A technology developed at Sandia to protect groundwater in sites that have been contaminated with radionuclides is being evaluated for use at the Fukushima site in Japan to prevent radioactive strontium from reaching the ocean.
Under a program funded by Tokyo Electric Power Co. (TEPCO), Sandia, Pacific Northwest National Laboratory (PNNL), and Savannah River National Laboratory will provide recommendations for permeable reactive barrier design, implementation, and monitoring at the Fukushima site in an effort to prevent contamination of groundwater.
Barrier technology at Hanford
The same technology, a calcium apatite-based permeable barrier has been in use at the DOE Hanford site for eight years, says its inventor, Sandia chemical engineer Robert Moore (6915). The Hanford barrier is sequestering mobile strontium-90 that threatens the Columbia River, a major source of water for three states.
Hanford, a decommissioned World War II-era nuclear production complex in southeastern Washington State, is contaminated from materials left over from reactor operations and decommissioning activities.
Jim Szecsody, a geochemist at PNNL, says PNNL contacted Robert in 2003 about his patented technology, wondering if it could work at Hanford.
“Before the barrier could be used, we had to figure out how to pump a solution to precipitate apatite without ‘flushing’ the mobile strontium-90 and making it more mobile,” Szecsody says.
A collaboration between Sandia and PNNL tested the calcium apatite barrier using small and then scaled-up lab experiments that looked at reaction rates and pumping rates, intended to customize the geochemistry and delivery for the Hanford site, using some of the calcium available in the subsurface sediments and less injected calcium, which resulted in less flushing of the strontium-90.
“In some ways, it’s like growing crops. You need to know what’s there before you can figure out what nutrients you need to add,” Szecsody says.
The customized barrier was then field-tested by Fluor, the prime contractor for cleanup of the Central Plateau at the Hanford Site. From 2005 to 2011 the barrier was placed along a 300-foot-long section of Columbia River shoreline in Washington state.
The results were impressive: After six years, monitoring wells placed between the barrier and the Columbia River indicated that the barrier sequestered more than 95 percent of the strontium, preventing it from traveling into the river.
The initial work was so successful that CH2M HILL Plateau Remediation Company began expansion of the barrier in 2012 to protect 2,500 feet of Columbia River shoreline, and additional barriers are being considered at Hanford.
How the barriers work
Robert says the barrier can be formed in a few ways, depending on the specific types of contamination and the characteristics of the soil. One way is by pumping an aqueous solution containing a calcium citrate compound and sodium phosphate into the ground.
As groundwater passes through the barrier, nano-size apatite crystals bind to contaminants and immobilize them, allowing groundwater to flow through the barrier, eliminating the need for groundwater treatment, Robert says.
“The indigenous soil bacteria biodegrade the calcium citrate compound, leaving calcium apatite, an insoluble and stable mineral which can immobilize contaminants,” Robert says.
The barrier approach shows several positive advantages over alternative technologies:
- The solution flows into areas with highest soil porosity, so more apatite forms in areas with more groundwater, where greater protection would be needed.
- Leaving the contaminants fixed underground eliminates the costly process of removing contaminated soil and disposing of it as hazardous waste.
- Once in place underground, the barrier requires no ongoing maintenance, eliminating operational expenses for equipment such as ion exchange and filters, though it can be monitored with optional equipment.
- Because there is less contamination exposed above ground, workers are not exposed to contaminants as they would be using conventional trenching and backfilling with a reactive media.
“The barriers work well in locations where conventional solutions are not feasible or are excessively expensive, such as deep underground and under large obstacles such as buried waste tanks and piping systems where conventional construction techniques are not possible,” says Robert.
For example, one such chemical reactive barrier is a 2,300-foot-wide chemically reduced barrier for chromate remediation at the Hanford 100D area. This technology was developed at PNNL and then upscaled to full-scale field tests. After 15 years, this 100D reactive barrier is still 80 percent to 90 percent effective. This barrier technology was then implemented at sites in other states.
Other types of remediation methods are expensive, can only be used in certain locations, and expose workers to contaminated soils and construction hazards. One commonly used current method involves excavating a trench perpendicular to the contaminated groundwater flow-path and then backfilling with the reactive media. Another method is high-pressure injection to force the reactive media into the soil. Both methods are disruptive and, in some instances, have actually altered the site geohydrology, resulting in a portion or all of the contaminated groundwater flowing around the barrier instead of through it.
Immobilizing contamination from Hanford’s tank farms
Sandia has signed a Government Use agreement with CH2M HILL to allow Washington River Protection Solutions (WRPS), a contractor charged with cleanup of Hanford’s tank farms, to use a tin(II) apatite barrier to help prevent the radionuclide technetium, a highly mobile radionuclide with a long half-life, from travelling into the environment.
The Hanford site has 177 underground storage tanks in its “tank farm,” many of which date back to World War II. Because many of the tanks have outlived their anticipated design life, some are leaking.
Some tanks are being grouted to prevent the movement of materials out of the tanks. Unfortunately, the grouting doesn’t prevent technetium from moving out of the tanks since few things bind to technetium.
“Technetium is a difficult problem to solve. It’s a long-term dose driver at the Hanford site because it has long half-life. It’s a challenge because technetium binds to nothing we’ve tried, except this barrier,” says Robert.
Robert says a stannous-treated tin(II)apatite barrier, which is particularly effective for technetium, might be used.
“With some research to determine soil pH, amounts of free calcium, and soil porosity, the same technology could be used in areas around Hanford’s tank farms to contain radionuclides from tank leaks,” Robert says.
So far, the results are promising.
“In tests performed recently by WRPS the a stannous-treated tin(II)apatite bound the technetium into the apatite crystal lattice immobilizing the radionuclide even when subjected to leach testing,” says WRPS senior scientist Jim Duncan.
Other potential uses
Barriers can also be used with a wide variety of radionuclides and heavy metals.
“The method could be used prophylactically to protect groundwater during drilling, hydraulic fracturing operations, or other excavation activities where the potential exists for groundwater contamination,” says Robert.
Thousands of sites throughout the world are contaminated with radionuclides, heavy metals, and natural contaminants that threaten groundwater, surface water, and food supplies.
A 2012 report from the US Geological Survey says approximately 50 percent of the population relies on groundwater as their primary drinking water supply. It is therefore of vital importance to keep contaminants out of groundwater.
One unanswered question is the longevity of the barrier; research is ongoing to assess how long contamination remains in the bound form.
“So far, the results indicate that the contaminants will remain sequestered for a long time,” says Robert.
The work was funded under Sandia’s Laboratory Directed Research & Development (LDRD) program.