Migration of radionuclides under the SX-tank farm at the Hanford nuclear waste complex involves interaction of variably water saturated sediments with concentrated NaOH–NaNO3–NaNO2 solutions that have leaked from the tanks. Constant Kd models for describing radionuclide retardation are not valid under these conditions because of strong competition for sorption sites by abundant Na+ ions, and because of dramatically changing solution compositions with time as the highly concentrated tank fluid becomes diluted as it mixes with infiltrating rainwater. A mechanistic multicomponent sorption model is required that can account for effects of competition and spatially and temporally variable solution compositions. To investigate the influence of the high ionic strength tank fluids on Cs+ migration, numerical calculations are performed using the multiphase-multicomponent reactive transport code FLOTRAN. The computer model describes reactive transport in nonisothermal, variably saturated porous media including both liquid and gas phases. Pitzer activity coefficient corrections are used to describe the high ionic strength solutions. The calculations take into account multicomponent cation exchange based on measured selectivity coefficients specific to the Hanford sediments. Solution composition data obtained from Well 299-W23-19, documenting a moderately concentrated leak from the SX-115 tank, are used to calibrate the model. In addition to exchange of cations Na+, K+, Ca2+, and Cs+, aqueous complexing and a kinetic description of precipitation and dissolution of calcite are also included in the calculations. The fitted infiltration rate of 0.08 m yr−1, and fitted cation exchange capacity of 0.05 mol kg−1 are consistent with measured values for the Hanford sediments. A sensitivity analysis is performed for Na+ concentrations ranging from 5 to 20 m to investigate the mobility of Cs+ interacting with a highly concentrated background electrolyte solution believed to have been released from the SX-108/SX-109 tanks. The calculations indicate that during the initial period of the tank leak when Cs+ is associated with high Na+ concentrations, there is little retardation of the Cs+ plume. However, as time increases the Na+ and Cs+ profiles become chromatographically separated due to differences in their selectivity coefficients and dilution of the tank leak plume with infiltrating rainwater. Eventually the two species become separated spatially, and Cs+ becomes highly retarded and remains essentially fixed in the sediments by cation exchange. For the 20 m Na+ simulated tank leak, the sorbed Cs+ profile is in close agreement with data obtained from the slant borehole and consistent with the estimated tank supernatant concentration. The simulations suggest that natural attenuation processes should result in strong fixation of Cs+ in the vadose zone in spite of the release of high Na+ concentrations during a tank leak event.