[en] This project was initiated to develop a strategy for infiltration of a Ca-citrate-PO₄ solution in order to precipitate apatite [Ca₆(PO₄)₁₀(OH)₂] in desired locations in the vadose zone for Sr-90 remediation. Laboratory experiments have demonstrated that infiltration of a Ca-citrate-PO₄ solution into sediments at low and high water saturation results in citrate biodegradation and formation of apatite. The citrate biodegradation rate was relatively uniform, in spite of the spatial variability of sediment microbial biomass, likely because of microbial transport processes that occur during solution infiltration. The precipitate was characterized as hydroxyapatite, and the Sr-90 substitution into apatite was shown to have an incorporation half-life of 5.5 to 16 months. One and two dimensional (1-D and 2-D) laboratory infiltration experiments quantified the spatial distribution of apatite that formed during solution infiltration. Slow infiltration in 2-D experiments at low water saturation show the apatite precipitate concentrated in the upper third of the infiltration zone. More rapid 1-D infiltration studies show the apatite precipitate concentrated at greater depth. (authors)