Air Entry Barriers: Mechanism for Creating High Permeability Pathways Above the Water Table

University Of Notre Dame, Notre Dame IN

Investigators

Abstract

0087228 Silliman A substantial body of literature exist which demonstrates the importance of the zone of transition, dominated by the capillary fringe, from the vadose zone into the region at and below the water table. However, there has been minimal study of the interplay of flow and transport mechanisms across this zone of transition in general, and specifically within the capillary fringe. It is here hypothesized that heterogeniety will lead to unique flow, transport and geochemical characteristics within the zone of transition related, in part, to low gas-phase permeabilities that can be present at high moisture contents. Air Entry Barriers (AEBs)and other impediments to flow and transport may form in situations in which fine-grained overlie coarse-grained sediments. During drainage, air may be prevented from entering coarse-grained sediments that are overlain by fines. During imbibition, air may be trapped within zones of coarse material overlain by fines. Such restriction on the movement of the air phase will have significant effect on and over time be affected by water-flow, chemical transport, geochemistry and microbiology in the zone of transition. The central hypothesis is that due to AEBs, the moisture content, hydraulic conductivity, and flux of fluid and chemicals within coarse-grained sediments within the ZT will depend on: (a) the pore water pressure within the coarse-grained sediments, (b) the continuity of the coarse sediments, (c) the position and continuity of the surrounding finer-grained sediments, (d) the history of fluctuation of the water table relative to both the coarse-grained sediments and the surrounding finer-grained sediments, and (e) the initial distribution of moisture content. This hypothesis will be addressed through laboratory experiments on flow and transport in homogeneous media, and in media consisting alternatively of simple layers, interconnected layers, and disconnected lenses of the coarse size fraction embedded within a fine size fraction. Discrepancy between pore water pressure and measured moisture contents, plus observation of preferential flow pathways will be used to identify the presence of AEBs.