doi:10.1016/j.gca.2005.05.020 Geochemistry of unsaturated soil systems: Aqueous speciation and solubility of minerals and gases in capillary solutions ARNAULT LASSIN, 1, *MOHAMED AZAROUAL, 1 and LIONEL MERCURY 2 1 BRGM, 3 av. C. Guillemin, BP 6009, 45060 Orléans, France 2 UMR-CNRS 8148 “IDES,” Université Paris-Sud, bat. 504, 91405 Orsay, France (Received December 20, 2004; accepted in revised form May 31, 2005) Abstract—It has been shown that the capillary state of aqueous solutions in the unsaturated zone (UZ) modifies chemical equilibria due to the decreasing capillary pressure of the whole system (isobaric scenario) or of the aqueous phase only (anisobaric scenario). Meanwhile, the role of salinity in capillary solutions has not been explicitly taken into account up to now, at least not in a manner independent to capillarity. A consistent way to do so is considered in this paper by calculating activity coefficients as a function of capillary pressure through the extrapolation of the Davies model. The integrated approach thus defined is applied to the interpretation of some laboratory experiments (taken from the literature) carried out under different capillary conditions. Calculations and measurements of the boehmite ↔ bayerite hydration equilibrium in an atmo- sphere of varying humidity agree very satisfactorily if the anisobaric scenario is selected. The solubility of reactive gases O 2 and CO 2 is found to increase in the pore water when the relative humidity decreases. Consequently, and in agreement with experimental measurements, the extent of the pyrite oxidation depends on the relative humidity. The proposed model refines the manner with which chemical equilibria and mineralogical assemblies may be interpreted. In particular, the different scenarios that may be envisioned (isobaric, anisobaric and dual) are considered in the light of the possible modes of precipitation of the minerals (precipitation within the capillary solution, epitaxial type precipitation). Finally, it should be noted that the geochemical approach proposed here for the UZ fully ties in with and conforms to the methods used in the water-saturated zone (SZ). Copyright © 2005 Elsevier Ltd 1. INTRODUCTION Thermodynamic and kinetic approaches are widely used in groundwater and surface water studies to quantify/predict the intensive/extensive levels of the acting physical and chemical processes. Geochemical calculation codes (Phreeqc, EQ3/6, etc.) make these approaches easier and more powerful so that they are applied to a very wide range of hydrogeochemical systems by the scientific community. The basic concepts of water-rock interactions were devel- oped for the water-saturated domain, and have been extended towards the unsaturated zone (UZ), in general by some param- eters which correct for the water volumetric saturation in such media. However, the whole theoretical SZ frame is then used, implicitly assuming that the chemical equilibria are not modi- fied by the capillary nature of the UZ solutions. Yet, the chemical consequences of unsaturated conditions are well known, in particular in studies of clay hydration isotherms (Barshad, 1955), of soil geochemistry (Tardy, 1982; Bourrié et al., 1983; Tardy and Nahon, 1985; Trolard and Tardy, 1987, 1989; Tardy and Novikoff, 1988; Zilberbrand, 1997, 1999; Mercury and Tardy, 1997a,b, 2001, 2004; Mercury et al., 2003), of paleoclimatic studies using noble gases (Mer- cury et al., 2004), of nuclear waste storage in unsaturated rocks (Bruton and Viani, 1992), and of conservation of buildings (Benavente et al., 2004). All these studies share a common theoretical background related to the influence of the chemical potential of capillary water on the solid-aqueous solution interactions, but use more or less different methods. The most recent method (Mercury and Tardy, 2001, 2004; Mercury et al., 2003) relies on the calculation of the effect of the internal pressure of capillary water on its thermodynamic and electrostatic properties. This method allows to extend, in a continuum, the thermodynamic description of the geochemistry from the SZ up to the UZ. This continuum is essentially provided by using the same pressure parameter to describe the water state in all configurations. The objective of the present article is to complete the capil- lary pressure approach tested up to now on dilute solutions, by addressing the role of salinity, i.e., of the osmotic contribution of the interactions. This is done by calculating the activity coefficients of the aqueous (capillary) solutes, as a function of both (but in a distinct manner) the capillary pressure of the solution and its salinity. The overall model thus becomes more suited to the resolution of a number of geochemical problems and is applied in this paper to some laboratory data reported in the literature. This article is set out in the following manner. In a first section, the fundamental physical and chemical aspects of the UZ are presented. In a second section, the approach is applied to real, simple case studies. The agreement between calcula- tions and observations is then discussed, underlining by the way the physical meaning of the theoretical concepts of the proposed approach. Finally, the entire reasoning is summarized and some perspectives are provided. 2. THEORY The activity of water (a w ) is the classical parameter used to describe the thermodynamic equilibrium between an under- * Author to whom correspondence should be addressed (a.lassin@ brgm.fr). Geochimica et Cosmochimica Acta, Vol. 69, No. 22, pp. 5187–5201, 2005 Copyright © 2005 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/05 $30.00 + .00 5187