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STOKES COUNTY, N.C. — On a small tributary of the Dan River, several miles upstream of the Rockingham County power plant owned by Duke Energy that contaminated the river with coal ash in February, sits another power plant, one of Duke’s largest.

It’s called the Belews Creek Steam Station, in southern Stokes County.

From this power plant, a coal-waste substance known as bromide, discharged legally into the tributary, has been causing trouble for the downstream town of Madison and the city of Eden, as well as buyers of Madison’s and Eden’s drinking water, such as Rockingham County and Henry County, Va., according to public-record emails between officials with the N.C. Department of Environment and Natural Resources.

In the treated drinking water of Madison and Eden, a little more than two years ago, lurked potentially unsafe levels of trihalomethane – levels that were near to or in excess of the safe limit set by the U.S. Environmental Protection Agency, according to the emails.

The substance, according to the EPA, poses a cancer risk: “Some people who drink water containing total trihalomethanes in excess of the MCL (safety limit) over many years could experience liver, kidney or central nervous system problems and increased risk of cancer.”

Bromide from the Belews Creek power plant was a primary cause of trihalomethane, according to the emails, local officials and Duke Energy.

The elevated levels of trihalomethane, conservationists say, present another cautionary tale of the risks posed by allowing coal-waste dumps to leach potentially toxic waste into North Carolina’s rivers, streams, lakes and groundwater.

“I think we’re just looking at the tip of the iceberg,” said Dean Naujoks, the executive director of the Yadkin Riverkeeper. “What else don’t we know about?”

Chlorine meets bromide

The public water systems run by Madison and Eden pick up raw water from the Dan River; treated water – scrubbed clean by municipal treatment plants – is what pours through faucets. They scrub the water clean, using, among other disinfectants, chlorine. When chlorine meets bromide, trihalomethane is the byproduct.

The levels of trihalomethane started to exceed unsafe levels in Madison and get close to unsafe levels in Eden because the water systems had to increase their use of chlorine to scrub the raw river water clean of rising levels of bromide flowing in the river from the Belews Creek power plant, according to the emails.

Madison, Rockingham County, and Dan River Water Inc. received notices of violation for the unsafe trihalomethane levels, according to the emails. Rockingham County purchases water from Madison; Dan River Water Inc., as well as Henry County, Va., purchase water from Eden, according to the emails. Although the treated water in Eden did not have exceeding levels of trihalomethane, the water Eden sold ultimately did have the substance because the longer the bromide and chlorine stay in contact, the more likely they are to form trihalomethane.

Upgrading systems

Several of the public-water systems had to deliver notices to their consumers letting them know that their “water contains unsafe levels of these cancer-causing chemicals,” wrote Jessica Godreau, chief of the public water supply section, a part of the N.C. Division of Water Resources, in an email dated March 8, 2012.

“They mentioned they are talking about a possible lawsuit against the department as perhaps their best course of action,” Godreau wrote.

Duke Energy agreed to assist Madison and Eden financially to upgrade their water-treatment systems, to make them capable of scrubbing water clean of bromide without producing unsafe levels of trihalomethane, according to Erin Culbert, a Duke spokeswoman.

“We felt participating in a solution was the right thing to do, and we’ve been partnering with Eden and Madison since 2011 to research and invest in treatment technologies at their water plants that will resolve the issue. In Eden, that involves helping to fund the city’s conversion to another disinfection method (chloramines). In Madison, that involved helping to install a new aeration technology that reduced organic content,” Culbert said.

“This is an industry issue that is also getting research focus from broader electric utility industry groups,” she said.

David Myers, the mayor of Madison, said that he is optimistic about working with Duke Energy: “As far as I know, yes, they have made things whole. They have made things right.”

Indirect result of cleaner air

The increased presence of bromide in the Dan River is, to a certain extent, an indirect result of a state clean-air law enacted in 2002. The Clean Smokestacks Act set limits on the amount of nitrogen oxides and sulfur dioxide puffed into the air by coal-fired power plants. Seven power plants in North Carolina have installed scrubbers required by the state law.

And air is cleaner.

Officials with the N.C. Department of Environment and Natural Resources said in those emails that in 2006, before the scrubbers were installed, air quality data reported 95,365 and 21,013 tons of emission, respectively, of sulfur dioxide and nitrogen oxides from the Belews Creek power plant. After the scrubbers were installed in 2008, another round of air quality emission data from 2010 reported 3,643 and 3,277 for sulfur dioxide and nitrogen oxides, respectively.

“There’s always going to be a byproduct,” said Tom Boyd, an environmental senior specialist who works in the N.C. Department of Environment and Natural Resources regional office in Winston-Salem. Scrubbers take more sulfur dioxide and nitrogen oxides out of the air but more bromide shows up in the water.

Bromide is hard to extract, Culbert said, “even through the sophisticated treatment processes we have in place, and there is no ‘off the shelf’ technology available. We continue researching viable options for the Belews Creek plant itself.”

One of the most alarming aspects of the trihalomethane trouble, according to Frank Holleman, senior attorney for the Southern Environmental Law Center, is that state and federal regulators have not imposed limits on how much bromide – a primary cause of the trihalomethane – can be discharged into state waterways. Even now, the Belews Creek power plant and several others owned by Duke Energy are not violating clean-water laws as they discharge unlimited amounts of bromide.

“This is what is happening when everything is going ‘right’,” Holleman said.

For now, Duke Energy must monitor the amount of bromide it discharges, DENR officials said.

“North Carolina does not have a state water quality standard for bromide, but we are closely following the research in this emerging issue,” said Susan Massengale, a DENR spokeswoman. “If it becomes apparent that a statewide standard is appropriate, we have a process for pursuing that.”

“At this point, we have not seen that other (Duke scrubber) systems have affected the water treatment plants of other communities,” she said.