Geophysical Constraints on the Hydrogeologicand Structural Settings of the Gulf of Suez Rift-RelatedBasins: Case Study from the El Qaa Plain, Sinai, Egypt

Mohamed Ahmed • William Sauck • Mohamed Sultan •

Eugene Yan • Farouk Soliman • Mohamed Rashed

Received: 27 May 2013 / Accepted: 17 October 2013 / Published online: 9 November 2013� Springer Science+Business Media Dordrecht 2013

Abstract Groundwater has been identified as one of the major freshwater sources that

can potentially meet the growing demands of Egypt’s population. Gravity data (from 381

ground gravity stations) were collected, processed, and analyzed together with the avail-

able aeromagnetic (800 line-km) data to investigate the hydrogeologic and structural

settings, areal distribution, geometry, and water storage of the aquifers in El Qaa coastal

plain in the southwest Sinai Peninsula, and to assess their longevity given projected

extraction rates. Findings include (1) complete Bouguer anomaly and total magnetic

intensity maps show two connected sub-basins separated by a narrow saddle with an

average basin length of 43 km and an average width of 12 km; (2) two-dimensional

modeling of both gravity and magnetic data indicates basin fill with a maximum thickness

of 3.5 km; (3) using anomalous residual gravity, the volume of water in storage was

estimated at 40–56 km3; and (4) progressive increases in extraction rates over time will

deplete up to 40 % of the aquifers’ volume in 200–230 years and will cause the water

quality to deteriorate due to seawater intrusion in 45 years. Similar geophysical exploration

campaigns, if conducted over the entire coastal plains of the Red Sea and the Gulfs of Suez

and Aqaba, could assist in the development of sound and sustainable management schemes

for the freshwater resources in these areas. The adopted techniques could pave the way

toward the establishment of sustainable utilization schemes for a much larger suite of

similar aquifers worldwide.

M. Ahmed � W. Sauck � M. Sultan (&)Geosciences Department, Western Michigan University, 1903 W. Michigan Avenue, 1187 Rood Hall,Kalamazoo, MI, USAe-mail: mohamed.sultan@wmich.edu

M. Ahmed � F. Soliman � M. RashedGeology Department, Faculty of Science, Suez Canal University, Ismailia, Egypt

E. YanArgonne National Laboratory, Argonne, IL, USA

M. RashedGeophysics Department, Faculty of Earth Sciences, King Abdulaziz University, Jeddah, Saudi Arabia

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Surv Geophys (2014) 35:415–430DOI 10.1007/s10712-013-9259-6

Keywords Gravity � Aeromagnetic � Sinai Peninsula � Groundwater � Sustainable

utilization

1 Introduction

Access to ‘‘potable’’ freshwater resources is a human right and a basic requirement for

economic development in any society. The characterization and understanding of the

geologic and hydrologic settings of, and the controlling factors affecting, freshwater sys-

tems are gaining increasing importance due to the challenges posed by increasing popu-

lations across the world. Such an understanding provides the foundation for the

development and the preservation of these resources in any region. These challenges are

amplified in arid and semiarid areas in the Middle East. One such area is in the country of

Egypt, a country that is seeking additional freshwater resources to support its increasing

population and is in need of such resources to pursue its plans for modernization and

development. The Nile River has been a vital surface freshwater resource for Egypt’s

population and has been used for the development of its agricultural and industrial sectors.

However, Egypt is currently using its total annual allocation of Nile River water, estimated

at 55 9 109 m3/year.

Given the scarcity of surface freshwater resources in Egypt, and the difficulties and

expenses entailed in channeling Nile River water to regions that are distant from the Nile

River valley, groundwater aquifers remain a viable alternative that could address Egypt’s

growing demands for freshwater resources. Egypt’s economy depends largely on the tourism

industry, which has been growing over the years. Along the Sinai coastlines, new cities,

resorts, and tourism facilities are being constructed to support this important industry

(Fig. 1a). The principal limitation on the development of such facilities is the paucity of

freshwater supplies. The identification and further development of groundwater aquifers in

these areas provide viable and cost-effective alternatives to the construction of desalinization

plants and extensive pipelines to channel freshwater from the distant Nile River valley.

