Entrapment, stability, and persistence of air bubbles in soil water

Abstract

Air bubbles in soil water affect both hydraulic conductivity and moisture content at a given capillary potential. Consequently changes in the volume of entrapped air, which are not included in the specification of relationships between hydraulic conductivity, moisture content, and capillary potential, will affect all soil-water interactions. Current understanding of the process of air bubble entrapment during infiltration suggests that, in nature, significant air entrapment will often occur. It is shown that infiltrating water can dissolve only a very small volume of air, much less than the amount usually entrapped. Air bubbles in saturated soils are unstable since their pressure must exceed atmospheric, resulting in a diffusive flux of dissolved air from bubbles to menisci contacting the external atmosphere. However, stable bubbles are possible in unsaturated soils. Bubbles which are constrained by pore architecture to non-spherical shapes are usually stable, and spherical bubbles can be stable when the magnitude of the capillary potential exceeds about 3 bars. An approximate analysis of the characteristic time of bubble equilibration indicates that, in an example, it is of order 104 sec, but it may be greater or less by at least a factor 10. Since the equilibration time will be often at least as large as the period of significant soil temperature changes, it cannot be assumed that the entrapped air in a field soil is in an equilibrium state. In such circumstances unstable bubbles may be quasi-permanent. It is suggested that the slow growth of entrapped bubbles may account for the anomalously slow release of water observed in some outflow experiments. Changes of entrapped air volume may also account for the reported dependence of soil-water characteristics on the magnitude of the steps of capillary potential.