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Module 1 — Understanding Groundwater
Some precipitation that falls to the ground surface infiltrates the ground and becomes
groundwater. Groundwater is defined as sub-surface water that fills openings and pore
spaces in soil and rock layers. Below the ground surface is an unsaturated zone, which water
travels through, to reach lower zones. The water table is the point at which the ground is
completely saturated. Below this level the pore spaces between every grain of soil and rock
crevice completely fill with water.
Aquifers and Aquicludes
The layers of soil and rock below the water table are classified in two broad categories:
Aquifers
Aquicludes.
Aquifers are water bearing layers (or formations) that yield water to wells in usable
amounts. Typical aquifers are made of sand, gravel or sandstone. These materials have large
enough connected pore spaces between grains that water moves freely. Coal and shale are
more tightly compacted but may also be suitable aquifer materials if they are fractured (or
cracked) enough to allow water to move through them.
Aquicludes are water bearing formations that cannot yield adequate water for wells.
Examples of these are clay and unfractured shale and coal. The pore spaces between grains
of these materials are so small that water moves through them extremely slowly.
Confined and Unconfined Aquifers
Unconfined aquifers are exposed directly to the atmosphere through openings in the
soil. The volume of water in unconfined aquifers is mainly dependent on seasonal cycles of
precipitation that refills the aquifer. A water table aquifer is an example of an unconfined
aquifer (see Figure 2, Types of Aquifers).
A confined aquifer is trapped below an upper confining layer of rock, clay or shale.
When a well is drilled into a confined aquifer, the water level in the well rises above the
upper boundary of the aquifer. Aquifers that are completely saturated with water and
under pressure are called artesian aquifers. The artesian aquifer shown in Figure 2, Types
of Aquifers, is an example of a confined aquifer. A flowing artesian well results when the
pressure in the aquifer raises the water level above the ground surface.