After reading this article you will learn about the classification of aquifers.
An aquifer is a geological unit that can store and transmit water. Unconsolidated sand, and gravel, sandstone, limestone, lava flows and fractured pletonic and metamorphic crystalline rocks are examples of typical aquifers.
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There are two kinds of aquifers:
(i) Unconfined aquifers, and
(ii) Confined aquifers.
Aquifers that extend continuously from the land surface downward through material of high permeability are known as unconfined aquifers. Recharge to the aquifers may be from percolation downward through the unsaturated zone, from later ground water flow, or from upward seepage through underlying material.
In case of confined aquifers subsurface material sometimes contains confining layers, having very low permeability, typically less than about 10–2 darcy. Ground water may move through confining layers, but the rate of movement is usually very slow. Confining layers can be subdivided into aquitards, aquicludes and aquifuges.
An aquitard is a leaky confining layer of low permeability that can store ground water and transmit it sluggishly from one aquifer to another.
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An aquiclude also has low permeability but is situated in i position to form the upper or lower boundary of a ground water flow system.
An aquifuge is an almost totally impermeable body of .rock or unconsolidated material.
Artesian water is confined ground water that is under high hydrostatic pressure. Artesian aquifers are overlain by an aquiclude Recharge to them may occur where the aquifer is exposed at the surface or by slow, downward percolation through an aquitard, water from such an aquifer will rise above the top of the confined aquifer where it is intersected by a well or other conduit as a result of hydraulic head.
The level to which water will rise in a well is the potentiometric surface. A potentiometric surface is included away from the recharge area because of frictional losses as water moves through the pore in an aquifer and because pressure is lost through leakage in fractures.
Where a layer of low-permeability material occurs as a lens within more permeable material, water moving downward through the unsaturated zone is intercepted and will collect on top of the impermeable lens.
As saturation develops above the impermeable lens, a perched aquifer is formed above the main water table. However, under pumping, the removal of water causes the water table to be drawn down around the well, producing a cone of depression in the water table.
Different types of aquifers are shown in Fig. 27.3:
Thermal Springs and Geysers:
In regions of unusually high geothermal gradients—especially those of recent volcanic activity—ground water becomes heated to high temperatures, creating thermal springs and geysers when the water discharges to the surface. If the water is hot enough, geysers may form when the water flashes into stream near the surface.
Ground Subsidence:
Withdrawal of large quantities of ground water or other fluids, such as petroleum, decreases the fluid pore pressure between grains. The decrease in pore pressure allows tighter grain-to-grain contact and leads to sediment compaction and subsidence of the ground surface.
Karst:
Although ground water is rarely seen, its effect on topography may be profound as a result of the dissolving of soluble rocks and the transportation of the dissolved material in solution. The effects of solution produce unique land forms that distinguish solution topography from other land forms.
Ground water is typically slightly acidic and reacts chemically with subsurface rocks, especially those that are highly soluble, such as carbonate rocks (limestone, marble), dolomite (magnesium carbonate), gypsum (calcium sulphate) and some evaporates.
Extensive solution of such rocks produces extraordinary landforms grouped under general category of karst, characterised by numerous closed depressions, caves and various collapse features.
There are number of karst landforms features known today.
These are:
1. Sinkholes are small, shallow, circular to oval, closed depressions formed by downward solution of limestone from the surface or by collapse of the roof of a solution cavity.
2. Sinkhole ponds and karst lakes are sinkholes that have filled with water.
3. Karst valley is a subterranean drainage system which develops in the underlying limestone and surface flow becomes increasingly diverted underground.
4. Caves are elongated cavities in limestone produced by solution, aided by mechanical erosion of subterranean flowing water.
5. Cockpits are large, bowl-shaped sinkholes up to about 1 km in diameter that occur in thick limestone’s of tropical climates.
The relative relationship of ground water storage versus hydrological cycle is shown in Fig. 27.4:
