A critical evaluation of recent developments in hydrogeochemical transport models of reactive multichemical components

Abstract

Many hydrogeochemical models have appeared in recent years for simulating subsurface solute transport. The hydrological transport of solutes can be described by a set of linear partial differential equations, and the chemical equilibria are described by a set of nonlinear algebraic equations. Three approaches are currently used to formulate the problem: (1) the mixed differential and algebraic equation (DAE) approach, (2) the direct substitution approach (DSA), and (3) the sequential iteration approach (SIA). An extremely important consideration in any approach is the choice of primary dependent variables (PDVs). Six types of PDVs have been employed in the existing models: (1) concentrations of all species, (2) concentrations of all component species and precipitated species, (3) total analytical concentrations of aqueous components, (4) total dissolved concentrations of aqueous components, (5) concentrations of aqueous component species, and (6) hybrid concentrations. Because of many possible combinations of PDVs and approaches, many hydrogeochemical transport models for multicomponent systems have been developed. This paper critically evaluates and discusses these models. The discussion and evaluation are conducted in terms of (1) how severe can the constraints be that a model imposes on computer resources, (2) which equilibrium geochemical processes can a model include, and (3) how easily can a model be modified to deal with mixed kinetic and equilibrium reactions. The use of SIA models leads to the fewest constraints on computer resources in terms of central processing unit (CPU) memory and CPU time; both DAE and DSA models require excessive CPU memory and CPU time for realistic two‐ and three‐dimensional problems. Only those models that use the first three types of PDVs can treat the full complement of equilibrium reactions simultaneously. DAE and SIA models can be modified with reasonable ease to handle mixed chemical kinetics and equilibria. DSA models require strenuous efforts to modify for treating mixed chemical kinetics and equilibria. Therefore SIA models using the third type of PDVs are recommended for their practicality and flexibility. DSA and DAE models should remain research tools for one‐dimensional investigations.

A three-dimensional reactive transport model for sediments, incorporating microniches

“…Given the model does not consider advection (v i = 0), transport of the ith component is driven by reaction and diffusion processes only (Eqn 5) [36,37] :…”

Nuclear Energy Advanced Modeling and Simulation (NEAMS) Waste Integrated Performance and Safety Codes (IPSC) : FY10 development and integration.

“…Widely popular is the assumption of local partial equilibrium, or fast and slow reactions (Lichtner 1996), in which fast reactions like most homogeneous reactions are taken at equilibrium, while slow reactions (including many mineral-water reactions) are kinetically mediated (Lichtner 1985;Ortoleva et al 1986;1987a;1987b;Yeh and Tripathi 1989;1991). This method is covered in detail in the summary text by Lichtner et al (1996) and follows much of the derivation in the previous section.…”

“… Sequential iterative approach (SIA) (recommended by Yeh and Tripathi 1989;1991); several schemes are in use but involves iterating between reaction and transport within a single time step. One method involves alternating reaction and transport terms as a source term from the previous iteration; this has shown convergence problems for some types of reaction schemes.…”

A bio-chemo-hydro-mechanical model for microbially induced calcite precipitation in soils

“…The first reactive transport studies were published in the mid-eighties by Lichtner [35]. A number of reactive transport models were later proposed [36][37][38][39].…”