In the Sinai Peninsula, highly productive wells are tapping aquifers that are formed of

fossil, modern, or mixtures of fossil and modern water (e.g., Sultan et al. 2011). These

include Quaternary alluvial, Lower Miocene sandstone, Jurassic to Cretaceous Nubian

sandstone, and Precambrian fracture basement aquifer systems. Determining the aquifer

geometry, volume of water in storage, and current and projected extraction rates is critical

for the development of sustainable management of these systems. We investigate these

parameters for one of most significant Gulf of Suez rift-related aquifers, the El Qaa Plain,

the main source of fresh water for the cities of El Tor, and Sharm El Sheikh (Fig. 1a). In

our investigations, we use (1) gravity and aeromagnetic data to map the aquifer areal

distribution and subsurface geometry, (2) gravity data to estimate the volume of water in

storage, and (3) the extracted information together with hydraulic gradient data to assess

the projected impacts of current management schemes on the longevity of these aquifers.

2 Geologic and Hydrogeologic Settings

The Gulf of Suez rift, the northward extension of the Red Sea rift (Said 1990), is an

inactive intercontinental rift (length 300 km; width 80 km) created by the stretching and

collapse of the continental crust (Garfunkel and Bartov 1977) between the Sinai micro

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plate and the African plate (Landon 1994; Moustafa 2002) (inset; Fig. 1a). The rift started

in early Miocene (Alsharhan 2003; Moustafa 1993; Patton et al. 1994) and was associated

with uplift; the rift shoulders were elevated by as much as 4 km, exposing the underlying

crystalline rock and the overlying thick (up to 2.5 km) sedimentary successions to

extensive erosion (Garfunkel and Bartov 1977). The rifting was largely accommodated by

Fig. 1 a Geologic map of the El Qaa Plain region showing the distribution of Precambrian outcrops and thePhanerozoic rock units along the Gulf of Suez eastern margin [modified from Klitzsch et al. (1987) andMoustafa (2004)]. Also shown are the rift boundary (thick black line), main cities (red circles), main road(purple line), field gravity data (polygon a), and aeromagnetic data (polygon b). Inset shows the transferzones (orange and blue lines), and the thermal springs (colored crosses). b Geologic cross section along lineX–X0 shown in Fig. 1a modified from Moustafa (2004). c Geologic cross section along line Y–Y0, shown inFig. 1a, showing the generally SW-dipping normal faults and NE-dipping bedding and the aquifers unitsmodified from Pivnik et al. (2003)

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extensional normal faults that strike north and northwest, forming a complex array of half-

grabens and asymmetric horsts (Pivnik et al. 2003). The uplift eroded the thick sedimentary

successions, whereas the extensional faults preserved these successions as subsided blocks

(half-grabens) under the Gulf of Suez and the marginal coastal plains. The subsided blocks

were subsequently filled by syn- and post-rift clastics shed from the uplifted blocks. The El

Qaa Plain basin, which is bounded to the east by a major extensional fault (Said 1990)

(Fig. 1a), is an example of this kind of basin. The rift-related basins are separated by

accommodation zones (Faulds and Varga 1998; Bosworth 1985; Lambiase and Bosworth

1995; Rosendahl et al. 1986). The latter are defined as zones of along-strike displacement

transfer between oppositely dipping rift-border and intrarift faults, also termed ‘‘transfer

zones’’ (Morley et al. 1990; Moustafa 1997, 2002), ‘‘interference accommodation zones’’

(Versfelt and Rosendahl 1989), or ‘‘hinge zones’’ (Alsharhan 2003). Two accommodation

zones (inset; Fig. 1a) were recognized north and south of the El Qaa Plain area, namely the

Morgan and the Zaafarana accommodation zones, respectively (Moustafa 1997).

