Modeling Sr-90 Exchange and Availability for Induced Co-Precipitation Remediation of Contaminated Vadose Zones.

Historical leakage of Sr-90 has contaminated perched groundwater and sediments at several DOE facilities.  One possible remediation strategy at many of the facilities that have alkaline vadose zone environments is to stimulate indigenous microorganism respiration to facilitate a succession of reactions that result in co-precipitation of Sr-90 in carbonate minerals.  Availability of the Sr-90 for co-precipitation is controlled by cation exchange reactions.  Thus, implementation of an immobilization remediation strategy requires an understanding of cation exchange on the subsurface sediments so that Sr-90 co-precipitation in carbonates can be modeled.  In this research cation exchange of Sr²⁺, Mg²⁺, Na⁺, Li⁺ and NH₄⁺ on a Ca-saturated vadose zone sediment at ionic strengths of 0.1 and 0.005 were measured.  Cation exchange constants for the divalent cations were near unity, suggesting the surface selectivity coefficients were indifferent.  In contrast to the divalent cations, Na⁺ and Li⁺ had a much lower selectivity than Ca²⁺, and NH₄⁺ was more preferentially adsorbed than Ca²⁺.  Modeling of the exchange constants is being done using Vanselow, Gaines-Thomas, and Gapon selectivity coefficients so that they can be used in reactive transport models.  The models will integrate cation availability with calcite precipitation kinetics to evaluate the potential for in-situ remediation of the exchangeable Sr-90.