An Injectable Apatite Permeable Reactive Barrier for In Situ 90Sr Immobilization

An injectable permeable reactive barrier (PRB) technology was developed to sequester Sr-90 in groundwater through the in situ formation of calcium-phosphate mineral phases, specifically apatite that incorporates Sr-90 into the chemical structure. This injectable barrier technology extends the PRB concept to sites where groundwater contaminants are too deep or where site conditions otherwise preclude the application of more traditional trench-emplaced barriers. An integrated, multiscale development and testing approach was used that included laboratory bench-scale experiments, an initial pilot-scale field test, and the emplacement and evaluation of a 300-feet-long treatability-test-scale PRB. The apatite amendment formulation uses two separate precursor solutions, one containing a Ca-citrate complex and the other a Na-phosphate solution, to form apatite precipitate in situ. Citrate is needed to keep calcium in solution long enough to achieve a more uniform and areally extensive distribution of precipitate formation. In the summer of 2008, the apatite PRB technology was applied as a 91-m-long (300 feet) PRB on the down gradient edge of a Sr-90 plume beneath the Hanford Site in Washington State. The technology was deployed to reduce Sr-90 flux discharging to the Columbia River. Performance assessment monitoring data collected to date indicate that the barrier is meeting treatment objectives (i.e., 90% reduction in Sr-90 concentration). The average reduction in Sr-90 concentrations at four downgradient compliance monitoring locations was 95% relative to the high end of the baseline range approximately 1 year after treatment, and continues to meet remedial objectives more than 4 years after treatment.