8/13/2019 WLSWI

1/15

Module-22

Water logging and Seawater Intrusion

22.1 Water logging:

In agricultural land, when the soil pores within the root zone of the crops get

saturate with the subsoil water, the air circulation within the soil pores gets totally

stopped. This phenomenon is termed as water logging. The water logging makes the soil

alkaline in character an the fertility of the land is totally destroyed and the yield of crop is

reduced.

Due to the heavy rainfall for a longer period or due to the continuous percolation

of water from the canals, the water table gets raised near the surface of the soil. Thenm

by capillary action the water rises to the root zone of crops and goes on saturating the

soil. If this condition goes on for a longer period, the soil becomes alkaline and is

damaging to the crops.

The height to which the soil water rises above the water table by capillary action is

known as Capillary fringe. !enerally, the height of capillary fringe varies from " m to

".# m. $hen the water table comes to ".# m below the surface of the soil, the land is said

to be water logged.

%gain, due to heavy rainfall and poor drainage system the water goes on

collecting in low lying areas. This stagnant water makes the land marshy. The weeds and

a&uatic plants gradually cover the whole area becomes unsuitable for cultivation.

22.2 CAUSES OF WATER O!!I"!:

The following are the main causes of water logging'

(") O#er Irrigation: In inundation irrigation since there is no controlling system of

water supply it may cause over irrigation. The e*cess water percolates and remains

"

8/13/2019 WLSWI

2/15

stored within the root zone of the corps. %gain, in perennial irrigation system if water is

supplied more than what is re&uired. This e*cess water is responsible for the water

logging.

$2% See&age 'ro( Canals: In unlined canal system, the water percolates through the

bank of the canal and gets collected in the low lying areas along the course of the canal

and thus the water table gets raised. This seepage is more in case of canal in banking.

$)% Inade*uate Sur'a+e ,rainage' $hen the rainfall is heavy and there is no proper

provision for surface drainage the water gets collected and submerges vast area. $hen

this condition continues for a long period, the water table is raised.

$% Ostru+tion in "atural Water Course: If the bridges or culverts are constructed

across a water course with the opening with insufficient discharge capacity, the upstream

area gets flooded and this causes water logging.

$/% Ostru+tion in Su-soil ,rainage: If some impermeable stratum e*ists at a

lower depth below the ground surface, then the movement of the subsoil water gets

obstructed and causes water logging in the area.

$0% "ature o' Soil: The soil having low permeability, like black cotton soil, does not

allow the water to percolate through it. +o, in case of over irrigation or flood, the water

retains in this type of land and causes water logging .

$% In+orre+t Metod o' Culti#ation: If the agricultural land is not levelled

properly and there is no arrangement for the surplus water to flow out, then it will create

pools of stagnent water leading to water logging.

$3% See&age 'ro( Reser#oir: If the reservoir basin consists of permeable zones,

cracks and fissures which were not detected during the construction of dam, these may

cause seepage of water. This subsoil water will move towards the lowlying areas and

-

8/13/2019 WLSWI

3/15

cause water logging.

$4% oor Irrigation /anagement' If the main canal is kept open for a long period

unnecessarily without computing the total water re&uirement of the crops, then this leads

to over irrigation which shall result in water logging.

$15% 0*cessive 1ainfall' If the rainfall is e*cessive and the water gets no time to get

drained off completely. Then a pool of stagnant water is formed which might lead to

water logging.

$11% Topography of the 2and' If the agricultural land is flat, i.e, with no country slope

and consists of depressions or undulations, then this leads to water logging.

$12% 3ccasional 4lood' If an area affected by flood every year and there is no proper

drainage system, the water table gets raised and this causes water logging.

22.) EFFECTS OF WATER O!!I"!:

The following are the effects of water logging'

(") Salini6ation o' Soil: Due to water logging the dissolved salts like sodium carbonate,

sodium chloride and sodium sulphate come to the surface of the soil. $hen the water

evaporates from the surface, the salts are deposited there. This process is known as

salinization of soil. 0*cessive concentration of salt makes the land alkaline. It does

not allow he plants to thrive and thus the yield of crop is reduced. This process is also

known as salt efflorescence.

