Uranium sequestration operations near the Columbia River in the Hanford Site’s 300 Area are expanding after a successful test project.
Soil had earlier been excavated in places down to a depth of 15 feet in the 300 Area to remove most of the uranium that has put the groundwater at risk of contamination.
The hope was that contamination already in groundwater would naturally attenuate, or dissipate, allowing the water to meet drinking water standards. Instead, the groundwater near the river at the Washington state facility has periodically risen along with river levels, rewetting the contaminated soil and enabling the uranium to spread to the groundwater.
Contamination levels have been particularly high in one three-acre area. In some places when the river is high, groundwater samples have been found to contain uranium at three times the drinking water standard, despite years of natural attenuation.
During World War II and the Cold War, uranium fuel for Hanford reactors was machined in the 300 Area. As a byproduct of the process, uranium dissolved in water was disposed of in cribs and trenches.
Uranium sequestration, a method to bind the material to the soil to keep it out of groundwater, was tested last year on a quarter of an acre of the three acres targeted for additional remediation. Phosphates were injected into the ground through wells and a drip system that used hoses buried about 6 feet deep in the soil. Subsequent checks of the groundwater in three monitoring wells showed it met drinking water standards, even after the river had been high, said Marty Doornbos, CH2M Hill Plateau Remediation Co. director of Hanford soil and groundwater projects.
Phosphate combines with uranium in a carbonate form to produce autunite, a stable uranium phosphate mineral that does not readily dissolve when the groundwater rises into the contaminated soil. Tests have shown that when phosphates are injected in the ground they immediately start combining with uranium and that over time the effect becomes stronger, according to CH2M.
The project now is being expanded to the remainder of the three acres, with work expected to cost $31.9 million, including for the test last year, CH2M said. With the weather improving after an unusually cold and snowy winter at Hanford, drilling has begun for 48 injection wells and 24 monitoring wells. The drip method, which did not distribute the phosphate as effectively as the wells during testing, will not be used for the remainder of the project. Phosphate will be injected into the ground through wells in August and September when the river is at its lowest to sequester the uranium in as much soil as possible, said Mike Cline, Department of Energy director for the Hanford soil and groundwater program.
The phosphate will be spread through the soil at a depth of 25 to 35 feet, which is just above the groundwater. Each well can spread phosphate over a radius of about 40 feet, with a sufficient number of wells planned so that the areas of treated soil will overlap.
The Department of Energy also plans to continue its plan of natural attenuation with the expectation that groundwater quality will improve once the problem of repeated recontamination is solved. The Columbia River is contaminated with uranium from non-Hanford sources, including fertilizer used at area farms. Across the river three irrigation outlets are estimated to be contaminating the river with 10 times as much uranium as is now entering the river from the 300 Area.
