Entrapment of Air Near the Water Table due to Air Entry Barriers and Heterogeneity

Laboratory experiments were used to create zones of coarse sand within an overall matrix of fine sand. Three forms of heterogeneity were investigated: (i) Zone I: A single zone of coarse sand with vertical continuity to the surface, (ii) Zone II: A single zone of coarse sand which terminated within the fine sand, but which was vertically continuous over a depth approximately equal to the capillary rise in the fine sand, and (iii) Zone III: A series of coarse lenses which were discontinuous and of short vertical continuity (relative to the height of capillary rise in the fine sand). Starting at the upper edge of the medium, the water table was lowered resulting in: (i) Zone I: drainage at a rate equal to the rate of decline in the water table, (ii) Zone II: the overall moisture content within the zone remained high until the overlying fine sand became partially desaturated, at which point the moisture content dropped rapidly (without further drop in the water table), and (iii) Zone III: maintained high moisture content until the fine sand immediately overlying each lens desaturated. The water table was then increased from the base of the tank to the upper surface of the medium resulting in: (i) Zone I: resaturation at a rate equal to the rate of rise in the water table, (ii) Zone II: partial resaturation, entrapping air in the upper portion of this zone, at a pressure substantially above the pressure of the surrounding water phase, and (iii) Zone III: no resaturation or coarse lenses, creating multiple air pockets below the water table.