Test: Well Hydraulics - Civil Engineering (CE) MCQ

Aquifer:

• An aquifer is a saturated formation of earth material from which water is yielded in sufficient quantity, due to the high permeability of earth material.
• Unconsolidated deposits of sand and gravel are good for aquifer formation.
• Groundwater is generally extracted from aquifers, so it is of much importance to us.

These are generally of 3 types:

1. Unconfined aquifers (Non-artesian aquifers)
2. Confined aquifers (Artesian aquifers)
3. Perched aquifer

Unconfined aquifer (Non-artesian aquifers):

•  When the water table serves as the upper boundary of the aquifer, the aquifer is known as an unconfined aquifer.
• The unconfined aquifer is also known as the water table aquifer and phreatic aquifer.
• An impervious layer is generally served as the bottom boundary of an unconfined aquifer and the top layer is not confined.

Confined aquifer (Artesian aquifers):

• A confined aquifer shall mean groundwater that is confined under pressure greater than atmospheric by overlying, relatively impermeable strata.
• An aquifer that is bounded by two impervious layers at the top and bottom of the aquifer is called a confined aquifer.
• The confined aquifer is also known as a pressure aquifer or artesian aquifer.

Confined aquifer:

• When some amount of water is present in between two impermeable layers, it is known as a confined aquifer.
• It is also known as the Artesian aquifer.
• The pressure in this aquifer is greater than hydrostatic pressure.
• Groundwater in a confined aquifer is under pressure and will rise up inside a borehole drilled into the aquifer. The level to which the water rises is called the piezometric surface.
• Hence, a confined aquifer is NOT associated with the phreatic zone because it is associated with an unconfined aquifer.

The specific yield S_γ of an aquifer is defined as the ratio of volume of water drained by gravity V_wy to the total volume V. It is expressed as percentage. The specific yield S_γ is an indication of the water yielding capacity of an unconfined aquifer.

The specific retention S_R of an aquifer is defined as the ratio of volume of water retained V_wR to the total volume V. It is expressed as percentage.

The storage coefficient is defined as the volume of water that an aquifer releases per unit surface area per unit change in the component of head normal to that surface.

The coefficient of transmissibility T equals the coefficient of permeability k multiplied by the aquifer thickness b.
T=bk.

The porosity n, specific retention S_R, specific yield S_γ is given by,
S_γ=(V_wγ/V)*100 and S_R=(V_wR/V)*100
V_w= V_wγ+ V_wR
But n=(V_w/V)*100
∴ n= S_R+S_γ.

The permeability can be determined by pumping out test. Pumping must continue at a uniform rate for a sufficient time to approach a steady state condition.

Unconfined aquifer: 

• An unconfined aquifers is one in which the water table forms the upper surface of zone of saturation.
• The term unconfined signifies the absence of any geological layer confining the zone of saturation (above the water table).

• The unconfined aquifer is in direct contact with atmosphere through the zone of aeration. At the water table, the hydraulic pressure head or ground water pressure head is equal to atmospheric pressure (i.e. zero).

• This aquifer is also known as water table aquifer or phreatic aquifer.

Confined aquifer:

• The confined aquifer a geological formation qualifying as an aquifer is overlained by a impermeable layer or an aquiclude. 
• The confining layers show very low intrinsic permeability (k).Here the overlying layer acts as layer confining the underlying aquifer.

• The water in the confined aquifer is not in direct contact with the
  atmosphere. This is because of the presence of confining layer overlying the confined aquifer.

Semiconfined aquifer:

• An aquifer partially confined by soil layers of low permeability through which recharge and discharge can still occur.

• Semi-confined aquifers can usually be located between aquifers and the ground surface with the water seeping into via the internal layers to move freely through the main aquifers.

Perched aquifer:

• A special case of unconfined aquifer is known as perched aquifer. A perched aquifer is formed when the infiltrated rain water is intercepted within the zone of Aeration by an impermeable layer and a local zone of saturation is formed.

• The upper surface of such local zone of saturation is known as perched water table. The perched aquifer occurs at higher elevation than the regional water table.
  
  

Different types of geological formations:

• Aquitard: These are porous but less permeable geological formations from which water cannot flow but instead sieves through it. A good example of aquitard is silty clay material. 
• Aquifers: These are porous and permeable geological formations from which sufficient discharge can be extracted. It generally comprises layers of sand and gravel and fractured bedrock.
• Aquiclude: These are porous but impermeable geological formations from which discharge cannot be extracted. A good example of aquiclude is clay.
• Aquifuge: These are neither porous nor permeable geological formations. A good example of it is a massive compact rock.