EXPERIENCES IN MOVING THE WIPP GEOCHEMISTRY MODEL FROM FMT TO EQ3/6

Geochemical modeling of the near-field environment of a repository containing brine requires the use of an accurate model for the activity coefficients of water and dissolved species. The WIPP (Waste Isolation Pilot Plant) geochemistry model (e.g., Xiong, 2005) is a Pitzer-based model built around the Harvie et al. (1984) model of the “sea salt” system. The WIPP model was developed for use with the FMT code (Babb and Novak, 1995). Recently it became desirable to port the model to EQ3/6 (e.g., Wolery and Jarek, 2003), which has been employed on the Yucca Mountain Project. The FMT thermodynamic database was translated into a format used by EQ3/6. Results from the two codes were then compared for a set of 14 test problems. Some expected differences resulted from the fact that FMT, unlike most geochemical codes, requires initial brines to be described in terms of elemental mole numbers (including contributions from the solvent, water). In contrast, EQ3/6 requires initial brines to be described in terms of parameters (usually molalities and pH) for a set of “basis” species. Differences can also result from different ways of adjusting for charge balance. Some unexpected differences were observed. These were traced to (1) the use in FMT of 0.39 as the value for the Debye-Hückel A_φ parameter instead of the correct 0.392, (2) the use in FMT of an older approximation (Pitzer, 1975) for the J(x) function instead of the later Harvie (1981) approximation, and (3) a difference in how the two codes handle missing values of the ψ parameter for cc’a and aa’c ion triplets. EQ3/6 was modified to handle missing ψ values in the more conventional manner used by FMT and to allow the use of the older J(x) approximation as an option (the WIPP density model was also added at this time). EQ3/6 can use the 0.39 value of A_φ by specifying that on the thermodynamic data file. By accounting for all of these factors, EQ3/6 and FMT produce essentially identical results, despite the fact that the codes use substantially different numerical methods. The effect on the WIPP geochemistry model of using the correct A_φ value and the Harvie (1981) approximation for J(x) was examined and found not to sensibly affect results of concern to WIPP performance assessment. This is due to the fact that the dominant radionuclide species have relatively low charge numbers.