Unfortunately, the pre-, syn-, and post-rift stratigraphic sections are not well defined in

the El Qaa Plain area (Fig. 1b). In contrast, the availability of seismic data allows for a

better definition of the stratigraphic section of the surrounding areas. In these areas, the

pre-rift stratigraphic section (Fig. 1b) consists of Precambrian basement that is uncon-

formably overlain by Cambrian to late Eocene sedimentary successions (Said 1990; Patton

et al. 1994; Schutz 1994). Thick sequences of Lower Cambrian–Lower Cretaceous clastics,

dominantly shallow marine to fluvial sandstones (Allam and Khalil 1989) also called the

Nubian Sandstone (Said 1962), are overlain by interbedded sandstones, limestones, and

shales of the Raha, Wata, Matulla, and Duwi formations (Ghorab 1961). These in turn are

overlain by thin Paleocene shales of the Esna Formation and early to middle Eocene

carbonates of the Thebes, Darat, Mokattam, Tanka, and Maadi formations (McClay et al.

1998; Patton et al. 1994). The pre-rift strata are unconformably overlain by late Oligocene–

early Miocene red beds and volcanics that were deposited in isolated, fault-bounded basins

(Sharp et al. 2000; Said 1962, 1990). These successions were followed by early to middle

Miocene, thick (up to 6 km) (Bosworth et al. 1998) syn-rift clastics of the Gharandal and

Ras Mallab groups (Sharp et al. 2000; McClay et al. 1998). The post-rift strata consist of

Pliocene clastics and Quaternary wadi and sabkha deposits (Moustafa 2004).

The El Qaa Plain basin has four aquifers (Quaternary alluvium, Lower Miocene clastics,

Nubian Sandstone, and Precambrian fractured basement; Fig. 1c) and three aquitard units

(massive basement, middle calcareous division, and Upper Miocene evaporites) (Gorski

and Ghodeif 2000). These aquifers are being recharged by modern precipitation and dis-

charge along deep-seated faults of the underlying Nubian Sandstone Aquifer waters (Sultan

et al. 2007). The Nubian Sandstone Aquifer is considered to be a ‘‘fossil’’ aquifer, a type of

aquifer believed to be largely recharged under previous wet climatic periods (Sturchio

et al. 2004). Regionally, the fossil groundwater in the Nubian Sandstone Aquifer moves

northward, toward the Mediterranean Sea, but locally in the El Qaa Plain, flow is west-

ward, toward the Gulf of Suez, and the aquifer discharges along the extensional faults

bounding the Gulf of Suez at high temperatures (up to 70 �C) within the coastal plain of the

Gulf of Suez and along its coastline (Sturchio et al. 1996).

Using geochemical and geophysical data and applying hydrologic models, it has been

demonstrated (Sultan et al. 2011) that the alluvial aquifers in Sinai, including those flooring

the El Qaa Plain, are receiving modern meteoric contributions in intervening dry climatic

periods such as the present time. Using isotopic and geochemical analyses along with field

and remote sensing data, Sultan et al. (2007) showed that discharge of the Nubian Sand-

stone Aquifer is occurring on a larger scale, primarily through deep-seated fault systems,

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and that ascending groundwater discharges into relatively thick alluvial aquifers proximal

to the fault complex that defines the Nile River and the Gulf of Suez, including the El Qaa

Plain.

3 Data Acquisition and Processing

3.1 Land-Based Gravity Data

Data from a total of 381 ground gravity stations were collected throughout four field

seasons (December 2003, May and December 2005, and September 2011) in the El Qaa

Plain area (Fig. 1a, polygon a; Fig. 2). The Worden Master gravimeter, which has a

sensitivity of 0.01 mGal, was used in the gravity data collection. Each surveying day

started and ended with base station readings. Most of the gravity readings were taken along

profiles trending N60E, were tied by northwest (NW)–southeast (SE) traverses, and cov-

ered an 85 km 9 20 km area. The station interval along a typical profile ranged from 0.5

to 1.0 km, and the profile separation ranged from 1.0 to 5.0 km. Accurate horizontal and

vertical positioning of the gravity stations was done using paired Magellan ProMark X or

paired Trimble global positioning systems (GPS).