(-) a+7 o' Aeration: The crops re&uire some nutrients for their growth which are

supplied by some bacteria or microorganisms by breaking the comple* nitrogenous

compounds into simple compounds which are consumed by the plants for their

growth. 5ut the bacteria re&uire o*ygen for their life and activity. $hen the aeration

6

8/13/2019 WLSWI

4/15

in the soil is stopped by water logging, these bacteria cannot survive without o*ygen

and the fertility of the land is lost which results in reduction of yield.

(6) Fall o' Soil Te(&erature: Due to water logging the soil temperature is lowered. %t

low temperature of the soil the activity of the bacteria becomes very slow and

conse&uently the plants do not get the re&uisite amount of food in time. Thus, growth

of the plants is hampered and the yield also is reduced.

(7) !rowt o' Weeds and A*uati+ 8lants: Due to water logging, the agricultural land is

converted to marshy land and the weeds and a&uatic plants are grown in plenty. These

plants consume the soil foods in advance and thus the crops are destroyed.

(#) ,iseases o' Cro&s' Due to low temperature and poor aeration, the crops get some

diseases which may destroy the crops or reduce the yield.

(8) ,i''i+ult9 in Culti#ation' In water logged area it is very difficult to carry out the

operation of cultivation such as tilting, ploughings, etc.

(9) Restri+tion o' Root !rowt' $hen the water table rises near to root zone the soil

gets saturated. The growth of the roots is confined only to the top layer of the soil. +o,

the crops cannot be matured properly and the yield is reduced.

22. CO"TRO OF WATER O!!I"! $I.E. A"TI WATER O!!I"!

MEASURES%:

(") 8re#ention o' 8er+olation 'ro( Canals' The irrigation canals should be lined with

impervious lining to prevent the percolation of water through the bed and banks of the

canals. Thus the water logging may be prevented.

Intercepting drains may be provided along the course of the irrigation canals in

places where the percolation of water is detected. The percolating water is

intercepted by the drains and the water is carried to other natural water course.

(-) 8re#ention o' &er+olation 'ro( Reser#oirs:- During the construction of dam, the

geological survey should be conducted on the reservoir basin to detect the zone of

permeable functions through which water may percolate. These zones should be

treated properly to prevent the seepage. If afterwards it is found that there is still

7

8/13/2019 WLSWI

5/15

leakage of water through some zone, then sheet piling should be done to prevent the

leakage.

(6) Control o' Intensit9 o' Irrigation: the intensity of irrigation may cause water

logging so, it should be controlled in a planned way so that there is no possibility of

water logging in a particular area.

(7) E+ono(i+al Use o' Water: If the water is used economically, then it may control the

water logging and the yield of crops may be high. +o, special training is re&uired to

be given the cultivators to realize the benefits of economical use of water. It helps

them to get more crops by eliminating the possibility of water logging.

(#) Fiing o' Cro& &attern:+oil survey should be conducted to fi* the crop pattern. The

crops having high rate of evapotranspiration should be recommended for the area

susceptible to water logging.

(8) ro#iding ,rainage s9ste(:+uitable drainage system should be provided in the low

lying areas so that the rain water does not stand for long days. % network of sub

surface drains is provided which are connected to the surface drains. The surface

drains discharge the water to the river of any water course.

(9) I(&ro#e(ent o' "atural ,rainage:+ometimes, the natural drainage may be

completely silted up or obstructed by weeds, a&uatic plants, etc. The affected section

of the drainage should be improved by e*cavating and clearing the obstructions.

(:) 8u(&ing o' !round water:% number of open wells or tube wells are constructed in

the water logged area and the ground water is pumped out until the water table goes

down to a safe level. The lifted ground water may be utilized for irrigation or may be

discharged to the river or any water course.

(;) Constru+tion o' Su(& well:+ump wells may be constructed within the water logged

area and they help to collect the surface water. The water from the sump wells may be

pumped to the irrigable lands or may be discharged to any river.

