Radionuclide levels dropping at Rarity Ridge Treatment Plant, but sludge shipments continue for now

 

Note: This story was last updated at 9:55 a.m.

The radioactive nuclide that was first detected in the city’s wastewater lines more than two years ago near the demolition project at the former K-25 Building doesn’t affect drinking water, and it’s not believed to pose any threat to residents or municipal employees, officials said this month.

The levels of the radionuclide, technetium 99, are dropping at several measuring spots in the sewer system in west Oak Ridge, but it’s not clear how long UCOR, the federal government’s cleanup contractor, might have to ship sludge from the Rarity Ridge Wastewater Treatment Plant to an out-of-state landfill.

Officials said UCOR has already hauled away about 80,000 gallons of sludge using a 5,000-gallon tanker truck about once every one or two months since 2014. The sludge, which is about 3 percent to 4 percent solid, comes from a part of the plant known as a digester, and the shipments vary depending upon how much is processed at the plant each month.

Officials don’t know yet when the shipments might end. The sludge is now being taken to the Perma-Fix Northwest treatment facility in Richland, Washington. The last shipment was this month.

UCOR said there should not be any long-term impacts that would require replacing sewage lines, basins, or equipment at the Rarity Ridge Wastewater Treatment Plant because of the technetium 99.

“The plant in effect decontaminates itself as uncontaminated sewage flows through the sewage network and treatment plant,” UCOR spokesperson Anne Smith said.

The modular plant, which is running at low capacity, can be expanded, and it is still relatively new, built somewhere around 2007-2008.

As they work through some of the challenges associated with the technetium 99, one official said workers haven’t been able to seek advice from anyone else who has dealt with a similar type of contamination.

“To my knowledge, it’s unprecedented,” said Ken Glass, environmental compliance officer for the Oak Ridge Public Works Department.

The technetium 99 was first identified in November 2013 in environmental sampling in storm drains south of the East Wing of the former mile-long, U-shaped K-25 Building. K-25 was once used to enrich uranium for nuclear weapons and commercial nuclear power plants, but the building has since been demolished.

The last six units of the 54-unit K-25 Building, which was once the world’s largest under one roof, contained technetium 99, or Tc-99, among other challenges for workers.

UCOR said the technetium 99, which can be mobile in the environment, is suspected to have migrated with water flows from the K-25 Building demolition project equipment and then infiltrated the city’s wastewater pipelines. When UCOR investigated after first finding the Tc-99 two years ago, the radionuclide was found in sampling conducted in inactive electrical duct bank lines and in the sanitary sewer network south of the East Wing.

The sanitary sewer lines in that part of the former K-25 site, now known as East Tennessee Technology Park, include sanitary sewage manhole 95, which leads to the city-owned Rarity Ridge Wastewater Treatment Plant, which is across the Clinch River from ETTP.

Officials said the Tennessee Department of Environment and Conservation is aware of the technetium 99 contamination, and there has been no effect on Oak Ridge’s discharge permit. The city hasn’t exceeded its limits, Glass said.

City employees said they have been advised to continue using their normal protective gear, such as work gloves.

“We don’t think it’s even affecting the health of our workers,” Oak Ridge City Engineer Roger Flynn said.

UCOR said technetium 99 is a low-energy beta-emitting radionuclide that was present in the gaseous diffusion process used to enrich uranium at K-25.

“The low-energy beta-emitting radionuclides do not present an external radiation exposure hazard because the outer skin layer shields the radiation,” UCOR spokesperson Mike Butler said. “They are only hazardous if inhaled or ingested.”

Measurements between February 2014 and November 2015 show large drops in the technetium 99, as measured in picocuries per liter at various points in the wastewater system, including at Rarity Ridge Lift Station One and in the effluent to the Clinch River. Levels in the effluent to the river have dropped from 41,383 picocuries per liter in February 2014 to 73 in November 2015.

A picocurie is one-trillionth of a curie, a standard measure for the intensity of radioactivity contained in a sample of radioactive material.

The technetium 99 levels remain the highest at the digester at the treatment plant, where UCOR picks up the sludge and where the radionuclides are more concentrated. The levels are going down there also, Glass said, but just at a slower pace.

“We do definitely see the levels coming down,” he said.

The digester levels have dropped from 522,000 picocuries per liter in February 2014 to 171,000 in November 2015.

Smith said the levels will have to drop more before UCOR can stop shipping the sludge to Washington.

“UCOR on behalf of DOE has agreed to dispose of the city sewage treatment plant sludge until such time as there is agreement that support to the city operations is no longer needed,” Smith said. “The specifics on the point at which the UCOR/DOE support will no longer be needed will be determined at a later date. However, in general, the level  of Tc-99 that would determine when UCOR/DOE support is no longer needed will be well below the current Tc-99 concentration, which was last measured as 171,100 picocuries per liter.”

 

It’s not clear how many shipments might remain.

“Overall Tc-99 levels continue to decline within the Rarity Ridge Treatment Plant, but it is premature to estimate the exact number of shipments (of sludge) or even the estimated time frame that will be needed to bring the facility back to normal operating parameters,” Butler said.

Smith said the sewage collection line where Tc-99 infiltrated the system has been isolated by pouring grout plugs into manholes and within sections of the line to prevent additional contamination from entering the network.

“Downstream sampling of the sewage network (which belongs to the city) continues from multiple manhole locations on a routine basis to identify any new significant infiltration points into the onsite sewage collection network,” Smith said.

UCOR said there has been environmental monitoring of the groundwater and Poplar Creek, the water body immediately adjacent to ETTP, as part of the historical environmental monitoring program at the site. The sampling work has been more frequent since the technetium 99 was found in the storm drains.

“The levels in Poplar Creek, which is a surface water stream upstream from the Clinch River, have remained at low background levels,” Butler said. “There are measured levels above historical background results in the onsite groundwater, and additional investigations are continuing.”

The Rarity Ridge Wastewater Treatment Plant can treat 600,000 gallons of waste per day from ETTP and the Rarity Ridge/Preserve at Clinch River residential development. In November, the average flow into the plant was 113,640 gallons per day, Glass said.

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