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ORIGINAL ARTICLE
Use of salinity and resistivity measurements to study the coastalaquifer salinization in a semi-arid region: a case study in northeastNile Delta, Egypt
Mohamed Attwa1 • Khaled S. Gemail1 • Mohamed Eleraki2
Received: 27 November 2015 / Accepted: 23 March 2016
� Springer-Verlag Berlin Heidelberg 2016
Abstract To understand the impact of sea level rise (SLR)
and aquifer intensive use would cause to the groundwater
level and saltwater intrusion, an integrated relationship
between salinity and electrical resistivity of a Quaternary
aquifer is established in the northern part of East Nile Delta
(El Sharkia area), Egypt. Historical data of groundwater
salinity are mapped and compared with the current water
samples to better understand the salinity spatial variability.
The salinity maps show that salinization has increased
sustainably. Additionally, the transition/dynamic zone
related to SLR and/or excessive pumping can be addressed.
In particular, the surface DC resistivity soundings were
carried out to demonstrate the vertical and horizontal
salinity distributions in the area. In the course of this study,
the 1D model generation using a hybrid genetic algorithm
(GA) was applied and tested using borehole information.
The constructed geoeletrical cross-sections emphasize and
delineate the extension of saltwater intrusion. Cleary, it is
found that due to excessive pumping from shallow wells
over the last decades, the subsurface resistivity and TDS
vertical distributions can change rapidly within a short
distance. Additionally, the results show that despite the
dominance of brackish and saltwater at the northern part of
the area, perched low conductive lenses are observed
reflecting a low level of groundwater salinization. To
obtain a link between water salinity and aquifer resistivity,
an empirical relationship was derived to predict the salinity
variations at different depths. In comparison with the
measured total dissolved solids (TDS), the predicted
salinity map appears realistic. These results demonstrate
the important role of the integration between resistivity and
salinity measurements for mapping the groundwater
salinization with depth, and call for further research to plan
and manage the area’s water resources.
Keywords Sea level rising (SLR) � Groundwater salinity �Resistivity soundings � Salinity prediction � Nile Delta �Saltwater intrusion
Introduction
Being one of the oldest and most serious environmental
problems, groundwater salinization poses critical chal-
lenges for the managing of agricultural and natural areas
(Nosetto et al. 2013). Further, the salinity of water is an
important parameter in groundwater evaluation research. In
Nile Delta, Egypt, climate changes over the last 100 years
have caused the Mediterranean Sea to rise 0.15 m (6
inches) (Batisha 2012). Accordingly, the intrusion of salt-
water on the lush terrain has created a major challenge. As
a result, the groundwater in Nile Delta aquifer is vulnerable
due to the impact of climate change and related sea level
rise (SLR). Due to low elevation in the Nile Delta region,
Egypt is considered one of the top five countries expected
to be mostly impacted with about 1 m SLR resulting from
global warming (Ghoneim 2009). Figure 1 shows the vul-
nerable zone with 0.5 and 1 m SLR compared with the
current sea level.
The groundwater quality in the Nile Delta area is
affected mainly by saltwater intrusion from the north, in
& Khaled S. Gemail
1 Environmental Geophysics Lab (ZEGL), Department of
Geology, Faculty of Science, Zagazig University,
Zagazig 44519, Egypt
2 Geology Department, Faculty of Science, Zagazig
University, 44519 Zagazig, Egypt
123
Environ Earth Sci (2016) 75:784
DOI 10.1007/s12665-016-5585-6
addition to surface contaminants. In the last decades, a
common problem of Nile Delta sandy aquifer is the
degradation of the groundwater quality due to seawater
intrusion from the north and excessive pumping in relation
to average natural recharge (McGranahan et al. 2007). In
East Nile Delta region, extensive land reclamation and
urbanization projects have been conducted since 1980 to
mitigate the environmental and resource stresses on the
overpopulated regions in the Nile Delta, principally at the
desert fringes and in nearby coastal zones (Farid and
Tuinhof 1991). These development projects require con-
tinuous monitoring to avoid large-scale environmental
deterioration, particularly in soil salinity and groundwater
resources (Abu Zeid 1991).
In hydrogeophysical investigations, many geophysical
tools can be useful in locating and delineating subsurface
aquifer conditions and water quality. The electrical resis-
tivity techniques including direct current (DC) sounding
and electrical resistivity imaging (ERI) surveys represent
the main tools for mapping subsurface boundaries, struc-
tures, and hydrologic features of the groundwater aquifer
(Gemail et al. 2004; Attwa et al. 2009, 2011; Nadeau et al.
2015; Gemail 2015). Later several studies have been dis-
cussed on various aspects of correlation between hydro-
geological, hydrochemical parameters and resistivity data
to assess the aquifer conditions (Attwa et al. 2010; Attwa
and Gunther 2013). In addition, other studies used this
integration approach for monitoring and vulnerability
mapping of saltwater and surface waste water in sandy
aquifers (Rottger et al. 2005; Kirsch 2009; Attwa 2012;
Gemail 2012; Attwa et al. 2014).
In East Nile Delta region, the groundwater aquifer is
naturally bounded northward by the Mediterranean Sea
through El Manzala Lake and eastward by the Suez Canal
(Fig. 2). In the eastern Nile Delta, the seawater has intru-
ded the aquifer up to a distance of about ca. 116 km
measured from the shoreline along the bottom boundary of
the aquifer (Sherif et al. 2012; Mazi et al. 2014). In present
work, El Sharkia area (northern part of East Nile Delta)
was considered as a case study to delineate the impact of
SLR and overpumping on the groundwater quality using
surface resistivity and total dissolved solids (TDS) mea-
surements at different depths. El Sharkia area is bounded
northward by the largest saltwater body in the Nile Delta,
El Manzala Lake (Fig. 2).