Gravity data reduction started with the differential processing of the daily GPS data for

accurate horizontal and vertical positioning of each gravity station. Following the accurate

positioning, a conventional processing regime was applied to the raw gravity measure-

ments including drift, tide, latitude, free-air, and Bouguer corrections. Terrain correction

was also applied using a 30-m horizontal resolution digital elevation model (DEM) data to

produce the complete Bouguer gravity map (Fig. 3a).

3.2 Aeromagnetic Data

A total of 800 line-km of aeromagnetic data in the northern El Qaa Plain area were acquired by

purchase (Fig. 1a, polygon b; Fig. 2). The aeromagnetic data were part of an airborne survey

conducted by the exploration division of the Egyptian Nuclear Material Authority in March

and April of 1998. A Scintrex Cesium magnetometer was used in data acquisition with in-

flight sensitivity of 0.001 nT, sampling at 0.1 s, and an average terrain clearance of 120 m.

Twenty-seven NW–SE-oriented paths covering a 65 km 9 15 km area with a flight path

separation of 1.0 km were acquired over the northern part of the El Qaa Plain area.

A conventional processing regime was applied to the raw total magnetic field data by

the Egyptian Nuclear Material Authority to produce residual magnetic anomalies (Fig. 3b).

The applied processing steps include the removal of the diurnal variation effects, heading

corrections, lag correction, removal of the International Geomagnetic Reference Field

(IGRF), and leveling (Elsirfe et al. 1998).

4 Discussion and Findings

4.1 Basin Geometry and Outlines

Both gravity and magnetic anomalies reveal the spatial distribution and relief of subsurface

basement rocks, which in turn mimic the subsurface basin depth and geometries in the

study area. Examination of the complete Bouguer gravity anomaly map [Fig. 3a; contour

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interval (CI): 2 mGal] shows a NW–SE-trending gravity low, parallel to the Gulf of Suez,

occupying the central and northern parts of the mapped area. This anomalous feature

indicates a thick sedimentary section (i.e., basin) overlying a complex trough of basement

rocks. Contour spacing is closer along the east-northeast (ENE) side of the basin, indi-

cating the location of the controlling fault of the half-graben. The gravity low anomaly

feature is expressed as two elliptical shapes, A: -45 mGal and C: -48 mGal, which

represent two sub-basins connected by a narrow saddle, B: -42 mGal (Fig. 3a). The two

basins are connected to the Gulf of Suez by a north–south (N–S) trending channel like

feature of low gravity (D: -35 mGal; Fig. 3a). Examination of the southern part of the map

Fig. 2 Location map showing the spatial distribution and the measurement time of the gravity stations(colored circles) and the locations of the flight paths for the aeromagnetic survey (black lines). Also shownare the locations of the 2D modeling profiles (purple lines)

420 Surv Geophys (2014) 35:415–430

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Fig. 3 a Complete Bouguer anomaly (CI: 2 mGal), and b total magnetic intensity (CI: 10 nT) maps of theEl Qaa Plain area

Surv Geophys (2014) 35:415–430 421

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shows a gravity high (9 mGal; Fig. 3a), which indicates the presence of shallow basement

rocks to the south, or the presence of a high-density intrusive body within the country rock.

