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Threat of land subsidence in and around Kolkata City
and East Kolkata Wetlands, West Bengal, India
P Sahu∗and PKSikdar
Department of Environment Management, Indian Institute of Social Welfare
and Business Management, Kolkata 700 073, India.
∗e-mail: paulamisahu@yahoo.com
This paper attempts to estimate the possible rate of land subsidence of Kolkata City including Salt
Lake City and the adjoining East Kolkata Wetlands located at the lower part of the deltaic alluvial
plain of South Bengal basin. Demand of groundwater for drinking, agricultural and industrial purposes
has increased due to rapid urbanization. The subsurface geology consists of Quaternary sediments
comprising a succession of clay, silty clay and sand of various grades. Groundwater occurs mostly under
confined condition except in those places where the top aquitard has been obliterated due to the scouring
action of past channels. Currently, the piezometric head shows a falling trend and it may be accelerated
due to further over-withdrawal of groundwater resulting in land subsidence. The estimated mean land
subsidence rate is 13.53 mm/year and for 1 m drop in the piezometric head, the mean subsidence is
3.28 cm. The surface expression of the estimated land subsidence is however, cryptic because of a time
lag between the settlement of the thick low-permeable aquitard at the top and its surface expression.
Therefore, groundwater of the cities and wetland areas should be developed cautiously based on the
groundwater potential to minimize the threat of land subsidence.
1. Introduction
Land subsidence is an environmental hazard which
is caused by overdraft of groundwater or oil extrac-
tion and results in gradual settling or sudden sink-
ing of the earth’s surface owing to subsurface move-
ment of the materials of the earth. More than 200
occurrences of land subsidence have been docu-
mented throughout the world during the past few
years (Ustun et al 2010). Land subsidence associ-
ated with groundwater level decline has been rec-
ognized as a potential problem in various parts of
the world. Important cities such as London, Venice,
Mexico, Jakarta, Tokyo, etc., have experienced land
subsidence due to over-extraction of groundwa-
ter for domestic and industrial purposes (table 1).
Land subsidence occurs when large amounts of
groundwater have been withdrawn from certain
types of rocks, such as fine-grained sediments. The
sediment compacts because the water is partly
responsible for holding up the ground. Decline
of water table or piezometric surface results in
vertical compression of the subsurface materi-
als (Bouwer 1977). Along with vertical compres-
sion, lateral compression may also take place due
to initiation or acceleration of lateral flow of
groundwater. This lateral movement also results in
subsidence of the land surface. Any flow or over-
draft of groundwater in unconsolidated material
should produce some movement of the land sur-
face. This movement is generally small, but may
become very significant where subsurface materials
Keywords. Kolkata; East Kolkata Wetlands; groundwater; land subsidence; groundwater potential; sustainable
development.
J. Earth Syst. Sci. 120, No. 3, June 2011, pp. 435–446
c
Indian Academy of Sciences 435
436 P Sahu and P K Sikdar
Tabl e 1 . Case histories of land subsidence in other countries.
Maximum
Cause Location subsidence (m) Period Source of information
Municipal and Su-Xi-Chang, China 0.018 1984–2005 Zhang et al (2010)
industrial Mashhad, Iran 0.09 1995–2005 Motagh et al (2007)
overdraft Shanghai 3.00 1921–2001 Chai et al (2004)
Po Plain, Italy 0.07 – Carminati and Martinelli (2002)
Jakarta, Indonesia 0.016 1991–1997 Hirose et al (2001)
Hanoi, Vietnam 0.150 1988–1995 Nguyen and Helm (1995)
Virginia, Atlantic City 0.004 1980–1983 Leaky and Martin (1993)
Venice, Italy 0.15 1930–1973 Gambolati and Freeze (1973)
Mexico City 8.00 1938–1968 Poland (1969)
Tokyo, Osaka 4.00 1928–1943 Poland (1969)
Taipei 1.00 Poland (1969)
London 0.18 1865–1931 Poland and Davis (1969)
Franklin 0.01 1980–1985 Leaky and Martin (1993)
Irrigation Koyna Closed Basin, Turkey 0.05 2006–2009 Ustun et al (2010)
overdraft San Joaquin Valley, California 8.50 – Lofgren (1969)
Santa Clara Valley 4.00 – Poland (1969)
Eloy Area, Arizona 2.30 1948–1967 Schumann and Poland (1969)
Oil removal Wilmington 9.00 Mayuga and Allen (1969)
Brown coal Latrobe Valley, Victoria, Australia 0.02 1970–1977 Gloe et al (1973)
withdrawal
are thick and/or compressible and the groundwa-
ter level declines appreciably (Sikdar et al 1996).