To monitor the groundwater salinity in EL Sharkia area,
hydrochemical investigations are considered and integrated
with surface resistivity measurements. Here, the salinization
history and current status of the Quaternary aquifer were
reviewed. The main objective is related to the vulnerability
classifications of the groundwater salinity in the area. Finally,
the relationship between the intrinsic aquifer resistivities and
the measured TDS values is studied in an attempt to predict
the aquifer salinity with depth. This information can be
analyzed, integrated and compiled with hydrogeological data
to establish successful policies of groundwater development
and monitoring programs, as the environmental impact of the
agricultural programs can be dramatic if they are not based
on sound ecological principles.
Geological and hydrogeological background
The Nile Delta aquifer is one of the largest aquifers in the
world and is characterized by direct hydraulic contacts with
the Mediterranean Sea from the north and Suez Canal in
Fig. 1 The land areas in the
Nile Delta that would be flooded
if the sea level rose of 1.0 m and
if the sea level rose up to 5.0 m
(after Ghoneim 2009). Note that
the red circles and orange
boundary refer to city locations
and El Sharkia political border,
respectively
784 Page 2 of 18 Environ Earth Sci (2016) 75:784
123
the east. Shata and Hefny (1995) indicated that in the Delta
area, as well as its fringes, the strata of hydrological
importance belong essentially to the Quaternary and Ter-
tiary with average thickness of 700 m. Of these strata, the
deltaic deposits (200–500 m thick) which belong to Mit
Ghamr Formation (Pleistocene age) constitute the bulk of
the main aquifer (Sherif 1999). The Nile Delta aquifer
consists of Mit Ghamr Formation, which is composed of
intercalation of unconsolidated clay, silt, sands and gravel
with occasional clay lenses (Fig. 3). The top boundary of
the deltaic deposits, which acts as a cap for the aquifer is
composed of semi-pervious clay, silt and fine sand layers
and is known as Bilqas Formation of Holocene age
(Fig. 3). Mit Ghamr aquifer and Bilqas aquitard are
hydraulically connected, but each one has its own hydraulic
head. The thickness of the Bilqas aquitard is only a few
meters in the southern part of the East Nile Delta (Zaghloul
et al. 1977), while its thickness can be greater than 70 m in
the northeast of Nile Delta.
The Nile Delta aquifer is a complex groundwater sys-
tem. In the eastern Nile Delta, the system is semi-confined
where the silty clay layer is well represented and acts as an
aquitard. In the other areas, where the confining layer is
missing, as in the eastern and southwest fringes desert of
the study area, the aquifer is under phreatic conditions. The
base of the aquifer slopes regionally in the northward and is
affected by faulting system (El-Dairy 1980). The lithology
of the aquifer system changes from more sandy and grav-
elly in the south to more silty and clayey in the north due to
change the depositional environment from fluviatile in the
south to more marine in the north (Shatta et al. 1979).
The aquifer is recharged by the infiltration from irriga-
tion network such as Ismailia Canal, excess of irrigation
water and little precipitation through the upper clay layer
(Gemail et al. 2011). It may also be recharged by any
possible flow coming from the Damietta Nile branch in
western parts (Fig. 2). The piezometric level decreases
gradually from more than 15 ? MSL in the south near
Cairo to near sea level in the north at El Manzala Lake
(RIGW 1992). According to the piezometric levels, the
groundwater flow direction is generally from south to north
and northeast directions; however, local discharge sites
may redirect groundwater flow to other sites, e.g. Suez
Canal and Damietta Branch (Ismael 2007; Elewa et al.
2013). On the other hand, the northern parts of the aquifer
are affected by saltwater intrusion due to the over pumping
of groundwater and SLR (Sherif 1999; Sherif et al. 2012).
When the groundwater is pumped from the aquifer which is
in hydraulic connection with El Manzala Lake, the gradi-
ents that are set up may induce a flow of seawater from the
Fig. 2 Location map of study site, northeast Nile Delta (El Sharkia area)
Environ Earth Sci (2016) 75:784 Page 3 of 18 784
123
sea toward the well. As a result, the groundwater quality in
the northern parts of the area has been deteriorated con-
siderably; any additional pumping should be practiced in
the middle parts (transition zone) and pumping from the
eastern and western parts should be reduced.
In the coastal basin of the northern part of East Nile
Delta, salinization of fresh groundwater is a conspicuous
aspect of deterioration in groundwater quality (Elewa et al.
2013). Seawater intrusion has been occurring in the area for
several decades and has affected irrigation and domestic
wells (Tantawi 1998).
In general, the shape and degree of the seawater intru-
sion in a coastal aquifer depend on several factors. Some of
these factors are natural and cannot be controlled while
others are manmade and could, thus, be managed. The
depth of the aquifer at the sea side, pumping and recharge
rates, and well depths and locations, are the most critical
factors to be considered (Sherif and Singh 1997). Several
numerical techniques were employed to assess and simu-
late the saltwater intrusion in the Nile Delta aquifer (e.g.
Amer and Farid 1981; Sherif and Singh 1997; Sherif 1999;
Sakr et al. 2004; Farid and Hassan 2005; Sherif et al. 2012;
Abdelaty et al. 2014). Most of these studies were rather
theoretical in nature as there were not enough records of
salinity of the aquifer at different depths. As Mabrouk et al.
(2013) mentioned, in the case of the Nile Delta, the tran-
sition zone is relatively large, characterized by the dynamic
relation between fresh and seawater. Therefore, the aim of
the present study is to delineate the relationship between
surface resistivity and salinity to predict the graduation in
the groundwater salinity due to the intensive pumping in
the transition zone of East Nile Delta aquifer in the El
Sharkia area.
Materials and methods
Hydrochemical data collection
The data of historical total dissolved salts (TDS) from 1983
to 2007 were collected from published sources (Sallouma
1983; Ismael 2007). The collected data were used for
constructing TDS change maps to follow the brackish and/
or dynamic zone of salinity distributions over several years.
