INTRODUCTION TO HYDROGEOCHEM
The purpose of HYDROGEOCHEM is to simulate transient and/or steadystate densitydependent flow fields and temperature distribution and to compute transient and/or steadystate distribution of reactive multispecies chemical concentrations in subsurface media.
HYDROGEOCHEM computes and predicts the distribution of pressure head, moisture content, flow velocity, and total head over a threedimensional plane in either completely saturated, completely unsaturated, partially unsaturated, or partially saturated subsurface media. It also computes and predicts the spatialtemporal distribution of multichemical components. The media may consist of as many types of soils and geologic units as desired with different material properties. Each soil type may be isotropic or anisotropic. The processes governing the distribution of chemical distribution include (1) geochemical equilibrium of aqueous complexation, reductionoxidation, sorption, and precipitation and dissolution, and (2) hydrological transport by flow advection, dispersion, and effect of unsaturation.
The generalized Richards' equation and Darcy's law governing pressure distribution and water flow in saturatedunsaturated media are simulated with the Galerkin finiteelement method subject to appropriate initial and four types of boundary conditions. The hydrological transport equations (a set of PDEs) are derived based on the principle of conservation of mass, and the geochemical equilibrium equations (a set of AEs) are derived based on the mass balance and mass action. The coupled set of PDEs and AEs are simulated with either the conventional finiteelement methods or the hybrid LangrangianEulerian finiteelement method with peak capturing scheme subject to appropriate initial and four types of boundary conditions. Hexahedral elements, triangular prism, and tetrahedral elements are used to facilitate the discretization of the region of interest.
HYDROGEOCHEM is a coupled model of hydrologic transport and geochemical reaction in saturatedunsaturated media. It is designed to simulate transient and/or steadystate transport of Na, aqueous components and transient and/or steadystate mass balance of Ns adsorbent components and ionexchange sites. Along the transport path, HYDROGEOCHEM computes the species distribution of N component species, Mx complexed species, My adsorbed species. Mz ionexchanged species, and Mp potentially precipitated species. The physical, hydrological and chemical settings are as follows.

Media: Heterogeneous and Anisotropic.

Flow Conditions: SaturatedUnsaturated Flows.

Hydrologic Processes: Advection, Dispersion and Diffusion.

Chemical Processes: Aqueous Complexation, Adsorption/Desorption (Surface Complexation, Constant Capacitance, and Double Layer Approaches), IonExchange, Precipitation/Dissolution, Redox, and AcidBase Reactions.

Source/Sink: Spatially and TemporallyDependent Element and Point Sources/Sinks.

Initial Conditions: Prescribed Initial Condition or the Simulated SteadyState Solution as the Initial Condition.

Boundary Conditions: Prescribed Total Analytical Concentrations on Dirichlet Boundaries, Prescribed Fluxes on FlowIn Boundaries, Natural Advective Fluxes on FlowOut Boundaries  All Boundary Values (Concentrations or Fluxes) are Spatially and TemporallyDependent.

Numerical Discretization: FiniteElement Methods with Quadrilateral Elements, Triangular Elements, or the Mixtures of These Two Types.

Approximation Options: Consistent Matrix or Mass Lumping, Nodal Quadrature or Gaussian Quadrature for Surface and Element Integrations.

Solvers: Direct Band Matrix Solver, Basic Point Iterations, and 4 PCG Methods (polynomial PCG, Incomplete Cholesky PCG, Modified Incomplete Cholesky PCG, and Symmetric Successive OverRelaxation PCG).

Time Stepping: Implicit Difference, CrankNicholson Central Difference, or MidDifference.

Solution Methods for Geochemical Reactions: NewtonRaphson with Full Pivoting to Solve the Jacobian Matrix Equation and Constraints on Species Concentrations.
HYDROGEOCHEM is the only commerciallyavailable model for the simulation of reactive multispeciesmulticomponent chemical transport through saturatedunsaturated media. It is not a path model; it is a true transport model coupled with homogeneous and heterogeneous geochemical reactions.
HYDROGEOCHEM SPECIAL FEATURES
The special features of HYDROGEOCHEM are its flexibility and versatility in modeling as wide a range of problems as possible. The model is designed to (1) treat heterogeneous and anisotropic media, (2) consider spatially and temporallydistributed as well as point sources/sinks, (3) accept the prescribed initial conditions or obtain initial conditions by simulating the steadystate version of the system under consideration, (4) deal with prescribed transient concentrations distributed over a Dirichlet boundary, (5) handle timedependent fluxes over variable boundaries, (6) deal with timedependent total fluxes over Cauchy boundaries, (7) include the offdiagonal dispersion coefficient tensor components in the governing equation for dealing with cases when the coordinate system does not coincide with the principal directions of the dispersion coefficient tensor, (8) provide two options for treating the mass matrix  consistent and lumping, (9) give three options (exact relaxation, under and overrelaxation) for estimating the nonlinear matrix, (10) include two options (direct solution with Gaussian elimination method and successive point iterations) for solving the linearized matrix equations, (11) include both quadrilateral and triangular elements to facilitate the discretization of the region, (12) automatically reset time step size when boundary conditions or sources/sinks change abruptly, and (13) include simultaneous chemical processes of aqueous complexation, precipitation/dissolution, adsorption, ion exchange, redox, and acidbase reactions.
(1) Geometry in terms of nodes and elements, and boundaries in terms of nodes and segments; (2) soil properties including (a) saturated hydraulic conductivities or permeabilities; (b) compressibility of water and the media, respectively; (c) bulk density; (d) three soil characteristic curves for each type of soil or geologic unit which are the retention curve, relative conductivity vs head curve, and water capacity curve; (e) effect porosity; and (f) dispersivities, and effective molecular diffusion coefficient for each soil type or geologic unit; (3) initial distribution of pressure head over the region of interest; (4) net precipitation, allowed ponding depth, potential evaporation, and allowed minimum pressure head in the soil; (5) prescribed head on Dirichlet boundaries; (6) prescribed fluxes on Cauchy and/or Neumann boundaries; (7) artificial withdrawals or injections of water; (8) number of chemical components as well as chemical species and their thermodynamic data base; (9) artificial sources/sinks of water and all chemical components; (10) prescribed total concentrations of all chemical components on Dirichlet boundaries; (11) prescribed fluxes of all chemical components on variable boundaries; and (12) initial distribution of all chemical component concentrations. All inputs in items 4 through 11 can be timedependent or constant with time.
(1) pressure head, total head, moisture content, and flow velocity over the twodimensional grid at any desired time; (2) water fluxes through all types of boundaries and amount of water accumulated in the media at any desired time; (3) distribution of total analytical concentrations, total dissolved concentrations, total sorbed concentrations, total precipitated concentrations, and free ion concentrations of all chemical components over a threedimensional grid at any desired time; (4) amount of waste fluxes through the variable boundary; and (5) equivalent kds as a function of time and space in the region of interest.
HYDROGEOCHEM Requirements: Pentium with 16 MB RAM and FORTRAN Compiler, any Workstation, e.g., IBM RS6000, DEC Alpha, Silicon Graphics, Sun SparcStation, and HP 9000 Series.
