs.

The Obama administration put off making a final decision for years, so environmental groups, led by Earthjustice, sued the EPA. The courts ordered the agency to reach a final decision by December 19, 2014. Which brings us to today.