All this trouble stems from the powdery, drab-colored substance that remains after a power plant burns coal to produce electricity.

Billions upon billions of the tiny particles gushed and twirled through the retired steam station’s broken pipe into the frigid Dan River, turning it a grayish hue during the next week for miles downstream.

Sheila Michael holds up a container of water holding sample of coal ash that was spilled into the Dan River as she joined other supporters of the Dan River who gathered to rally against a coal ash spill on the Dan River at Island Ford Landing in Eden, NC on March 22, 2014. &quotI'm pretty disgusted by the whole thing, I think we all are going to have to pay for it, &quot Michael said. &quotIt (the water sample) looks like sewage to me, it's sickening.&quot

Heavier than water, some of the ash particles remained on the move as the swift current kept them suspended and swept them along, mile after mile.

But huge amounts also settled in places where the meandering river slowed, at bends in the stream and near the various natural and man-made rock formations that dot the stream.

“Areas that are a little more deep, that’s where ash accumulates,” said Avner Vengosh, a Duke University professor known for his research into coal ash and its effect on rivers, lakes and other aquatic settings.

Many media accounts indicated, in the days and weeks to follow, that the river was “coated by toxic sludge” for 70 miles downriver, like Pepto-Bismol on a tummy ache.

But that was not exactly what happened.

View Graphic - Dan River spill sampling and cleanup locations

Instead, the river was dotted, peppered and lined in specific, discrete locations with varying amounts of ash — lots here, none there and a little somewhere else.

Environmental regulators and other experts would find later that the majority of what remained in traceable amounts settled out in the first 25 river miles downstream, before reaching the relatively large Schoolfield Dam, a hydroelectric plant on the edge of Danville, Va.

That’s better than the alternative of a pastelike sludge here, there and everywhere. Unfortunately, it doesn’t necessarily mean no harm, no foul for a river that already suffered from decades of industrial pollution.

Experts say that the ash’s initial impact on the river probably was as a smothering blanket that killed some of the river’s “benthic” creatures — the mussels and other small invertebrates living in the first few inches of muck that makes up the river’s bottom. There are no mysterious chemical processes at work there. The critters need porous river mud to survive, not the thick, foreign layer that swamped some of the places where they were burrowed.

This sign, stapled to a tree at Abreu-Grogan Park in Danville, Va., advises citizens of a coal ash spill in the Dan River (background) that occurred about 20 miles upstream in February 2014. Photographed at the park on Wednesday, April 2, 2014.

River-watchers reported finding substantial numbers of the creatures dead, particularly Asiatic clams, downstream from the spill. But scientists documented no great die-off, and some said later that winter often leads to substantial numbers of invertebrate deaths.

“In terms of die-offs or direct mortality, there were none we have been made aware of that have been linked directly to the spill,” said Sara Ward of the U.S. Fish and Wildlife Service’s Raleigh office.

Everybody has an opinion, but the simple fact is that nobody really knows how grievous a wound the river has suffered.

Some coal ash researchers, including Wake Forest University biologist Dennis Lemly, said they believe the ash will lurk in the river mud for a long but hard to predict timespan. When a fierce enough storm comes along, the mud will be stirred up to recontaminate the submerged environment with such potentially harmful materials as selenium, mercury and arsenic, all found in coal ash.

Then the ash will settle once again into the river bed somewhere downstream, to await the next severe weather.

What is coal ash

Or it will settle into the river’s sediments, where chemicals will leach out of the ash into the muck and then into the water, Lemly and other environmental scientists say.

Lemly spent decades before the spill proving the link between coal ash contaminants and genetic damage to fish and other aquatic creatures, starting at Duke Energy’s Belews Creek Steam Station, about 30 river miles upstream from the Dan River spill.

In the 1970s and early 1980s, he and his fellow researchers showed that Duke Energy’s discharges of coal ash wiped out entire species of fish in the man-made lake that received its effluent from a holding basin.

His research would show that in parts of the lake’s cloistered environment, selenium leached out of the ash to permeate the underwater domain. Over time, the element — ironically, a vital nutrient in small doses — affected the fishes’ reproductive processes to cause birth defects and render increasing numbers unable to spawn a new generation.

But Duke Energy changed the plant’s ash-handling practices to dispose of most of the waste as a dry substance in landfills. Today, the lake has re-emerged as a popular venue for anglers in search of game fish.

More recently, Lemly documented similar biological processes under way at Sutton Lake, a former Progress Energy plant near Wilmington that came under the Duke Energy banner in 2012 when the two utilities merged.

Duke University scientist Vengosh focused his research extensively on the streams damaged by the huge coal ash spill near Kingston, Tenn., in December 2008. The largest in U.S. history, it unleashed about 100 times more coal ash than the spill in Rockingham County.