Land subsidence may not be noticeable because it
can occur over large areas rather than in a small
spot. That does not mean that subsidence is not
a big event – for example, in the United States,
states such as California, Texas and Florida have
suffered damage to the tune of hundreds of millions
of dollars over the years due to land subsidence.
Land subsidence is either individual or a combined
effect of inelastic compression of the confining bed
and/or elastic compression of the solid matrix of
the aquifers. Land subsidence due to groundwater
overdraft is essentially irreversible in case of inelas-
tic compression of the overlying confining clay bed.
It can be stopped only by halting the decline of
groundwater level. However, rebound of the land
surface is generally insignificant even if the ground-
water levels are restored to the height, prior to
subsidence (Bouwer 1977).
The decline of artesian pressure due to with-
drawal of water from confined aquifers generally
results in decrease in pore-water pressure and con-
sequent increase in effective stress in the subsoil.
The distribution of stress at the interface of a
confining bed and confined aquifer is as follows:
σ=σ+P,
where σ= total vertical stress, σ= effective stress,
and P= pore-water pressure.
The increased effective stress is accommodated
by the solid matrix of the aquifer which undergo
elastic compression as long as artesian pressure
continues to decline.
Apart from the elastic compression, inelastic
compression of the confining beds of low permeabil-
ity and silty clay beds included within the confined
beds also results in land subsidence due to reduc-
tion of artesian pressure by pumping of ground-
water. The reduction of artesian pressure in the
aquifer creates a hydraulic gradient between the
aquifers and the adjacent clay beds resulting in
the leakage of the pore water from the adjacent
clay bed. This draining out of water reduces pore
pressure of the clay bed which then undergoes con-
solidation. The inelastic compression, leading to
vertical shortening of the confining beds or com-
paction of the clay or silty beds occurring within
the confined aquifers, has a greater role in land sub-
sidence than elastic compression. Sikdar et al (1996),
Bhattacharyya et al (2004) and Bhattacharyya
and Patra (2007) used Dominico’s (1972) equation
to calculate land subsidence. Now-a-days remote
sensing techniques are used to determine land sub-
sidence due to groundwater extraction. For exam-
ple, C-band ERS-1/2 and ENVISAT radar images
have been applied to investigate the urban subsi-
dence due to groundwater abstraction. The Persis-
tent Scatterer InSAR results are interpreted and
compared to investigate the effect of groundwater
extraction to urban subsidence. The GIS software
is used to interpret the Persistent Scatterer InSAR
results (Chang et al 2005). Sneed et al (2001)
have used global positioning system (GPS) and
Threat of land subsidence in and around Kolkata and EKW 437
interferometric synthetic aperture radar data to
estimate land subsidence due to groundwater
abstraction. Hirose et al (2001) have applied digi-
tal elevation modelling derived from interferogram
using JERS-1/SAR L-band data acquired from
1992 to 1998 to estimate land subsidence. The sub-
sidence data thus acquired was verified by level
surveying using GPS. Chatterjee et al (2007) have
used differential synthetic aperture radar interfer-
ometry (D-InSAR). D-InSAR data processing was
carried out using DIAPASON software developed
by the French Space Agency (CNES). The prob-
lems of identifying and separating the deformation
fringes due to temporal decorrelation in the data
pairs and atmospheric effects was rectified by fil-
tering the noisy interferograms using an adaptive
filter.
Extensive work carried out by Biswas and Saha
(1985, 1986, 1992) and Biswas (1990) highlighted
the progressive decline of the piezometric head of
groundwater in central and south-central parts of
Kolkata. Their observation, for the first time, indi-
cated that the progressive decline of the piezo-
metric head of this area may pose problems of land
subsidence in future.
Sikdar et al (1996) found that during 1956–
1993, the decline of piezometric level was high in
Gobra–Tiljala area with a magnitude of 8.29 m.
