DEFINITION OF MOVEMENT OF GROUNDWATER

Most of the water under the ground is constantly in motion,

although rates are typically only centimeters per day. The rates

of movement are controlled by the amount of open space in

the bedrock or regolith, and how the spaces are connected.

Porosity is the percentage of total volume of a body that

consists of open spaces. Sand and gravel typically have about

20 percent open spaces, while clay has about 50 percent. The

sizes and shapes of grains determine the porosity, which is

also influenced by how much they are compacted, cemented

together, or deformed.

In contrast, permeability is a body’s capacity to transmit

fluids or to allow the fluids to move through its open pore

spaces. Permeability is not directly related to porosity. All the

pore spaces in a body may be isolated (high porosity), but the

water may be trapped and unable to move through the body

(low permeability). Permeability is also affected by molecular

attraction, the force that makes thin films of water stick to

objects, instead of being forced to the ground by gravity. If

the pore spaces in a material are very small, as in a clay, then

the force of molecular attraction is strong enough to stop the

water from flowing through the body. When the pores are

large, the water in the center of the pores is free to move.

After a rainfall, much of the water stays near the surface,

because clay in the near-surface horizons of the soil retains

much water due to molecular attraction. This forms a layer

of soil moisture in many regions and is able to sustain seasonal

plant growth.

Some of this near-surface water evaporates and is used

by plants. Other water runs directly off into streams. The

remaining water seeps into the saturated zone, or into the

water table. Once in the saturated zone it moves slowly by

percolation, from high areas to low areas, under the influence

of gravity. These lowest areas are usually lakes or streams.

Many streams form where the water table intersects the surface

of the land.

Once in the water table, the paths that individual particles

follow varies. The transit time from surface to stream may

vary from days to thousands of years along a single hillside.

Water can flow upward because of high pressure at depth and

low pressure in streams.

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