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Hydrogeochemical and vertical electrical soundings for groundwater

investigations, Burg El-Arab area, Northwestern Coast of Egypt

Mohamed G. Atwia a,, Mohamed M. Abu-Heleika b, Mohamed M. El-Horiny a

a Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egyptb Geology Department, Faculty of Science, El-Minia University, El-Minia 61111, Egypt

a r t i c l e i n f o

Article history:

Received 9 February 2012

Received in revised form 10 October 2012

Accepted 1 November 2012

Available online 28 November 2012

Keywords:

Hydrochemistry

Vertical electrical soundings (VESs)

Burg El-Arab

Egypt

a b s t r a c t

An integrated geological, hydrochemical, and geoelectrical investigation of shallow groundwater occur-

rence in Burg El-Arab area, northwesterncoastal zone of Egypt is carried out. Groundwater of oolitic lime-

stone and clastic aquifers is the principal source of water supply for agriculture in the area. The purpose

of this study is to describe the hydrogeologic characteristics of aquifers and to provide a general evalu-

ation of the chemical quality of water in aquifers. Chemical analysis was used to evaluate the chemical

characteristics of groundwater and assessment of water quality. Electrical soundings were employed

to delineate different water bearing formations and the configuration of the interface between them.

Thirty-four water samples were collected and chemically analyzed from the two main aquifers in the

area. Groundwaters of oolitic limestone aquifer are dominated by NaCl and have average TDS of approx-

imately 2830 mg/l. Groundwater samples from clastic aquifer are slightly weakly mineralized (TDS

approximately 2700 mg/l) and dominated by CaSO4. Thehydrochemical data indicate that the groundwa-

ter is of meteoric origin. The variation in the chemistry of water is thought to be related to the weathering

of minerals of the water-bearing sediments, mixing with marine water, and leaching of fertilizers in the

newly reclaimed areas. Groundwater of the area can be used for irrigation under special circumstances

management as the sodium hazard is medium while the salinity hazard ranges from high to very high.

Thirty-four profiles of vertical electrical soundings (VESs) were obtained in Burg El-Arab area to exam-ine the variations of subsurface geology and associated groundwater chemistry. Resistivity and thickness

of aquifers, resistivity of the unsaturated zone and depth to the confining bed have been delineated from

the interpretation of electrical sounding data. The range of electrical resistivity values have been assigned

to different layers by calibrating electrical resistivity with the borehole data. Results of the vertical elec-

trical soundings and the hydrochemistry of the groundwater samples show that the brackish groundwa-

ter is dominated in the study area whereas the fresh groundwater is found as isolated patches in oolitic

limestone aquifer.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

The rapid expansion of land reclamation projects in desert

areas, growing population and civilization have resulted in an

increasing consumption and demand of water for irrigation and

domestic uses. Northwestern Coast Zone of Egypt, especially Burg

El-Arab area, is considered as a favorable district for further devel-

opment due to the following reasons. The Egyptian government

has already established good infrastructure in this area to encour-

age agricultural and industrial sectors to invest in it. Moreover,

Burg El-Arab is very close to Alexandria (about 50 km) the second

largest and important city in Egypt. So, a large number of its pop-

ulation will be attracted to move and settle in this new area.

Surface water resources in Egypt are limited and fully used through

much of the Nile Valley and Nile Delta regions. Consequently,

groundwater is an important source of water for new reclaimed

desert areas.

Surface water sources in Burg El-Arab area are limited in time

and space and are represented by Bahig and El-Nasr canals

(Fig. 1). Groundwater of oolitic limestone and clastic shallow

aquifers is the most important alternative source of water for agri-

culture in the region. Groundwater used for irrigation in Burg El-

Arab began in the late 1960s with drilling number of shallow wells

(

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Most of the reclaimed desert areas in Burg El-Arab employ flood-

irrigation system. A continuous increase of groundwater abstrac-

tion from different water wells in the area has led to degradation

of water quality, a rapid rising of water levels in some areas, and

extended of sea water invasion.

The main purpose of the present study is to provide information

on the subsurface lithology and groundwater occurrence for hydro-

geologic interpretation using an integration of the geoelectric

resistivity measurements, lithologic information from boreholes,

and hydrochemical data obtained from irrigation wells.

