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
