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

[00:87 . 228]大量的文献表明，以毛细条纹为主的过渡带从渗透带进入地下水位及以下区域的重要性。然而，总的来说，对整个过渡区，特别是毛细条纹内的流动和输送机制的相互作用的研究很少。本文假设，非均质性将导致过渡区内独特的流动、输运和地球化学特征，部分原因是在高含水量时可能存在低气相渗透率。在细颗粒覆盖粗颗粒沉积物的情况下，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，空气可能会被阻止进入被细颗粒覆盖的粗颗粒沉积物。在吸胀过程中，空气可能被困在细颗粒覆盖的粗颗粒区域内。这种对气相运动的限制将对过渡带的水流、化学运输、地球化学和微生物学产生重大影响，并随着时间的推移受到影响。中心假设是，由于AEBs， ZT内粗粒度沉积物中的水分含量，水力导电性以及流体和化学物质的通量将取决于：(a)粗粒沉积物内部孔隙水压力，(b)粗粒沉积物的连续性，(c)周围细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历史，(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输运的实验室实验来解决，以及在由简单层、相互连接层和嵌入细粒颗粒中的粗粒颗粒的分离透镜组成的介质中。孔隙水压力与实测含水率之间的差异，加上对优先流动路径的观察，将用于识别aeb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.