Integrated Geophysical and Chemical Study of Saline Water Intrusion

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Integrated Geophysical and Chemical Study of Saline Water Intrusion

Saline water intrusion in many coastal areas has

resulted in the contamination of ground water and consequently

environmental problems (Ginsberg and Levanton

1976; Frohlich et al. 1994). Ground water abstraction intensifies

migration of contaminants to the subsurface, activates

salt water encroachment into pumped aquifers from neighboring

ones, and sea water intrusion into coastal wells (Kalimas

and Gregorauskas 2002). Todd (1959) indicated that

the ratio of chloride and bicarbonate ions in ground water is

directly related to the extent of sea water intrusion in coastal

aquifers. Yechieli (2000) studied the interface between fresh

and saline water in the Dead Sea area using in situ profiles

of electrical conductivity (EC) of water. Nowrooji et al.

(1999) opined that the resistivity sounding method is a powerful

tool for delineating the fresh water/salt water interface

in the eastern shore of Virginia and mapped the subsurface

zones intruded by saline water. Albouy et al. (2001)

described the utility of both electrical resistivity and electromagnetic

methods for coastal ground water studies because

of the large contrast in resistivity between fresh water-bearing

and saline water-bearing formations.

In this paper, geophysical resistivity studies and chemical

analyses of ground water for Na + , Mg +2 , Cl – , EC, total

dissolved solids (TDS), and Cl – /HCO – 3 from different tube

wells were carried out in the Digha-Shankarpur coastal

tract of India (Figure1) where two tube wells had abnormally

high TDS of 1400 ppm and chloride content of 360

to 380 mg/L. The aim of the research was to assess the utility

of data integration for delineating the regions contaminated

by saline water, as well as to demarcate areas or

possible channels through which mixing of saline water

Figure 1. Layout map of Digha-Shankarpur area, West Bengal, India.

672

K. Choudhury, D.K. Saha GROUND WATER 42, no. 5: 671–677

and fresh water was taking place. Another important objective

was to delineate the areas suitable for ground water

development. Seventy-five electrical resistivity soundings

were carried out in the Digha-Shankarpur coastal belt of

West Bengal (Figure 1) to determine the resistivity variation

in the vertical downward direction up to a depth of

~300 to 350 m. Resistivity profiling for 4 km was also carried

out to study the variation of resistivity along horizontal

profiles at different depths that could be correlated with

saline water intrusion. In addition, periodic chemical analyses

of ground water samples were carried out to constrain

the resistivity interpretation and distinguish the effects of

lithology from water quality.

Hydrogeology and Physical Setting

Except for a few sand dunes, the area is more or less

flat with a gentle slope toward the sea and forms part of the

vast alluvial tract of the Bengal Basin. The shoreline was

formed from reworked Upper Tertiary Age unconsolidated

clay, silt, and sand deposited in the Recent Age. Singhal

(1963) reported the presence of scattered saline water pockets

in the area. In the recent past, the sea started advancing

toward the land, endangering the township of Digha

(Niyogi and Chakraborty 1966). Goswami and Bose (1981)

classified the coastal tract into several geomorphologic

groups such as active/abandoned/inactive marine coastal

plain and alluvial upland of fluvial origin.

The annual rainfall in the area generally ranges from

1400 to 1600 mm, the major portion of which occurs

between June and October. On a regional scale, shallow

Location of VES Points

Location of Resistivity Profiling Points

Geoelectrical Section

Location of Abandoned Dug Well

81° 37�

Centre of Seismic Profile

Canal

Road

Coast Line

Location of Tube Well

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