RSICC CODE PACKAGE PSR-494

1. NAME AND TITLE

MINTEQ: Code System for Calculating Aqueous Geochemical Equilibria.

2. CONTRIBUTORS

Pacific Northwest Laboratory, Richland, Washington, through the Energy Science and Technology Software Center, Oak Ridge, Tennessee.

3. CODING LANGUAGE AND COMPUTER

Fortran 77; DEC VAX11 (P00494DVX1100).

4. NATURE OF PROBLEM SOLVED

MINTEQ is a geochemical program to model aqueous solutions and the interactions of aqueous solutions with hypothesized assemblages of solid phases. It was developed for the Environmental Protection Agency to perform the calculations necessary to simulate the contact of waste solutions with heterogeneous sediments or the interaction of ground water with solidified wastes. MINTEQ can calculate ion speciation/solubility, adsorption, oxidation-reduction, gas phase equilibria, and precipitation/dissolution ofsolid phases. MINTEQ can accept a finite mass for any solid considered for dissolution and will dissolve the specified solid phase only until its initial mass is exhausted. This ability enables MINTEQ to model flow-through systems. In these systems the masses of solid phases that precipitate at earlier pore volumes can be dissolved at later pore volumes according to thermodynamic constraints imposed by the solution composition and solid phases present. The ability to model these systems permits evaluation of the geochemistry of dissolved traced metals, such as low-level waste in shallow land burial sites. MINTEQ was designed to solve geochemical equilibria for systems composed of one kilogram of water, various amounts of material dissolved in solution, and any solid materials that are present. Systems modeled using MINTEQ can exchange energy and material (open systems) or just energy (closed systems) with the surrounding environment. Each system is composed of a number of phases. Every phase is a region with distinct composition and physically definable boundaries. All of the material in the aqueous solution forms one phase. The gas phase is composed of any gaseous material present, and each compositionally and structurally distinct solid forms a separate phase.

5. METHOD OF SOLUTION

MINTEQ applies the fundamental principles of thermodynamics to solve geochemical equilibria from a set of mass balance equations, one for each component. Because the mass action constraints are nonlinear in the components, the problem reduces to the solution of a system of nonlinear equations. In MINTEQ when a solid enters the equilibrium assemblage, the system of mass balance equations is modified to incorporate the additional solubility constraint. The system is then solved directly using the Newton-Raphson technique. MINTEQ is composed of four submodels. The speciation submodel computes the activities of complexed and uncomplexed cationic and anionic species, neutral ion pairs, and the activities of cationic and anionic redox species. These activities are then fed to the solubility submodel, which calculates ion activity products for solids and minerals. The results from these calculations are used by the mass transfer submodel to calculate the mass of solid that precipitates or dissolves. In the fourth or adsorption submodel MINTEQ models adsorption onto solid surfaces via several mechanisms: an activity Kd, an activity Langmuir isotherm, an activity Freundlich isotherm, an ion exchange model, a constant capacitance surface complexation model, and a triple-layer surface complexation model. The calculations completed by each submodel are dependent on the thermodynamic data stored in the MINTEQ database.

6. RESTRICTIONS OR LIMITATIONS

MINTEQ assumes that the system is at equilibrium and will not explicitly consider kinetics. The equilibrium constants in the MINTEQ data base are given for 25 degrees C. MINTEQ should not be used to model solutions at temperatures greater than 100 C.

7. TYPICAL RUNNING TIME

NESC compiled and executed the sample problem in 129 CPU seconds on a DEC VAX11.

8. COMPUTER HARDWARE REQUIREMENTS

255 Kbytes were required on a DEC VAX11/785.

9. COMPUTER SOFTWARE REQUIREMENTS

MINTEQ was tested under VMS 4.2 and VMS 4.5 using the Vax Fortran 77 compiler at the National Energy Software Center at Argonne National Laboratory in 1988. It was not retested or modified when it was transferred to RSICC and released in August 2001. The files were downloaded to PC and compressed to provide a convenient way to transfer them.

10. REFERENCES

a) included in documentation:

S.R. Peterson, C.J. Hostetler, W.J. Deutsch, and C.E. Cowan, "MINTEQ User's Manual," NUREG/CR-4808, PNL-6106 (February 1987).

A.R. Felmy, D.C. Girvin, E.A. Jeene, "MINTEQ - A Computer Program for Calculating Aqueous Geochenical Equilibria," PB84-157148, EPA-600/3-84-032 (February 1984).

b) background reference:

S.R. Peterson, B.E. Opitz, M.J. Graham and L.E. Eary, "An Overview of the Geochemical Code MINTEQ: Applications to Performance Assessment for Low-Level Wastes," PNL-6112, UC-70 (March 1987).

A.R. Felmy, S.M. Brown, Y. Onishi, S.B. Yabusaki and R.S. Argo, "MEXAMS, The Metal Exposure Analysis Modeling System," EPA-600/ 3-84-031 (1984).

11. CONTENTS OF CODE PACKAGE

Included are the reference documents in 10.a and software on one-3.5" HD diskette containing a PkWare® self-extracting executable for WINDOWS® housing the source, data, and sample problem files. No MINTEQ executable file is included with package.

12. DATE OF ABSTRACT

August 2001.

KEYWORDS: ENVIRONMENTAL IMPACTS; WASTE MANAGEMENT