Among other things, Vengosh’s studies suggest the arsenic in coal ash also might threaten fish and wildlife. He and other researchers at the university’s Nichols School of the Environment found that threat about a foot deep in the sediment of the river bottom, where “pore water,” drawn from the muck, contained arsenic at many times the federal limits.

The creatures that live in these bottom sediments get repeated doses of arsenic, which can concentrate in their tissue and possibly cause problems for fish and wildlife that prey on them, Vengosh said.

Don’t expect the Dan River spill to cause a “Silent Spring” — a sudden massive extinction that wipes out entire species in a few years.

The impacts of such environmental wounds are more subtle, Vengosh believes.

“I see it not like a catastrophic event but as a long-term process that involves continued mobilization of the coal ash,” he said.

The spill near Eden dumped significantly less ash into the water than its Tennessee counterpart. And unlike the situations at the Belews and Sutton plants that Lemly studied, the ash went into a river, not a lake.

The Dan is refreshed by as much as a billion gallons of fresh water daily as it loops between northern North Carolina and southern Virginia, in contrast with the slower turnover of water in a lake.

Experts on the other side of the coal ash debate say that this continual refreshment should prevent most spots in the river from being saturated with ash in the way necessary to trigger an epidemic of genetic damage.

“The two environments are considerably different,” coal ash researcher Ken Ladwig said of rivers in comparison with lakes and ponds. “In regards to a free-flowing river system, it’s not a given that they’re going to identify significant ecological issues.”

Ladwig, who has researched coal ash for 30 years, works at the nonprofit Electric Power Research Institute that is financed partly by the utility industry. His years of research led him to conclude that it is wrong to label coal ash a “toxic sludge.” In reality, coal ash is a siltlike material with tiny amounts of potential pollutants like arsenic and selenium, a commodity that’s only a threat when improperly stored in huge quantities, he said.

“It has toxic elements in it, but so does pretty much everything else,” Ladwig said.

“You don’t refer to the toxic computer monitor on your desk.”

During the months after the spill, EPA and state regulators confronted the complexities of this substance, which contained potentially harmful elements, but also dispersed so quickly and widely into the river. They had to balance the realistic threat coal ash posed against the risk of stirring up more dangerous PCBs and other toxic industrial chemicals also buried in the river muck from a century of heavy manufacturing along the Dan in both states.

Regulators repeatedly tested sediment and the water itself at 29 sites along more than 70 miles of river between the spill and Kerr Reservoir.

They determined that once the plume of pollution subsided after the first week, most of the coal ash settled in various spots along the first 25 miles downstream from the Eden plant. But they discovered only three spots that contained enough ash to warrant removal by dredging.

Digging about 6 inches into the river bottom — the zone where most invertebrates live — they found that after a few months the percentage of ash in the river sediments at many places dropped into the single digits and kept dropping.

The various state and federal agencies decided that it made more sense to let the ash remain diluted, submerged and dispersed in these places rather than to dredge it up, along with earlier deposits of the other industrial waste.

Of the sites they decided were suitable for dredging or other types of removal, the regulators found by far the most heavily polluted was just upstream from Danville, Va., near the Schoolfield Dam, with a city park nearby that easily could be converted to a work area for the removal. Also, two smaller deposits were removed at the retired Eden plant and a short distance downstream, where Town Creek flows into the Dan River.

In Danville, about four months after the spill, a contractor for Duke Energy converted the riverside Abreu-Grogan Park to a coal ash cleanup facility that ultimately removed several thousand tons of a mixture of ash and natural sediment.

Environmentalists protested when regulators allowed Duke Energy to stop dredging at that point, but some knew from Day One that likely would be the outcome.

“Long term, it’s going to be very hard for Duke to ever get all that ash back out of the river,” Rick Gaskins, the executive director of the Catawba Riverkeeper Foundation, told state legislators in Raleigh at a hearing about the spill, conducted just two weeks after it occurred. “It’s kind of like the Humpty Dumpty thing: You can’t put it back together again.”

Ultimately, the utility recovered 10 percent or less of what spilled from the Dan River pond those four days in February before the leak was plugged.

Regulators, the utility and residents alike only can hope that someday, history will chalk up the Dan River spill as a close call that, ultimately, spared the river much harm.

Research after the coal ash disaster in Kingston, Tenn., showed that creatures living in or near the river bed were unharmed if sediments in their parts of the stream contained less than 40 percent coal ash, said John Kennedy of the Virginia Department of Environmental Quality.

“We’re not anywhere close to that threshold,” Kennedy said about the 40 percent ash level in the Dan River. “We’re going to let the data speak for itself. But the early indications are that, maybe, we have avoided a large-scale environmental impact.”

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