#

8/13/2019 WLSWI

6/15

22. Seawater intrusion

+ea water intrusion is the movement of sea water into fresh water a&uifers due to naturalprocess or human activities. +ea water intrusion caused by decrease in the ground water

levels or by rises in the sea water levels. $hen you pump out the fresh water rapidly, you

lower the height of fresh water rapidly< you lower the height of the freshwater in the

a&uifer forming a cone of depression. In coastal a&uifers, sea water originates from two

primary sources

") Connate water

-) +ea water

Connate water is the water remaining in the rock formation from the time of deposition.

+ea water most often occurs in a&uifers, by entrance through submarine outcrops.

Intrusion is the increase of salinity in an a&uifer resulting from the acts of man. The most

common cause is localized over pumping of wells, penetrating coastal a&uifers.

+ea water intrusion is common in coastal areas, especially in over populated towns.

In the land hydraulic system, when the fresh ground water is withdrawn by pumping

wells at a faster rate than it replenished, a drawdown of the water table occurs with

resulting decrease in the overall hydrostatic pressure.

=nder natural conditions, the seaward movement of freshwater prevents saltwater from

encroaching coastal a&uifers, and the interface between freshwater and saltwater is

maintained near the coast or far below land surface. This interface is actually a diffuse

zone in which freshwater and saltwater mi*, and is referred to as the zone of dispersion

(or transition zone) !roundwater pumping can reduce freshwater flow toward coastal

discharge areas and cause saltwater to be drawn toward the freshwater zones of the

a&uifer. +altwater intrusion decreases freshwater storage in the a&uifers, and, in e*tremecases, can result in the abandonment of supply wells. +altwater intrusion occurs by many

mechanisms, including lateral encroachment from coastal waters and vertical up coning

near discharging wells. 4reshwater and saltwater mi* in the zone of dispersion by the

processes of diffusion and mechanical dispersion. % circulation of saltwater from the sea

to the zone of dispersion and then back to the sea is induced by mi*ing within this zone.

8

8/13/2019 WLSWI

7/15

!round-water 'low &atterns and te 6one o' dis&ersion in an ideali6ed;

o(ogeneous +oastal a*ui'er.

Causes o' sea water intrusion'

+ea water intrusion is generally caused by

> 2ocalized over pumping from the wells

> 5oreholes and

> Infiltration galleries

It is a manmade problem caused by e*cessive drainage of lowlying coastal areas

or, in some cases the over pumping of fresh ground water from a&uifers that are in

hydraulic connection with an e*tensive salt water body, such as a sea, or a salt

lake or an ocean.

The destruction of natural barriers, that separate fresh and salt water

(e.g.' construction of salt water canals), which would enable salt water to advance

inland and percolate into fresh water a&uifer.

Commercial and urban development of recharge areas, which reduces the

permeable land surface.

2owering the water tables by drainage canals

lowered rates of groundwater recharge in sewered or urbanized areas, also could

lead to saltwater intrusion, but the impact of such stresses on saltwater intrusion,

at least currently likely is small in comparison to pumping and land drainage

9

8/13/2019 WLSWI

8/15

+ea water intrusion depends on several factors.

Climatic conditions

%&uifer properties

Changes in seaware natural flow Tidal effects

?uman activities such as discharge and recharge wells

Statisti+s:

%lmost two thirds of the world@s population lives within 7AA km (-#A miles) of the ocean

shoreline< Bust over half live within -AA km ("-# miles), an area only taking up "A of

the earth@s surface. /ost of these coastal regions rely on groundwater as their main source

of fresh water for drinking, industry, and agricultural purposes. %s the world@s population

continues to grow at an alarming rate, fresh water supplies are constantly being depleted,

bringing with it issues such as saltwater intrusion and increasing the importance of

groundwater monitoring and management.

+altwater intrusion is a maBor concern commonly found in coastal a&uifers around the

world. +altwater intrusion is the induced flow of seawater into freshwater a&uifers caused

by groundwater development near the coast. $here groundwater is being pumped from

a&uifers that are in hydraulic connection with the sea, induced gradients may cause the

migration of salt water from the sea toward a well.

