Forms of subsurface water & Aquifer Aquitard, Aquiclude, Aquifuge - definition

Sources of Groundwater

Groundwater is derived from precipitation and recharge from surface water. It is the water that has infiltrated into the earth directly from precipitation, recharge from streams and other natural water bodies and artificial recharge due to action of man. Infiltration and further downward percolation from sources like rain, melting of snow and ice, rivers and streams, lakes, reservoirs, canals and other watercourses are the usual main sources that contribute to the groundwater of a region.

Forms of Subsurface Water

Classification of subsurface Water

Water in the soil mantle is called subsurface water and is considered in two zones

1. Saturated zone
2. Aeration zone

Saturated Zone

This zone, also known as groundwater zone, is the space in which all the pores of the soil are filled with water. The water table forms its upper limit and marks a free surface, i.e. a surface having atmospheric pressure.

Zone of Aeration

In this zone, the soil pores are only partially filled with water. The space between the land surface and the water table marks the extent of this zone. Any flow of water in this zone is in unsaturated soil matrix condition. The zone of aeration has three subzones. 

Soil Water Zone 

This lies close to the ground surface in the major root band of the vegetation from which the water is transported to the atmosphere by evapotranspiration. 

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Capillary Fringe 

In this subzone, the water is held by capillary action. This subzone extends from the water table upwards to the limit of capillary rise.

Intermediate Zone 

This subzone lies between the soil water zone and the capillary fringe. The thickness of the zone of aeration and its constituent subzones depend upon the soil texture and moisture content and vary from region to region. The soil moisture in the zone of aeration is of importance in agricultural practice and irrigation engineering. The present chapter is concerned with the saturated zone only.

Infiltration and Percolations

Infiltration and percolation are both processes that move water in the pores of the soil The main difference between these two processes is that infiltration occurs closer to the surface of the soil. Infiltration denotes the entry of rainwater or ponded water in t the earth surface. The interface is the essence of infiltration process. The descending motion of the infiltered water through soil and rock is called percolation. Infiltration delivers water from the surface into the soil and plant rooting zone while percolation moves it through the soil profile to replenish groundwater supplies or become part o the sub-surface runoff process. Thus, the percolation process represents the flow water from unsaturated zone to the saturated zone.

Saturated Formation

All earth materials, from soils to rocks have pore spaces. Although these pores are completely saturated with water below the water table, from the groundwater utilization aspect only such material through which water moves easily and hence can be extracted with ease are significant. On this basis, the saturated formations are classified into four categories:

• Aquifer
• Aquitard,
• Aquiclude
• Aquifuge

1. Aquifer

An aquifer is a saturated formation of earth material which not only stores water but yields it in sufficient quantity. Thus, an aquifer transmits water relatively easily due to its high permeability. Unconsolidated deposits of sand and gravel form good aquifers.

2. Aquitard

It is a formation through which only seepage is possible and thus the yield is insignificant compared to an aquifer. It is partly permeable. A sandy clay unit is an example of aquitard. Through an aquitard appreciable quantities of water may leak to an aquifer below it..

3. Aquiclude

It is a geological formation which is essentially impermeable to the flow of water. It may be considered as closed to water movement even though it may contain large amounts of water due to its high porosity. Clay is an example of an aquiclude.

4. Aquifuge

It is a geological formation which is neither porous nor permeable. There are no interconnected openings and hence it cannot transmit water. Massive compact rock without any fractures is an aquifuge.

The definitions of aquifer, aquitard and aquiclude as above are relative. A formation which may be considered as an aquifer at a place where water is at a premium (e.g. arid zones) may be classified as an aquitard or even aquiclude in an area where plenty of water is available.

Confined & unconfined Acquifers

The availability of groundwater from an aquifer at a place depends upon the rates of withdrawal and replenishment (recharge). Aquifers play the roles of both a transmission conduit and a storage. Aquifers are classified as unconfined aquifers and confined aquifers on the basis of their occurrence and field situation. An unconfined aquifer (also known as water table aquifer) is one in which a free water surface, i.e. a Water table exists . Only the saturated zone of this aquifer is of importance in groundwater studies. Recharge of this aquifer takes place through infiltration of precipitation from the ground surface. A well driven into an unconfined aquifer will indicate a static water level corresponding to the water table level atthat location.

A confined aquifer, also known as artesian aquifer, is an aquifer which is confined between two impervious beds such as aquicludes or aquifuges. Recharge of this aquifer takes place only in the area where it is exposed at the ground surface. The water in the confined aquifer will be under pressure and hence the piezometric level will be much higher than the top level of the aquifer. At some locations: the piezometric level can attain a level higher than the land surface and a well driven into the aquifer at such a location will flow freely without the aid of any pump. In fact, the term artesian is derived from the fact that a large number of such freeflow wells were found in Artois, a former province in north France. Instances of free-flowing wells having as much as a 50 m head at the ground surface are reported.

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A confined aquifer is called a leaky aquifer if either or both of its confining beds are aquitards.

Water Table

A water table is the free water surface in an unconfined aquifer. The static level of a well penetrating an unconfined aquifer indicates the level of the water table at that point. The water table is constantly in motion adjusting its surface to achieve a balance between the recharge and outflow from the subsurface storage. Fluctuations in the water level in a dug well during various seasons of the year, lowering of the groundwater table in a region due to heavy pumping of the wells and the rise in the water table of an irrigated area with poor drainage, are some common examples of the fluctuation of the water table. In a general sense, the water table follows the topographic features of the surface. If the water table intersects the land surface the groundwater comes out to the surface in the form of springs or seepage.

Sometimes a lens or localised patch of impervious stratum can occur inside an unconfined aquifer in such a way that it retains a water table above the general water table. Such a water table retained around the impervious material is known as perched water table. Usually, the perched water table is of limited extent and the yield from such a situation is very small. In groundwater exploration, a perched water table is quite often confused with a general water table.

Effluent & Influent Streams

The position of the water table relative to the water level in a stream determines whether the stream contributes water to the groundwater storage or the other way about. If the bed of the stream is below the groundwater table, during periods of low flows in the stream, the water surface may go down below the general water table elevation and the groundwater contributes to the flow in the stream. Such streams which receive groundwater flow are called effluent streams (gaining streams) . Perennial rivers and streams are of this kind. If, however, the water table is below the bed of the stream, the stream-water percolates to the groundwater storage and a hump is formed in the groundwater table. Such streams which contribute to the groundwater are known as influent streams (losing streams). Intermittent rivers and streams which go dry during long periods of dry spell (i.e. no rain periods) are of this kind.