HYDROGEOCHEM 2 - reactive multispecies-multicomponent chemical transport model - hydrologic transport and mixed geochemical kinetic/equilbrium reactions in saturated/unsaturated media
HYDROGEOCHEM 2 DESCRIPTION
HYDROGEOCHEM 2 Introduction
HYDROGEOCHEM 2 Reactions
HYDROGEOCHEM 2 Applications
HYDROGEOCHEM 2 Limitations
HYDROGEOCHEM 2 Requirements
HYDROGEOCHEM 2 INTRODUCTION
HYDROGEOCHEM 2 is a modification of HYDROGEOCHEM 1.0 (Yeh et al., 1991), a general purpose computer program written in FORTRAN 77 which was designed to solve coupled hydrologic transport and geochemical equilibrium problems. The modification includes replacement of the EQMOD chemical equilibrium subroutines by a mixed chemical Kinetic and Equilibrium model (KEMOD) to deal with species whose concentrations are controlled by either thermodynamics or kinetics.
HYDROGEOCHEM 2 is a coupled model of hydrologic transport and geochemical reaction in saturated-unsaturated media. HYDROGEOCHEM 2 comprises two basic modules: the transport module and the geochemical reaction module. The transport module is designed to simulate: (1) transient and/or steady-state transport of Ns aqueous components, (2) transient and/or steady-state mass balance of Ns adsorbent components and NSITE ion-exchange sites. The geochemical reaction module is designed to compute the species distribution of N = (Na+ Ns) component species, Mx-Kx equilibrium-controlled complexed species, (My - Ky) equilibrium-controlled adsorbed species, Ky kinetic-controlled adsorbed species, (Mz - Kz) equilibrium-controlled ion-exchanged species, Kz kinetic-controlled ion exchanged species, (Mp - Kp) equilibrium-controlled potentially-precipitated species, and Kp kinetic-controlled precipitated species. The two modules are solved iteratively with three options: (1) a complete iteration, (2) an operator splitting, and (3) a predictor-corrector method. The transport module includes advection, dispersion/diffusion, and slight deformation. In the geochemical reaction module, nine types of reactions are included to generate the aforementioned eight types of product species.
HYDROGEOCHEM 2 REACTIONS
Elementary equilibrium complexation reaction: reactants are any number of aqueous components, and the product is an equilibrium-controlled complexed species.
Elementary kinetic complexation reaction: reactants are any number of aqueous components, and the product is a kinetic-controlled complexed species.
Elementary equilibrium adsorption/desorption reaction: reactants are any number of aqueous and adsorbed components, and the product is an equilibrium-controlled adsorbed species.
Elementary kinetic adsorption/desorption reaction: reactants are any number of aqueous and adsorbed components, and the product is a kinetic-controlled adsorbed species.
Elementary equilibrium ion-exchange reaction: reactants are one of the aqueous and complexed species and of the ion-exchanges species, and the products are one of the equilibrium-controlled ion-exchanges species and one of the aqueous and complexed species.
Elementary kinetic ion-exchange reaction: reactants are one of the aqueous and complexed species and one of the ion-exchanges species, and the products are one of the kinetic-controlled ion-exchanged species and one of the aqueous and complexed species.
Elementary equilibrium precipitation/dissolution reaction: reactants are aqueous components, and the product is an equilibrium-controlled precipitated species.
Elementary kinetic precipitation/dissolution reaction: reactants are aqueous components, and the product is a kinetic-controlled precipitated species.
Parallel kinetic reaction: reactants, and the products can involve all species.
HYDROGEOCHEM 2 is designed for generic application to reactive transport problems controlled by both kinetic and equilibrium reactions in subsurface media. Input to the program includes the geometry of the system, the spatial distribution of finite elements and nodes, the properties of the media, the potential chemical reactions, and the initial and boundary conditions. Output includes the spatial distribution of chemical concentrations as a function of time and space and the chemical speciation at user-specified nodes.
The program must be run with a consistent set of units. Units of mass (M), length (L), and time (T) are indicated in the input description. The unit of length should be decimeter (dm), and the unit of mass for any chemical species should be mole. The density of water and solid should be expressed in kg/dm3 (liter). The ion-exchange capacity is in equivalents/Mass of solid. The corresponding concentration unit of all species (aqueous, sorbed, and precipitated species) is mole/liter of fluid (Molar); the corresponding unit for the sorption distribution coefficient is dm3/kg (= ml/g). Any units of time may be used as long as the same unit is used throughout the input file.
HYDROGEOCHEM 2 APPLICATIONS
Treat heterogeneous and anisotropic media.
Consider spatially- and temporally-distributed sources/sinks as well as point sources/sinks.
Accept the prescribed initial conditions or obtain initial conditions by simulating the steady-state version of the system under consideration.
Deal with prescribed transient concentrations distributed over a Dirichlet boundary.
Handle time-dependent fluxes over variable boundaries.
Include the off-diagonal dispersion coefficient tensor components in the governing equation for dealing with cases in which the coordinate system does not coincide with the principal directions of the dispersion coefficient tensor.
Provide two options for treating the mass matrix (consistent and lumping).
Give three options for estimating the nonlinear matrix (exact relaxation, under-relaxation, and over-relaxation).
Include six options for solving the linearized matrix equations (direct solution with the Gaussian elimination method, successive point iterations, and four preconditioned conjugate gradient methods).
Include both quadrilateral and triangular elements to facilitate the discretization of the region.
Automatically reset the time-step size when boundary conditions or sources/sinks change abruptly.
Include chemical processes of aqueous complexation, precipitation/dissolution, adsorption, ion-exchange, redox, and acid-base reactions.
Handle multiple adsorption sites and multiple ion-exchange sites.
Include colloid transport.
HYDROGEOCHEM 2 LIMITATIONS
Inability to simulate co-precipitation (solid solution).
Inability to simulate microbiological reactions which may transform pollutants in the subsurface environment.
The need to import hydrologic variables of flow velocity, moisture content, and pressure head generated by a subsurface flow model.
The assumption of isothermal conditions.
Applications limited to two-dimensional problems.
Applications limited to single-fluid phase flows.
HYDROGEOCHEM 2 is the only commercially-available model for the simulation of reactive multispecies-multicomponent chemical transport controlled by both kinetic and equilibrium reactions.
HYDROGEOCHEM 2 Requirements: 486/Pentium (Pentium recommended) with 16 MB RAM and Fortran compiler, any Workstation, e.g., IBM RS6000, DEC Alpha, Silicon Graphics, Sun SparcStation, and HP 9000 Series.