HyPiT: A thermodynamic database for modeling geochemical systems up to high temperatures, pressures and salinities

Modeling water-rock interactions in underground environments requires the use of thermodynamic databases that account for the complexity of groundwater chemical compositions under relevant pressure and temperature conditions. A key issue concerning saline groundwaters is their high ionic strength, which are often not reliably calculated and calls for the use of specific thermodynamic approaches. Among such approaches, the Pitzer model appears to be one of the most promising. However, its applicability to underground fluids is limited by the fact that the values of its numerous parameters are only known for a subset of chemical systems and, more particularly, are often missing for trace elements and at temperatures other than 25 °C. This paper describes a new way to develop a relevant thermodynamic database to account for the trace elements in these types of deep environments. It combines two in-house databases: Thermoddem, which relies on the B-dot Extended Debye-Hückel activity model, and PhreeSCALE, which uses the Pitzer formalism. The resulting Hy brid Pi tzer- T hermoddem (HyPiT) thermodynamic database (TDB) integrates a simplified Pitzer model to account for major and trace elements. Based on a series of recent works ( Simoes et al., 2017a , 2017b , 2016 ), the Pitzer binary interaction parameters, β (0) and β (1) and their first derivative with respect to temperature (d β (0) /dT and d β (1) /dT) are determined for simulating the first order interactions between trace elements and major species. The HyPiT database is successfully applied to different geochemical systems relevant to underground environments. They include (i) the solubility of gypsum in NaCl and sea-type brines of varying ionic strength at temperatures between 0 and 25°C, (ii) the solubility of barite in NaCl brines at 25, 60 and 80°C, (iii) the solubility of calcite in 0.1 and 4 M NaCl brines for temperatures and pressures up 250°C and 1450 bars, respectively, (iv) the solubility of amorphous silica up to 150°C in various single electrolyte solutions, namely HCl, NaCl, MgCl 2 , HNO 3 , NaNO 3 , Na 2 SO 4 , and MgSO 4 . Particular attention is also given to the modeling of iron-bearing systems such as the speciation of Fe III in LiCl solutions up to 20 mol/kg. • We considered all the comments made by the reviewer. As the revisions are minor, we have nothing to highlight here. • We are grateful to the reviewer and the associate editor for their support.