File: ABSTRACT.TXT MINTEQA2 Model System Abstract Center for Exposure Assessment Modeling (CEAM) National Exposure Research Laboratory - Ecosystems Research Division Office of Research and Development (ORD) U.S. Environmental Protection Agency (U.S. EPA) 960 College Station Road Athens, Georgia 30605-2700 706/355-8400 _____________________________________________________________________________ Summary MINTEQA2 is a equilibrium speciation model that can be used to calculate the equilibrium composition of dilute aqueous solutions in the laboratory or in natural aqueous systems. The model is useful for calculating the equilibrium mass distribution among dissolved species, adsorbed species, and multiple solid phases under a variety of conditions including a gas phase with constant partial pressures. A comprehensive data base is included that is adequate for solving a broad range of problems without need for additional user-supplied equilibrium constants. The model employs a pre-defined set of components that includes free ions such as Na+ and neutral and charged complexes (e.g., H4SiO40, Cr(OH)2+). The data base of reactions is written in terms of these components as reactants. An ancillary program, PRODEFA2, serves as an interactive pre-processor to help produce the required MINTEQA2 input files. Documentation Several modifications important to the user have been made since the last publication of a MINTEQA2/PRODEFA2 user manual (Allison et al.,1991; version 3.0, file USERMANU.PDF). Two documents supplement the original user's manual file (USERMANU.PDF) and are included--along with the original user's manual--with the release of versions 4.01 and 4.02 of the MINTEQA2 model system. The file - SUPPLE1.PDF supplements the basic information in the version 3.11 user's manual and describes significant changes since version 3.0 - SUPPLE2.PDF outlines the development of the data base of diffuse-layer sorption reactions for hydrous ferric oxide (HFO) to be used in equilibrium speciation calculations for the Hazardous Waste Identification Rule (HWIR) The file SUPPLE1.PDF presents modifications in the geochemical speciation model MINTEQA2 and its associated user interface, PRODEFA2, for versions 4.01 and 4.02. Version 3.11 incorporated changes that were never formally described in model documentation. Important version 3.11 changes that apply to versions 4.01 and 4.02 are also described in this file. The basic model theory described in the version 3.11 model documentation is still valid and provides a basic description of the model. The information in the file SUPPLE1.PDF is intended to supplement that provided in the version 3.11 user manual. Significant changes in the MINTEQA2/PRODEFA2 model since the publication of the version 3.11 user manual include: Incorporation of the Gaussian model for computing trace metal complexation with dissolved organic matter, modifications to minimize the occurrence of violations of Gibbs phase rule, modifications to allow direct simulation of a titration in one model run, modifications to allow selected output to be written for easy importing to a spreadsheet, modifications to improve model execution speed and convergence, and modifications to improve the thermodynamic data base used by the model, including the addition of beryllium (II), cobalt (II and III), molybdenum (VI), and tin (II and IV) compounds. Also, errors in thermodynamic constants associated with certain metal-organic reactions in earlier versions have been corrected in version 4.01, thermodynamic constants for inorganic species have been reviewed and updated, and all reference citations for equilibrium constants have been included in the revised data base. These and other revisions are described in the file SUPPLE1.PDF. The file SUPPLE2.PDF outlines the development of the data base of diffuse-layer sorption reactions for hydrous ferric oxide (HFO) to be used in equilibrium speciation calculations for the Hazardous Waste Identification Rule (HWIR). The sorption of contaminant metals and metalloids is important in that it retards contaminant transport in the subsurface. In HWIR, the results of the speciation modeling are used to compute the contaminant sorption distribution coefficient, Kd, a transport model parameter that must be included to account for this retardation. The contaminant metals and metalloids of interest in HWIR are: arsenic (As), antimony (Sb), barium (Ba), beryllium (Be), cadmium (Cd), chromium (Cr), cobalt (Co), copper (Cu), lead (Pb), mercury (Hg), molybdenum (Mo), nickel (Ni), selenium (Se), silver (Ag), thallium (Tl), tin (Sn), vanadium (V), and zinc (Zn). Although not a metal, cyanide (CN) is also of interest as a transportable contaminant. A consistent set of sorption reactions are presented for all HWIR contaminants of interest. The corresponding MINTEQA2 data base is also presented in this file. All MINTEQA2 user manuals are included in the INSTALMT.EXE installation file and can be found in the "docs" sub-directory after installation. Refer to the file README.TXT in the "docs" sub-directory for further information on the storage format and software requirements of the user manual files. _____________________________________________________________________________