Water samples were collected from 34 wells (Fig. 2) represented

the study area during the years 20082009 and were analyzed later

to determine the chemical characteristics of groundwater. Theinvestigation also included reviewing the published literatures

and geologic maps, obtaining data from the existing wells, and con-

ducting the electrical resistivity surveys. Information derived from

this study will aidin planning futureland use and developing water

supplies for irrigation use.

2. Location and climate

Burg El-Arab is one of the big cities located in the Western

Mediterranean coast zone (Fig. 1). It is bounded by latitudes 30

45031 000N and longitudes 29 28029 450E and characterized

by Mediterranean climate with high relative humidity (annual

mean 66%). Summer temperatures are mild with 32 C as a maxi-mum while winter temperatures average 1321 C. The average

annual rainfall is about 150200 mm/year, most of which occurs

during the wet seasons (OctoberApril). Potential evaporation is

about 1400 mm/year in coastal areas of the Mediterranean Sea

(FAO, 1984).

3. Physiography

Burg El-Arab area and its surroundings can be subdivided into

four distinct geomorphologic units. From north to south, these

units are: the coastal plain, piedmont plain, the tableland and the

hydrographic basin (Fig. 1). The average altitude of this area is

around 30 m above sea level with a maximum elevation of about80 m, sloping gradually to the north towards the sea (El-Shazly

et al., 1975).

The coastal plain extends from the coastal line southward to

the piedmont plain or to the tableland constituting a varying

number of parallel ridges. The area is characterized by the develop-

ment of successive ridges running parallel to the present coast

(El-Bayomi, 2009). Four distinct ridges have been distinguished:

the coastal (first) ridge, El-Max-Abu Sir (second) ridge, Gebel Mari-

ut (third) ridge, and Khashm El-Ish (fourth) ridge (Fig. 1). The

ridges are made of white oolitic and pseudo-oolitic calcareous sand

grains. These ridges are separated from each other by sabkha-la-

goonal depressions (Embabi, 2004).

The piedmont plain occupies the area between Gebel Mariut

ridge in the north and Mariut tableland in the south. Alluvialdeposits of piedmont plain are spread over the surface of flat

Fig. 1. Location map and physiographic features of Northwestern Coast of Egypt.

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terrain forming thin sheets mixed with aeolian sand (Guindy,

1989). Thicknesses of piedmont plain materials are variable, but

in some localities between Bahig and El-Hammam locality it might

reach to about 10 m.

Mariut tableland constitutes a prominent geomorphic unit

bounding the coastal and piedmont plains of Burg EL-Arab area

(Fig. 1). It is composed mainly of limestone and rises to an eleva-

tion of 6080 m above sea level (Shata, 1971).

The hydrographic basin is represented in the area by Abu-Mina

basin which is a shallow erosional depression and located in the

south-eastern part of the study area. The surface of the basin is

covered by alluvial deposits of Holocene age (CEDARE, 2009).

4. Geology and hydrogeology framework

The study area is generally covered by sedimentary rocks

belonging to the Holocene, Pleistocene and late Neogene. However,

the depression areas may be occupied largely by alluvial deposits

and dune sand accumulations.

The Pleistocene sediments can be tentatively differentiated into

two units. The first one is the oolitic limestones which have a wide

exposure in the coastal strip of the Mediterranean Sea. These sed-

iments are composed of detrital limestone associated with calcar-

eous clayey soil (Abdel Mogheeth, 1968). Economic gypsum

deposits are known in El-Gharbaniate area in the lagoon separating

the third and the fourth ridges (Adindani et al., 1975). El-Sharabi

(2000)described the gypsum in this region as elongated lenses of

variable widths and thickness that gradually changes at the outeredges to gypsiferous limestone and limestone. The second unit

(40 m thickness) is mainly in Abu-Mina basin and composed of flu-

viomarine facies of sands, clays and gypsiferous clays (Mohamed

et al., 1979).

