Integrated Geophysical and Chemical Study of Saline Water Intrusion

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Integrated Geophysical and Chemical Study of Saline Water Intrusion

Integrated Geophysical and Chemical

Study of Saline Water Intrusion

by Kalpan Choudhury 1 and D.K. Saha 2

Abstract

Surface geophysical surveys provide an effective way to image the subsurface and the ground water zone without

a large number of observation wells. DC resistivity sounding generally identifies the subsurface formations—the

aquifer zone as well as the formations saturated with saline/brackish water. However, the method has serious ambiguities

in distinguishing the geological formations of similar resistivities such as saline sand and saline clay, or water

quality such as fresh or saline, in a low resistivity formation. In order to minimize the ambiguity and ascertain the

efficacy of data integration techniques in ground water and saline contamination studies, a combined geophysical survey

and periodic chemical analysis of ground water were carried out employing DC resistivity profiling, resistivity

sounding, and shallow seismic refraction methods. By constraining resistivity interpretation with inputs from seismic

refraction and chemical analysis, the data integration study proved to be a powerful method for identification of the

subsurface formations, ground water zones, the subsurface saline/brackish water zones, and the probable mode and

cause of saline water intrusion in an inland aquifer. A case study presented here illustrates these principles. Resistivity

sounding alone had earlier failed to identify the different formations in the saline environment. Data integration

and resistivity interpretation constrained by water quality analysis led to a new concept of minimum resistivity for

ground water-bearing zones, which is the optimum value of resistivity of a subsurface formation in an area below

which ground water contained in it is saline/brackish and unsuitable for drinking.

Introduction

Geophysical resistivity surveys are regularly used for

studies related to ground water investigations. Resistivity

profiling delineates the lateral changes in resistivity that can

be correlated with steeply dipping interfaces between two

geological formations in the subsurface. DC resistivity

sounding determines the thickness and resistivity of different

horizontal or low dipping subsurface layers including

the aquifer zone. However, there are some serious limitations

in such investigations as they fail to distinguish

between formations of similar resistivities such as saline

clay and saline sand, and the causes of low resistivity due to

1 Geophysics Division, Northern Region, Geophysical Survey of

India, Lucknow, 226024, India; fax 91–0522–2370467; kalpan_gsi

@yahoo.com

2 Central Geophysics Division, Geological Survey of India, 27,

J.L. Nehru Road, 4th Floor, Calcutta–700016, India; fax

91–2249–6956; gsicgd@cal2.vsnl.net.in

Received May 2002, accepted September 2003.

Published in 2004 by the National Ground Water Association.

water quality (fresh or saline). Ambiguity regarding low

resistivity also arises from the enhanced mobility of ions in

areas of high geothermal activity. Scale limitations involving

electrode spacings, depth of investigation, and required

resolution is also a drawback for resistivity soundings.

Again, some combination of resistivity and thickness of

subsurface formations can produce an identical anomaly

and hence give rise to ambiguity. An integration of geophysical

methods (seismic and resistivity) combined with

data interpretation largely resolves the uncertainty. Chemical

analyses of ground water samples are helpful in studying

the hydrogeological conditions and saline contamination of

aquifer zones. This also discriminates between the lithology

and water quality effects when the two cannot be differentiated

by a resistivity survey alone. The objective of the present

research was to examine the utility of integration of

data obtained from different geophysical methods and

chemical analyses of water samples for ground water and

saline contamination studies. The geophysical methods

selected for the study were DC resistivity sounding, resistivity

profiling, and seismic refraction. An example is discussed

to illustrate the utility of such an approach.

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Vol. 42, No. 5—GROUND WATER—September–October 2004 (pages 671–677)


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.

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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


ground water generally occurs under water table conditions

in the depth range of 6 to 12 m below ground level (bgl).

However, in the study area, the depth to the water surface

of the shallow ground water zone varies from < 1 m to > 4

m. The water is potable, but often dries out in summer. A

number of shallow tube wells operate on the beach during

the postmonsoon period. The deeper aquifer between 130

to 200 m bgl is being tapped for drinking and irrigation purposes.

Ground water in the area occurs within the sand layers

grading from fine to coarse. There are eight tube wells

operating for ~10 hrs/day in the study area, five of which

are close to the Digha Beach. The total amount of ground

water withdrawal in the area is ~7.2 million L/day. The producing

tube well B/5 and the other, ~70 m to its northeast,

yielded saline water during the summer months. Excessive

withdrawal of ground water had given rise to saline water

contamination.

Data Acquisition and Integrated Interpretation

Vertical electrical soundings (VES) employing a

Schlumberger configuration were conducted in the area

using a powerful transmitter (3 kW) with a normal operating

range of 2 to 3 amperes and a precision receiver unit

capable of measuring signals in microvolts. (Both instruments

are manufactured by Scintrex, Canada.) In the present

VES technique, direct current is injected into the

ground through a pair of current electrodes and the resulting

potential difference between two other intermediate

points is measured using nonpolarized electrodes after neutralizing

the self-potential. With increasing separation

between the source and the nonpolarized electrodes, apparent

resistivity values contain more information about the

deeper layers. In the present survey, a Schlumberger electrode

configuration with maximum current electrode separation

of 2 km was employed. An analysis of apparent

resistivity variation with change in electrode spacing helps

to deduce the depth and resistivity distribution of various

subsurface units, which, in turn, are interpreted in terms of

various geological formations. Based on parametric sounding

and velocity refraction surveys conducted near some

tube wells where the lithology is known, litho-resistivity

and litho-velocity relationships were obtained (Table 1).

The findings are largely similar to the one obtained earlier

by the present authors in the urban delta west of the Calcutta

megacity (Choudhury et al. 2000). The Schlumberger

VES curves were interpreted first by master curves (Orellana

and Mooney 1966) and subsequently by inversion and

the very fast simulated annealing technique (Shalivahan

2000).

The EC of the ground water samples collected from

locations near the sounding points was measured with the

help of a portable conductivity meter for estimation of

salinity of ground water, as well as for the interpretation of

resistivity data. The EC of ground water from the shallow

abandoned dug wells (where saline water is present) varies

from 3.40 to 3.60 mmhos/cm which indicates the saline

nature of at least some of the shallow subsurface waterbearing

zones. The EC of the deep aquifer ranges between

1.200 and 1.260 mmhos/cm. As the variation in EC of

ground water in different areas is insignificant, it can be

assumed that variation in resistivity is mainly due to variation

in lithology. This is a significant inference derived

from the analyses of water samples and shows the utility of

data integration. Resistivity profiling was carried out along

three traverses with a Wenner array having three spacings

of 50, 100, and 200 m. In the array, four electrodes with

equal distance between two adjacent electrodes are used.

The whole array is moved along the profile to measure the

apparent resistivity at various locations along the profile,

which essentially indicates lateral variation in resistivity

caused by the different geological formations. The higher

the spacing between two consecutive electrodes, the greater

the probing depth. The seismic refraction survey is based

on the measurement of the travel time of seismic waves

refracted at the interfaces between subsurface formations of

different velocity. Seismic energy is generated at the shot

point, travels downward, and then moves along the higher

velocity layers before returning to the surface. This energy

is detected at the surface using geophones. The depth profile

of the refractor can be found from the observed travel

times and shot geophone distances of the refracted signals.

Resistivity Sounding and

Seismic Data Integration

A limited seismic survey carried out in the area could

distinguish between saline clay and saline sand, which otherwise

was not possible by resistivity techniques alone.

Seismic refraction profiling in the area mostly indicated

three subsurface layers with velocities of the order of 880,

1480, and 2550 m/sec representing unconsolidated sand,

saturated sand, and clay, respectively. A classic example of

data integration and combined geophysical interpretation of

subsurface formations using seismic refraction and resistivity

soundings is illustrated in Figure 2. The resistivity

Table 1

Relationship Between Lithology and TDS of Ground Water, Resistivity, Conductivity, and Velocity

Mean TDS of Resistivity Conductivity Velocity

Lithology Ground Water (ppm) (� m) (mmhos/cm) (m/sec)

Saline/brackish water zone 1800 1.1–4 9.09–2.50 —

Saturated clay/silt 4–7 2.50–1.43 2400–2500

Saturated silty/clayey sand/fine sand 310 7–17 1.43–0.56 1600–1850

Saturated predominantly medium/coarse sand above 17 < 0.56 1450–1600

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

673


sounding VES–5 indicated a thick low resistivity zone of

1.6 Ω m below the unsaturated sand layer, which could be

due to saline clay or saline sand (Figures 2a and 2c). A seismic

refraction survey in the area resolved this zone into two

distinct velocity layers of 1488 and 2545 m/sec, which represent

saturated sand and saturated clay, respectively (Figures

2b and 2d). The combined interpretation is shown in

Figure 2e, where the resistivity interpretation is constrained

by seismic survey results. Interpreted depths from most of

such seismic surveys were used as a priori information for

precisely interpreting resistivity sounding data using the

inversion method. Depths of the various geological formations

inferred from such data integration tallied well with

the lithologs obtained from tube wells. Thus, such an integrated

approach of resistivity and seismic surveys can be

effectively adopted for delineating the subsurface formations

and the saline water zones.

Integration of Resistivity

Sounding and Chemical Data

Three geoelectrical cross sections (AA�, BB�, and

CC�) were prepared for the area from the interpretation of

resistivity sounding results duly constrained by analyses of

water samples. Only one section along the beach is presented

here to illustrate the nature of such integrated data

interpretation for identifying sea water intrusion. The E-W

interpreted section AA� (Figures 1 and 3) along the beach

of the Shankarpur area showed a thick saline zone in the

shallow subsurface. The saline zone having a resistivity

value of 3.8 Ω m or less starts right at the surface and continues

to a depth of 100 m, which indicates the intrusion of

saline water even at greater depth toward the east. Fine sand

saturated with fresh water is interpreted at varying depths in

674

h2 = 22.1 m.

Figure 2. Integrated geophysical interpretation using resistivity

sounding and seismic refraction survey. ρ 1 , ρ 2 , and ρ 3

are the interpreted resistivities of the first, second, and third

layer, respectively. V1, V2, and V3 represent the corresponding

velocities.

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

the area and constitutes the aquifer zone, which has a resistivity

between 10 and 15 Ω m (Figure 3).

Integration of Resistivity

Profiling and Chemical Data

High TDS (1400 ppm) and chloride content (360

mg/L) of ground water in tube well B/5 were found by our

chemical analysis of ground water samples from the tube

well. Resistivity profiling (PR1) with adjacent electrode

separations of 50, 100, and 200 m were carried out near

tube well B/5 (Figures 1 and 4) to ascertain the nature and

mode of saline water intrusion. All the arrays brought out a

conductive zone (1.0 to 3.5 Ω m) in the central part of the

profile. The lowering of resistivity was due to the intrusion

of saline water from the adjacent formations as was confirmed

by the high TDS and chloride content in the water

samples of tube well B/5. In all probability, high pumping

of the tube wells in the summer months caused such a steep

rise. The ambiguity associated with the cause of low resistivity

has been nicely resolved by the chemical analyses of

ground water, which indicated high chloride content to be

the contributing factor.

Another resistivity profiling (PR2) with a Wenner configuration

was carried out near the Champa sea water canal

in the northern part of the study area with electrode spacing

of 50, 100, and 200 m (Figures 1 and 4). All the spacing

indicated a conductive zone (1.3 to 3 Ω m), which is interpreted

to be due to saline water percolation from the adjacent

sea water canal. This has been confirmed by the high

concentration of Cl – (450 mg/L) in ground water samples.

Thus, there is a good correlation between sea water intrusion

and the resistivity profiles in both areas. We recommend

such surveys for any study related to saline water intrusion.

Resistivity Contour Maps

and Integrated Interpretation

One of the objectives of the study was to apply the data

integration technique to determine the lower limit of resistivity

of the ground water zone below which water cannot

Figure 3. Interpreted geoelectrical section along AA� (Figure

1).


Figure 4. Resistivity profiles PR1 and PR2 showing the low

resistivity zones caused by saline contamination.

be used for drinking. Ground water having a maximum

TDS value of 1000 ppm is considered to be of optimum

acceptable quality (Klimentov 1983; Todd 1959). Further,

it is known that the higher the value of TDS of ground

water in an aquifer zone, the lower its resistivity. In any

alluvial terrain of subsurface sand and clay, fine grain sand

shows the minimum resistivity among all the aquifer materials.

Thus, the fine grain sand formation containing high

TDS (1000 ppm) water gives rise to the minimum resistivity

value of an acceptable ground water zone. High TDS

(990 ppm) of ground water was found from the fine sand

formation encountered in the producing well B/4 near

VES–8.The sounding data indicated a resistivity of 10 Ω m

for the ground water zone. Therefore, this is the minimum

resistivity value among all the acceptable and potable

ground water-bearing formations in the area. Any subsurface

formation showing resistivity < 10.0 Ω m should be

avoided for ground water development. Thus, it is again

seen that the combined data analysis (resistivity and chemical

analysis of ground water) proved to be helpful.

A mean resistivity contour map in the depth range 0 to

5 m was prepared for the entire area from the interpreted

sounding data (Figure 5). Barring some patches in the southwestern

part and a few pockets elsewhere, the area contains

brackish/saline water in the depth range as borne out by the

presence of low resistivity zones of < 4.0 Ω m associated

with high TDS (1450 ppm) and chloride content (410

mg/L). It is interesting to note that the shallow subsurface

near the sea water Champa canal in the east central part

exhibits very low resistivities of the order of 1 to 2 Ω m corroborated

by high chloride content of ground water, typical

of saline water contamination. The resistivity distribution

pattern at the 5 to 10 m level is similar to the shallower level

of 0 to 5 m, indicating the presence of widespread saline and

brackish zones. In fact, saline zones are more pervasive at

the 5 to 10 m depth level encompassing vast areas. Percolation

of saline water from the Champa sea level canal is also

prominent at this depth as is characterized by resistivity of

1 to 2 Ω m. These contaminated zones are, however, very

much restricted at the depth level of 40 to 50 m where only

five saline zones could be identified from resistivity data.

Mean resistivity contour maps in the depth range 80 to

100 m and 130 to 150 m do not indicate the presence of any

saline water zone as the resistivity values are on the high

side. A high resistivity value (30 Ω m or more) interpreted

in some areas indicates the presence of a good water-bearing

zone comprising a coarse grain sand formation with low

TDS water. Hence, it is inferred that the saline water found

at the shallower levels are not able to percolate below in

most places due to the presence of intervening clay formations.

Thus, the integrated interpretation using resistivity,

seismic, and chemical analysis data could clearly delineate

the various subsurface geological formations and the saline

water zones.

Chemical Analysis of Ground Water Samples

Ground water samples were collected from eight deep

tube wells in the study area during the premonsoon

(April–May) and postmonsoon (October–November) periods

for three years for providing input to the integrated data

interpretation and for studying the spatial-temporal variations

of the quality of ground water in the area. The samples

were analyzed for major chemical parameters/elements, i.e.,

Na + , Mg +2 , Cl – , TDS, Cl – /HCO – 3 , and EC, which is the

inverse of resistivity. Besides, chemical analyses of ground

water samples were also carried out from a few dug wells in

the study area for providing input to constrain the interpretation

of resistivity data. The mean value of the parameters

in the premonsoon and postmonsoon periods in three tube

wells is presented in Figure 6. This shows that the premonsoon

values of the parameters increased as compared to

postmonsoon values. Resistivity of the water samples also

increased in the postmonsoon period. Analysis of water

samples from tube well B/5 indicated that the Na + value in

the premonsoon period was 90 ppm, which went down to 30

ppm in the postmonsoon period, chloride decreased from

180 to 65 ppm, and Cl – /HCO – 3 reduced from 0.76 to 0.36

ppm. TDS decreased from 980 to 790 ppm in the postmonsoon

period. It is interesting to note that one premonsoon

observation of TDS just before the commencement of the

resistivity surveys showed a value of 1400 ppm. Resistivity

of the water samples increased from 18 Ω m (conductivity

0.555 mmhos/cm) in the premonsoon period to 30 Ω m

(conductivity 0.333 mmhos/cm) in the postmonsoon period.

Further, it is observed that tube well B/5 recorded the maximum

increase in the values of the chemical parameters

among all the measuring wells. This could be attributed to

saline water intrusion into the aquifer, which, in turn, was

due to the high rate of pumping. This is also supported by

the results of resistivity profiling that indicated the presence

of a subsurface saline water zone close to the tube well.

Conclusions

Data integration of surface geophysical surveys provided

a powerful method to image the subsurface. A large

number of observation wells or tube wells were not

required to access the aquifer. Resistivity techniques fail to

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

675


Figure 5. Mean resistivity contour map in the depth ranges of 0 to 5 m, 5 to 10 m, 40 to 50 m, 80 to 100 m, and 130 to 150 m

showing the extent of saline zones.

distinguish between two formations having similar resistivity,

but the ambiguity is minimized when resistivity sounding

interpretations are constrained by seismic refraction

results. Such a combined geophysical survey and data integration

can be used as a subsurface mapping tool for delineating

the various geological formations, aquifer zones, and

zones of saline water.

Resistivity profiling coupled with resistivity sounding,

periodic chemical analysis of ground water samples, and

data integration was found to be a highly effective method

for determining the fresh water areas and the saline watercontaminated

zones, as well as the mode and cause of

saline water intrusion. Such integrated research also

evolved a new concept of minimum resistivity of a subsurface

formation in an area below which ground water contained

in it is brackish/saline and unsuitable for drinking.

676

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

Such data integration was successfully applied in a

coastal region in India to identify one narrow saline water

zone/channel, which caused high TDS and high chloride in

the ground water. Further, the integrated study delineated

subsurface saline-contaminated zones close to a sea water

canal and potable ground water zones at different depth

levels.

Acknowledgments

The authors gratefully acknowledge the help rendered

by the Chemical Division of Central Headquarters of Geological

Survey of India in the analysis of ground water samples.

Thanks are due to N.R. Biswas for drafting the

figures. The authors express their deep gratitude to Mary P.

Anderson, E Zia Hosseinipour, and the associate editor for

reviewing this paper and offering valuable suggestions.


Figure 6. Chemical analysis of ground water samples during

premonsoon and postmonsoon periods; ρ is the resistivity of

water samples.

Editor’s Note: The use of brand names in peer-reviewed

papers is for identification purposes only and does not constitute

endorsement by the authors, their employers, or the

National Ground Water Association.

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