One of the particularities of the unsaturated zone (UZ) is that it places in physical contact (i) the atmospheric air; (ii) a solution of variable composition (that depends on both depth and interaction time with the solid matrix), either percolating as free water or held in pores by capillarity; and (iii) minerals and organic matter. Geochemically, the coexistence of two types of water (percolating vs. capillary), each having its own spatial and time dynamics of interactions with solids, organics, and air is a basic characteristic of the soil systems. The transport models have very early integrated the physical parameters associated with the unsaturated state of the medium. Meanwhile, most attempts to develop a model coupling chemistry and transport through the UZ have assumed that properties of water and minerals are the same as those in the Saturated Zone (SZ). Recently, different tentative efforts have been done to calculate explicitly the chemical potential of the capillary water and its influence on chemical properties of the capillary water--mineral--soil atmosphere system (e.g., Zilberbrand, 1997, 1999; Mercury and Tardy, 1997a,b, 2001, 2004; Mercury et al., 2003, 2004). The parameter proposed to control the capillary state within the UZ is the relative humidity of the close-contacting air. Mercury et al. (2003, 2004) determined the effect of capillarity on the aqueous speciation and the solubility of both minerals and gases in capillary solutions in general terms. Lassin et al. (2005) incorporated the osmotic aspects related to saline capillary solutions and tested this frame against some laboratory studies.