The residual magnetic anomalies map (Fig. 3b; CI: 10 nT) shows a NW–SE magnetic

low parallel to the Gulf of Suez. The magnetic low consists of two (A: -100 nT, and C:

-70 nT; Fig. 3b) near-circular magnetic anomalies separated by a narrow saddle (B: -40

nT; Fig. 3b). These anomalous features represent two subsurface basins with a thick

sedimentary section overlying deep basement rocks. These anomalous features are

equivalent to those shown in the complete Bouguer anomaly map (Fig. 3a). The difference

in the geometries between gravity and magnetic anomalies is attributed to one or more

parameters, including induced and remnant magnetization(s); differing magnetic suscep-

tibility of rocks having the same density; and the depth, strike, and magnetic inclination

and declination of the source bodies.

The gravity/magnetic field analytic signal maps, which are based on derivatives (hor-

izontal and vertical), were used to outline the areal distribution of the sources causing the

gravity/magnetic anomalies. Analytic signal represents a combination of the three deriv-

atives [horizontal (X and Y) and vertical (Z)]. Features striking perpendicularly, or at high

angles, to the direction of the applied derivatives were enhanced. The analytic signal was

used to define and delineate the main boundaries of the features causing the gravity and

magnetic anomalies, regardless of the structural dip of these features and independent of

the direction of the induced and/or remnant body magnetizations. The boundaries of these

features are located directly beneath the analytic signal maxima (Nabighian 1972; Roest

et al. 1992; MacLeod et al. 1993; Saibi et al. 2006; Doo et al. 2009).

Examination of the analytic signal maps of both gravity (Fig. 4a) and magnetic

(Fig. 4b) data generally shows the boundaries of the subsurface basins located beneath the

maximum amplitude of the analytic signal. These boundaries, in the case of the gravity

map, represent the subsurface faults (Fig. 4, black line) controlling the NE margin of the

basins, where high-density basement is juxtaposed next to lower-density basin fill. The two

investigated basins are bounded to the NE by normal faults striking NW–SE, parallel to the

Gulf of Suez direction. Curiously, neither the magnetic nor the gravity analytic signal maps

show the master half-graben fault as a continuous feature (although it probably is). These

maps also have broad areas of diffuse medium gradient; this may be related to choices of

parameters for the analytic signal processing rather than having real geologic significance.

Both maps do show a feature just N of El Tor that may be one or even a pair of intersecting

local faults. Unfortunately, the magnetic map does not extend far enough S to cover the

entire feature. This does coincide with the sudden SE termination of the tilted ridge, as well

as the location of the high-discharge Moses’ Spring. Figure 4 also gives an estimate of the

subsurface basin geometries (i.e., length and width). The average NW–SE length of the

basin is estimated to be 43 km. The basin width varies along the main NW–SE extension

of the basin. The average width at the north is estimated to be 12 km, which decreases to

7 km at the center of the basin, then reaches its maximum of 14 km at the southern parts of

the basin.

Calculation of the depth to the basement rocks is an important step in defining the

spatial variations in the thickness of the sedimentary cover and delineating the structural

relief of the underlying basement and its effect on the overlying sedimentary units. In this

study, a two-dimensional (2D) modeling was carried out simultaneously for both gravity

and magnetic data. The 2D modeling was carried out after removing the linear regional

field from each of the selected profiles. A trial and error technique was used to adjust the

initial model to match the observed anomalies, where the configuration of the model layers

was iteratively modified for best fit between the observed and the computed anomaly data.

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Fig. 4 The analytic signal map of a gravity field, and b magnetic field data over the El Qaa Plain. Also shownare the subsurface faults (black lines) controlling the distribution and geometries of the subsurface basins

Surv Geophys (2014) 35:415–430 423

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Fig. 5 2D modeling of both gravity (red) and magnetic (blue) data along the profiles A–A0, B–B0, C–C0, andD–D0. The profile locations are shown in Fig. 2. D density and k magnetic susceptibility

424 Surv Geophys (2014) 35:415–430

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An understanding of the subsurface geology, as well as knowledge of the physical prop-

erties (e.g., density, magnetic susceptibility), and the total magnetic intensity, inclination,

and declination of the magnetic field for the investigated sequences are needed to develop

sound 2D models.

