Types of Aquifers

Types of Aquifers

When the upper limit of an aquifer is the top of the zone of saturation itself, groundwater is said to occur under water table conditions. The water in the openings at the water table is under atmospheric pressure, just as though it were in an open tank. The aquifer itself is called a water-table aquifer. The water pressure at any level within a water-table aquifer is equal to the depth from the water table and may be expressed as hydraulic head in feet of water.

The water table does not stay the same at all times. When water is added to the zone of saturation, the water table rises. During drought, when groundwater is discharged through springs, streams, or wells, it falls.

Some zones of saturation include both layers with openings in which groundwater can occur and impermeable layers through which it cannot penetrate. Only the permeable layers are aquifers. When an aquifer is sandwiched between impermeable layers, it and the water in it are confined. The upper layer prevents the groundwater from being open to atmospheric pressure, and it occurs within the openings under artesian conditions. Such an aquifer is called an artesian aquifer.

When a well is drilled through the upper confining layer into an artesian aquifer, water rises in the well to a point somewhere above the top of the aquifer. This point is at the piezometric level of the aquifer, a term used to describe an imaginary surface similar to the real water surface (or water table) of a water-table aquifer. Pressure at any point within an artesian aquifer is equal to the depth from the piezometric level and also is expressed as hydraulic head in feet of water.

Sometimes the pressure within an artesian aquifer is great enough for the piezometric level to be above the surface of the earth above it. When a well is drilled into such an artesian aquifer, a flowing artesian well results. The water rises above the surface under its own pressure.