In the second stage, hydrochemical data collection was
carried out in urban areas of El Sharkia area (Fig. 2)
through many field trips. Based on field observations and
during such trips, it is important to note that the private
water well systems (i.e. drilled well with a submersible
pump and pressure tank) are most common and widely
used for agriculture and drinking purposes, especially at the
central and northern part of the area. Therefore, there is no
control on the pumping rate, which, consequently, causes
vertical upconing of saline water from the deeper parts of
the aquifer. Further, another point to consider is that the
salinization of groundwater wells is a common phe-
nomenon at the northern part of the area. In turn, a general
water crisis is inevitable resulting in an upward shift in
agriculture and drinking water demands. Accordingly, in
2014, 50 groundwater samples were collected near the well
Fig. 3 Hydrogeological cross-sections across East Nile Delta (after Elewa et al. 2013) showing the Quaternary aquifer units; Bilqas (Holocene
aquitard) and Mit Ghamr (Pleistocene aquifer) formations (Rizzini et al. 1978)
784 Page 4 of 18 Environ Earth Sci (2016) 75:784
123
head and before the water went through tanks/treatment
units. To monitor the saltwater intrusion and to delineate
the current dynamic zone, the majority of groundwater
samples were collected from the northern and central parts
of the area (Fig. 4, left). At the southern part of the area
(Tenth of Ramadan city), two groundwater samples, nos.
50 and 52 (Fig. 4, left), were taken from published sources,
El-Sayed et al. (2012) and Embaby et al. (2014),
respectively.
During the field work, and on the spot, the group team
measured pH value and electrical conductivity (EC) of
collected water samples. In addition, the available water
table and well depth information was recorded (Table 1).
Locations of water samples from the study area are shown
in Fig. 4, left. The chemical analyses of TDS, Cl-, HCO3-
and Cl-/HCO3- values are listed in Table 1. Finally, the
historical and current TDS maps were compared to high-
light the distinct increase in salinity, which can reflect the
influence of sea water intrusion from El Manzala Lake and
aquifer intensive use on groundwater salinization.
DC resistivity data acquisition/inversion
In this study, 34 Schlumberger resistivity soundings
(Fig. 4, right) were measured and used to demonstrate the
vertical and horizontal distributions of saltwater intrusion
over the area in comparison with the available geological
information from the observed boreholes and the measured
TDS, mentioned above. The measured DC soundings
location was concentrated at the northern and central parts
of the area to discriminate the current transition/brackish
zone. Further, the link between the surface resistivity
measurements and the measured TDS of collected
groundwater samples were examined in an attempt to
establish an empirical relationship to predict and/or map
the current TDS with depth over the study site. The max-
imum current half-spacing (AB/2) was 400 m. In addition,
nine soundings (nos. 35–44) were collected from published
sources (El-Mahmoudi 1989; Abd El-Gawad 1997) to
cover the southern part of the area, where the Quaternary
freshwater aquifer is dominant.
Figure 4 (right) shows the distribution of the DC
sounding locations over El Sharkia area. Practically, to
overcome the non-uniqueness or equivalence problems, the
available borehole and salinity data were used to decide the
number of layers for the inversion process to derive lay-
ered-earth models at the nearest DC soundings. Further, the
calibration process was used to define the geoelectric
boundaries of the subsurface layers and to acquire general
information about the salinity distributions.
For the inversion process, the genetic algorithm (GA)
for 1D model was used to obtain the best fit model. The
Fig. 4 Locations of (left) collected water samples and (right)
measured DC resistivity soundings at the investigated site, El Sharkia
area. Note, (left) the water samples nos. 50 and 52, at Tenth of
Ramadan, were collected from El-Sayed et al. (2012) and Embaby
et al. (2014), respectively. Additionally, (right) nine soundings
(35–44) were collected from published sources (El-Mahmoudi 1989;
Abd El-Gawad 1997) to cover the southern part of the area
Environ Earth Sci (2016) 75:784 Page 5 of 18 784
123
Table 1 Observed wells information, chemical analysis, Cl-/HCO3- ion ratio, palatability of drinking water (WHO 2007) and water genesis
(Custodio 1987) of collected groundwater samples (for location see Fig. 4, left)
Sample
nos.