The Pliocene sediments are not exposed at the surface in Burg

El-Arab area. It underlies the oolitic calcareous bars of the Pleisto-

cene sediments in the coastal zone and overlain by clastic sedi-

ments of the Pleistocene age in Abu Mina basin. The Pliocene

sediments overlie Moghra Formation along the western Mediterra-

nean coastal area. The Pliocene sediments may be subdivided into

two units. The upper unit is dominated by loose sands and a few

thin layers of limestone. The maximum thickness of the upper unit

reaches about 100 m. The lower unit (about 70 m thick) is domi-

nated by dark mudstone and shales. A simplified geological section

for the study area is presented inFig. 3.The main aquifer units in Burg El-Arab area are a consolidated

detrital oolitic limestone (Pleistocene) in the coastal zone areas

and Pleistocene clastic sediments in Abu Mina basin area. There

are several less important aquifers (e.g. unconsolidated coastal

dune aquifer (Holocene), and multilayered aquifer consisting of

alternation of sand and clay (upper Pliocene).

The oolitic limestone aquifer is well developed in the coastal

and piedmont plains and extends towards the tableland. These

sediments are thin generally towards the south and east until they

disappear at an average distance of 25 km south of the Mediterra-

nean coast (Shaaban, 2001). The thickness of this aquifer varies

from20 m at the central of coastal plain to about 50 m at the coast.

Groundwater occurs under water table conditions in the oolitic

limestone aquifer. The hydraulic conductivity of this aquifer isabout 19 m/day (Hilmy et al., 1977). Most recharge to the oolitic

Fig. 2. Map showing the locations of boreholes, vertical electrical soundings, and the different selected profiles in the study area.

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Fig. 3. Cross-section showing geological units (after RIGW (1991)).

Fig. 4. Equipotential lines (groundwater level contours) based on piezometric data in 2008.

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limestone aquifer takes place through the precipitation falling di-

rectly on the area. Some recharge may occur through excess irriga-

tion water, seepage from irrigation canals, and the Mediterranean

Sea. The groundwater discharged naturally to Mallahet Mariut,

coastal marshes, and northward to the Mediterranean Sea. It is also

discharged through evaporation and pumping. The depth of water

level from the ground surface varies from 3 m to 25 m. Ridges and

depressions in Burg El-Arab area control the groundwater flow pat-

tern. The flow of groundwater in this aquifer is due north and

northeast (Fig. 4).

Pleistocene clastic aquifer in Abu Mina basin is mostly built up

of fluviomarine facies of sands, clays and gypsiferous clays.

Groundwater occurs in this aquifer under semi-confined condi-

tions as the clastic sediments are covered with alluvial deposits

of Holocene age. The clastic sediments have a thickness varying be-

tween 40 and 60 m. Transmissivity ranges from less than 500 m2/

day to about 5000 m2/day (CEDARE, 2009). Recharge takes place

through infiltration of rainwater, lateral inflow from the main

aquifer system in the Nile Delta, lateral seepage from El-Nasr canal,

and downward percolation of excess irrigation water. It is dis-

charged by evaporation, evapotranspiration, and through a number

of wells. In the basin, the inflow to the groundwater is principally

from the southwest following the regional topography configura-

tion (Fig. 4). The water level is at a depth of 17 m. The coastal

aquifers mostly contain slightly brackish water.

5. Methodology

5.1. Chemical analyses

Thirty-four water wells were samples in 2008 and 2009 for

chemical analysis of water; 24 samples for the oolitic limestone

aquifer (wells nos. 124) and 10 samples (wells nos. 2534) from

the Pleistocene clastic aquifer (Fig. 2). In addition to that three sur-face water samples were also collected from Mariut Lake, Bahig

and El-Nasr canals, and analyzed to identify the chemical charac-

teristics of surface water. These wells are used for irrigation water

and were being pumped during the sampling period. The parame-

ters analyzed or measured were: pH, temperature (C), electrical

conductivity (lS/cm), total dissolved solids TDS (mg/l), sodium

(Na+), potassium (K+), calcium (Ca2+), magnesium (Mg2+), bicarbon-

ateHCO3, chloride (Cl), sulfateSO24 and nitrateNO

3. Analy-

sis of the water samples was carried out following the methods

described inHach (1990).

5.2. Electrical resistivity method

For this study, a total of 34 VES profiles (Fig. 2) were obtained in

2008 and 2009, using the control 42 resistivity meter (Model E85)

with the Schlumberger configuration (A-MN-B). For each VES

profile the distance between the potential electrodes (MN) was

Fig. 5. Distribution of groundwater salinity in the Pleistocene aquifers, Burg El-Arab area.

