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Workshop on "Sustainable development and management of ground water in West Bengal and

future of irrigation vis-a-vis arsenic contamination in ground water” Central Ground Water Authority & Central Ground Water Board

Eastern Region, Kolkata-700 091 19th February 2013

A CASE STUDY ON THE CHANGING PATTERN OF WETLANDS IN WEST

BENGAL AND ITS POSSIBLE IMPACTS ON GROUNDWATER RESOURCES

Sayan Sinha, Lipi Kundu Basu, Anindya Halder* and Tapan Chakraborty

Geological Studies Unit, Indian statistical Institute, Kolkata-700108

INTRODUCTION

Water is the most valuable natural resource and is indispensable for all economic and social

development. However, with ever increasing population, industrial growth, water pollution and

atmospheric changes, the water availability per capita is shrinking day by day. Growing water

scarcity and increasing water demands are expected to reduce diversion of water for agriculture in the

future. Efforts are needed to utilize the available limited water resources efficiently. Water resources

not only provides water for irrigation but also provide water for a range of other productive uses such

as home gardens, livestock, fishing and aquatic products, and micro enterprises such as brick making.

In the lower gangetic plain of West Bengal, wetlands are used as multiple systems and have

significant impacts on livelihoods of the local people through the most important benefit arising from

fisheries as well as wetland cultivation and jute retting. We know that wetlands are the one of most

important natural source of water and it maintains the food chain, ecological balance and absorb

pollution. Wetlands are known to play important role in ground water recharge, flood control,

reduction of soil erosion and detoxifying surface water from many of its toxic and metal ion

concentration (Greb et al., 2006)But the wetlands are threatened by the continuous expansion of the

urbanization.

ROLE OF GROUND WATER IN WETLANDS:

Groundwater hydrology is fundamentally important to wetland science. The absence of groundwater

beneath a wetland, may prevent the build-up of saturated conditions at the surface and groundwater

inflow in many wetlands, and is a critical component of their budget. For most wetlands, the temporal

pattern of drainage to and from groundwater storage provides a key control on the water table regime.

Where upward groundwater flow occurs, water table can be maintains close to the wetland surface

even in times of droughts. In contrast, in the absence of groundwater flow, rapid water table draw

down can occur if precipitations fall during times of high evapotranspiration. Alternatively, a reduced

flux of water, comprising groundwater recharge from a wetland, can produce a rapid or sustained

water table fall. Ground water also tends to be chemically distinct from surface waters with higher

concentrations of many elements, particularly K+

and Ca2+, and lower concentration of phosphate in

comparison with other water sources (Wassen et al. 1990). Ground water fed wetlands therefore tend

to be more nutrient rich than those fed by solely surface water and may enable ground water fed

wetlands to support distinctive vegetation communities characterized by higher biomass and plant

height, in comparison with adjacent areas that do not receive ground water inflow.

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Fig.1 A schematic diagram illustrating the role of ground water storage and fluxes in the hydrology

of wetlands.

Ground fed wetlands may also be vulnerable to pollution where ground water receives recharge from

urban or intensively farmed areas. In urban areas, these pollutants can consist of heavy metal,

hydrocarbons and de-icing salts.

In this backdrop, we investigated the status of surface wetlands in West Bengal. Our investigation

was concentrated in the peri-urban areas of situated in the floodplains of Hoogly and the interfluves

regions between Damodor and Hoogly. We present here the preliminary findings of this

investigation.

WETLANDS IN HOOGLY-DAMODAR INTERFLUVE

LOCATION

SRTM section across in the area between the Damodar-Darakeswar River in the west and Hoogly

River in the east is a topographically low-lying area with depressions up to 13 m deep. The area

dominantly comprises of interfluves channels (like Saraswati R., now completely covered up by

anthropogenic activity), levees of the main channels and large bodies of permanently waterlogged

marshes. The bottom of the depressed areas lies close to the mean sea level or very close to the water

level in the Hoogly River. In the field the 2.5 m thick dug sections near Dankuni Railway Station

(22°40’06.06”N 88°17’49.74” E) show multiple layers of grey hydromorphic soils and peat layers

alternating with silty fine sand indicating long standing position of these areas close to water table.

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Silty sand layers probably denote periodic records of flood from flanking interfluve channels. Based

on micro-topographic analysis, shallow boreholes, resistivity logs Geological Survey of India has

also reported thick units of carbonaceous mud, coaly layers, peats and extensive wetlands in

Dankuni-Hindmotor area (unpublished report GSI, 1973, 2000).

Fig.2 A SRTM DEM visualization of the study area. A section through the study area shows a

depressed area that forms large wetland bodies. Note the cross section of the Hoogly River and the

base of the wetland are in the same level.

Ground investigations and Google Earth images indicate that filling up of more than 2 m deep water

bodies continues unabated in this region.

Fig.4 2 m section in the Markardah site showing fly-ash horizon as a filled material used for the

filling up the wetlands.

Fig.3 Two

historical images

of Google Earth

showing filling

up the wetlands

in the Dankuni

area.

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Fig.5.1 Google Earth image

of Kalyani Township.

Fig.5 Google Earth image of the

state of West Bengal

Fig.5.2 Google Earth image

of Dankuni-Baidyabati

region.

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Fig. 6. Classified images of the study area in the year of 2000 and 2010 in the Dankuni-Baidyabati

region. Classified image 2010 shows the remarkable changes of land use in this area.

In order to estimate the loss of wetlands, optical signals in the imageries were divided into 48

different classes based on the tonal character and texture of the images. These classes can be broadly

grouped into 4 categories namely the urban habitat, the rural habitat, agricultural fields, rivers-

waterbodies-wetlands. Supervised classification of a temporal succession of LANDSAT images

using these broad classes was performed using the standard methods available in ERDAS software.

