On Equilibrium Constants for Aqueous Geochemical Reactions in Water Unsaturated Soils and Sediments

Usually, equilibrium constants for aqueous geochemical reactions in the unsaturated zone are assumed to be equal to those for free water solutions (at atmospheric pressure) considered in classical water chemistry. This paper shows that high negative pressures in pore water may essentially change these constants in dry soils and sediments. The influence of negative capillary pressures on equilibrium constants for some important reactions occurring in the upper part of the unsaturated zone is analyzed. It is shown that values of these constants at low water contents may differ from those normally used by orders of magnitude. Sediment drying usually decreases the equilibrium constant for salt dissolution-precipitation reactions (makes precipitation easier) and for silicate weathering (delays it), whilst in the case of dedolomitization, orthoclase-albite transition and some types of cation exchange the equilibrium constant grows and these processes in dry soils and sediments have to be enhanced. This revised version was published online in July 2006 with corrections to the Cover Date.