Migration of 45Ca and 90Sr in a clayey and calcareous sand: Calculation of distribution coefficients by ion exchange theory and validation by column experiments

The transient transport of trace radiocalcium and radiostrontium in a clayey and calcareous sandy soil is mainly governed by: (1) cation exchange with the major cations on clay minerals; (2) calcocarbonic equilibria which control the concentration of calcium in the leaching solution; and (3) the initial composition of the leaching solution. If there are no changes in the concentration of the major cations and pH and ionic composition are stable, it is possible to calculate a distribution coefficient (Kd) as a function of the major cations. In a constant physicochemical environment, this coefficient can be introduced in a linear transport model. Chromatographic experiments have been performed in which pulse injections of radiostrontium or radiocalcium are made into sand columns fed with various leaching solutions. The results are compared: (1) with the simulations based on a complete speciation model; and (2) with predictions from a linear model including the previously calculated Kd. The shape and the location of radionuclide peaks are well simulated for a large range of ionic composition (10−3−10−1 M for a mixture of CaCl2 and NaCl). The distribution of clay minerals in the porous medium is of great importance. The local equilibrium assumption only holds when clay minerals particles are uniformly distributed on the sand grains. Kinetic effects occur when clays are in the form of aggregates. These effects are evaluated by means of a first-order transfer model and by comparison with tracer tests at different flow rates.