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

大量文献表明了从包气带到地下水位及地下水位以下的过渡带的重要性，该过渡带以毛细管边缘为主。然而，对这一过渡区的流动和传输机制的相互作用，特别是在毛细管缘内的相互作用，研究得很少。这里假设，非均质性将导致过渡带内独特的流动、输送和地球化学特征，部分与高含水率时可能存在的低气相渗透率有关。在细粒覆盖粗粒沉积物的情况下，可能会形成空气进入障碍(AEB)和其他流动和运输障碍。在排水过程中，可以防止空气进入覆盖有细粉的粗粒沉积物。在吸气过程中，空气可能会被困在覆盖着细粉的粗料区域内。这种对空气相运动的限制将对过渡带的水流、化学输送、地球化学和微生物学产生重大影响。中心假设是，由于AEBs，ZT内粗颗粒沉积物中的水分含量、水力传导性以及流体和化学物质的通量将取决于：(A)粗颗粒沉积物内的孔隙水压力；(B)粗沉积物的连续性；(C)周围细颗粒沉积物的位置和连续性，(D)相对于粗粒沉积物和周围细粒沉积物的地下水位波动的历史，以及(E)含水率的初始分布。这一假设将通过对均匀介质中的流动和传输的实验室实验来解决，在介质中交替地由单层、相互连接的层和嵌入在细粒度中的粗粒级的不相连透镜组成。孔隙水压力和测量的水分含量之间的差异，加上对优先流动路径的观察，将被用来识别AEBs的存在。

0087228SillimanA 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.