PROCEEDINGS, TOUGH Symposium 2006 Lawrence Berkeley National Laboratory, Berkeley, California, May 15–17, 2006 IMPLEMENTATION OF A PITZER ACTIVITY MODEL INTO TOUGHREACT FOR MODELING CONCENTRATED SOLUTIONS Guoxiang Zhang, Nicolas Spycher, Eric Sonnenthal, and Carl Steefel Lawrence Berkeley National Laboratory, Berkeley, University of California 1 Cyclotron Road, MS 90-1116 Berkeley, California, 94720, USA e-mail: gxzhang@lbl.gov ABSTRACT TOUGHREACT (Xu et al., 2006) is a general- purpose reactive geochemical transport numerical simulator. It deals with multiphase flow, solute transport and geochemical reactions including aqueous complexation, mineral dissolution/ precipitation and cation exchange. Making use of an extended Debye-Hückel ion activity model, this simulator can handle solutions concentrated to slightly above ~1 molal with caution, and only for NaCl-dominant waters at ionic strengths up to ~4 molal. However, brines produced under natural and artificial conditions are often more concentrated. To handle such brines, a Pitzer activity model was implemented in TOUGHREACT, based on the standard Harvie-Moller-Weare (HMW) formulation that accounts for all binary and ternary combinations of interaction terms. The vapor-pressure-lowering effect caused by the low water activity in brines was also accounted for by this code. The extended version was verified and tested using published results from laboratory experiments and benchmarked against other computer codes. This new version of TOUGHREACT is being applied to the investigation of boiling and evaporation within and around the proposed high-level nuclear waste emplacement tunnels at Yucca Mountain, Nevada. An example application is presented. Processes considered in the example include evaporation of porewater to near dryness, formation of highly concentrated brines, precipitation of deliquescent salts, and generation of acid gases. 1. INTRODUCTION Concentrated aqueous solutions are in general defined as aqueous solutions with ionic strength higher than 1 molal. Such solutions may be produced in many natural and artificial processes, and exist in many natural and contaminated environments. Scientists need to deal with these solutions when solving environmental problems related to water evaporation/boiling, seawater intrusion (Harvie and Weare, 1980; Harvie et al., 1984; Krumgalz, 2001), leakage of toxic solutions and electrolytic fluids from storage tanks (Lichtner, 2001; Lichtner at al., 2004; Steefel et al., 2003; Zhang et al., 2005), and acid mine drainage (Blowes et al., 1991). Concentrated aqueous solutions are significantly different from dilute solutions in flow, transport, and geochemical processes, because of their nonidealities, (e.g., large density, viscosity, and complicated thermodynamic activities). Thus, numerical modeling of these solutions remains a challenge, since most geochemical reactive transport models are based on dilute aqueous solutions, which are not applicable to concentrated aqueous solutions (Pitzer, 1991; Oldenburg and Pruess, 1995; Zhang et al., 2005). TOUGHREACT (Xu, et al., 2006) was developed by introducing geochemical transport to the framework of a multiphase flow code, TOUGH2 (Pruess et al., 1999). It can simulate nonisothermal multiphase groundwater flow, diffusive and advective transport of gases (including vapor) and solutes, geochemical equilibrium and kinetics, including aqueous speciation, and mineral dissolution and precipitation. This code can handle dilute solutions (ionic strength up to 1 molal) and concentrated NaCl dominant solutions (up to ~ 4 molal) by using an extended Debye-Hückel ionic activity model (HKF model, Helgeson et al., 1981). TOUGHREACT has been used to perform many numerical simulations of reactive geochemical transport processes, at various scales and a wide range of geochemical conditions for studies of geothermal energy (Xu et al., 2004), CO 2 sequestration (Xu et al., 2005), high-level nuclear waste disposal (Spycher et al., 2003 and Sonnenthal et al., 2005) and groundwater contamination. However, models of some saline geothermal systems, injection of CO 2 into deep brines, contamination involving concentrated solutions, and geochemical processes in high-level nuclear waste repositories often involve concentrated aqueous solutions with ionic strengths much higher than 1 molal. Most of these processes take place in complex flow and transport systems, such as nonisothermal multiphase flow, double porosity and dual permeability media, and complicated geochemical systems. The sophisticated capabilities of TOUGHREACT could satisfy the requirements of these applications if a suitable ionic activity model were employed. The incorporation of such an ionic activity model into TOUGHREACT is the objective of the present study. - 1 -