The density and magnetic susceptibility data used in this study were compiled from

previously published studies (Setto 1991; Omran 1982; Ismail 1998; Selim 2002; Sultan

et al. 2009; Rabeh and Miranda 2008; Rabeh et al. 2009) in areas surrounding, and with

similar geologic conditions to, the El Qaa Plain. The parameters of the geomagnetic field

were extracted from the National Oceanic and Atmospheric Administration (NOAA)

National Geophysical Data Center (NGDC) website (available at www.ngdc.noaa.gov/

geomagmodels/IGRFWMM.jsp). We created a three-layer, 2D model with two clastic

layers overlying a layer of basement rock with densities (from top to bottom) of 2.47 gm/cm3

(syn- and post-rift clastics), 2.52 gm/cm3 (pre-rift sediments), and 2.67 gm/cm3, respec-

tively, and magnetic susceptibilities of 0 cgs, 0 cgs, and 2.3 9 10-3 cgs, respectively,

where the total magnetic field, inclination, and declination were found to be 42,187 nT,

40.9�, and 2.4�, respectively.

Four 2D gravity and magnetic profiles (location; Fig. 2), trending NE–SW (A–A0, B–B0,and C–C0) and WNW–ESE (D–D0), covering the anomalous features (A, B, C, and D;

Fig. 3) were selected for modeling. All of the selected profiles show (1) a uniform

thickness (*1.5 km) of the pre-rift section; (2) uniform geometry (east dipping) of the pre-

rift sediments; and (3) bounding SW-dipping faults at the contact of the sedimentary

section with the basement rocks. Examination of the cross sections A–A0 and C–C0 (Fig. 5)

reveals a dipping and thickening of the syn- and post-rift sedimentary sections toward the

east. The thickness of the syn- and post-rift section increases from the west to the east (A–

A0: 1.5 km; C–C0: 2 km). Inspection of cross section B–B0 reveals a thick (* 1.5 km) syn-

and post-rift section at the central part of the profile that decreases in thickness toward the

eastern and western margins. Cross section D–D0 shows only the pre-rift section with a

maximum thickness (*0.7 km) at the central part that also decreased toward the east and

west margins of the profile.

4.2 Volume of Water

While an infinite number of geometries and density distributions can give rise to a single

residual gravity map of an area, a remarkable property of the gravity method is that the

total anomalous mass is precisely determined by the areal integration of the residual

gravity anomaly. The total anomalous mass (DM; missing in this case) is calculated using

Gauss’s theorem (Ramsey 1949) and is directly related to the integral of the residual

gravity anomalies (Dg(x, y)) over each unit cell (dxdy) included in the selected basin:

DM ¼ 1

2pG

Z1

�1

Z1

�1

Dg x; yð Þdxdy

If we assume that the missing mass is entirely due to two layers, air-filled pore space in

the unsaturated zone and water-filled pores below, it is possible to calculate the mass, and

hence the volume, of water in the basin. The adopted model depicts a basin of an area (S)

filled with alluvium sediments, with the lower surface of the alluvium bounded by bedrock.

Above the water table, the alluvium sediments with a thickness (h) have a matrix density

(q: 2.67 gm/cm3), and the pore spaces within the sediment are filled with air [qair: 0 gm/cm3;

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density contrast (Dq2): -2.67 gm/cm3], whereas below the water table, the pore spaces are

saturated with water (qw: 1 gm/cm3; Dq1: -1.67 gm/cm3). The total anomalous missing

mass (DM) is the sum of the missing mass below the water table (DM1: saturated zone) and

the missing mass above the water table (DM2: unsaturated zone).

DM ¼ DM1 þ DM2

DM2 ¼ Dq2hS

DM1 ¼ DM�Dq2hS

In this study, we used the -36 mGal contour line of the complete Bouguer anomaly

map (Fig. 4) to define the basin boundary. The basin area (S), the integral of the residual

gravity anomalies within that basinR1�1R1�1 Dg x; yð Þdxdy

� �, and the total mass (DM) were

estimated at 518.5 km2, -2.76 9 104 (m/S2) m2, and -6.59 9 1013 kg, respectively.