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gradually increased in steps starting from0.5 m to 50 m to obtain a

measurable potential difference. The half current electrode separa-

tion (AB/2) was usually increased in steps starting from 1.5 m to

300 m.

The field geoelectric data were processed and interpreted auto-

matically by using two geoelectrical softwares, i.e. the ATO com-

puter program of Zohdy (1989) and RESIX-P computer program

of Interpex (1990). An automatic computerized interpretation

method (Zohdy, 1989) has been used to obtain true resistivity

and depth from the measured apparent resistivity data at each site.

The program generates a large number of layers for each sounding

data set. To reduce the number of layers an edge-preserving and

smoothing technique is applied. RESIX-P (Interpex Ltd., 1990)

which is an interactive, graphically oriented, forward and inverse

modeling program was used for interpreting sounding data in

terms of a layered model. Sounding curves were entered as appar-

ent resistivities versus spacing (AB/2) for Schlumberger sounding

array. Estimated resistivities and thickness of layers based on bore-

hole data were used as input parameters for starting model in the

inverse modeling. Inverse modeling produces a model that best fits

the data in a least squares sense using ridge regression by itera-

tively adjusting the parameters of the starting model.

Because the electric resistivity of sediments depend on lithol-

ogy, water contact, clay content, and salinity (Choudhury et al.,

2001), it is important to correlate the VES results with the litholog-

ical and hydrological data obtained from available boreholes in the

study area. Two boreholes W-1 and W-2, close to the VES stations

VES-19 and VES-28, respectively, were used as representative

examples for the correlation.

6. Results and discussion

6.1. Hydrogeochemistry

The results of hydrochemical analyses of groundwater samples

are given inTable 1and the areal distribution of dissolved-solids

concentration is shown in Fig. 5. Chemical characteristics of

groundwater in oolitic limestone vary depending on the rock type

and the geographic conditions of the aquifer in relation to both sea

level and fresh surface water (Bahig canal). The relationship be-

tween TDS values of groundwater samples and well depths reveals

that the groundwater salinity is not clearly affected by changing

well depths (Atta et al., 2007). Groundwater in this aquifer is of so-

dium-chloride type and water salinity ranges from a relatively

fresh water zone (TDS < 1000 mg/l) near to El-Gharbaniat area to

a slightly saline (TDS 10003000 mg/l) in most water samples.

Close to the natural outlet of oolitic limestone aquifer, Mallahet

Mariut and Mediterranean Sea, the groundwater is mostly moder-

ately saline (3000 and 10,000 mg/l TDS) with TDS varying between

3000 and 5000 mg/l. The sequence of both anions and cations in

the water samples has the following order.

Cl> SO24 > HCO

3 and Na> Mg2 > Ca2

Groundwater of Pleistocene clastic aquifer in Abu-Mina basin is

generally slightly to moderately saline and has a TDS ranging be-

tween 1000 and 5000 mg/l (Fig. 5). The concentration of dissolved

solids in groundwater near to El-Nasr canal is generally fresh water

(TDS < 1000 mg/l). In parts of Abu Mina basin the groundwatermay exceed 4000 mg/l due to the rise of the piezometric level

and the exposure of groundwater to increased evaporation.

Groundwater in this aquifer contains sulfate as the dominant anion

and calcium as predominant cation which is attributed to the

occurrence of gypsum in water-bearing formation, the contact of

water with limestone, and widespread of fertilizer use to improve

soil properties (e.g. MgSO4H2O). Most groundwater samples are

characterized by the following sequence for both anions and

cations

SO24 > Cl

> HCO3 and Ca

2> Na > Mg2

The water samples plotted on Pipers diagram (Piper, 1944),

Fig. 6 shows that two groups of water samples can be distinguished

Fig. 6. Trilinear diagram of groundwater in the study area.

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in the diamond shape-field. The first group that includes most of

the water samples from the oolitic limestone aquifer occupies

the lower right hand side of the diamond shape. Its chemical prop-

erties are dominantly characterized by primary salinity and there-

fore, sodium-chloride water type prevails. The second group which

represents the Pleistocene clastic aquifer water samples occupies

the upper right hand side. All waters have secondary salinity char-

acteristics, where chloride and sulfate salts are dominant due to

leaching and sea water intrusion.

