BALANCE is a USGS computer program for calculating mass transfer for geochemical reactions in ground water. BALANCE is designed to help define and quantify chemical reactions between ground water and minerals. Data required to run BALANCE are:

Implicit in this treatment is the assumption that only these selected phases participate in the chemical reactions that determine the composition of the final water.

BALANCE calculates the mass transfer (amounts of phases entering or leaving the aqueous phase) necessary to account for the observed changes in composition between the two solutions. The purpose of BALANCE is to derive balanced reactions of the form:

A "reaction model" is defined by the selected phases and the calculated amount of each phase necessary to satisfy the equation. In general, many reaction models can account for an observed change in water chemistry. BALANCE alone cannot determine if any one, unique set of phases governs the reactions in the ground-water system.

BALANCE models are not constrained by any thermodynamic criteria and may imply reactions that are thermodynamically impossible. Methods for identifying reaction models that do not satisfy thermodynamic or other criteria are presented in Plummer and others (1982). In the ideal case, all but one reaction model can be eliminated, leaving one unique chemical model consistent with the available data.

BALANCE is designed specifically for mineral-water interactions, but essentially, BALANCE solves any set of linear equations formulated by the user. The report includes discussions of several processes which can be formulated as linear equations:

In general, the number of phases must equal the number of elements in order to solve the set of equations. Although the calculated mass transfer for one or more phases might be zero, indicating that the phase(s) did not participate in the reaction, the phase(s) must still be included in the input to BALANCE in order to perform the calculations.

a1 + a2 = 1

For a mixing problem, the number of phases (other than solutions) that must be included is equal to the number of elements minus one, P = J - 1.

A redox species is defined as a species of any element which can occur in more than one oxidation state in natural aqueous environments. The rules for determining the operational valence of aqueous species are:

- (1)the formal elemental valence for aqueous redox species,
- (2) use zero for non-redox species,
- (3) use the sum of the operational valences of species which associate to form redox complexes,
- (4) assign zero to the valences of H and O in aqueous species,
- (5) use zero for H+,
- (6) use -2.0 for H2 (aq) and +4.0 for O2 (aq).

Using these definitions, a linear equation ensuring the conservation of electrons can be formulated. The redox state is included in BALANCE input as an "element."

BALANCE accesses an external data base file containing default data for phase names, standard abbreviations for the constituent elements in the phase, the stoichiometric coefficients for the phase elements and RS, the operational valence of the phase if redox is considered.

BALANCE includes an internal editor, BALNINPT for creating/modifying the required BALANCE input data files.

Both source and executable codes are included with the package.

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