An infrastructure failure at a Tennessee power plant last month that sent 1 billion gallons of sludge into the surrounding area -- flooding fields, destroying homes and polluting water -- has raised serious questions about the true cost of coal.

On Dec. 22 a portion of an earthen wall surrounding a coal ash waste pile at a power plant in Kingston, Tenn., collapsed, and 1 billion gallons gushed out of the holding facility, covering about 300 acres and destroying three homes. The ash and water mixture, which local authorities and others are currently trying to clean up, contains heavy metals, including arsenic, mercury and lead.

According to the Environmental Protection Agency, these substances can harm public health and threaten the safety of drinking water, but the agency does not currently monitor the disposal of coal ash waste, which is stored nationwide in large holding facilities like the one at the Tennessee Valley Authority power plant.

Of the 600 domestic coal ash disposal sites, about 45 percent are wet ponds of coal ash, where the waste is stored mixed with water, as it was at the TVA power plant, according to the Utility Solid Waste Activities Group, a trade association. None of these are subject to federal oversight.

These sites could pose health problems, even without catastrophic spills like the one in Tennessee, said Sen. Barbara Boxer, D-Calif., chairwoman of the Environment and Public Works Committee.

"A 2007 U.S. EPA report found 67 ash impoundments or landfills in 23 states that had caused or were suspected of causing contamination, including to ground or surface waters," Boxer said Thursday at an EPW Committee hearing. "EPA knew of dozens of other sites but lacked sufficient information to single out the cause."

The potential for similar spills provides more fuel for coal critics, who say the cost to human health and the price to clean up these massive messes should be put on coal's tab.

"Here we are creating huge dumps of coal ash waste, and the costs aren't included in the system," Sen. Tom Udall, D-N.M., said at Thursday's hearing. "So I hope we take our regulatory responsibilities seriously, and I hope we look at the full costs in terms of our energy sources."

Among those pushing for a price adjustment that reflects the external costs of coal is Friends of the Earth, an environmental organization whose leadership says coal's upfront price tag is misleadingly cheap.

"There are real healthcare costs for those affected by pollution, and there are environmental cleanup costs, and global warming is going to be a huge cost," said Nick Berning, press secretary for Friends of the Earth. "When these costs are included, coal is extremely expensive."

Congress could adjust the price of coal to include these externalities through a number of mechanisms, including a tax on coal or a system that would place a price on coal emissions, Berning told United Press International.

But others, including Sen. James Inhofe, R-Okla., ranking member of the EPW Committee, say such a price adjustment would harm the economy and undercut the nation's primary energy source.

"Certain extremist groups will try and use this to further their own political agendas, namely to eradicate the use of coal in this country," Inhofe said at Thursday's hearing. "We all know that would be a disaster for energy security, for jobs and for the health of our economy. We know how to use coal in a clean manner."

One of the main methods of preventing air pollution from coal-fire power plants, however, increases the amount of leftover coal ash waste. Federal and state laws have pushed many power plants to install scrubbers, which remove significant amounts of pollutants, such as mercury and sulfur dioxide, from their emissions streams. However, that leaves more of these substances in the waste that gets stored in large holding facilities, which potentially can contaminate water and cause health problems if leaks occur.

As clean air laws come into force and more stringent ones are passed, it will mean increasingly toxic waste at the plants, said Stephen Smith, executive director of Southern Alliance for Clean Energy, a non-profit organization that has threatened to sue the TVA if it doesn't clean up the mess from its power plant.

"This problem is only going to get worse," Smith told policymakers Thursday. "As we tighten air pollution standards, we will end up with more coal ash waste."

As a result, Smith and others want the Environmental Protection Agency to start monitoring coal ash. The EPA declined to do so in 2000, ruling the material did not constitute a hazardous waste.

However, that doesn't mean coal ash isn't monitored at all, said Jim Roewer, executive director of the Utility Solid Waste Activities Group.

"The states have programs in place," he told UPI. "It's regulated already."

Replacing those regulations with federal oversight would be less efficient than the current system, he said.

"States are closer to the issue, and they can deal with their own unique situations," he said.

Nevertheless, the EPA has been reviewing its position toward coal ash for some time and has been considering some proposals submitted by environmental organizations and citizen groups that would prohibit the placement or disposal of the waste in ground or surface water and impose guidelines for managing coal ash, said Tisha Petteway, an EPA spokeswoman.

"EPA is currently analyzing the peer reviews (of these proposals) as well as (public) comments, and will consider this information as it continues to follow up on its regulatory determination for coal combustion wastes disposed of in landfills and surface impoundments," Petteway told UPI.