This award will enable Professor H. Magdi Selim, Louisiana State University, to collaborate with Professor Hannes Fluhler, Swiss Federal Institute of Technology, Zurich. They will study the mobility and retention of major ions in a soil solution, and they will develop a model for describing solute interactions and transport during transient and water-unsaturated flow in soil and geological strata. Laboratory and field experiments will measure ion-exchange selectivity coefficients and the affinity of exchange as a function of counter ions. The major ions to be considered include calcium, magnesium, potassium, and sodium. The counter ions are nitrate, chloride, and perchlorate. The research will try to determine whether the ion retention mechanisms are due to diffusion or other physical and chemical interactions. The studies, performed on Swiss and United States soils, complement each other. The research will be done in the field and laboratory, on different kinds of soils, and the various measurements will help determine the necessary parameters for validating a soil transport model based on ion exchange processes for multiple ions. Most current models do not incorporate multiple ion interactions, so the complete set of phenomena known to affect the mobility of solutes is ignored. Those models that do consider multiple ion interactions deal primarily with idealized systems of uniform and water-saturated soils. The two collaborators have successfully worked together in the past and the proposed research is an extension of their previous research.