PH EC (ls/cm)
TDS
(ppm)
Water
table (m)
Well
depth (m)
Cl-
(epm)
HCO3-
(epm)
Cl-/HCO3-
(range)
Palatability
class
Water
genesis
1a 8.08 3770 2639 2 15 18.69 3.28 5.7 Unacceptable
2 8.27 4680 3276 2 10 20.2 2.84 7.1 Unacceptable
3 7.52 2166 1516 9.6 2.76 3.4 Unacceptable Inland water
4 7.39 3450 2415 1 10 11.1 2.24 4.9 Unacceptable Inland water
5 7.43 4583 3208 1.5 35 Unacceptable
6 7.29 4820 3374 1.5 40 24.24 2.65 9.1 Unacceptable
7a 8.13 664 465 2 12 Good
8 7.6 1550 1085 2 23 6.26 1.84 3.4 Poor Inland water
9 7.52 1045 700 3 23 5.35 1.6 3.3 Unacceptable
10 7.29 30,030 21,020 15 48 170.7 1.56 109.4 Unacceptable Seawater
11a 7.28 24,180 20,000 15 30 146.5 2.76 54 Unacceptable Seawater
12 7.42 26,110 18,277 30 131.3 3.28 40 Unacceptable Seawater
13a 7.32 2626 1838 2 3 Unacceptable
14 7.01 14,780 10,346 2 10 87.87 2.32 37.8 Unacceptable Seawater
15a 7.01 23,970 16,779 15 20 130.3 1.44 90.5 Unacceptable Seawater
16a 7.45 1333 933 2 10 7.37 0.92 8 Poor
17a 7.14 1269 888 15 40 4.24 0.8 5.3 Fair
18 7.38 2837 1986 7 35 18.18 2.2 8.3 Unacceptable
19 7.27 18,810 13,167 6 40 125.2 1.16 108 Unacceptable Seawater
20 7.04 22,080 14,456 15 30 108.1 1.44 75 Unacceptable Seawater
21 7.1 21,730 15,211 102 1.92 53 Unacceptable
22 7.28 1734 1214 3 15 8.28 1.88 4.4 Unacceptable
23 7.38 1048 702 2 8 3.13 1.8 1.7 Fair Inland water
24a 7.58 1333 933 2 9 6.57 1.8 3.65 Poor Inland water
25 7.23 1179 790 2 3 Fair
26 7.42 887 568 2:3 24 3.33 2.2 1.5 Good Inland water
27a 7.53 2205 1500 2 13 12.63 4.64 2.7 Unacceptable Inland water
28 7.55 5637 3946 3:5 9 26.77 7.16 3.7 Unacceptable Inland water
29 7.38 599.76 366 40 1.52 0.88 1.7 Good Inland water
30 6.94 1192 799 9 36 3.23 4.27 0.7 Fair Inland water
31 7.64 876 561 30 Good
32 7.7 1218 853 60 Fair
33 7.22 7297 5108 3 15 36.36 5.2 7 Unacceptable Inland water
34 7.08 13,060 9142 18 30 Unacceptable
35 7.6 1616 1134 12 24 6.06 4.4 1.4 Poor Inland water
36a 7.63 2841 1989 1.5 22 15.15 5.12 3 Unacceptable Inland water
37 7.8 1503 1052 7 17 5.65 3.2 1.8 Poor Inland water
38 7.3 2957 2070 2 30 17.8 1.56 11.4 Unacceptable
39 7.74 1171 785 12 5 4.446 2.44 1.8 Fair Inland water
40 7.53 2586 1810 10 45 11.11 4.64 2.4 Unacceptable Inland water
41a 7.69 780 500 25 40 6.3 0.88 7.15 Fair
42 7.09 10,860 7602 3 13 53.53 8.48 6.3 Unacceptable
43a 7.33 1678 1080 3 28 8.48 1.16 7.3 Poor Inland water
44 7.65 818 525 10 40 3.03 3.6 0.8 Good Inland water
45 7.46 868 556 120 3.03 4.2 0.72 Good Inland water
46 8.07 1578 1105 Poor
784 Page 6 of 18 Environ Earth Sci (2016) 75:784
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details of the algorithms can be found in Basokur et al.
(2007). As a first step, the DC sounding data were inverted
using conventional inversion methods; based on a linear
filtering as 1D forward modeling and damped least-squares
(Levenberg–Marquardt) algorithm to solve the inverse
problem (deGroot-Hedlin and Constable 1990). Then, the
search space in the GA was adapted by taking into account
the results of damped least-squares, i.e. the resistivity range
was defined in the GA based on the conventional 1D
inversion results in the form of control points. In the 1D
implementation, the parameter search space is defined at
the first stage and it progressively shrunk through succes-
sive generations. The algorithm produces some equivalent
solutions at final generation. The number of layers was
derived from the borehole information.
Figure 5 shows the calibration examples between the
interpreted resistivity layers of the selected sounding no.
25, applying GA, and the corresponding borehole data. The
Table 1 continued
Sample
nos.
PH EC (ls/cm)
TDS
(ppm)
Water
table (m)
Well
depth (m)
Cl-
(epm)
HCO3-
(epm)
Cl-/HCO3-
(range)
Palatability
class
Water
genesis
47 7.53 375.36 229 48 1.82 1.04 1.8 Excellent Inland water
48 7.65 888.42 588.6 18 3.23 1.48 2.2 Good Inland water
49 7.36 1108 742 30 4.34 1.4 3 Fair Inland water
50 8.2 1550 804 9.28 3.6 2.6 Fair Inland water
51a 7.27 3428.5 2400 8 9 11.1 5.08 2.2 Unacceptable Inland water
52 8.6 406 260 2.495 0.5 4.9 Excellent Inland water
53 7.9 730 476 46 3.03 2.24 1.4 Fair Inland water
54 7.36 1259.7 882 46 5.56 2.72 2 Fair Inland water
Note, the chemical analysis of water samples nos. 50 and 52 were collected from El-Sayed et al. (2012) and Embaby et al. (2014), respectivelya Selected water samples for deducing the empirical relationship in order to predict the TDS
Fig. 5 The interpreted resistivity layers constrained with the geolog-
ical data and TDS measurements at sounding no. 25 (for location, cf.
Fig. 4, right) using Genetic algorithm (GA); a variations of mean and
best misfit versus generation, b a comparison between the measured
and calculated data at the best misfit value; c a comparison between
the model response and borehole information (water sample no. 28,
for location, cf. 4, left)
Environ Earth Sci (2016) 75:784 Page 7 of 18 784
123
population size and the number of generations were both
60. In addition, the variation of mean and best misfits
versus generation was illustrated (Fig. 5a). In the first
generation the parameter sets are scattered over the
parameter space. In succeeding generations, the majority of
the models generate relatively small error energies and
consequently they accumulate around the global and local
minima. In view of that, Fig. 5a shows that the average and
smallest misfit values decreases very rapidly. It can be
noted that the smallest misfit at each generation cannot be
reduced below a certain value. In addition, a good fitting
between the measured and theoretical data can be observed
at the best misfit value (Fig. 5b). The correlation between
the final inversion results using GA and the borehole
information are shown in Fig. 5c. Similarly, the other
soundings were inverted using GA with the same number
of populations and generations. It can be observed that
there is a good correlation between the inversion results
using GA and the borehole information. This indicates that
the large number of forward calculations increases the
chance of acquiring the best model corresponding to the
borehole data. Then, the results obtained from the inversion
of DC soundings were used to construct geoelectrical
cross-sections by stitching the 1D inversion results.
