Aquifer Properties part (1)

Aquifer Properties part (1)

We know by now that an aquifer refers to a geological formation, which can store

and transmit groundwater in sufficient quantity, so that the water can be

economically utilized from the aquifer. Now in this section we will be discussing

about the properties of a formation, which makes it an aquifer.

1-Porosity

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 (Equation-1). Generally, it is expressed as percentage. Thus:

                      n =(Vv/V) x 100 ----- Equation-1.

Where

n = porosity

vv = volume of voids

v = total volume of rock

A look at equation-1 reveals that porosity is ratio of two volumes and does not

have any unit, rather it is dimensionless.

An aquifer can have either primary or secondary porosity. The flow

chart in Fig.6 clearly shows that the primary porosity is formed during genesis of

the rock, while secondary porosity is formed after genesis of the rock. The

porosity of loose sand is the best example of primary porosity, while porosity

imparted to hard rock because of fracturing is an example of secondary porosity.

Primary porosity is a function of sorting, packing, shape and fabric of the grains,

while secondary porosity is a function of intensity of fracturing, degree of

solution of the hard rock etc. It must be mentioned explicitly that if all the grains

are spherical then in such case porosity is not a function of the diameter of the

sphere. Thus if you just magnify the grain size while sorting, packing etc. of the

grains remain same, and then the porosity remains same.

2-Effective porosity

Effective porosity of a formation refers to the fraction of porosity available for

fluid flow. It is measured as a ratio of interconnected pore space/voids available

for fluid flow to the total volume of the rock or formation (Equation-2).

Generally, it is expressed as percentage. Thus:

                            e =Vi/Vx 100 ----- Equation-2.

Where

e = effective porosity

vi = volume of interconnected pore space/voids

v = total volume of rock

A look at equation-2 reveals that effective porosity is ratio of two volumes and

does not have any unit, rather it is dimensionless.

3-Specific Yield

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 (Meinzer 1923) (Equation-3).

                            Sy =Vw/V ----- Equation-3.

Where

Sy = Specific yield

Vw = Volume of water drained under influence of gravity

V = total volume of rock

A glance at equation-3 reveals that Specific yield is ratio of two volumes and

does not have any unit, rather it is dimensionless.

It is to be noted that specific yield is also an approximate estimate of the volume

of water required to saturate an aquifer by flow under influence of gravity.

4- Specific Retention

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 (Meinzer 1923) (Equation-4).

                              Sr =Vr/V ----- Equation-4.

Where

Sr = Specific retention

Vr = Volume of water retained against the force of gravity

V = total volume of rock

A look at equation-4 reveals that Specific retention is ratio of two volumes and

does not have any unit, rather it is dimensionless.

Here if we add specific yield and specific retention we get equation-5. In this

equation, it is clear that Vy: the volume of water yielded plus Vr: the volume of

water retained is a measure of total voids or pores in the formation. Hence, it is

clear that specific yield added with specific retention equal to porosity of the

formation.

          S𝑦 + S𝑟 =(V𝑦+V𝑟)/𝑉= n (porosity) ----- Equation-5

5- Storage Coefficient and Storativity

Storage coefficient helps in estimation of the storing capability of an aquifer.

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 (Equation-6).

                        Sc =Vw/(A×Δh)-----Equation-6

Where:

Sc refers to storage coefficient

Vw refers to the volume of water either taken in or released out by the aquifer

A refers to the surface area of the aquifer

Δh refers to change in hydraulic head

A glance at equation-6 reveals that Storage coefficient does not have any unit

and is dimensionless.

Storage coefficients for confined aquifer are also referred to as Storativity.

6-Specific Storage

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

(Equation-7).

                         Ss =Vw/(V×Δh)----Equation-7

Where

Ss refers to specific storage

vw refers to the volume of water either taken in or released out by the aquifer

v refers to the volume of the aquifer

Δh refers to change in hydraulic head

A look at equation-6 reveals that it has unit of per meter and dimension of L-1

In case of confined aquifers, the relationship between Storativity and Specific

storage is given by equation-8 below:

                           S = Ss × b -------Equation-8

Where

S is storage coefficient

Ss is specific storage

b is the saturated thickness of the aquifer

End of part 1