DESCRIPTION OF DATA INPUT
The database file is used to define static data for the thermodynamic model. Although any keyword data block can occur in the database file, normally, it contains the keyword data blocks: EXCHANGE_MASTER_SPECIES, EXCHANGE_SPECIES, SOLUTION_MASTER_SPECIES, SOLUTION_SPECIES, SURFACE_MASTER_SPECIES, SURFACE_SPECIES, and PHASES. These keyword data blocks define master species and the stoichiometric and thermodynamic properties of all of the aqueous phase species, exchange species, surface species, and pure phases. Two database files are provided with the program, a database file derived from PHREEQE (Parkhurst and others, 1980) and a database file derived from WATEQ4F (Ball and Nordstrom, 1991). These files are described in more detail in Attachment B and the PHREEQE-derived database file is listed. The elements and element valence states that are included in phreeqc.dat are listed in table 1 along with the PHREEQC notation and the default formula used to convert mass concentration units to mole concentration units.
Initial conditions are defined with SOLUTION, EXCHANGE, SURFACE, EQUILIBRIUM_PHASES, and GAS_PHASE keywords. Solution compositions and speciation calculations are defined with the SOLUTION keyword data block. The composition of an exchange assemblage is defined with the EXCHANGE keyword data block; the composition of a surface assemblage is defined with the SURFACE keyword data block; and the identity and amount of each phase in a pure-phase assemblage is defined with the EQUILIBRIUM_PHASES keyword data block. The composition of a fixed-total-pressure multicomponent gas phase is defined with the GAS_PHASE keyword data block. Multiple solutions, exchange assemblages, surface assemblages, pure-phase assemblages, and gas phases can be defined.
Reactions are defined by allowing a solution or mixture of solutions to come to equilibrium with one or more of the following entities: an exchange assemblage, a surface assemblage, a pure-phase assemblage, or a multicomponent gas phase. In addition, mixtures, irreversible reactions, and reaction temperatures can be specified for reaction calculations. An entity in a reaction can be defined implicitly or explicitly. For implicit definitions, a solution or mixture (SOLUTION or MIX keywords) must be defined within the simulation, then the first of each kind of entity defined in the simulation will be used in the reaction simulation. That is, the first solution (or mixture) will be equilibrated with the first defined of each of the following entities in the simulation: exchange-assemblage (EXCHANGE), gas phase (GAS_PHASE), pure-phase-assemblage (EQUILIBRIUM_PHASES), surface assemblage (SURFACE), irreversible reaction (REACTION), and reaction temperature (REACTION_TEMPERATURE). Alternatively, "USE keyword number" can be used to explicitly define an entity to be used in the reaction calculation from any previously defined entities. (See examples 3, 6, 7, 8, and 9). "USE keyword none" can be used to eliminate an entity that was implicitly defined (See examples 8 and 9.) Any combination of entities can be used to define a reaction. The composition of the solution, exchange assemblage, surface assemblage, pure-phase assemblage, or gas phase can be saved after a set of reaction calculations with the SAVE keyword.
Advective, 1-dimensional transport can be modeled with the TRANSPORT keyword and a combination of the EQUILIBRIUM_PHASES, EXCHANGE, GAS_PHASE, MIX, REACTION, REACTION_TEMPERATURE, SOLUTION, and SURFACE keywords. Logically, a sequence of n reaction cells are defined. An initial solution corresponding to numbers 1 through n must be defined for each cell. In addition, gas phases and exchange, pure-phase, and surface assemblages may be defined for each cell with their numbers corresponding to the cell numbers. The infilling solution is always solution number 0. Advection is modeled by "shifting" solution 0 to cell 1, the solution in cell 1 to cell 2, and so on. At each shift, the solution in each cell is equilibrated with the gas phase and assemblages that are present in the cell. To facilitate definition of the initial conditions the keywords EQUILIBRIUM_PHASES, EXCHANGE, GAS_PHASE, MIX, REACTION, REACTION_TEMPERATURE, SOLUTION, and SURFACE allow simultaneous definition of a range of cell numbers. The SAVE keyword also allows a range of solution, gas phase, or assemblage numbers to be saved simultaneously.
Inverse modeling is defined with the INVERSE_MODELING keyword. Previous definitions of solution compositions with SOLUTION input and possibly new reactants with PHASES or EXCHANGE_SPECIES input are needed for inverse modeling.