Efficient numerical techniques for modeling multicomponent ground-water transport based upon simultaneous solution of strongly coupled subsets of chemical components

An iterative solution technique for reactive transport problems is develope d, which we call the selective coupling method, that represents a versatile alternative to traditional uncoupled iterative techniques and the fully co upled global implicit method. The chemical formulation studied allows a com bination of equilibrium and kinetic reactions, and therefore is a more vers atile model formulation than a purely equilibrium-based system. However, th is is a very challenging system for obtaining an efficient numerical soluti on. Techniques that sequentially compute the concentrations of aqueous comp onents possibly ignore important derivatives in the Jacobian matrix of the full system of equations. The selective coupling method developed here allo ws only the strongly coupled components to be solved together, and the tran sport iteration consists of solving groups of components simultaneously. We also develop a method denoted as coupled normalization to reduce the compu tational work and memory requirements for particular types of reactive tran sport problems. These approaches can result in computational savings relati ve to the global implicit method by achieving a similar iteration count whi le reducing the cpu time per iteration. More importantly, the memory requir ements of the selective coupling technique are controlled by the maximum nu mber of coupled components, rather than by the total number of components. For complex aqueous chemical systems and grids with a large number of nodes , memory efficiency is the characteristic that makes the selective coupling method particularly attractive relative to the global implicit method. A s eries of example cases illustrate the efficiency of the new approach. These test problems are also used to address the implementation issues surroundi ng the most efficient strategy for coupling the aqueous components when car rying out the chemical transport iteration. In-depth knowledge of the behav ior of the chemical system is required to select an appropriate solution st rategy. Published by Elsevier Science Ltd.