Sorption

Sorption includes a number of geochemical and chemical reactions, such as adsorption of solutes to the aquifer material surface by electrostatic forces (called cation exchange). If these processes occur sufficiently fast compared with the water flow velocity they can be described by an equilibrium sorption isotherm.

Different equilibrium sorption isotherms have been identified from experimen­tal results with different sediments, soils and rock types, see, for example, Fetter, 1993. MIKE SHE AD includes three of the most commonly applied iso­therms - namely the linear, Freundlich and Langmuir equilibrium sorption iso­therms.

Sorption processes that are slow compared with the water flow velocity must be described by a kinetic sorption isotherm. In MIKE SHE AD the three equi­librium sorption isotherms have been extended to include a kinetically con­trolled sorption process so that a certain part of the sorbed matter is “transferred” to another part of the soil material.