Modeling reactive chemical transport

Numerous issues need to be addressed before mechanistic-based reactive chemical transport modeling can justifiably replace the widely employed `ad hoc' reactive contaminant transport modeling. The most important issues among others are: (1) mathematical formulation to use `true' chemical transport velocities (retarded velocities) rather than fluid velocities in computations, (2) algorithm development to circumvent numerical problems specifically associated with reactive transport, and (3) reaction pathway identification to obtain theoretical rate laws. The use of fluid velocity to transport chemical species generates incorrect moving fronts. There are special requirements of numerical algorithms for reactive transport. These include the prevention of loss of peaks due to advection, the capture of new peaks due to reactions, the minimization of numerical spreading, the elimination of oscillation, and the removal of grid orientation problems. Reaction laws need to be explored such that the parameters representing the reactions are free from state variables. Microscopic level reactions and their parameters must be free of empiricism before they are scaled up to macroscopic modeling.