Multicomponent transport with coupled geochemical and microbiological reactions: Model description and example simulations

A reactive transport code (FEREACT) has been developed to examine the coupled effects of two-dimensional steady-state groundwater flow, equilibrium aqueous speciation reactions, and kinetically-controlled interphase reactions. The model uses an iterative two-step (SIA-1) solution algorithm to incorporate the effects of the geochemical and microbial reaction processes in the governing equation for solute transport in the subsurface. This SIA-1 method improves upon the convergence behavior of the traditional sequential iterative approach (SIA) through the inclusion of an additional first-order term from the Taylor Series expansion of the kinetic reaction rate expressions. The ability of FEREACT to simulate coupled reactive processes was demonstrated by modeling the transport of a radionuclide (cobalt, ⁶⁰Co²⁺) and an organic ligand (ethylenediaminetetraacetate, EDTA^4-) through a column packed with an iron oxide-coated sand. The reaction processes considered in this analysis included equilibrium aqueous speciation reactions and three types of kinetic reactions: adsorption, surface dissolution, and biodegradation.