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Chapter: Environmental Engineering - Sources of water supply

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Sources of water supply: Aquifer and their types

An Aquifer is an water bearing stratum or formation capable of transmitting water in quantities sufficient to permit development.

 

AQUIFER AND THEIR TYPES

 

AQUIFER = An Aquifer is an water bearing stratum or formation capable of transmitting water in quantities sufficient to permit development.

 

AQUICLUDE = It is an impermeable stratum that may contain large quantities of water but whose transmission rates are not high enough to permit effective development.

 

AQUIFUGE = It is a formation that is impermeable and divide of water.

 

AQUIFERS may be considered as falling into two categories.

i.                   Unconfined or Non-Artesian Aquifer

 

ii.                 Confined or Artesian Aquifer, depending on whether or not the water table or free water surface exists under atmospheric pressure.

 

i.                   UNCONFINED AQUIFER OR NON ARTESIAN AQUIFER

 

The top most water bearing stratum having no confined impermeable over burden (Aquiclude) lying over it, is known as an unconfined aquifer or non-Artesian aquifer.

 

The ordinary gravity wells of 2-5m dia which are constructed to tap water from the top most water bearing strata i.e from unconfined aquifer are known as unconfined or Non Artesian wells. The water levels in these wells will be equal to the level of water table. Such wells are therefore known as water table wells.

 

ii.                 CONFINED or ARTESIAN AQUIFERS

 

When an aquifer is confined on its upper and under surface by impervious rock formations and is also broadly inclined so as to expose the aquifer somewhere to the catchment area at a higher level for the creation of sufficient hydraulic head, it is called a confined or an artesian Aquifer. A well excavated through such aquifer yields water that often flown out automatically under the hydrostatic pressure thus even rise or gush out of the surface for a reasonable height.

 

 

PERCHED AQUIFER

 

It is a special case which is sometimes found to occur within a confined Aquifer. If within a zone of saturation an impervious deposit is found to support a body of saturated material then this body of saturated material which is a kind of Aquifer is known as perched Aquifer. The top surface of the water held in this perched aquifer is known as perched water table.

SPECIFIC CAPACITY = Specific capacity of a well is the measure of the effectiveness of the well and is defined as the yield of the well per unit draw down. Therefore

SPECIFIC CAPACITY = Q/L = yield/ draw down

 

Specific capacity of a well is not constant but decreases as discharge increases

 

WATER TABLE:- The uppermost layer of soil or top soil at ground level is generally pervious. The rainwater which is directly percolated through this top soil is contained by it. The upper surface of free water in top soil is termed as water table or ground water level. The water table is the surface of a water body which is constantly adjusting itself towards an equilibrium condition. If there were no recharge or outflow from the ground water in a basin, the water table would eventually become horizontal.


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