Two‐dimensional transport model for variably saturated porous media with major ion chemistry

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Abstract
We present the development and demonstrate the use of the two‐dimensional finite element code UNSATCHEM‐2D for modeling major ion equilibrium and kinetic nonequilibrium chemistry in variably saturated porous media. The model is intended for prediction of major ion chemistry and water and solute fluxes for soils under transient conditions. Since the solution chemistry in the unsaturated zone is significantly influenced by variations in water content, temperature, and CO2 concentrations in the soil gas, all these variables are also calculated by the model. The major variables of the chemical system are Ca, Mg, Na, K, SO4, Cl, NO3, alkalinity, and CO2. The model accounts for equilibrium chemical reactions between these components such as complexation, cation exchange, and precipitation‐dissolution. For the precipitation‐dissolution of calcite and dissolution of dolomite, either equilibrium or multicomponent kinetic expressions are used which include both forward and back reactions. Other dissolution‐precipitation reactions considered include gypsum, hydromagnesite, and nesquehonite. Since the ionic strength of soil solutions can often reach high values, both modified Debye‐Hückel and Pitzer expressions were incorporated into the model to calculate single ion activities. The need for an iterative coupling procedure between the solute transport and chemical modules is demonstrated with an example which considers root water uptake and irrigation using moderately saline water. The utility of the model is further illustrated with two‐dimensional simulations with surface and subsurface irrigation from a line source.