MODELING MULTIPLE REACTIVE SOLUTE TRANSPORT WITH ADSORPTION UNDER EQUILIBRIUM AND NONEQUILIBRIUM CONDITIONS

A coupled transport-chemistry hydrogeochemical model is presented. The model has been formulated for groundwater flow systems with multiple physicochemical interactions among the chemical components in the aque ous and solid phases (acid-base, complexation, precipitation-dissoluti on, oxidation-reduction and adsorption-desorption). A nonequilibrium s urface complexation model has been introduced to account for adsorptio n nonequilibrium resulting from external mass transfer, intraparticle mass transfer or chemical kinetics. The model formulation and a series of example problems are included. The results show that nonequilibriu m adsorption entails a different time distribution of kinetically cont rolled adsorbate concentrations from the local equilibrium assumption. The effect of the pH on the evolution of precipitation-redissolution fronts, and the development of contamination plumes with greater conce ntrations that in the incoming solution may be derived from competitiv e adsorbate transport are also shown.