The Capillary Fringe and Soil Pore-Size Distribution

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Introduction to Basic Ground-Water Flow

By the earthDr!

Capillary Fringe and Soil Pore-Size Distribution in Relation to the Water Table and Vadose Zone

A basic discussion dealing with soil genesis and morphology is warranted to get a better understanding of subsurface transport. Looking at this first figure, we see numerous soil particles of varied shape and size. This figure displays soil particles both above and below the water table. The zone of soil above the water table is known as the unsaturated zone or the vadose zone. Note that the soil particles in the

unsaturated zone are coated with layers of water. The zone of soil below the water table is known as the saturated zone. The zone immediately above the water table is known as the capillary fringe.

No matter how tightly (densely) the soil particles are packed, there will always be void spaces between them. The void spaces between the soil particles are known as the soil pores. Below the water table the pore spaces are filled with water. Above the water table the pore spaces are filled with varied amounts of air and water. These soil pores are interconnected amongst themselves. The soil pores are analogous to water pipes. Just like waterpipes, the diameter of soil pores can vary. The diameter of soil pores range from large to small. Large pores are called macropores. Medium-size pores are called mesopores. Small pores are called micropores. In some soils, the flow of water goes primarily through the macropores. The macropores are often lined by clay minerals. This lining of clay minerals is called clay skins. This is analogous to the lining of an artery with plaque or the armoring of a stream bed with detritus. This lining or armoring of the larger soil pores limits/slows water flow or water drainage from the micropores to the macropores.

Back to the capillary fringe: The capillary fringe is an interesting zone within the soil profile. Looking at the figure below, the capillary phenomenon is detailed. I remember many years ago, my skin being pricked and then, my blood flowed up a small diameter glass tube that was applied by the lab technician to the skin abrasion. The soil macropores are even smaller than the inner diameter of these capillary

tubes. As depicted, the smaller is the inner diameter of the tube, the greater is the rise of water. In a very large pipe, there would be no measurable capillary rise of water. The capillary fringe is immediately above the water table. Under ideal conditions, when a well is installed in the soil, water will not rise above the level of the water table. There is no measurable rise of water in the well since the diameter of the well is large and has no capillary effect. However, the capillary effect of the small-diameter soil pore sizes will cause water to rise above the water table. This water is not free water. This water is essentially trapped within these small capillary pores. The water at and below the water table is free water, that is not trapped by the capillary pores that exist below the water table. 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. At the top of the capillary fringe, saturation by water is now limited to only the micropores. Also, please note that all the soil particles are surrounded by water. Even in the desert when the soil is baked by the sun, soil particles continue to be surrounded by water. Water is just limited; it is not nonexistent. It would take baking the desert soil in an oven to remove the remaining layers of water coating the soil particles.

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