Knowing the matrix density, porosity, and depth to water, one can calculate the unsaturated

zone mass (DM2). We used a porosity (/) of 20 % and an average depth to water table of

35 m (Sultan et al. 2009). The masses missing due to the unsaturated zone (DM2) and the

saturated zone (DM1) were estimated at -9.69 9 1012 kg and -5.62 9 1013 kg, respec-

tively. Knowing the missing mass in the saturated zone, the volume of water Vw ¼ DM1

qw

� �in the basin can be calculated. We estimated the volume of water to be approximately

56 9 109 m3. More details about volume of water calculations are shown in Table 1. If the

porosity of the unsaturated zone is 30 % instead of 20 %, then it accounts for more of the

missing mass, and the mass of water required for the mass balance will be correspondingly

less.

It is possible that part of the missing mass could be due to the presence of evaporites in

the early syn-rift sedimentary section. Based on the reported occurrences of evaporites in

the Gulf of Suez and its surroundings, the evaporites, if present, are likely to be halite (q:

2.2 gm/cm3) or anhydrites (q: 2.85 gm/cm3; Telford et al. 1990). In either case, the cal-

culations conducted assuming that all the missing mass is solely related to water-filled pore

spaces would have to be corrected to account for contributions from the presence of

Table 1 Volume of water calculations

Parameter Value Remarks

S (m2) 5.185 9 108 Basin areaR1�1R1�1 Dg x; yð Þdxdy[(m/S2) m2] -2.762 9 104 Integral of the residual gravity anomalies

DM (kg) -6.590 9 1013 Total missing mass

/ (%) 0.20 Porosity

h (m) 35 Depth to water

DV2 (m3) 3.630 9 109 Volume of unsaturated zone

q (kg/m3) 2.670 9 103 Matrix density

qair (kg/m3) 0 Density of air

Dq2 (kg/m3) -2.670 9 103 Density contrast in unsaturated zone

DM2 (kg) -9.691 9 1012 Mass of unsaturated zone

DM1 (kg) -5.621 9 1013 Mass of saturated zone

qw (kg/m3) 1.000 9 103 Density of water

Dq1 (kg/m3) -1.670 9 103 Density contrast

Vw (m3) 5.621 9 1010 Volume of water

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evaporites. The corrections for the presence of the anhydrite would increase the calculated

water volume, while the corrections for the presence of the halite would decrease the water

contribution to the negative anomaly. Inspection of published geologic maps (Klitzsch

et al. 1987) and available sparse borehole information did not reveal the presence of

significant evaporite successions in the El Qaa Plain basin. A minimal figure for water

volume can be obtained by presuming the presence of a 50 m thickness of halite, which

will reduce the volume of water to 40 9 109 m3 if we also assume the water table to be at

45 m (rather than 35 m) and the unsaturated zone porosity (/) to be 25 % (instead of

20 %).

4.3 Sustainable Utilization of El Qaa Plain Aquifers

Extraction rates from the Quaternary aquifer in the El Qaa Plain are on the rise. They were

0.18 9 106 m3/year in 1930 (Attia 1930), 1.1 9 106 m3/year in 1972 (Gilboa 1972),

9.5 9 106 m3/year in 2000 (Gorski and Ghodeif 2000), and 11.0 9 106 m3/year in 2011.

Given these progressive extraction rates over the past 80 years, we estimate (using a

second-order polynomial function) that extraction in 2020, 2050, and 2100 will reach

15 9 106, 30 9 106, and 65 9 106 m3/year, respectively.

