Simulating Cation Transport during Water Flow in Soil: Two Approaches

Two numerical models were used to simulate observed ternarycation transport in soil columns during unsteady, unsaturated flow with variable total solution concentration ( C T ). Transport of Ca, Na, and K cation species was simulated for constant‐flux infiltration of aqueous electrolyte (NaCl‐KCl) solutions in columns of water‐unsaturated clay soil that was initially saturated with Ca. A simplified transport model where selectivity coefficients and C T were assumed to be constants and steady flow was imposed described observed spatial distributions of concentrations for all species in both solution and exchange phases. A detailed numerical model that combines transient, unsaturated flow and transport that includes variable C T and binary exchange selectivity coefficients, which vary with C T and with ion concentration within the solution phase, provided an equally good description of concentrations in solution and exchange phases but was also able to simultaneously provide excellent descriptions of volumetric water content (θ) and C T distributions. Thus approximations of constant C T , constant binary exchange selectivities, and steady liquid flow were sufficient to describe cation transport observed during experiments with variable C T and transient, unsaturated liquid flow in soil columns.