Results and discussion
Salinity data analysis
The chemical analysis of the water samples is listed in
Table 1. The pH of the groundwater samples varies
between 6.9 and 8.6. Accordingly, all the studied ground-
water samples were neutral to slightly basic with pH[ 7.
Moreover, the EC of the groundwater in the study area
ranges between 223 and 30030 ls/cm (Table 1), ground-
water samples nos. 47 and 10, respectively (Fig. 4, left). In
addition, the historical and current TDS data were mapped
to monitor the changes in TDS over time (Fig. 6a–d).
The constructed maps indicate that there is, in general,
TDS variability from 1 year to another. Examination of
historical TDS change maps of 1983–2007 (Fig. 6a–c)
show that most groundwater salinity (TDS) increases
steadily towards the north of the area. According to Win-
slow and Kister’s classification (1956) for salinity, all the
studied water samples were categorized into fresh, brack-
ish, saline and very saline. Notably, excursions in the TDS
are seen at the peripheries of seawater intrusion from El
Manzala Lake. Figure 6a–c shows a trend of salinization
and the formulation of an enlarged zone of very saline
water ([10,000 ppm) in 2007, which can be attributed to
the accelerated shoreline regression and SLR (Elewa et al.
2013). It can be also noted that, in general, the saline zone
(i.e. 3000–10,000 ppm) excursion has a limited increase
over years.
The inspection of TDS change maps (Fig. 6a–c) shows
that the saline–fresh water interface is not a sharp contact,
but mixing brackish water forms a transitional zone
between both water bodies. The brackish water and/or
dynamic zone (1000–3000 ppm) occurred as narrow strips
between freshwater and saline water zones. The significant
change in TDS was encountered within this zone and can
be considered the result of changes in land tenure, agri-
culture and over pumping. Similarly, Carnoda et al. (2004)
and Somay and Gemici (2009) stated that the development
of agriculture or public water supply, i.e. aquifer intensive
use, can contribute to salinization of extracted groundwa-
ter. The change in transition zone extension over the years
can be observed at the western and central parts.
The TDS data from collected groundwater samples at
different depths (Fig. 6d) were classified into four salinity
zones: very high saline water zone with TDS more than
10,000 ppm to freshwater zone with less than 1000 ppm.
Because the measured TDS data are intensive and redun-
dant (Table 1), the detailed TDS changes within each zone
can be observed. The salinization trend (Fig. 6d) map
envisaged the impact of saltwater intrusion to be degraded
from fresh at the middle central and southern parts to
extremely (very) saline at the northern parts, i.e. El Man-
zala Lake. A narrow strip between very saline and brackish
zones can be noticed at the northern parts of the area. In
comparison to the historical results mentioned above
(Fig. 6a–c), it can be seen that the saline zone, recently,
extends to the western part of the area (Fig. 6d).
The collected groundwater samples were categorized
according to WHO’s classification (WHO 2007) for
palatability of drinking water into: excellent, good, fair,
poor and unacceptable. Table 1 shows that most of good
(300–600 ppm) to excellent (\300 ppm) water quality for
domestic usage (Table 2) is concentrated at the southern
parts of the area. On the other hand, unaccept-
able ([1200 ppm) water quality for drinking (Table 2) is
concentrated at the northern and central parts.
In comparison with historical TDS maps, Fig. 6d shows
a significant change in TDS within the transition/dynamic
zone, which can be attributed to intensive use of ground-
water aquifer. At the northern part (Fig. 6d), shallow
freshwater groundwater samples (i.e. \1000 ppm) was
observed within the transition zone (e.g. groundwater
samples nos. 23, 25 and 26; Table 1), ranging from good to
poor (900–1200) quality (WHO 2007). As a result of the
lack of freshwater at these sites, it was observed that such
shallow groundwater was used for irrigation and frequently
for drinking. Further, the freshwater aquifer was observed
at the eastern new reclaimed land (water samples nos. 7, 9
and 17, for location cf. Fig. 4, left) reflecting an increase of
784 Page 8 of 18 Environ Earth Sci (2016) 75:784
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Fig. 6 a–d TDS monitoring over several years in El Sharkia area,
East Nile Delta. e Hydrogeochemical index map of Ionic ratio Cl-/
HCO3-. Note that all collected water samples were categorized
according to Winslow and Kister’s (1956) and Custodio’s (1987)
classifications for salinity and water genesis, respectively
Environ Earth Sci (2016) 75:784 Page 9 of 18 784
123
recharge rate from Ismailia Canal with a decrease in dis-
charge rate from overpumping of groundwater. In com-
parison with the constructed TDS maps (Fig. 6), it is
obvious that the most parts of the central and eastern parts
have become brackish (i.e. 1000–3000 ppm). Towards the
south, the salinity maps are spatially identical and rela-
tively alike, presumably due to the presence of Ismailia
Canal (for location, cf. Fig. 2) as a natural response, in
general, to high water levels at these recharge sites.
Because chloride is a fingerprint element for seawater
intrusion (Hem 1985; Richter and Kreitler 1993; Somay
and Gemici 2009), the ratio of Cl-/HCO3-, consequently,
can be a good indicator for salinization due to the sea-
water encroachment. Here, the ionic ratios of Cl-/HCO3-
are calculated and listed in Table 1. It is obvious that the
Cl-/HCO3- values range from 0.7 to 109.4, water sam-
ples nos. 30 and 10, respectively. According to Custodio
(1987), values of ionic ratio index between 0.7 and 5
were given for inland water, but the hydrogeochemical
index of 20–109.4 was given for seawater. Accordingly,
the inspection of the constructed Cl-/HCO3- map
(Fig. 6e) shows a steady increase in salinization toward
the north. Consequently, higher values of Cl-/HCO3- at
the northern parts can be attributed to seawater intrusion,
whereas the appearance of lower values of Cl-/HCO3-
ratio (i.e. 0.1–5 index) in the southern part or even more
mid parts of the area (Fig. 6e), pinpoints to a decrease in
TDS values and/or inland water genesis. Further, the
medium ratio of Cl-/HCO3- (i.e. 5–20 indeces) can be a
good indicator for mixing of groundwater with saltwater
at these locations. In this case, the increase in ground-
water salinity is probably due to: (1) overpumping of
slightly brackish groundwater which induces vertical
upconing of saline water from the deeper parts of the
aquifer, as Sherif and Singh (1997) mentioned, and/or (2)
overuse of fertilizers, where salts accumulation from
agricultural practices is highly expected, as Elewa et al.
