Coal ash, what’s left over after coal is burned to generate electricity, is one of the largest waste streams in the U.S., with hundreds of millions of tons of it lying in hundreds of sites across the country.

However, a lot of that ash, which can contain a host of toxic metals, isn’t just sitting around in a landfills or disposal pits, it’s also been a cheap source of fill material, with 2 million tons of it being used for that purpose in 2021 alone, according to the American Coal Ash Association, a trade group. EarthJustice, an environmental group, citing the association’s numbers, says 180 million tons of coal ash has been used for fill since 1980. Ash has been used on everything from a golf course in Virginia to playgrounds in Tennessee and much of an entire Indiana town.

“Coal ash structural fill has been used to construct stable base layers for roads, bridges, airfields and large buildings across the state. … The availability and often low cost of coal ash made the product attractive to developers and landowners, especially during the 1980s and 1990s,” North Carolina’s Department of Environmental Quality says on its website. The state is one of few that has actually attempted to track where ash was used as structural fill.

And, in a draft risk assessment published last month by the U.S. Environmental Protection Agency as part of a proposed broader revision of its coal ash management rules, the agency now says using coal ash as fill may create elevated cancer risk from radiation.

“This is the first time EPA has identified the threat from radioactivity from ash use as fill,” said Lisa Evans, a senior attorney at EarthJustice, an environmental law group. “Which is really important because ash has been used as fill for 100 years. … We didn’t really worry about the radioactivity until EPA pointed it out in this draft risk assessment.”

Coal ash, according to the EPA assessment, is recognized as “a type of technologically enhanced naturally occurring radioactive material,” meaning that naturally occurring radium has “has been concentrated or altered, such as through combustion, in a way that increases the potential for exposure.”

In addition to groundwater contamination, risk depends on a host of factors, such as how much coal ash was mixed with soil and how much soil was used to cover the ash, called “coal combustion residuals” (CCR) in agency jargon.

“EPA also found greater potential for risk from gamma radiation as CCR comes to be located closer to the ground surface due to a reduction in shielding,” the EPA wrote in the assessment. “An additional sensitivity analysis identified potential for further risk if CCR becomes mixed with surface soil. Accumulation of CCR can result in elevated cancer risk from incidental ingestion of arsenic and radium, in addition to direct exposure to gamma radiation from radium.”

More than 150 groups sent a letter to the EPA Dec. 11 urging the agency to take several steps to protect the public from risks associated with using ash as fill.

They want EPA to quantify the full range of health risks, from radiation in particular, investigate where ash was placed near residential areas and require clean up, craft a rule that prohibits the use of ash as structural fill and issue a public advisory recommending an immediate halt to use of ash as fill in residential areas. The letter notes that EPA found cancer risks even when small amounts of ash (1 to 2% of the soil mix) are used.

“EPA action to remedy these hazardous sites and prevent further dangerous use of toxic coal ash is needed because EPA’s current regulation of coal ash fill is grossly inadequate,” the groups wrote, noting that there are no restrictions on ash placement for volumes less than 12,400 tons. “For larger volumes, the lack of enforceable safeguards and oversight is equally disastrous. In most states, coal ash fill can be placed directly next to or under dwellings, drinking water wells, aquifers and playgrounds. Further, there is often no requirement to even cover the toxic waste.”

A spokesperson for the agency said it would “review the letter and respond through the appropriate channels.”