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HYDROGEOCHEM 2 is a modification of HYDROGEOCHEM V1.0 (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.
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