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

Abstract

An iterative solution technique for reactive transport problems is developed, which we call the selective coupling method, that represents a versatile alternative to traditional uncoupled iterative techniques and the fully coupled global implicit method. The chemical formulation studied allows a combination of equilibrium and kinetic reactions, and therefore is a more versatile model formulation than a purely equilibrium-based system. However, this is a very challenging system for obtaining an ecient numerical solution. Techniques that sequentially compute the concentrations of aqueous components possibly ignore important derivatives in the Jacobian matrix of the full system of equations. The selective coupling method developed here allows only the strongly coupled components to be solved together, and the transport iteration consists of solving groups of components simultaneously. We also develop a method denoted as coupled normalization to reduce the computational work and memory requirements for particular types of reactive transport problems. These approaches can result in computational savings relative to the global implicit method by achieving a similar iteration count while reducing the cpu time per iteration. More importantly, the memory requirements of the selective coupling technique are controlled by the maximum number 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 eciency is the characteristic that makes the selective coupling method particularly attractive relative to the global implicit method. A series of example cases illustrate the eciency of the new approach. These test problems are also used to address the implementation issues surrounding the most ecient strategy for coupling the aqueous components when carrying out the chemical transport iteration. In-depth knowledge of the behavior of the chemical system is required to select an appropriate solution strategy. Published by Elsevier Science Ltd.

Conditional Probability Density Functions of Concentrations for Mixing-Controlled Reactive Transport in Heterogeneous Aquifers

“…A common method to estimate spatial distributions of chemical species concentrations and associated reaction rates is to employ numerical modeling. A series of mathematical formulations are available in the literature, which are included in a variety of codes (Rubin 1990;Yeh and Tripathi 1991;Friedly and Rubin 1992;Lichtner 1996;Steefel and MacQuarrie 1996;Tebes-Stevens et al 1998;Clement et al 1998;Saaltink et al 1998;Parkhurst and Appelo 1999;Robinson et al 2000;Molins et al 2004). All these methodologies are based on the idea that reactive transport problems can be reformulated mathematically in terms of chemical components, defined as linear combinations of reactive species concentrations.…”

The Development and Application of Reactive Transport Modeling Techniques to Study Radionuclide Migration at Yucca Mountain, NV

“…Characterization and pmformance assessment studies for the potential high-level 237Np as a radionuclide of concern for nuclear waste repository at Yucca Mountain have identified the proposed repository (Viswanathan et al, 1998). This chapter serves to test the methods for a practical application that is of interest to the Yucca Mountain Project.…”

“…The predictions utilize existing Yucca Mountain databases and provide a means of checking model abstractions and the current databases. The reactive transport equations solved by FEHM are described in greater detail in Viswanathan et al(1998), and thus we will only provide a brief summary in this section. …”

“…The finite element grid for the entire cross section is shown in Figure 3-3b and c. The unstructured grid captures the complex stratigraphy at the site scale, while also allowing transport near the potential reposito~to be captured at a grid spacing of about 3 m resulting in a mesh with 7070 spatial nodes. For details on the hydrologic models, parameters, and infdtration fluxes used to investigate 237Np transport, see Viswanathan et al(1998). In the present study, the nonisothermal effects examined in that paper are ignored in favor of an isothermal model, so that the chemical transport processes and numerical schemes can be examined more directly.…”

Early-stage hypogene karstification in a mountain hydrologic system: A coupled thermohydrochemical model incorporating buoyant convection

“…The capabilities of FEHM for simulating transitions of heat transfer regimes in a system with time-evolving permeability were demonstrated by Chaudhuri et al [2009]. Multispecies reactive solute transport has been modeled with either explicit or implicit coupling to fluid flow and heat transport in previous work [Robinson et al, 2000;Viswanathan et al, 2009]. We enhanced the capabilities of FEHM by incorporating porosity-permeability alteration by dissolution/precipitation of calcite, and adapted FEHM to simulate fracture/fault permeability.…”