4resh water is less dense than salt water, therefore it floats on top. The boundary between

salt water and fresh water is not distinct, the transition zone or interface is brackish with

salt water and fresh water mi*ing.

The key to controlling saltwater intrusion is to maintain the proper balance between water

being pumped from the a&uifer and the amount of water recharging it. Constant

monitoring of the saltwater interface is necessary in determining the proper management

techni&ue.

:

8/13/2019 WLSWI

9/15

Te !9en-

8/13/2019 WLSWI

10/15

Fres water (ust eit te a*ui'er to &re#ent seawater intrusion. It is the e*cess in

elevation of fresh water what pushes saltwater back.

A distriuted s(all drawdown is &re'erale to a lo+alised large drawdown at e&ual

pumping rate, provided the total pumping rate stays well below the total freshwater flu*.

!hyben?erzberg e&uation provides an empirical first appro*imation to the depth of

seawater in natural (more or less static) coastal a&uifers

The final form of the e&uation is

$here, Hf F 4resh water density

Hs F +alt water density

h Fwater level above sea level z F thickness of the fresh water below the sea level

I EFFECTS OF SAT WATER I"TRUSIO"

+altwater intrusion from rising sea levels will reduce the &uality and &uantity of

freshwater supplies. This is a maBor concern, since billions of people already lack access

to freshwater.

egative ramifications on human activities such as,

agriculture

livestock

commercial fishing operations

Intrusion caused by pumping may have severe impacts

"A

8/13/2019 WLSWI

11/15

> +alinisation of freshwater wells

> +alinisation of coastal lagoons, and springs

> +alinisation of soils through evaporation near the capillary fringe

> Death of deep root trees

En#iron(ental i(&a+ts asso+iated wit o#ere&loitation in +oastal a*ui'ers

reventive measures of sea water intrusion:

""

4resh water

+alt water

+ea

Coastallagoon (ofen

brackish)

+alt water+alt water

=

+alt water

+ea4resh water

4resh water

+alt water

+ea

Coastallagoon (ofen

brackish)

4resh water

+alt water

+ea

Coastallagoon (ofen

brackish)

+alt water+alt water

=

+alt water

+ea4resh water

+alt water+alt water

=

+alt water

+ea4resh water

8/13/2019 WLSWI

12/15

1. Control te water alan+e:+ome water always has to flow out to the sea to

e&uilibrate the saltwater wedge, however, reducing the outflow facilitates intrusion. If

one would take care to never draw down the freshwater heads below the e&uivalent

freshwater head of the sea saltwater intrusion would be effectively stopped. This also

re&uires distributing the drawdown, many small wells instead of a big one.

Distribution of abstractions in space is only an option of the overall budget is still

positive.

1educing freshwater outflow leads to an increase in saltwater penetration

2. 8er'or( seawater intrusion arriers:

They are purported to prevent saltwater from flowing inland. They can be of several

types'

J egative hydraulic barrier

J ositive hydraulic barrier

J 2owk barrier

J /i*ed

egative hydraulic barrier'

They work by pumping at the shore, so as to intercept inflowing seawater. Therefore,

they are easy to design and manage. In fact, a traditional way of (not) managing coastal

"-

Salt waterFresh water Salt waterFresh water

8/13/2019 WLSWI

13/15

a&uifers consists of pumping boreholes until they get salinised. $hile these wells are

pumped, intrusion does not proceed further.

The (ain &role(with these type of barriers is that wells end up pumping as muchfresh water than salt water, thus contaminating freshwater resources. To reduce this

effect, it is best to concentrate pumping at depth.

ositive hydraulic barrier'

The increased use of groundwater has caused the saltwater interface. 1echarge wells,

recharge basins and barrier wells have proven to be very useful in maintaining the proper

e&uilibrium between pumping and groundwater recharge. Therefore, proper groundwater

monitoring techni&ues and groundwater management, combined with groundwater

conservation are needed to keep saltwater intrusion under control.

"6

4resh water

Inland pumping

(protected)

4resh water+alt water

4resh water

Inland pumping

(protected)

4resh water+alt water

8/13/2019 WLSWI

14/15

8/13/2019 WLSWI

15/15

"#