The canary in the coal mine is singing. The coal industry, which fuels around 33 percent of electricity generation in the United States, is collapsing, hastened by competition, low natural gas prices and sluggish growth in electricity demand. In addition, the coal industry has been battered by new federal standards issued by the Environmental Protection Agency that limit the amount of mercury and other air pollutants, including toxic metals like arsenic. The new rules are forcing utility companies to shutter their coal-fired power plants and launch new plants that combust natural gas.

In April, Peabody Energy, America's largest coal company, filed for Chapter 11, a move coal opponents have hailed as the death knell for the entire industry. But Peabody isn’t alone: Around 44 percent of America's coal came from companies in bankruptcy, according to an analysis published earlier this year by SNL Energy, a market intelligence firm.

Feds: No more new coal mining leases, for now

An announcement made by Sally Jewell in January hasn’t helped matters for the nation’s coal barons. In a major blow to the industry, the Interior Secretary implemented a federal moratorium on the issuing of new coal mining leases on public lands across the U.S.—some 570 million acres—as her department conducts a review of the program, the first in more than three decades.

“Given serious concerns raised about the federal coal program, we’re taking the prudent step to hit pause on approving significant new leases so that decisions about those leases can benefit from the recommendations that come out of the review,” Jewell said following the announcement. “During this time, companies can continue production activities on the large reserves of recoverable coal they have under lease, and we’ll make accommodations in the event of emergency circumstances to ensure this pause will have no material impact on the nation’s ability to meet its power generation needs.”

Those concerns include a lack of transparency, coal’s climate impact and loopholes that allow coal companies to avoid paying taxpayers the full 12.5 percent of the value of surface-mined coal and 8 percent of underground coal in royalties, which is required by the Mineral Leasing Act of 1920. “We have an obligation to current and future generations to ensure the federal coal program delivers a fair return to American taxpayers and takes into account its impacts on climate change,” said Jewell.

“The president is right to stop this handout to big coal companies, which has cost American taxpayers more than $30 billion over the past three decades,” said Rhea Suh, president of the Natural Resources Defense Council.

End of fossil fuel era?

While the federal freeze—which could last up to three years—won’t impact existing leases, which generated almost $1.3 billion for the federal government in 2015, the move has fueled the ongoing speculation that the U.S. coal industry is burning its last embers. That belief has grown as coal struggles against the rise of inexpensive, less carbon-intensive natural gas.

“Peabody Energy's bankruptcy is a harbinger of the end of the fossil fuel era,” said Jenny Marienau, U.S. divestment campaign manager with the environment nonprofit 350.org. “Peabody is crashing because the company was unwilling to change with the times—they doubled down on the dirtiest of all fossil fuels, and investors backed their bet, as the world shifted toward renewable energy. They have consistently put profit over people, and now their profits have plummeted. Our world has no place for companies like Peabody.”

Last fall, Fortune's senior editor-at-large Brian Dumaine argued that “coal power is on the way out—divestment or not.” He wrote:

Market forces have already savaged the U.S. coal industry. Cheap natural gas, which emits roughly half the carbon of coal, has become a favorite of utilities in the U.S. The amount of electricity produced from coal has declined from 50 percent in 2005 to 36 percent today. As a result, the Dow Jones U.S. Coal index has fallen 93 percent during the past five years, compared with a 75 percent rise in the S&P 500.

Big polluters

While environmentalists and renewable energy advocates may be cheering coal’s potential demise, coal-fired power plants continue to pollute the nation’s lakes and waterways with a host of highly toxic substances, including carcinogens and neurotoxins such arsenic, lead, mercury, cadmium and chromium. The plants also discharge nitrates, which cause algae blooms that lead to low-oxygen dead zones in lakes, rivers and coastal waters.

According to a 2015 report, “Selling Our Health Down the River," which was co-authored by EIP, Sierra Club, Earthjustice, Clean Water Action and Physicians for Social Responsibility, U.S. power plants discharge more than 5.5 billion pounds of pollutants into the nation’s waterways every year, contaminating more than 23,000 miles of rivers and 185 water bodies. The pollution renders these waterways unsafe for drinking or fishing, poisons the fish and other wildlife that depend on them to survive, and even threatens child development.

"Coal-burning power plants are pouring poisonous heavy metals into our waterways. These toxic substances—like mercury, lead and arsenic—are putting at risk the health of our children and the developing brains of our babies," said Barbara Gottlieb, director of environment and health at Physicians for Social Responsibility.

A recent report published by the Environmental Integrity Project, a Washington DC-based environmental nonprofit, took a close look at America’s coal plants and ranked the top 10 worst polluters for a range of toxic metals.

“Coal plants are the single largest industrial source of toxic water pollution in the U.S., releasing more than five billion pounds of pollutants every year into rivers, lakes and small streams,” said EIP, in a press release. “These discharges include large quantities of arsenic, cadmium, chromium, lead, mercury and selenium, which are hazardous in very small concentrations.”

Image: EIP

The report, “Toxic Wastewater from Coal Plants,” is based on self-reported data for 2015 recorded in the Toxic Release Inventory, a publicly available database maintained by the Environmental Protect Agency that tracks toxic chemical releases and pollution prevention activities as reported by industrial and federal facilities. The report looked at 216 coal plants that discharged wastewater to rivers, lakes or tidal waters in 2015.

Among the report’s findings:

The Tennessee Valley Authority’s Cumberland power plant in Cumberland City, Tennessee, discharged 120 pounds of mercury to the Cumberland River in 2015.

The Elmer Smith plant in Owensboro, Kentucky, released 1,112 pounds of lead to Blue Lake.

The DTE Monroe Power Plant in Monroe, Michigan, dumped nearly a ton of arsenic into Lake Erie, and the SWEPCO Pirkey Plant Plant in Harrison County, Texas, released the same amount into the Brady Branch Reservoir.

"Based on our review, EPA, states, and power companies have a lot to do to close monitoring gaps, upgrade wastewater treatment, and revise permits to get the new standards in place,” the report’s authors write.

New rules must be enforced

Last fall, the EPA finally updated energy industry regulations that have not been revised since 1982. The new rules reduce the amount of toxic pollutants power plants can legally discharge by 1.4 billion pounds per year—which would reduce toxic pollution from coal-fired plants by 90 percent.

But issuing new rules is one thing; enforcing them is another. Many plants will have to install or upgrade wastewater treatment plants to comply with the new federal regulations. Adding to the problem is the fact that more than half of the power plants evaluated in the report have state permits that are expired, and over a third have permits that expired more than two years ago. EIP warns that the new rules “will be undermined by power plants that are badly behind in installing pollution control equipment, a staggering backlog of expired state permits, and weak monitoring.”

Image: EIP

If the EPA and the states don't update coal plant permits, increase facility monitoring and require the installation of better wastewater treatment systems, EIP warns, the new regulations meant to protect environmental and public health could be delayed, or even worse, derailed altogether.

“These limits on toxic water pollution from coal-fired power plants are already nearly 30 years overdue, ” said Eric Schaeffer, EIP’s executive director. “Unless EPA and states move promptly to make these plants install modern wastewater treatment systems, the delay will stretch well into a fourth decade—and our streams, lakes and rivers will continue to be a dumping ground for some of the deadliest toxins known to man.”

Here are America's top 10 coal plant water polluters for mercury, lead and arsenic, according to the EIP report:

[Editor's note: A previous version of this article incorrectly stated that the coal industry currently fuels around 45 percent of electricity generation in the United States. The correct figure is 33 percent. Thanks to Dr. Catherine Thomasson for pointing out that error.]