(2013) mentioned. Where Cl-/HCO3- ratio ranges
between 5 and 20, it can also be noted that the water
samples were categorized into fair to unaccept-
able (Table 1), according to WHO (2007).
Resistivity data analysis
Based on the inversion results of 43 DC soundings, geo-
electrical cross-sections were constructed in NW–SE and
NE–SW trends (Figs. 7, 8). The interpretation of such
cross-sections was constrained by the available geological
information, hydrogeological data and the measured TDS
values (Table 1). Along these sections, the topmost zone,
representing the surface layer, shows a wide range of
resistivity (2–510 Xm). The variations of the surface soil
resistivity can be attributed to local soil conditions around
the measured soundings. The relatively higher values
reflect dry sand and gravel soil (orange color) that is
dominant in the southern parts (e.g. sounding no. 37,
Fig. 8c) and far from the cultivated lands. It is obvious that
high features/topography (*150 m asl.) can be noted
towards the south (Fig. 8).
Most of the expected faults shown in NE–SW sections
(for location, cf. Fig. 8a) were confirmed by the structural
geological study/map carried out by El-Dairy (1980) in the
East Nile Delta. From a geological point of view (e.g.
Zaghloul et al. 1977; Rizzini et al. 1978), the geoelectrical
cross-sections indicate that the thickness of the near surface
low resistivity layer (\10 Xm) corresponding to the clay
(Bilqas aquitard) increases towards the northern parts
(Fig. 7b–d) or even mid parts (Fig. 7f) and disappears
towards the southern parts (e.g. soundings nos. 36 and 37;
Fig. 8), for sounding locations, cf. Figs. 7a and 8a. The
Table 2 The measured and
predicted TDS values at
selected water samples and DC
sounding points (for location,
cf. Fig. 4)
Water sample nos. DC sounding nos. The measured
TDS (ppm)
True resistivity
values (Xm)
The predicted
TDS (ppm)
1 1 2639 15 2689.256
7 7 465 51 638.9051
11 18 20,000 2 21569.57
13 9 1838 20 1963.09
15 10 16,779 3 14307.91
16 12 933 37 962.1586
17 13 888 51 638.9051
27 24 1500 23 1678.938
36 4 1985 20 1996.265
41 32 500 50 655.9913
43 31 1080 27 1398.294
51 34 2400 16 2507.714
Note, the laboratory measurements of the collected water samples were listed in Table 1. The deduced
empirical relationship (polynomial relation) for predicting TDS was shown in Fig. 10
784 Page 10 of 18 Environ Earth Sci (2016) 75:784
123
Fig. 7 NW-SE stitched resistivity sections (b–f) based on 1D inversion results showing the salinity distributions and freshwater aquifer (for
location, cf. a)
Environ Earth Sci (2016) 75:784 Page 11 of 18 784
123
Fig. 8 NE-SW stitched resistivity sections (b–e) based on 1D inversion results showing the salinity distributions and freshwater aquifer (for
location, cf. a)
784 Page 12 of 18 Environ Earth Sci (2016) 75:784
123
maximum thickness of this layer (dark brown color) is
about 30 m at the northern part, Fig. 7c.
In comparison with the measured TDS close to the DC
soundings (for location, cf. Fig. 4), three main types of
aquifers can be differentiated, in general, into freshwater
(\1000 ppm), brackish/saline water (1000–10,000) and
saltwater ([10,000 ppm). The cross-sections indicate that
shallow high resistivity layers ([35 Xm) can be recognized
at the northwestern parts at soundings nos. 15, 22 and 23,
for locations, cf. Figs. 7a and 8a. In comparison with the
geological information and salinity distributions, such layer
can be interpreted as perched freshwater sand (pink color),
which rests on a thin clay to sandy clay layer (Figs. 7c, d,
8b). The maximum thickness of this layer is *13 m at
sounding 23, Fig. 7d. In general, such perched water is
recharged directly from irrigation canals and sewage open
drains which are common in the northern parts of the area.
Additionally, a medium resistivity (16 Xm) layer can be
recognized at sounding no. 34 (powder blue color), which
corresponds to perched brackish water sand (*7 m thick),
and lies over low resistivity (7 Xm) layer corresponding to
clay (Fig. 8e) in a lens-like form. Based on the field
observations, this area is characterized by overusing of
fertilizer with salts accumulation where the agriculture
activities are dominant. Accordingly, the existence of such
perched brackish water zone can be related to such agri-
culture practices, as Elewa et al. (2013) discussed.
From a hydrochemical point of view, the geoelectrical
cross-sections show three main geoelectrical layers of very
low (\3 Xm), low to medium (5–33 Xm) and high
([35 Xm) resistivties. In comparison with the measured
TDS (Table 1), it is clear that very low resistivity values
(\3 Xm, e.g. soundings nos. 11 and 18) are concentrated at
the northern parts, which can be attributed to saltwater
intrusion ([10,000 ppm, water samples nos. 11, 16 and
18). In addition, the very low resistivity layer (blue color)
corresponding to saltwater intrusion can be recognized
below clay layer (dark brown color) at the northern parts of
the area (Fig. 8b–d). Because the thickness of the clay
layer increases at the northern part, the average depth of
saltwater intrusion is about 15 m. Further, it is clear that
the saltwater intrusion was not detected at the southern
parts of the area where it is deeper than 100 m depth,
Fig. 8b, c and e. At the southern and southeastern parts of
the area, the very low resistivity layer corresponding to
saltwater intrusion (Figs. 7e, 8e) was detected at about 8 m
depth (sounding no. 34, AB/2 = 400). It is clear that the
lateral migration of saltwater intrusion forms a tongue-like
form, which is a typical form of seawater intrusion, as
shown in Figs. 7 and 8.