Analysis of the satellite image of 2010 shows that of this 17.81% of the land is covered by the

wetlands and water bodies, while agricultural land comprises 34.85%, urban and rural habitation

comprises 14.34 and 32.99% of the area respectively. Similar analysis carried on LANDSAT image

of 2000 shows that the area of wetland in the same region was 22.79%, and agricultural land, urban

habitation and rural habitation comprises 22.66%, 12.43% and 42.11%. Thus a conservative estimate

of change indicates that during this period we have loosed 43.684 km2 (or 4368.4 hectare) of wetland.

This loss is about 21.74% of the wetland we had in this area in the year 2000 (Fig. 6).

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Fig. 7: Changing land character near Baidyabati Fig. 8: Changing land character near

NH2

Figure 7. shows that the wetland areas are converted to agricultural land. Rice, potato, oil seeds are

main cultivable crops of this area. Whereas Figure 8. shows that the Wetland areas are converted to

small scale industrial hub. Small-boats (Donga, local name) are of common use vehicle for

transportation purposes.

DEGREDETION OF WETLANDS IN HOOGLY ACTIVE FLOODPLAIN

The Kalyani township (22.9800° N, 88.4800° E) stretch across the eastern margin of the Hoogly

river. This area situated in the active floodplain of the Hoogly river. Different floodplain features like

meander scrolls, meander cut-off, ox bow lake, small water bodies are dominant characters in this

region. The distance between Hoogly river and Kalyani township is just 1.5 km. The bottom of the

low lying areas lies close to the mean sea level or very close to the water level in the Hoogly river.

Urbanization had been growing at very high speed at this region. The use of groundwater is

increasing every day. Small waterlogged lands were being encroached for urban constructions. The

groundwater level also decreases at alarming rate. The impacts of ongoing or proposed such activities

will negatively affect the wetland functions and related sustainable maintenance of the subsurface

groundwater aqifers. The conversion of wetlands to other uses is not sustainable.

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-

Fig.9 Geomorphic map of kalyani region in1990

Fig.10 Geomorphic map of kalyani region in 2010

Urban Settlement 1990

Legend

Side Bar

Main Channel

Meander Cutoff

Mid Channel Bar

Ox Bow Lake

Water Body

Floodplain

Meander Scrolls

Legend

Side Bar

Main Channel

Meander Cutoff

Mid Channel Bar

Ox Bow Lake

Urban Settlement 2010

Water Body

Floodplain

Meander Scrolls

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In the year 1990 kalyani township contained only 4.22 sq. km. area approximately. Whereas 2010 the

town ship comprehend 17.69 sq. km approximately. Between these twenty years 13.47 sq. km. area

added to the township.

We are trying to develop a new methodology borrowing the idea of pattern recognition for detection

of changes in the the landform in this area. The tentative idea is discuss below….

PROPOSED CHANGE DETECTION APPROACH:

Let X1 and X2 be two co-registered multispectral remotely sensed images of size P×Q acquired on the

same geographical area at different times t1 and t2. Let B be the number of spectral channels of each

image. The proposed technique consists of two main steps (see Figure. 11.). (i) Jeneration of input

pattern and (ii) Clustering.

(i) Generation of input pattern:

The multispectral difference image X∆ is according to the well known and widely used change

vector analysis technique by computing the vector difference of images X1 and X2. Consider the gray

level values associated with (i,j) pixel in band b are X1b (i,j) and X2b (i,j)for date t1 and t2,

respectively. The difference gray value X∆b (i,j) in band b associated with (i,j) pixel is computed as

X∆b(i,j) = X2b (i,j) - X1b (i,j) i,j ; and b = 1,2,…….B. (1)

Hence any multisprectral difference vector (i,j) corresponding to the (i,j) pixel is given

by

(I,j) = { X∆b (i,j) b = 1,2,….B} (2)

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will be considered a pattern for the clustering step.

(ii) Clustering:

From the previous step we got the multispectral difference vector X, i, j, corresponding

to the each pixel (i,j) and this set of multispectral difference vector will constitute set of

patterns for the clustering step. Here appropriate clustering technique will be followed to

jenerate two clusters. (a) corresponding to change class (wetland changes) and (b)

cooresponding to unchanged class (no wetland change).

Fig.12. Different Image (Band_1) HOOGLY-DAMODAR INTERFLUVE

REFERENCES:

1. Mitz and Grosselink (1986) wetland. Van Nostrand Reinhold, New York.

2. Greb et al (2006) Geological Soc America Sp Publ.399, 1-40.

3. IUCN (1990) Directory of wetlands of International importance. Ramsar Convention Bureau,

Gland Switzerland, 799 pp.

4. Ghosh D and Sen S (1987) Ecological history of Kolkata, Wetland Conversion,

Environmental Conservation, 14,219-225.

5. Gilvear. D.J. and Bradley.C. , 2009. Hydrological Dynamics II :Goundwater and

Hydrological Connectivity. Chap. 7 Maltby and T. Barker (ed.) The Wetland handbook.1st ed

Blackwell Publishing, pp.1058.

6. Singh, A., 1989. Digital change detection techniques using remotely sensed data.

International Joural of Remote Sensing 10 (6). 989-1003

7. Roy Chowdhury S. (1973) A Report On The Geomorphology And Geology Of Quaternary

Alluvium In The Singur Magra Tract Hoogly Dist, West-Bengal. Unpublished report

Geological Survey of India No. UE4316

8. Author unknown (2000) Geoenvironmental,Geohydrological And Geotechnical Appraisal Of

Bali-Dankuni Area,District Hugli,West Bengal. Unpublished report of Geological Survey of

India No. UE 13326.

Baidyabati

Dankuni

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