However, during 1993–1999, it was of the order
of around 1 m as obtained from annual reports of
the Central Ground Water Board. In sharp con-
trast to this, in Kasba–Gariahat–Dhakuria region
of south Kolkata, the piezometric drop was found
to be 6.82 m during 1958–1994 (Sikdar et al 1996),
whereas the piezometric level changes from 9.07 m
below ground level (bgl) in April 1994 to 15.18 m
bgl in April 1999 in the same region and hence
a total drop of 12.93 m in piezometric level is
observed during 1958–1999 in this region of south
Kolkata (Bhattacharyya et al 2004; Bhattacharyya
and Patra 2007). It should be noted that the pre-
monsoon month April is chosen as the reference
month for comparison and this is also in accor-
dance with the recent literature on land subsidence
(Agarwal 2002) which states that land subsidence
generally occurs in the pre-monsoon period when
the water table was the deepest and recharge to
groundwater is the least or negligible.
Sikdar et al (1996) reported that due to
higher rate of groundwater withdrawal, land sub-
sidence rate in Kolkata ranges from 7.14 to
13.78 mm/year. The maximum subsidence was
estimated in Gobra–Tiljala area in south-central
Kolkata, where the subsidence is estimated to be
more than 13 mm/year. The cumulative subsidence
between 1956 and 1993 in this area works out to
be 0.51 m. The total and the average annual rate
of subsidence were also worked out for a period of
42 years from 1958 to 2000 by Bhattacharyya et al
(2004). According to this study, apart from exces-
sive subsidence rate of 20.46 mm/year at Salt Lake
area which consists of landfills, the subsidence
rate usually varies from 6.52 to 13.0 mm/year.
Land subsidence rate measured in Ultadanga is
about 18 mm/year and its effect is exhibited as
prominent cracks found in some buildings in some
blocks of Salt Lake (Bhattacharyya and Patra
2007). Chatterjee et al (2007) have identified areas
in northern part of Kolkata in and around Machhua
Bazar, Calcutta University and Raja Bazar Science
College, which had been undergoing subsidence
during the period of observation from 1992 to 1998
with an estimated rate of 5 to 6.5 mm/year. There-
fore, it is scary to visualize a scenario of Kolkata
City with damaged buildings, subsiding roads
and leaning high-rise apartments. Any attempt
to estimate the possible land subsidence rate of
Kolkata City and EKW should be done against this
backdrop.
This paper attempts to assess the possible men-
ace of land subsidence not only of Kolkata and
Salt Lake cities – the main business, commercial
and financial hub of eastern India and the north-
eastern states of India, but also of East Kolkata
Wetlands (EKW) – a freshwater peri-urban inland
wetland ecosystem located at the lower part of the
deltaic alluvial plain of South Bengal basin and
east of Kolkata City (figure 1). Salt Lake City
covering an area of 12.35 km2was a low-lying
saucer shaped basin and a conglomerate of sev-
eral salt lakes, which were lower than the adjoin-
ing drainage channels by an average of 3 m. This
swampy area was filled by dredged silt and fine
sand from the bed of Hooghly River. The EKW
is well known for its resource recovery systems,
developed by local people through the ages, using
wastewater from the city. EKW was declared as a
Ramsar site (No. 1208) on 19 August 2002 (source:
www.ramsar.org/brochure.html), therefore attain-
ing an international status. Freshwater wetlands
are characterized by a hydrological cycle in which
the groundwater within the wetland may be recy-
cled from local and/or distant areas and will com-
prise varying quantities of water derived from
precipitation, surface water bodies and return flow
of groundwater used for irrigation in nearby irri-
gated land. Many wetland hydrology studies have
described the wetland water table or developed an
annual water budget (Owen 1995; Cooper et al
1998) to establish the link between wetland hydro-
logy and ecology (Drexler et al 1999) or have
utilized hydrogeology and isotope composition of
groundwater to understand the hydrological pro-
cesses prevalent in the wetland (Sikdar and Sahu
2009). These studies often fail to represent the
threat of land subsidence due to ever-increasing
438 P Sahu and P K Sikdar
Figure 1. Spatial distribution of calculated land subsidence rate in and around Kolkata, Salt Lake City and EKW.
human-induced pressure on groundwater for drink-
ing, agricultural and industrial purposes.