In comparison with the listed TDS values in Table 1, the
geoeletrical cross-sections indicate that the saltwater
intrusion lies below the low to medium resistivity layer,
which can be attributed to brackish/saline water aquifer.
The depth of such layer ranges between *5 m at the
northern parts and more than 160 m at the southern parts
(Figs. 7, 8). It can be noted that the brackish/saline water
aquifer is deeper than 200 m depth towards the south and,
accordingly, it was not detected at sounding no. 37
(Fig. 8c). The brackish/saline water aquifer is dominant at
the central parts at various depths and with different
thicknesses, which can be attributed presumably to the
overpumping of groundwater causing an increase in verti-
cal upconing of very saline water. Therefore, mixing of
saline water with fresh groundwater at these locations is
highly expected producing brackish/saline aquifer. Similar
to saltwater intrusion behavior, the distribution of the
brackish/saline water layer appears, in general, in a wedge-
like form (Figs. 7, 8).
Towards the south, southwest and southeast directions,
high resistivity layer ([35) can be recognized, which cor-
responds to freshwater aquifer (pink color). The increase of
freshwater thickness can be attributed, probably, to the
high hydrostatic pressure of freshwater from the Ismailia
Canal (for location, cf. Fig. 2), which is a natural response
to high water level at these recharge sites (Hamza et al.
2008). The low elevation traditionally cultivated land
adjacent to Ismailia Canal and those adjacent to the
reclaimed lands (i.e. New Salhia) are important locations
for groundwater discharge. Figure 8 indicates that a con-
siderable thickness ([50 m) of the freshwater aquifer can
be detected at the south-central parts (for location, cf.
Fig. 8a). The average depth of the freshwater aquifer at the
south-central and southern parts is about 15 and 100 m,
respectively. The inspection of the constructed cross-sec-
tions indicates that the freshwater-brackish/saline water
interface can be detected at the central and south eastern
parts (soundings nos. 13, 30, 34, 35 and 41), Figs. 7f and
8b, c and e. Therefore, the central part of the area, in
general, can be considered as a dynamic/dispersion zone,
Mabrouk et al. (2013) discussed this issue in detail.
According to the obtained results of the inverted DC
soundings at the northern part, 3D resistivity visualization
model was constructed (Fig. 9) using inverse distance-ani-
sotropic modeling method of rockworks software package
(Rockkware 2009). During the 3D gridding process, the
input gridding space (X, Y and Z) were applied to produce a
relatively fine mish grid (20, 20 and 2 m, respectively). The
input data of this model was created by sampling of the true
resistivities and depths of the subsurface layers at each
measured sounding point. The sampling rate was low at
shallow depths and increased with depth to explore the
shallow vertical variations in resistivity which reflects the
salinity graduations. The model shows a clear picture about
the vertical and horizontal resistivity variations in the
northern part of the area; the resistivity distributions and
Environ Earth Sci (2016) 75:784 Page 13 of 18 784
123
depths were constructed in the form of horizontal and ver-
tical slices with different depths. The constructed resistivity
model (Fig. 9) shows that the saltwater intrusion forms a
tongue-like form. In addition, at *75 m depth the saltwater
intrusion can be detected at the northern part of the area.
The transition zone between the saltwater intrusion at the
northern part and freshwater aquifer can be well noticed
(e.g. soundings no. 28 and 30). The irregular boundaries
between freshwater and saltwater intrusion, i.e. transition
zone, can be attributed to the recharge from irrigation canals
and local discharges sites as a results of intensive pumping
practices from Quaternary aquifer.
Based on the available borehole information, previous
hydrogeological and hydrochemical studies, 1D inversion
results and salinity measurements of collected water sam-
ples, a 3D schematic model was constructed as presented in
Fig. 10. Notably, larger values of groundwater flow, in
general, are associated with the local flow systems, for
example, the seepage from Ismailia Canal, drainage of
groundwater to the main surface drains (e.g. Bahr El Baqr
drain), groundwater flow towards the energetic pumping
stations, and finally the evaporation from the inland El
Manzala lagoon near the northern boundary of the Rolling
Plain (Fig. 10). Additionally, groundwater flow of
Fig. 9 3D visualization resistivity model shows the resistivity distributions with different depths at the northern part of the area
784 Page 14 of 18 Environ Earth Sci (2016) 75:784
123
freshwater towards the El Manzala Lake prevents saltwater
from entering freshwater coastal aquifers (Barlow 2003).
Resistivity-salinity relationship
To obtain a spatial distribution of groundwater salinity
and the influence of over pumping, a relation between the
measured TDS and the intrinsic aquifer resistivity (q)
values was studied. Figure 11 shows a polynomial,
inverse first order, relationship of both intrinsic aquifer
resistivities (q) versus the measured TDS of twelve data
points. The selected TDS data for such relation showed a
wide salinity variation, ranging from fresh to very saline,
and they were located close to DC sounding points
(Table 2). It is noticeable that the TDS was strongly
correlated (R2 = 0.98) with q derived from DC sounding
inversion using GA. Consequently, the prediction of TDS
from individual q seems to be applicable at different
depths. According to the least-squares fit between the
aquifer resistivities and the measured TDS values, the
coefficients ‘‘a’’ and ‘‘b’’ were calculated. On the basis of
the deduced relationship, the predicted TDS values were
calculated at each well and listed in Table 2. Furthermore,
the mean-squared logarithmic error of fit was calculated
using
e2 ¼ 1
N
XN
i¼1
ln TDSm ið Þ � ln TDSc ið Þ½ �2; d ¼ ee; ð1Þ
where N is the number of soundings (i.e. 12 points), TDSm(i) and TDSc (i) are the measured and predicted TDS using
the deduced empirical relationship, and d is the error factor.Here, by applying Eq. (1) the d value was 1.2. The results
listed in Table 2 have shown a very close semblance with
those obtained from TDS measurements.