Results of chemical analyses were also analyzed using the Sul-

lins (1946)classification method. These data can be used to deter-mine the origin of the groundwater. The data showthat most of the

water samples are of Na2SO4water type of meteoric origin (Fig. 7).

Stable isotopes measurements of water samples taken from wells

in Burg El-Arab area confirm that the main source of recharge in

these aquifers are local precipitation during the rainy seasons

(Nada and Al-Gamal (1996)andRashad (2009)).

Nitrate values ranged from 3.1 to 34.3 mg/l with mean of

16.6 mg/l (as NO3 ) for the groundwater samples from oolitic lime-

stone aquifer and 4.466.4 mg/l (mean 24.55 mg/l) for samples

from the Pleistocene clastic aquifer (Table 1). The relatively high

nitrate concentration in Burg El-Arab area is due to extensive use

of nitrate fertilizers and leakage of sewage from septic tanks.

6.2. Water quality

Most of the water sampled is unsuitable for domestic uses on

account of high TDS, sodium, chloride, sulfate and nitrate content.

The groundwater of the area is only evaluated according to its suit-

ability for irrigation uses. The quality of water used for irrigation is

an important factor in productivity and in quality of the irrigated

crops. TheUS salinity laboratory staff (1954)developed an irriga-

tion classification system based on sodium adsorption ratio (SAR)

and specific conductance.

Irrigation classification for water samples from aquifers in Burg

El-Arab was determined using the salinity Staffs classification

system (Fig. 8). Most of the water in Abu Mina basin has very high

salinity hazard and medium sodium hazard. This type of water

can be used for irrigation if some leaching occurs. Plants with mod-erate salt tolerance (e.g. olive, cucumber, tomato, sunflower,

wheat, etc.) can be grown in most instances without special prac-

tices for salinity control. Groundwater in Pleistocene oolitic lime-

stone has very high salinity hazard and high sodium hazard.

This water may not be suitable for irrigation on soils with poor

drainage without management for salinity control.

6.3. VES curves and borehole data

Fig. 9 shows the VES curve for the site VES-19 that is at the

coastal plain area, in oolitic limestone aquifer. The lithological var-iation, deduced from the borehole W-1, is shown in the left side of

Fig. 8. Classification of groundwater in Burg El-Arab area for irrigation use.

Fig. 7. Sulins graph for groundwater classification, Burg El-Arab area.

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Fig. 9. Based on the correlation between the VES results and the

lithological information from the borehole, three major resistivity

layers are recognized in the coastal plain area (Fig. 9). The topmost

layer, consisting of calcareous loam mixed with organic matter, is

represented in the geoelectrical column by two units of which

the upper one has a thickness of less than 1.5 m and an average

resistivity of 22 Ohm-m. The underlying layer has a thickness of

about 7 m with an average resistivity of 2 Ohm-m. This decrease

in the resistivity from the surface downwards is attributed the in-

crease in moisture content. The second resistivity layer shows

resistivities below 50 Ohm-m and is interpreted to coincide with

a water saturated oolitic limestone aquifer. The third geoelectric

Fig. 10. Representative examples of the VES (VES-28) calibration with the lithology of the nearest well (well W-2).

Fig. 9. Representative examples of the VES (VES-19) calibration with the lithology of the nearest well (well W-1).

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zone is the lowermost interpreted zone, which extends from the

base of the overlying oolitic limestone to the maximum depth of

investigation. The third layer which has a low resistivity

(

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show that the main aquifer materials are composed of silt and fine

to coarse-grained sand in the upper, and of gravels in the deeper

part (Fig. 10). The first geoelectric zone extends from the ground

surface to a depth which ranges from 1 to 2 m and attains rela-

tively high resistivity (264 Ohm-m). The relatively high resistivity

of this zone can be attributed to fine-grained sediments. The sec-

ond geoelectric zone underlies the first zone and extends to a

depth of 50 m, with resistivity value of 16 Ohm-m. Interpretation

of this zone resistivity, in the light of correlation with the borehole

information, indicates water saturated of aquifer materials which

composed of coarse-grained sand and gravel. The third layer which

has a low resistivity (

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Fig. 14. Contour map of earth resistivity of the main aquifers in the study area.