Abstraction of groundwater from areas prone to
land subsidence may cause adverse environmen-
tal impact on the wetland’s ecosystem. Appropri-
ate site selection for well fields of newly-planned
urban centres and industrial estates within close
vicinity of any major wetland is a strong pollu-
tion preventive measure that ensures environmen-
tal soundness of any developmental programme.
Therefore, it is imperative to estimate the possi-
ble rate of land subsidence and chalk out a sus-
tainable groundwater management plan based on
groundwater potential and estimated rate of land
subsidence of different areas, especially in a frag-
ile wetland to minimize the adverse environmental
impacts of groundwater development.
This study, for the first time, indicates the threat
of possible land subsidence due to unrestricted
groundwater abstraction in the EKW and also out-
lines a groundwater management plan for sustain-
able development of groundwater in an important
fresh water wetland ecosystem of eastern India.
This study also compares the rate of land subsi-
dence calculated by different workers for various
parts of Kolkata and Salt Lake with this study.
2. Methodology
Consolidation of clay as a result of dissipation of
pore water pressure is the major reason of land sub-
sidence caused by groundwater pumping. There-
fore, land subsidence depends on the subsurface
geological profile. The subsurface geology of the
area in and around EKW is completely blanketed
by the Quaternary fluviatile sediments comprising
a succession of silty clay, sand and sand mixed with
occasional gravel. In some places along with silty
clay, sticky clay is also present at the top of the
lithological column. Tertiary clay/silty clay under-
lies this Quaternary sequence at an average depth
of 296 m (Sikdar 2000; Mukherjee et al 2007). This
Threat of land subsidence in and around Kolkata and EKW 439
formation continues up to a depth of at least 614 m
below the ground surface. Therefore, the Quater-
nary aquifer of the area is sandwiched between two
aquitards made of silty clay/clay and is more or less
continuous in nature. A fence diagram (figure 2),
correlating the lithologs of favourably located
boreholes and vertical electrical sounding data
shows a three-dimensional view of the subsurface
disposition of the sediments underlying the area.
This fence diagram reveals that the top silty clay
bed is underlain by a sandy sequence grading from
fine to coarse. Thin intercalations of silty clay are
also present within the sandy sequence. In the west-
ern part of the area, top silty clay bed is very thick,
generally 40 m and above. But the silty clay layer
is conspicuously absent at few places in east, north,
south and central parts of the area due to the
scouring action of earlier channels. In these areas,
sands of various grades are observed throughout
the entire geological column.
Figure 2. Fence diagram depicting the subsurface geology of the area.
Tabl e 2 . Stratification of normal Kolkata deposit (after Dastidar and Ghosh 1967).
Coefficient of volume
Depth compressibility,
range (m) Description mv(cm2/kg)
0–5 Firm grey silty clay 0.014
5–15 Soft grey clay with wood stumps 0.04
15–20 Bluish grey clay with kankar 0.01
20–25 Laminated brown clay, silt 0.01
25–30 Stiff mottled grey and yellow clay with kankar 0.01
>30 Mottled silty clay laminated with parting of –
golden brown silty sand
440 P Sahu and P K Sikdar
Tabl e 3 . Estimated amount of land subsidence.
Decline of Estimated land subsidence
piezometric Top clay Total Per year Per meter drop in
Sl. no. Location Period head (m) thickness (m) (m) (mm) piezometric level (cm)
1 Pratap Nagar, Bhangar-I 2001–2005 0.28 8 0.005 1.12 1.79
2 Salt Lake Sector-III 1981–2004 2.06 40 0.062 2.69 3.00
3 Santoshpur–Garfa 1981–2005 9.61 26 0.29 12.01 3.02
4 Salt Lake 2001–2004 1.64 32 0.05 16.40 3.05
5 Bagbazar 2001–2005 0.79 23 0.024 5.93 3.04
6 Jadavpur–Babubagan 1994–2005 2.40 18 0.072 6.55 3.00
7 Tiljala–Picnic Garden 2001–2005 1.96 18 0.059 14.70 3.01
8 Bantala 2001–2005 4.23 40 0.127 31.70 3.00
9 Rajarhat–Salangari 2001–2005 8.76 10 0.175 43.80 1.99
10 Kashipur–Dakshin–Kashipur 2001–2005 1.34 20 0.040 10.05 2.99
11 Nayabad, Sonarpur 2001–2005 4.72 10 0.094 23.60 1.99
12 Tiljala–Tangra 2001–2005 1.04 50 0.031 7.80 2.98
13 Ultadanga–Maniktala–Narkeldanga 1956–2005 8.32 40 0.249 5.09 2.99
14 Beleghata 1956–2005 9.71 40 0.291 5.94 3.00
15 Gobra–Tiljala 1956–2005 10.96 50 1.096 22.30 10.00
16 Kasba 1958–2005 8.91 18 0.321 6.80 3.60
Tabl e 4 . Classification of land sub-
sidence rate.
