Saltwater Intrusion Questions and Answers

Q: When water is pumped from wells in some coastal areas, a problem arises known as :

The problem is saltwater incursion, usually called saltwater intrusion.In coastal aquifers fresh groundwater floats on denser seawater, and its pressure keeps the seawater back. When wells pump out fresh water faster than it is recharged, the water table falls and seawater moves inland and upward into the aquifer, making the wells saline. Coastal Gujarat and Tamil Nadu suffer from this.Pumping does not deposit sand or reduce the permeability of the aquifer, and artesian recharge refers to replenishment of a confined aquifer under pressure, not a problem caused by pumping.

Q: Which one of the following is the correct expression for Ghyben-Herzberg relation with respect to fresh water-seawater boundary.

The Ghyben-Herzberg relation says that for every metre of fresh water head above mean sea level, the fresh water zone extends about 40 metres below it.Fresh water (density about 1.000 g/cm\(^{3}\)) floats on sea water (about 1.025 g/cm\(^{3}\)) in a coastal aquifer. Balancing the pressures at the interface gives\[ \begin{aligned} z &= \frac{\rho_f}{\rho_s - \rho_f}\,h \\ &= \frac{1.000}{0.025}\,h = 40h \end{aligned} \]where h is the water-table height above sea level and z the depth of the interface below sea level. The 14 m figure has no basis, and the options with 40 m or 10 m of head giving 40 m of fresh water have the ratio wrong by factors of 40 and 10. This is why over-pumping that lowers the water table by 1 m lets salt water rise by about 40 m.

Q: "Salt water intrusion" in the coastal area is :

Salt water intrusion is an entry of saline water in a freshwater aquifer.In coastal aquifers fresh groundwater normally floats on and pushes back the denser sea water. When wells pump out fresh water faster than it is recharged, the water table falls and the boundary moves inland, so sea water enters the aquifer and the wells turn brackish. Over-pumping along the coasts of Gujarat and Tamil Nadu has caused this.The movement of sea water up a river in a delta, or through an estuary on to land, is tidal or surface salinity ingress, not intrusion into an aquifer. Salinity from waterlogging is a problem of irrigated soils, where rising groundwater brings salts to the surface.

Q: The saline water-fresh water interface in coastal aquifers was thoroughly studied by :

The interface was studied by Ghyben-Herzberg.Working independently around 1900, W. Badon Ghyben and A. Herzberg showed that, because seawater is about 1.025 times as dense as fresh water, the fresh-salt water interface in a coastal aquifer lies at a depth below sea level about 40 times the height of the water table above sea level:\[ z = \frac{\rho_{f}}{\rho_{s} - \rho_{f}}\, h \approx 40\, h \]Darcy established the law of flow through porous media, Dupuit the assumptions for unconfined flow, and Boussinesq the equation for unsteady groundwater flow; none of them is linked with this interface.

When water is pumped from wells in some coastal areas, a problem arises known as :

A.Saltwater incursion
B.Sand deposition
C.Permeability decrease
D.Artesian recharge

Exam Year(s)

MH SET - 2018, SET Exam - 2018,

Answer: A

The problem is saltwater incursion, usually called saltwater intrusion.

In coastal aquifers fresh groundwater floats on denser seawater, and its pressure keeps the seawater back. When wells pump out fresh water faster than it is recharged, the water table falls and seawater moves inland and upward into the aquifer, making the wells saline. Coastal Gujarat and Tamil Nadu suffer from this.

Pumping does not deposit sand or reduce the permeability of the aquifer, and artesian recharge refers to replenishment of a confined aquifer under pressure, not a problem caused by pumping.

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Which one of the following is the correct expression for Ghyben-Herzberg relation with respect to fresh water-seawater boundary.

A.For every meter of fresh water head above mean sea level the thickness of fresh water zone resting on the salt water is 14 metres
B.For every 40 metres of fresh water head above mean sea level the thickness of fresh water zone resting on the salt water is 40 metres
C.For every meter of fresh water head above mean sea level the thickness of fresh water zone resting on the salt water is 40 metres
D.For every 10 metres of fresh water head above mean sea level the thickness of fresh water zone above salt water is 40 metres

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: C

The Ghyben-Herzberg relation says that for every metre of fresh water head above mean sea level, the fresh water zone extends about 40 metres below it.

Fresh water (density about 1.000 g/cm\(^{3}\)) floats on sea water (about 1.025 g/cm\(^{3}\)) in a coastal aquifer. Balancing the pressures at the interface gives

\[ \begin{aligned} z &= \frac{\rho_f}{\rho_s - \rho_f}\,h \\ &= \frac{1.000}{0.025}\,h = 40h \end{aligned} \]

where h is the water-table height above sea level and z the depth of the interface below sea level. The 14 m figure has no basis, and the options with 40 m or 10 m of head giving 40 m of fresh water have the ratio wrong by factors of 40 and 10. This is why over-pumping that lowers the water table by 1 m lets salt water rise by about 40 m.

Discuss in Forum

"Salt water intrusion" in the coastal area is :

A.A flow of saline water on the land through estuary
B.An entry of saline water in a freshwater aquifer
C.An increase in salinity due to waterlogging
D.An entry of marine water in river in delta region

Exam Year(s)

MH SET - 2019, SET Exam - 2019,

Answer: B

Salt water intrusion is an entry of saline water in a freshwater aquifer.

In coastal aquifers fresh groundwater normally floats on and pushes back the denser sea water. When wells pump out fresh water faster than it is recharged, the water table falls and the boundary moves inland, so sea water enters the aquifer and the wells turn brackish. Over-pumping along the coasts of Gujarat and Tamil Nadu has caused this.

The movement of sea water up a river in a delta, or through an estuary on to land, is tidal or surface salinity ingress, not intrusion into an aquifer. Salinity from waterlogging is a problem of irrigated soils, where rising groundwater brings salts to the surface.

Discuss in Forum

The saline water-fresh water interface in coastal aquifers was thoroughly studied by :

A.Darcy
B.Dupuit
C.Ghyben-Herzberg
D.Bousinesq

Exam Year(s)

MH SET - 2024, SET Exam - 2024,

Answer: C

The interface was studied by Ghyben-Herzberg.

Working independently around 1900, W. Badon Ghyben and A. Herzberg showed that, because seawater is about 1.025 times as dense as fresh water, the fresh-salt water interface in a coastal aquifer lies at a depth below sea level about 40 times the height of the water table above sea level:

\[ z = \frac{\rho_{f}}{\rho_{s} - \rho_{f}}\, h \approx 40\, h \]

Darcy established the law of flow through porous media, Dupuit the assumptions for unconfined flow, and Boussinesq the equation for unsteady groundwater flow; none of them is linked with this interface.

Discuss in Forum

Saltwater Intrusion Questions and Answers

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