Test: Aquifers - Civil Engineering (CE) MCQ

The geological formations which are porous and contain a good amount of water but does not yield water freely to wells due to its lesser permeability is called aquitard. The water yield from such a formation is insignificant. Sandy clay is an example of the aquitard.

The geological formations which are both porous and permeable hence sufficient quantity of water can be extracted from them. Aquifers vary in depth and thickness and in general, confined and non-confined aquifers are two main categories of the aquifer.

When the saturated formations are drained under the action of gravity drainage, the volume of water drained is less than the volume of void space. The water contained in these voids cannot be drained out by force of gravity. The water which is always retained by these interstices due to molecular attraction is called pellicular water.

Field capacity can be expressed as the percentage of the total volume of the material drained. Specific retention or field capacity is given as – F.C = volume of water held against gravity drainage/total volume of the material drained x 100
The summation of Specific yield and field capacity is equal to the porosity.

The gravity wells which are constructed to tap water from the unconfined aquifer are known as unconfined or non-artesian wells. Such wells are also known as water table wells or gravity wells since the water level in these wells is equal to the level of the water table.

The geological formations which are porous but have no permeability are termed as an aquiclude. Water cannot be extracted from such formations. A clay layer is an example of aquiclude.

The geological formations which are very dense and contain no water in voids and are neither porous nor permeable are termed as aquifuge. Granite rock is an example of aquifuge.

Yield is the volume of groundwater extracted by gravity drainage from a saturated water-bearing material. It is known as the specific yield when it is expressed as the ratio of the volume of the total material drained. Specific Yield = (Volume of the water obtained by gravity drainage/ total volume of drained material) x 100.

Specific retention can be defined as the amount of water held between the grains due to molecular attraction on the walls of the interstices. If the effective size of grain decreases, the surface area between the interstices will increase causing more specific retention and less specific yield. In fine soils like clay, the specific retention would be more and it results in small specific yield.

A well excavated through confined aquifer yields water that often flows out automatically under the hydrostatic pressure. It may even rise or gush out of the surface for a reasonable height. Flowing well is a type of artesian well where water gushes out automatically.