Land subsidence
rate (mm/year) Class
>20 Very high
15–20 High
10–15 Medium
5–10 Low
0–5 Very low
The elastic deformation of the aquifer is insignif-
icant compared to that of the inelastic deforma-
tion. This is because, as the piezometric head
lowers, the aquifer is recharged with water due to
(i) the expansion of water molecules remained in
the aquifer, (ii) the compression of aquifer matrix,
and (iii) the compression of clay layers above and
below the aquifer. Therefore, in this paper, land
subsidence only due to inelastic compression of the
confining bed has been determined numerically by
applying the consolidation theory widely used in
soil mechanics.
In this paper, the vertical settlement for one-
dimensional strain during consolidation has been
calculated by using Dominico’s (1972) equation
which is as follows:
C=mv·Δσ·Z·H,
where C= amount of land subsidence (m), mv=
coefficient of volume decrease (cm2/kg), Δσ=
effective stress per metre fall of piezometric head
(kg/cm2), Z= drop in piezometric level (m) and
H= thickness of the confining bed (m).
The coefficient of volume compressibility (mv)of
the top aquitard (table 2) used in this study has
been taken from the work of Dastidar and Ghosh
(1967). The mvvalue represents the compressibil-
ity of a stratum as expressed by the volumetric
strain per unit increase of effective stress. A perusal
of table 2 reveals that the top 15 m of the subsur-
face formation is relatively soft whereas a low mv
value of clay layer between 15 and 30 m indicates
its comparatively stiff nature. Below 30 m depth,
the compressibility of the layers is even less than
0.01 cm2/kg due to increase in overburden pres-
sure. Therefore, volume of pore water squeezed out
from the layer decreases with depth. This results
in increase in pore water pressure due to increased
loading, and results in a time lag between actual
settlement of the layer and fall in the piezomet-
ric surface. This time lag depends on the hydraulic
conductivity and/or thickness of the layer. If the
layer is of low hydraulic conductivity and rela-
tively thick, which is the case for Kolkata, it will
take some time before this excess pore water is
Threat of land subsidence in and around Kolkata and EKW 441
squeezed out and hence land subsidence to occur.
So clay/other lithotypes present below 30 m, do
not play any significant role in land subsidence
and therefore, are not taken into account during
land subsidence calculation. For Salt Lake City,
the 3 m thick filled material at the upper part of
the lithological column was not considered during
calculation of land subsidence.
In order to determine the subsidence, past water
level data were collected from various published
and unpublished papers and reports (Chaterji
et al 1964; Biswas and Saha 1986; Sikdar et al
1996; Bhattacharyya et al 2004; Bhattacharyya
and Patra 2007). About 85 deep wells were mon-
itored for water level variation during 2003–2005.
The monitoring wells of these earlier studies and
the current study are different and hence instead
of calculating the drop in piezometric head for
individual observation wells, it had been calculated
sector-wise for a period varying between 1956 and
2005 as available. The entire area has been divided
into 16 sectors. The arithmetic mean of the water
level of the deep wells of each sector was calcu-
lated for a particular year and assigned to the cen-
tre of the sector. This value represents the average
piezometric head of the sector. In this way, average
piezometric heads of all the 16 sectors for the differ-
ent time periods were assigned. The average fall in
the piezometric head was worked out (table 3) and
the value was assigned to the centre of each sector.
