FLUID FLOW MECHANISM WITHIN A CAPILLARY FRINGE

Capillary fringe is a small, relatively thin part of water body that is usually approximated within one of the Dupuit assumptions or just simply neglected. There are many studies that deal with capillary fringe but most of them use only mathematical modeling. All mathematical simulations are based on generalizations and simplifications of physical processes and usually apply empirical correspondence of the hydraulic conductivity and the amount of water in the void space represented by the retention curve.

However, there are some recent studies performed in physical models known as “sand boxes” that bring substantial evidence of lateral movement within the capillary fringe. This approach is based on observations conducted in a physical model consisting of small glass spheres. This experimental arrangement simulates nearly ideal conditions. The virtually isotropic and homogenous well-graded glass spheres are an excellent porous medium for the observation of basic phenomena at the saturated-unsaturated zone interface.

The most characteristic feature of capillary fringe is the sub-atmospheric pressure caused by the surface tension meniscus. Surface tension results in a thin layer of water at the groundwater-soil-air interface with physical properties significantly different from those of remaining water body below. Under studied conditions, the meniscus is identical to the capillary fringe upper boundary as indicated from retention curve. The analysis of surface tension at fluid-air interface led to hypothesis formulations that interface is a single continuous meniscus occurring within each unconfined HG structure. The lateral sub-atmospheric flow caused by the surface tension meniscus invoked the idea of comparing capillary fringe area to a sub-atmospheric pipe line flow.

The principal aim of this study was to assess theoretical ability of dissolved substances to be transported within the capillary fringe, including water flow over the hydraulic barrier. Flow pattern was visualized by application of a color tracer directly to the specific place of interest in the model area. No effects such as density driven flow were observed. The flow pattern observed under these experimental conditions indicates that hydraulic barrier efficiency might be affected by the situation in the vicinity of pumped well.