The aquifer receives recharge from (1) infiltration from modern precipitation over the El

Qaa Plain and adjacent Sinai mountains, and (2) discharge from the underlying Nubian

Sandstone Aquifer that is occurring primarily through deep-seated fault systems where

ascending groundwater discharges into relatively thick alluvial aquifers bounded by the

fault complex within the El Qaa Plain (Sultan et al. 2007). A reasonable assumption to

make is that the system was at near steady-state conditions in the 1930s, given the small to

negligible extraction rates at the time. Under steady-state conditions, the recharge from the

ascending Nubian Sandstone Aquifer waters and from modern precipitation is balanced by

discharge to the Gulf of Suez.

If we adopt the modeled extraction rates described above, we estimate that in a period of

200–230 years, the volume of the El Qaa Plain Quaternary aquifer will be depleted by

40 %. The installation of water wells in a number of western states in the USA is regulated

so that the maximum allowed depletion does not exceed 40 % of the aquifer by volume

(McGuire et al. 2000). Depletion of aquifers beyond these limits could have significant

effects on aquifer recovery, stream and river systems, and riparian ecosystems. These

estimates are worst-case scenarios, given that the underlying assumption ignores the

increase in groundwater extraction in the Quaternary aquifer, which will potentially reduce

groundwater discharge, induce more ascending inflow from the underlying Nubian

Sandstone Aquifer, and increase groundwater lateral inflow to the El Qaa Plain Quaternary

aquifer from the surrounding areas. These potential gains, if considered, would partially

compensate for the increased groundwater loss from well extraction, resulting in an

increase in the estimated period for aquifer depletion. These estimates also ignore the

potential impact of increased extraction on seawater intrusion, which will affect the water

quality of the El Qaa Plain.

The mean groundwater level in the El Qaa Plain exceeds 20 m above mean sea level

(amsl). The eastern section of the plain has a gentle gradient (\ 0.0003) compared to the

western section (0.003) (Gorski and Ghodeif 2000). A first-order estimate for the maxi-

mum drawdown that can maintain a hydraulic gradient similar to that of the present is

approximately 10 m amsl (average for entire area). If we were to adopt the modeled

extraction rates described above, we estimate that the water levels will drop by 10 m in

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45 years. At this time, the aquifer will be threatened by seawater intrusion and its water

quality will be undermined.

5 Summary and Implications

We applied an integrated approach including the analysis of ground gravity, as well as

aeromagnetic data to identify the structural and hydrogeologic settings, the areal distri-

bution and geometry of aquifer units, water volume, and water depletion rates in one of the

Gulf of Suez rift–related basins, El Qaa Plain basin in SW Sinai. Analysis and interpre-

tation of both gravity and magnetic data revealed a NW–SE-trending basin, parallel to the

Gulf of Suez, with an average length of 43 km and an average width ranging from 7 km

(central section) to 14 km (southern section). The 2D modeling of both gravity and

magnetic data indicates basin fill with a maximum thickness of 3.5 km. Using anomalous

residual gravity techniques, the volume of water in storage was estimated to range from

40 km3 (conservative limit) to 56 km3 (upper limit) in the selected basin.

Our findings indicate that while, over time, the progressive increase in extraction rates

could alleviate the increasing demands for freshwater supplies for the rapidly growing

tourism industry in the region, it will have some serious adverse effects on the aquifer. The

aquifer will be depleted by up to 40 % of its volume in 200–230 years and will cause the

water quality to deteriorate due to seawater intrusion in 45 years. The adopted techniques

used in this analysis could be readily extended to investigate the setting and sustainable

utilization of several similar fossil aquifers on the west side of the Gulf of Suez and along

the Red Sea coastal areas. Moreover, these adopted techniques could pave the way toward

the establishment of sustainable utilization schemes for a much larger suite of similar fossil

aquifers worldwide.

Acknowledgments Research is supported by a NATO Science for Peace (Grant SFP 982614) awarded toWestern Michigan University. We thank Dr. Kamal Ghodeif for providing static water level measurements.We also thank Dr. Khaled Mamoun, Mr. Islam Nagi, and Mrs. Lamees Mohamed for assisting in thecollection of the 2011 gravity data.

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