THEREDA – Thermodynamic Reference Database

Part of the process to ensure the safety of radioactive waste disposal is the predictive modeling of the solubility of all relevant toxic components in a complex aqueous solution. To ensure the reliability of thermodynamic equilibrium modeling as well as to facilitate the comparison of such calculations done by different institutions, it is necessary to create a mutually accepted thermodynamic reference database. To meet this demand several institutions in Germany joined efforts and created THEREDA (Moog et al., 2015). THEREDA is a suite of programs at the base of which resides a relational databank. Special emphasis is put on thermodynamic data along with suitable Pitzer coefficients, which enable the calculation of solubilities in high-saline solutions. Registered users may either download single thermodynamic data or ready-to-use parameter files for the geochemical speciation codes PHREEQC, Geochemist's Workbench, CHEMAPP, or TOUGHREACT. Data can also be downloaded in a generic JSON format to enable the import into other codes. The database can be accessed via the world wide web: http://www.thereda.de (last access: 1 November 2021). Prior to release, the released part of the database is subjected to many tests. Results are compared to results from earlier releases and among the different codes. This is to ensure that by additions of new and modification of existing data no adverse side effects on calculations are caused. Furthermore, our website offers an increasing number of examples for applications, including graphical representation, which can be filtered by components of the calculated system.

Granite Hydrolysis to Form Deep Brines

Reaction path calculations suggest that water fixation by zeolite and chlorite formation can account for much of the high salinity of deep brines in contact with deep granites, as well as their Ca/Na ratios, which reflect the rock-dominated chemistry of such brines. Resultant brines, undiluted by the influx of shallower fresher waters, are likely to be at equilibrium with laumontite, chlorite, calcite, dolomite, anhydrite/gypsum, K-feldspar, quartz, plagioclase, and possibly halite. The growth of laumontite and chlorite consumes water, causing the concentration of residual salts to increase during the formation of such brines. In these analyses, the major trends suggest that these fundamental processes drive this outcome naturally. Predicted phase assemblages and end-point water compositions are relatively unaffected by the chemistry of the starting/reacting fluid. Additionally, mineralogical and mineral compositional variations both appear to have no major impact on brine formational trends. More precise analysis involves the use of Pitzer coefficients and considers Br/Cl exchange in the alteration phases. Explicit consideration of silicate dissolution points to water availability as a key control over granite alteration. Diffusion-limited water availability appears to lead to stagnant systems dominated by the increasing brine density and Ca/Na ratios with depth. Alteration phases tend to decrease permeability and porosity, further isolating such systems from the flow of shallower dilute fluids.