REMEDIATION OF GROUNDWATER CONTAMINATED WITH STRONTIUM-90 IN THE N-AREA, HANFORD SITE, WASHINGTON

The U. S Department of Energy has started remediating the strontium-90 plume in the N-Area along the Columbia River at the Hanford Site, Washington. Waste associated with nuclear reactor operations, which included strontium-90, was discharged to cribs and trenches in the area. Strontium-90 has leached into the groundwater and reached the river. An apatite sequestration barrier is currently being tested for full-scale implementation to stop strontium-90 from entering the river. This technology is expected to be accompanied by phytoextraction.

This N-Area plume is approximately 800 m (2600 ft) long and extends 900 m (3000 ft) inland from the river with a strontium-90 concentration up to 8,000 pCi/L, well above the 8 pCi/L drinking water standard. This contamination represents a risk to human health and the environment.

The barrier technology uses chemical compounds (calcium citrate and sodium phosphate) to form an apatite barrier which sequesters the strontium-90. The Washington State Department of Ecology is evaluating the performance of this barrier to prevent strontium-90 from entering the Columbia River.

The stratigraphy and hydrogeology of the formations impact the design and construction of this barrier technology. The movement of strontium-90 is also controlled by the various facies and hydrogeological parameters of the Hanford Formation of Pleistocene age and the underlying Ringold Formation of Miocene to Pliocene ages. These formations consist of unconsolidated sand and gravel and semi-consolidated gravel and mud units, respectively, representing migrating channel deposits from the ancestral Columbia River. The technology design must also take into account groundwater velocity fluctuations within the near-river environment due to the varying river stages influenced by an upstream dam.

Monitoring results from the initial 91 m (300 ft) long field-scale barrier showed about a 90% reduction of the strontium-90 concentration before it reached the river. The barrier is now being extended to 275 m (900 ft) long and further testing is underway. The decision to extend the barrier for the entire length of the plume and to apply phytoextraction as part of the treatment train is under consideration in order to stop all migration of strontium-90 to the Columbia River by 2016.