Aquifer Properties part (2)

Aquifer Properties part (2)

7-Hydraulic Conductivity

It refers to aquifer’s ability to transmit or conduct water. It is defined as volume rate of water of given kinematic viscosity moving through unit cross sectional area per unit hydraulic gradient (Equation-9). It is to be noted that the unit cross sectional area mentioned above is at right angle to the direction of groundwater flow.

K =Q/(A×(Δh/Δl))---------Equation-9

Where:

K is hydraulic conductivity

Q is the volume rate of the water

A is the cross sectional area

(Δℎ/Δ𝑙) is the hydraulic gradient

It can be observed from equation-9 that the unit of hydraulic conductivity is

m/day with dimension of LT-1

It is to be noted that hydraulic conductivity is a function of the porous media and

the fluid passing through it.

8- Intrinsic Permeability

Intrinsic permeability (k) is fundamental property of the aquifer, which

determines its ability to transmit any fluid through it. It is a function of media

only (equation-10).

k = C × d2--------Equation-10

Where:

C is a constant dependent on factors like distribution of grain size, sphericity

and roundness of grains, nature of their packing etc.

d is diameter of the grains.

The dimension of intrinsic permeability is L2 and the popularly used unit is”Darcy”, where 1 Darcy ≈ 10-8 cm2 (CGWB 1982).

The relationship between hydraulic conductivity and intrinsic permeability can

be understood with help of equation-11.

K =(k × ρ ×g)/μ -------Equation-11

Where:

K is hydraulic conductivity

k is intrinsic permeability

ρ is density

g is acceleration due to gravity

μ is kinematic viscosity

9-Transmissivity

It is yet another property, which refers to aquifer’s ability to transmit or conduct

water. It is defined as volume rate of water of given kinematic viscosity

conducted under influence of unit hydraulic gradient through unit saturated

width of the aquifer at right angle to the direction of groundwater flow (After

Theis 1935) (Equation-12).

T =Q/(w×(Δh/Δl)) -------Equation-12

Where:

T is Transmissivity

Q is the volume rate of the water

w is the saturated width of the aquifer

Δℎ/Δ𝑙 is the hydraulic gradient

It can be observed from equation-12 that the unit of transmissivity is m2/day with dimension of L2 T-1

As in case of hydraulic conductivity, transmissivity is also a function of the

porous media and the fluid passing through it.

10- Relationship between hydraulic conductivity and transmissivity

Let us examine how the two fundamental aquifer parameters concerned with

transmission of groundwater through aquifer are related by dividing equation-9

by equation-12 as shown below in equation-13:

(𝐾=𝑄/(𝐴×(Δℎ/Δ𝑙))) /(𝑇=𝑄/(𝑤×(Δℎ/Δ𝑙))) ---- Equation-13

Now the area A given at numerator in equation-13 is visualized with help of a

simple schematic saturated cross section of the aquifer at right angle to the

groundwater flow direction

A schematic saturated cross-section of the aquifer at right angle to the

groundwater flow direction. Here ‘w’ is saturated width while ‘b’ is saturated

thickness.

The area A in Fig is saturated width ‘w’ multiplied by saturated thickness ‘b’.

We substitute this in equation-13 and we get equation -14 as given below:

T = K × b -----Equation-14

Where:

T is Transmissivity

K is hydraulic conductivity

b is saturated thickness of the aquifer

Summary

           1.      An aquifer refers to a geological formation, which can store and transmit

groundwater in sufficient amount for economic utilization.

2.    On the basis of their geological settings and distinct hydrological regime, we

have mainly four types of aquifer: unconfined, confined, semi confined and

perched aquifer.

3.      Porosity of a formation is measure of void spaces in the formation. It is

expressed as ratio of the volume of voids to the total volume of the rock or

formation.

4.      Effective porosity of a formation is measured as a ratio of interconnected

pore space/voids available for fluid flow to the total volume of the rock or

formation.

5.      Specific yield of a rock or formation is measured as the ratio of volume of

water that after saturation is yielded/drained under influence of gravity to the

volume of the rock or formation.

6.      Specific retention of a rock or formation is measured as the ratio of volume

of water that after saturation is retained against the force of gravity to the

total volume of the rock or formation.

7.      Storage coefficient is a general term, which refers to volume of water either

taken in or released out by the aquifer per unit surface area per unit change in

hydraulic head.

8.      Specific storage is defined as the volume of water that an aquifer takes in or

releases per unit volume of the aquifer per unit decline in hydraulic head.

9.      Hydraulic conductivity is defined as volume rate of water of given kinematic

viscosity conducted under influence of unit hydraulic gradient through unit

cross sectional area at right angle to the direction of groundwater flow.

10.   Transmissivity is defined as volume rate of water of given kinematic

viscosity conducted under influence of unit hydraulic gradient through unitsaturated width of the aquifer at right angle to the direction of groundwater

flow.

End part 2