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Encyclopedia > Capillary fringe

The capillary fringe , or tension-saturated zone, is the subsurface layer in which water molecules seep up from a water table by capillary action to fill pores. Pores at the base of the capillary fringe are filled with water due to tension saturation. This saturated portion of the capillary fringe is less than total capillary rise because of the presence of a mix in pore size. If pore size is small and relatively uniform, it is possible that soils can be completely saturated with water for several feet above the water table. Alternately, the saturated portion will extend only a few inches above the water table when pore size is large or non-uniform. Capillary action supports an unsaturated zone above the saturated base within which water content decreases with distance above the water table. In soils with a wide range in pore size, the unsaturated zone can be several times thicker than the saturated zone. Capillary action or capillarity is the ability of a narrow tube to draw a liquid upwards against the force of gravity. ... The vadose zone, also termed the unsaturated zone, is the portion of Earth between the land surface and the water table, and is thus not considered groundwater (vadose is Latin for shallow). It comprises the unsaturated portion of the soil, regolith or bedrock, as well as the saturated capillary fringe...

Some workers restrict their definition of the capillary fringe only to the tension-saturated base portion and exclude it wholly from the vadose zone. This is more common among workers addressing solute transport and water flow. Others define the capillary fringe as including both the saturated and unsaturated portions. This is the preferred definition among workers dealing with the remediation of salt affected soils as well as those those dealing with the vapor phase of soil processes and bioremediation. It is not uncommon to see the capillary fringe treated as a boundary condition separating the water table from the unsaturated zone, without defining it as a significant part of either.

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Tubarc Action

Capillarity is being replaced by Tubarc action because the tube theory employed does not allow lateral flow important for unsaturated hydraulic flow that takes place on porosity (US Pat. 6,766,870 [1]

From the patent . . .

Random irregular porous systems utilized for unsaturated flow employ general principles of capillary action, which require that the tube geometry fit properly to the porosity, which is generally analogous to dimensions associated between capillary tubes and the voids in the random porosity. Random porosity has an irregular shape and a highly variable continuity in the geometrical format of the void space, which does not fit to the cylindrical spatial geometry of capillary tubes. This misunderstanding still holds true due to the fact that both capillary tubes and porosity voids are affected by the size of pores to retain and move fluids as unsaturated conditions. Consequently, an enhanced porosity for unsaturated flow that deals more clearly with the spatial geometry is required. This enhanced porosity becomes highly relevant when moving fluids between different locations by unsaturated conditions if reliability is required in the flow and control of fluid dynamic properties.

Specialized scientific literature about unsaturated zones also recognizes this shortcoming. For example: "Several differences and complications must be considered. One complication is that concepts of unsaturated flow are not as fully developed as those for saturated flow, nor are they as easily applied." (See Dominico & Schwartz, 1990. Physical and Chemical Hydrogeology. Pg. 88. Wiley). Concepts of unsaturated flow have not been fully developed to date, because the "capillary action" utilized to measure the adhesion-cohesion force of porosity is restrained by capillary tube geometry conceptions. The term "capillary action" has been wrongly utilized in the art as a synonym for unsaturated flow, which results in an insinuation that the tube geometry conception captures this phenomenon when in truth it does not.

  Results from FactBites:
The Capillary Fringe and Soil Pore-Size Distribution (576 words)
As depicted, the smaller is the inner diameter of the tube, the greater is the rise of water.
The capillary fringe is composed of various pore-size diameters.
At the base of the capillary fringe the soil is completely saturated by water: from the macropores to the micropores.
  More results at FactBites »



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