Fig. 15. Thickness contour map of the water saturated oolitic limestone (A), and combined geoelectrical layer (B) based on interpretation of VESs.

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for oolitic limestone and clastic sediments is shown in Fig. 14. In

general, low resistivities of oolitic limestone occur in the northern

part, towards Mariut lake, toward the center, the values increase

continuously. A comparison of Figs. 14 and 5 suggest that the

occurrence of low resistivities is probably, in parts, caused by high

concentration of TDS. The water bearing formation in Abu Mina ba-

sin has relatively lower resistivities ranging from 3 to 23 Ohm-m

all over the basin (Fig. 14). There is a general southward decrease

of the resistivities values which may be due to the type of the

water present and/or the presence of clay lenses in the aquifer.

The variation of the water-bearing formations thickness in the

study area is shown inFig. 15. The maximum thickness of the oo-

litic limestone is observed in the northwestern part of Burg El-Arab

area. The thickness of the clastic sediments in Abu Mina basin var-

ies from 26 to 58 m. The minimum value (26 m) is found at the

western part of the investigated area, whereas the maximum

thickness (58 m) is found at VES nos. 30 and 31 near El-Nasr canal.

7. Conclusions and recommendations

Two aquifers can be identified in the study area, oolitic lime-

stone aquifer and clastic sediments aquifer. The water is used

mainly to meet the demands of the agricultural sector in the area.

The oolitic limestone aquifer consists of oolitic and pseudo-oolitic

calcareous sand grains with occasional pockets of gypsum and

clays. The thickness of this aquifer ranges from 20 to 50 m; satu-

rated thickness increase towards the north. The oolitic limestone

aquifer acts as a phreatic aquifer. The production rate of wells in

the area ranges from 5 to 50 m3/h. The Pleistocene clastic aquifer

occurs at Abu mina basin in the south-eastern part of the study

area. The water-bearing sediments are mainly of deltaic deposits

of gravelly sand whose thickness ranges from 40 to 60 m. This

aquifer is bounded by Holocene alluvial deposits above and by Pli-

ocene alternating sand and clay below. Groundwater occurs in this

aquifer under semi-confined conditions. Significant rises in aquifer

heads and changes in the flow system have occurred in response to

man-made artificial surface water canals (e.g. Bahig and El-Nasr

canals), exploitation of groundwater, and the accumulation of irri-

gation return flow which finally joins the underlying groundwater.

The water from the oolitic limestone aquifer is a sodium chlo-

ride type. Dissolved solids in samples collected from the aquifer

ranged from 761 to 5110 mg/l. Water from the clastic aquifer con-

tained dissolved-solid concentrations ranging from 1065 to

5610 mg/l and is a calcium sulfate water type. Water quality of

the clastic aquifer has been deteriorating as a result of over pump-

ing from the aquifer and the exposure of shallow groundwater to

increased evaporation. Most of the water samples are unfit for

domestic uses on account of high the concentration of TDS, chlo-

ride, sulfate, and nitrate. It can, however, be used for irrigation pur-

poses under special circumstances management for salinity control

as the sodium hazard is medium while the salinity hazard rangesfrom high to very high.

Data from this integrated hydrochemical and geoelectrical

studies indicate that water quality from the water wells and geo-

physical surveys varies from fresh to slightly brackish. Based on

the resistivity values and geoelectric cross-sections, the area is

dominated by slightly brackish groundwater and the fresh water

is found as isolated patches.

Based on the results of this study, it is recommend that:

1. Drip and sprinkler irrigation systemshould be introduced to the

farmers, which in future may result in a more sustainable use of

the existing groundwater resources.

2. Installing a complete network of irrigation and drainage sys-

tems, especially in Abu Mina basin, which will enable optimum

use of water with minimum losses.

3. Locations future production wells, especially in lowland must

be carefully considered by water managers and planners if the

adverse impacts of proposed wells withdrawals on the hydro-

geologic flow system are to be controlled.

4. Electrical resistivity measurements should be carried out prior

any drilling operations for delineating the most appropriated

sites in regarding to their salinity and thickness of the aquifers.

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