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VICAIRE - MODULE 3
Groundwater Hydrology - Chapter 2
Summary: Porous Media - The Liquid Phase
This chapter presents important features of water in porous media. First, the basic properties of the water are
introduced: density, viscosity, compressibility and capillarity. A special attention was given to the dynamic and
kinematic viscosity. Water particles are subject to an upward attraction due to surface tension of the air-water
interface and the molecular attraction of the liquid and solid phases. This phenomenon explains the presence of
the capillary water in the small pores, which are interconnected.
Groundwater is part of the hydrologic cycle. The main source of the subsurface water consists in infiltration of the
precipitations fallen on the land surface.
Subsurface formations containing water may be divided vertically into a zone of aeration (or unsaturated zone) in
which the pores contain both air and water and an underlying zone of saturation in which the pores are filled with
water. The boundary between the two zones is called water table. The underground water may exist in the
following forms:
Bound water, as chemically bounded water (crystallized and constitutional water) and physically bounded
water (hygroscopic and pellicular water);
Free water as capillary water and gravitational water; this kind of water presents an extreme importance in
the groundwater hydrology;
Gaseous water.
The unsaturated (or the aeration) zone is divided into three sub-zones:
The root zone (soil water zone) extends from the ground surface down through the major root zone; the
moisture distribution in this zone is affected by precipitation, irrigation, air temperature and humidity and by
the possible presence of a close water table. The root zone contains beside the molecular water (the
bounded water) capillary water and gravitational water. Hygroscopic and pellicular water remains adsorbed
to the surface of soil grains; capillarity water is suspended in the canals formed by interconnected pores of
small diameter, while gravitational water is drained under the influence of gravity through the big pores in
communication. While the capillarity water moves in any direction (laterally, upward or downward)
depending on the local conditions, the gravitational water moves downward. Temporarily, during excessive
precipitation or flooding, the surface layers of the soil may be very close to saturation.
The capillary zone or fringe zone extends from the water table up to the limit of capillary rise. Because of
the irregularities in the size of the openings, capillary water does not rise to the same height above the
water table; thus, it forms an irregular fringe.
The vadose zone (called also intermediate zone) extends from the lower edge of the root zone to the upper
limit of the capillary zone.. In the vadose zone the water is permanently held in place by elctrostatic and
capillary forces; these forces cannot retain the surplus of water coming from the root zone, which moves
downward (percolation) through the vadose zone as gravitational water.
The moisture content of a porous medium is a very important index giving an image about the quantity of water
existing in the pore space. The moisture content may be defined from both the mass and volumetric point of view.
The mass humidity or water content (
) is the ratio of the mass of water Mw to the mass of solids Ms in the soil
dried at a temperature of 105°C, usually expressed as percentage:
In many cases, a volumetric humidity or volumetric water content ( ) is required, defined as the ratio of pore
water volume Vw to the total volume V of the soil sample, expressed as percentage too:
The relationship between mass humidity (
) and volumetric water content (
) is the following:
If only a part of the pores are full of water, the soil is unsaturated or partially saturated. The most used relation to
express the degree of saturation ( ) or saturation ratio is: