Physical or Chemical Nonequilibrium Solute Transport

Physical or Chemical Nonequilibrium Solute Transport

HYDRUS codes can simulate either nonequilibrium chemical or nonequilibrium physical solute transport. It cannot currently handle both physical and chemical nonequilibrium transports simultaneously (HYDRUS-1D can).

1. Chemical nonequilibrium transport is implemented using either a one-site or two-site sorption model, with one of the sorption sites having kinetic and other instantaneous sorption.

2. Physical nonequilibrium model uses the mobile-immobile water concept.  

HYDRUS decides which of the two nonequilibrium models to use based on the input solute transport parameters. When the immobile water content (ThImob) is specified with a nonzero value, then HYDRUS uses a physical nonequilibrium model and the “Frac” parameter is interpreted as a dimensional fraction (between 0 and 1) of the sorption sites in contact with mobile water. When the immobile water content (ThImob) is specified with a zero value and the “Frac” parameter is not equal to 1, then HYDRUS uses a chemical nonequilibrium model. Then the “Frac” parameter is interpreted as a dimensional fraction (between 0 and 1) of the sorption sites classified as type-1, i.e., sites with instantaneous sorption. The rest of the sorption sites (1-Frac) is classified as type-2, i.e., sites with kinetic sorption. For both models, the “Alpha” parameter represents the firs-order mass transfer coefficient; either for nonequilibrium adsorption when chemical nonequilibrium is considered or for mass exchange between mobile and immobile regions when physical nonequilibrium is considered.