It is a continium-approach parameter. In a porous medium the cross-sectional area includes both the portion occupied by the flowing liquid and the portion occupied by solid and air. It is determined by the pore-size distribution of the channels participating in the transmission of a bulk solution. The proportionality constant is called the first-order rate constant. It is the macroscopic outcome of the actual movements of individual tracer particles through the pores and the various physical and chemical phenomena that take place within the pores.
Hydrodynamic dispersion includes hydraulic dispersion and molecular diffusion .
In this case, the interfacial tension between any pair of fluids is zero, and all the fluids dissolve in each other. In groundwater hydrology, this type of flow is referred to as hydrodynamic dispersion. This net mass transfer is usually caused by gradients in concentrations of a chemical species. It is the ratio of adsorbed content to the concentration in an aqueous solution for the linear equilibrium isotherm. Retardation factor is a dimensionless parameter characterizing the retarding effect of adsorption on solute transport.