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Bedrock Regional Aquifer Systematics Study

Aquifers Explained ---

Ground water movement

Both the rate and the direction of ground water movement depend upon the gradient of the water table and the properties of the materials present. The hydraulic gradient in the simplest cases for an unconfined aquifer is approximately the slope of the water table.

The ability of a particular material to allow water to move through is called its hydraulic conductivity, which is expressed in units such as meters per day.

The hydraulic conductivity of an earth material is based on

  • properties of the material (diameter, size of pores, interconnectivity of pores)
  • properties of the fluid moving through (viscosity and density)

The percentage of void (empty) space in soil or rock is called porosity and depends on the nature and extent of its primary (intergranular) and secondary (fracture) openings. The following table shows porosity and hydraulic conductivity of some porous materials, such as clay, have a very low hydraulic conductivity. Although clay has a great deal of pore space because of its small, flat particles, the individual openings are very small and hold water tenaciously.

Porosity and hydraulic conductivity of selected earth materials
material porosity(%) Hydraulic conductivity (m/day)
clay 45 0.041
sand 35 32.8
gravel 25 205.0
gravel and sand 20 82.0
sandstone 15 28.7
dense limestone or shale 5 0.041
granite 1 0.0041

The term permeability is also used as a measure of the ability of an earth material to transmit fluid, but only in terms of the properties of that material (not the properties of the fluid).


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