Fig. 10 Schematic 3D model shows the freshwater/saltwater condi-
tions over the study area. Note, the freshwater, brackish water and
saltwater zones were deduced from the constructed TDS maps
(Figs. 6d, 11) and geoelectrical cross-sections (Figs. 7, 8). Note the
water flow direction was derived from Elewa et al. (2013)
Fig. 11 Correlation between Intrinsic resistivity derived from DC
soundings inversion versus TDS of some collected water samples
listed in Table 1
Environ Earth Sci (2016) 75:784 Page 15 of 18 784
123
The deduced polynomial relationship (Fig. 11) was
applied to predict the TDS values for the aquifer of the
investigated area in comparison with the measured TDS
values listed in Table 2. Further, the predicted TDS values
were mapped to show the salinity distributions over the
area investigated (Fig. 12). It is noticeable that the calcu-
lated TDS map (Fig. 12) is, in general, in the same order of
the average measured values from the laboratory mea-
surements (Fig. 6d). Compared with the overall measured
TDS values, the predicted TDS values prove the reliability
of use of the inverted sounding data to predict TDS with
depth using the deduced polynomial equation over the
study area. Field investigations and experiences revealed
the existence of an extensive dispersion zone, i.e. brackish
water, between the freshwater in the aquifer and the
intruded seawater. Moreover, such zone (i.e. transi-
tion/dynamic) causes a serious problem especially in semi-
arid condition in the form of increasing salinity. Accord-
ingly, the overusing of groundwater during submersible
irrigation increases the groundwater salinization. All of
these results and comparisons proved the accuracy of the
interpretation here and the validity of this method to pre-
dict the TDS using the deduced empirical relationship for
the area. Further, the constructed cross-sections (Figs. 7, 8)
show the measured TDS at different wells in comparison
with DC soundings inversion results. Accordingly, the TDS
can be predicted at different depths using the aquifer
intrinsic resistivity.
Summary and conclusions
The goal of this study was to evaluate the integration of
salinity and geophysical measurements to determine the
groundwater salinization of the coastal aquifer in a semi-
arid area. The present study showed that the utility of
combining DC resistivity with hydrochemical measure-
ments is very valuable in providing a clear picture of the
hydrogeological conditions and groundwater salinization.
The 1D inversion using GA appears to be robust and
reliable to characterize the geological and hydrogeological
conditions of the coastal aquifers. Furthermore, the power
law relation between the aquifer resistivity and the mea-
sured TDS is an effective tool to predict and distribute the
aquifer salinity at different depths for the semi-arid areas.
Here, the DC resistivity measurements were integrated
with the available borehole hydrological and hydrochemi-
cal data in the northern part of East Nile Delta (El Sharkia
area), Egypt. The main implications of the results at the
investigated area are:
1. The TDS change maps indicated that there is a
dynamic/transition zone of brackish water at the
Fig. 12 The predicted TDS
map based on the deduced
polynomial of inverse first order
relationship (Fig. 10) between
the measured TDS and intrinsic
resistivities derived from DC
inversion results
784 Page 16 of 18 Environ Earth Sci (2016) 75:784
123
central part of the area. Random and excessive
groundwater pumping in this zone affect the ground-
water quality by upward flowing of brackish water/
saltwater, i.e. upconing feature.
2. A dramatic increase of saltwater intrusion over time
was observed close to the low-lying coastal area of El
Manzala Lake. Accordingly, a continual prediction and
management of the potential impact of SLR is
necessary.
3. At the central part of the area, it was observed that the
aquifer resistivity values can change rapidly within a
short distance reflecting the rapid changes in salinity.
Thus, the area needs an achievable pumping regime
and continuously monitoring system to manage and
prevent the aquifer intensive use and saltwater
intrusion.
4. The constructed geoelectrical cross-sections and 3D
schematic illustration showed that the northern part
suffered from saltwater intrusion in a tongue-like
form. However, going to the south-central and south
parts of the area, the seawater intrusion disappears.
Moreover, a transition zone corresponding to brackish
water was a clearly identified reflection of the
groundwater quality degradation owing to intensive
aquifer exploitation.
5. The DC resistivity measurements identified a zone of
perched freshwater that overlies the brackish water as
well as a lateral heterogeneity within the Quaternary
deposits, which formed a shallow lens-like feature.
6. The groundwater salinity can be predicted at different
depths using the inverse first order polynomial rela-
tionship between intrinsic aquifer resistivity and mea-
sured salinity much faster than conventional drilling
and water sample analysis methods. However, the
derived equation is not expected to apply to other areas
but the methodology will.
7. Although the general conclusions drawn were fruitful,
further work including geophysical tomography is
required to: (1) image the saltwater intrusion and
shallow freshwater lenses and (2) study the mechanism
of TDS lateral rapid changes in the transition brackish
zone. Moreover, an unplanned outcome is that the
transition/brackish zone (i.e. dynamic zone) of
decreasing resistivity might be used to follow the
development of salinization over time.
Acknowledgments We would like to thank the Management of
Supporting Excellence (MSE) at Egyptian Ministry of Higher Edu-
cation, Continuous Improvement and Qualification for Accreditation
Project (PCIQA) for financial support of this work through Applied
Scientific Research project (Project ASRP1-010-ZAG). Authors
would also like to express appreciation to Khaled Gemail, the prin-
ciple investigator (PI) and Zagazig Environmental Geophysics Lab.
(ZEGL) members for great support during all stages of this work.
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