Traditionally, a two-parameter partial differential equation has been used to describe the one-dimensional convective-dispersive equilibrium transport of chemicals in field soils. The parameters in this equation include the dispersion coefficient and a distribution coefficient, the latter accounting for interactions between the chemical and the solid phase of the soil. The resulting model is relatively easy to use for linear equilibrium adsorption. More complex conceptual models have been introduced in attempts to better characterize the simulated system for conditions involving nonequilibrium transport. These models are all based on the assumption that, either for physical or chemical reasons, adsorption does not proceed at an equal rate in all parts of the soil medium. The resulting transport equations contain several parameters which must be quantified before actual predictions can be made in the field. Estimates for these parameters can be obtained by analyzing effluent curves from column displacement experiments. Since several parameters have to be estimated simultaneously, elaborate curve-fitting techniques are often needed. CFITIM is an accurate and easy to use least-squares computer program which may be used for that purpose. Depending upon the exact form of the transport model, the program allows up to five different parameters to be estimated simultaneously.
The report comes with a user manual giving detailed information about the different nonequilibrium models, and how to prepare the data input files for various applications.
M. Th. van Genuchten. 1981. Non-Equilibrium Solute Transport Parameters from Miscible Displacement Experiments, Version 1.0. Research Report No. 119, U.S. Salinity Laboratory, USDA, ARS, Riverside, California.