The coefficient of volume compressibility (mv)val-
ues of Dastidar and Ghosh (1967) for the top 30 m
depth of the stratigraphic column was assigned
according to the subsurface lithology available for
each sector. The vertical settlement was then calcu-
lated by applying the Dominico’s (1972) equation.
By numerical integration of these vertical settle-
ments, the total settlement of the ground in various
parts of the area was estimated (table 3).
3. Results and discussion
The estimated land subsidence varies between 1.12
and 43.8 mm/year (table 3) with a mean of
13.53 mm/year. The estimated land subsidence for
1 m drop in the piezometric head varies from 1.79
to 10 cm with an average of 3.28 cm. Categorization
of the rate of land subsidence is given in table 4.
The spatial distribution of the calculated land sub-
sidence is depicted in figure 1. A perusal of the map
indicates that maximum part of the area falls under
the category of medium land subsidence rate.
A comparative study of land subsidence rate
for a given period and their respective average
drop of piezometric head in parts of Kolkata and
surrounding area calculated by the present author
and other workers is summarized in table 5.
Tabl e 5 . Comparative study of land subsidence in parts of Kolkata and surrounding areas calculated by different workers at different times.
Present authors Bhattacharyya et al (2004) Sikdar et al (1996)
Average Calculated land Average Calculated land Average Calculated land
piezometric subsidence piezometric subsidence piezometric subsidence
Sl. no. Location Period head drop (m) (mm/year) Period head drop (m) (mm/year) Period head drop (m) (mm/year)
1 Tiljala 2001–2005 1.04 14.7 1956–1993 8.3 13.8 1956–1993 8.3 13.8
2 Kasba 1958–2005 8.91 6.8 1956–2000 10 9.2 1958–1994 6.8 8.2
3 Beleghata 1956–2005 9.71 5.9 – – – 1956–1993 7.3 12.2
4 Maniktala 1956–2005 8.32 5.1 – – – 1956–1993 6.3 10.5
5 Salt Lake 1981–2004 2.06 16.4 1956–2000 9.5 20.5 – – –
6 Tangra 2001–2005 1.04 7.8 – – – 1956–1993 7.3 12.2
7 Bagbazar 2001–2005 0.79 5.9 – – – 1958–1993 4.6 8.3
8 Jadavpur–Babubagan 1994–2005 2.40 6.6 – – – 1957–1993 6.1 10.6
9 Santoshpur–Garfa 1981–2005 9.61 12.0 1958–2000 11 6.5 – – –
10 Gariya – – – 1957–2000 7 7.4 1957–1993 6.1 10.6
442 P Sahu and P K Sikdar
Figure 3. Cross-section along AAshowing the position of
piezometric surface with respect to ground surface and
bottom surface of top silty clay layer.
The different values of land subsidence estimates
(table 5) by various authors are due to supply of
treated surface water in various parts of the city
by the government resulting in lesser groundwa-
ter pumping. Therefore, the piezometric head has
recovered in majority of the areas, and land subsi-
dence has slowed down. In areas such as Tiljala and
Santoshpur–Garfa, land subsidence has increased
because of the construction of new high-rise build-
ings with one or more tubewells fitted with medium
to heavy-duty motor pumps.
To understand the interrelation between the
depths of the piezometric surface in different sea-
sons and base of the top silty clay bed, two cross-
sections A-Aand B-B(figure 1) are drawn. These
sections, A-Aalong the east–west and B-Balong
the north–south, are shown in figures 3 and 4,
respectively.
These sections reveal that the groundwater
occurs in a confined condition with the piezomet-
ric head above the base of the top confining clay
bed. In areas such as Bantala and Bamanghata
(figure 1), the piezometric head is only 0.6 to
1.0 m (figure 3), and 1.6 m (figure 4) above the
base of the confining bed, respectively. Therefore,
groundwater abstraction in these areas should be
restricted. Excessive groundwater withdrawal from
Figure 4. Cross-section along BBshowing the position of
piezometric surface with respect to ground and bottom
surfaces of the top silty clay layer.
these parts of the aquifer will result in further drop
of the piezometric head. The piezometric head in
such a condition may ultimately fall below the base
of the confining bed, which will lead to change in
the aquifer condition from a confined system to an
unconfined one. This will result in loss of water
from the overlying aquitard into the underlying
aquifer. If this water contains toxic material, then
the underlying fresh water aquifer may get pol-
luted. Again loss of water from the aquitard will
result in volumetric compression of the aquitard,
which will be manifested at the surface in the form
of land subsidence.
