Abstract:UNSATCHEM-2D is a two-dimensional finite element code for modeling major ion equilibrium and kinetic non-equilibrium 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 flow equation incorporates a sink term to account for water uptake by plant roots. The heat transport equation considers the heat transport by conduction and by convection with flowing water. The diffusion in both liquid and gas phases and convection in the liquid phase are considered as CO2 transport mechanisms. CO2 production model is described. The major variables of the chemical system are Ca, Mg, Na, K, SO4, Cl, NO3, H4SiO4, 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, nesquehonite, and sepiolite. Since the ionic strength of soil solutions can vary considerably with time and space and often reach high values, both modified Debye-Huckel and Pitzer expressions were incorporated into the model to calculate single ion activities.
The program may be used to analyze water and solute movement in unsaturated, partially saturated, or fully saturated porous media. UNSATCHEM-2D can handle flow regions delineated by irregular boundaries. The flow region itself may be composed of nonuniform soils having an arbitrary degree of local anisotropy. Flow and transport can occur in the vertical plane, the horizontal plane, or in a three-dimensional region exhibiting radial symmetry about the vertical axis. The water flow part of the model can deal with prescribed head and flux boundaries, as well as boundaries controlled by atmospheric conditions.
The governing flow and transport equations are solved numerically using Galerkin-type linear finite element schemes. The program is written in ANSI standard FORTRAN 77. Computer memory is a function of the problem definition. This report serves as both a user manual and reference document. Detailed instructions are given for data input preparation. Selected example input and output files are also provided.
References:Simunek, J. and D. L. Suarez. 1993. UNSATCHEM-2D Code for Simulating Two-Dimensional Variably Saturated Water Flow, Heat Transport, Carbon Dioxide Production and Transport, and Multicomponent Solute Transport with Major Ion Equilibrium and Kinetic Chemistry, Version 1.1. Research Report No. 128, USDA-ARS U. S. Salinity Laboratory, Riverside, California.
The program and manual are available upon request from: