Integrated Geophysical and Chemical
Study of Saline Water Intrusion
by Kalpan Choudhury1 and D.K. Saha2
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
1Geophysics Division, Northern Region, Geophysical Survey of
India, Lucknow, 226024, India; fax 91–0522–2370467; kalpan_gsi
@yahoo.com
2Central 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.
Vol. 42, No. 5—GROUND WATER—September–October 2004 (pages 671–677)
671
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
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
81° 37⬘
Centre of Seismic Profile
Location of VES Points
Location of Resistivity Profiling Points
Geoelectrical Section
Location of Abandoned Dug Well
Figure 1. Layout map of Digha-Shankarpur area, West Bengal, India.
672
K. Choudhury, D.K. Saha GROUND WATER 42, no. 5: 671–677
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
Lithology
Mean TDS of
Ground Water (ppm)
Saline/brackish water zone
Saturated clay/silt
Saturated silty/clayey sand/fine sand
Saturated predominantly medium/coarse sand
1800
310
Resistivity
(⍀ m)
Conductivity
(mmhos/cm)
Velocity
(m/sec)
1.1–4
4–7
7–17
above 17
9.09–2.50
2.50–1.43
1.43–0.56
< 0.56
—
2400–2500
1600–1850
1450–1600
K. Choudhury, D.K. Saha GROUND WATER 42, no. 5: 671–677
673
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
h2 = 22.1 m.
Figure 2. Integrated geophysical interpretation using resistivity sounding and seismic refraction survey. ρ1, ρ2, and ρ3
are the interpreted resistivities of the first, second, and third
layer, respectively. V1, V2, and V3 represent the corresponding velocities.
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.
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
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
Figure 3. Interpreted geoelectrical section along AA⬘ (Figure 1).
K. Choudhury, D.K. Saha GROUND WATER 42, no. 5: 671–677
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.
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
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
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.
K. Choudhury, D.K. Saha GROUND WATER 42, no. 5: 671–677
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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
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