From the above discussion, it is imperative that
groundwater of the cities and wetland areas should
be developed cautiously to avoid the threat of land
subsidence. Therefore, groundwater development
should be carried out based on the groundwater
potential and the perceived threat of land subsi-
dence in the area. Sahu and Sikdar (2009) have
categorized the area into five groundwater poten-
tial zones namely very poor, poor, medium, good
and excellent (figure 5). Detailed study ground-
water potential zones of Kolkata including Salt
Threat of land subsidence in and around Kolkata and EKW 443
Figure 5. Spatial distribution of groundwater potential of the area.
Lake and EKW is described in Sahu and Sikdar
(2009).
Land subsidence can have several negative eco-
nomic and social implications such as changes
in groundwater and surface water flow patterns,
restrictions on pumping in land subsidence-prone
areas, localized flooding, failure of well casings as
well as shearing of structures. To minimize such
environmental effects, groundwater management
should be considered in subsidence-prone areas
(Don et al 2006). To chalk out appropriate ground-
water management plans for sustainable develop-
ment of groundwater, a cross-operation between
spatial distribution of groundwater potential and
land subsidence rate has been carried out using
the GIS software ILWIS 3.3 Academic Version. The
resultant cross map (figure 6) of the entire area has
been divided into three classes and the manage-
ment options recommended for each of the classes
are shown in table 6.
4. Conclusion
The results described in this paper reveal that the
estimated land subsidence rate of the area ranges
between 1.1 and 43.8 mm/year with an average of
13.5 mm/year. Visible evidences of land subsidence
have not yet been recorded because of the pres-
ence of a thick layer of very low permeable mate-
rial at the top of the geological column. The
surface expression of the estimated land subsidence
is however, cryptic because of a time lag between
the settlement of clay/silty clay and its surface
expression. For example, although in Kolkata no
surface expression of land subsidence is reported,
the effect of land subsidence is exhibited as promi-
nent cracks have been found in a few buildings
in some blocks of Salt Lake (Bhattacharyya and
Patra 2007). The calculated land subsidence how-
ever needs ground verification by recalibrating the
reduced levels of the existing benchmarks. Again,
444 P Sahu and P K Sikdar
Figure 6. Cross map between land subsidence and quality-based groundwater potential of the area.
Table 6. Management plans for zones with different land subsidence rates and different groundwater potential.
Ground water Land subsidence
potential Very high to high Medium Poor to very poor
Excellent Groundwater abstraction should be Some ground water Further ground water
to good minimized and treated surface water abstraction possible abstraction possible
supply system to be introduced;
introduction of roof-top
rainwater harvesting.
Medium Groundwater abstraction should be Some ground water Further ground water
minimized and treated surface water abstraction possible abstraction possible
supply system to be introduced;
introduction of roof top rainwater
harvesting and artificial recharge.
Poor to Groundwater abstraction should be Status quo to Groundwater abstraction
very poor minimized and treated surface water be maintained should be minimized and
supply system to be introduced; treated surface water supply
introduction of roof top system to be introduced;
rainwater harvesting and introduction of roof top
artificial recharge. rainwater harvesting.
Threat of land subsidence in and around Kolkata and EKW 445
to measure the actual compaction of sediments,
compaction recorders or extensometers should be
installed in areas of heavy pumping. A piezome-
ter should be installed close to each extensometer
to record the variation in depth of the piezomet-
ric surface. This in turn would help to understand
the field relationship between the head decline
and land subsidence (Sikdar et al 1996). Develop-
ment of groundwater should be done cautiously to
avoid the threat of land subsidence. Groundwater
development should be carried out based on the
groundwater potential and the perceived threat of
land subsidence in the area.
Acknowledgements
The authors convey their thanks to the Director,
Indian Institute of Social Welfare and Business
Management for his help and encouragement dur-
ing the research work. PS is thankful to Council
of Scientific and Industrial Research (CSIR), India
for the financial assistance.
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MS received 29 June 2010; revised 2 February 2011; accepted 4 February 2011
