Sorption, ion exchange and equilibrium chemistry in advective-dispersive solute transport

Abstract

A method of incorporating specific classes of generic chemical reactions into advective-dispersive solute transport is presented. The reactions comprise a hybrid system governed by local chemical equilibrium in which both solute-solid and homogeneous aqueous interactions occur simultaneously. The chemical environment may be defined either by the linear sorption of a single species and two soluble complexation reactions which also involve that species, or binary ion exchange and one complexation reaction involving a common ion. In each case, a total of three solutes are transported. A set of three partial differential equations is developed, only one of which is nonlinear. These equations are solved numerically in two dimensions through the modification of an existing finite-element ground-water flow/transport algorithm. Example runs are presented that demonstrate the individual and combined effects of linear sorption, ion exchange and aqueous reactions on the simultaneous transport of several solutes in chemical equilibrium.