Groundwater constitutes the foremost source of freshwater in El-Tor city, South Sinai, Egypt. In this context, a hypothetical mechanism amalgamating one-dimension (1D) Schlumberger geoelectric resistivity inversion, geohydrological analysis, borehole data is conducted to define aquifer geometry states in the study area. Furthermore, pumping tests along with hydro-chemical analysis is employed in the area.
Qualitative and quantitative modelling of the inverted resistivity data efficiently delineated the fresh aquifer with true resistivity ranging between 71 to 110 Ohm.m, and thickness ranging between 55 to 90 m. Furthermore, aquifer loss in relation to the pumping regime was characterized as well. The eastern section of the freshwater aquifer characterized by thickness increase (90 m), tortuosity (1.31), hydraulic conductivity (63 m/day), transmissivity (5077 m 2 /day), formation resistivity factor (4.5), and storativity (0.224). Moreover, it characterized by decreased values of electrical conductivity (0.048~0.356 mS/m), electric anisotropy (1), mean resistivity (255 ohm.m), porosity (37%), and total dissolved solid (TDS) (496 ppm).
Schlumberger resistivity, Electric and hydraulic parameters, TDS, Pumping regime
Due to their strategic locations, coastal cities have been given significant attention throughout the decades. This attention has been outstandingly emulated in founding new settlements, tourism, industrial, cultiva­tion, as well as mega-economic progressive pro­jects. Thus, the demand of the freshwater in these cities considerably increased. Since Sinai Peninsula in general and El-Tor in particular are areas where no widespread aquifer damage has been manifested, a convenient groundwater quality protection program can be effective in preserving natural groundwater quality. On account of the absence of naturally occurring and/or recharged surface water, it is highly vital to particularly concern about the status of the groundwater. Of the most important interests is the quality and quantity of the groundwater, coupled with any natural or anthropogenic hazards, causing groundwater contamination. El-Tor is the administrative capital of South Sinai Governorate. Worth mentioning, the city was named after the Arabic name of the mountain in which the prophet Moses received the tablets from God. Furthermore, El-Tor is a rapidly develop­ing city with a number of different tourist accommodations and amenities. The study area (El-Tor) is located in the middle sector of the El-Qaa plain, south-east Gulf of Suez, Sinai (Figure 1). The area is bounded by longitude and latitude 563000 to 566000 N and 3117500 to 3119750 E, respectively covering an area of about 4 by 3 km (12 km 2 ). Furthermore, the screened groundwater bores in the El-Qaa Quaternary alluvial aquifer yield up to 16,000 m 3 /day. Although, the water is slightly brackish with total dissolved solids (TDS) of 500 ppm, it’s mainly employed for drinking in El-Tor as well as Sharm El-Sheikh [1]. In this context, a number of electrical conditions, conjoined with aquifer hydraulic parameters were investigated for the freshwater aquifer in the study area. Moreover, the effect of the pumping regime (pumping rate, well loss, well specific capacity, and well efficiency) in aquifer loss was analyzed.
As mentioned by Rushdi Saidin [3], Gulf of Suez is the north-western arm of the Red Sea rift system which is bounded by a major border extensional fault. Not only, the El-Qaa plain is a secondary rift basin trending NNW-SSE parallel to the main rift system of the Gulf. But also, there is a conspicuous syncline plunging S-E in El Qaa plain northern zone. Sultan, et al. [4] established that the El-Qaa Quaternary alluvial aquifer is considered the main freshwater source in the El-Tor region. Moreover, the aquifer is characterized by the presence of Quaternary alluvial fans surrounded by igneous and metamorphic Pre-Cambrian rocks. Praiseworthy, there are about ten main wadis (dry ephemeral riverbed) dissevering the basement terrain and draining the water via the El-Qaa plain. The water quality data from El-Qaa aquifer designated that there is a distinct fresh/saline water transition zone across the aquifer upon which fresh surface water runs off from the wadis, recharges as well as mixes with saline groundwater [4-7]. Although seawater intrusion phenomenon is a very usual and is a widespread environmental problem in the El-Qaa plain, it has significant contribution to the salin­ity of the groundwater [4,8-11]. Noteworthy, saltwater intrusion may transpire due to human activities and/or by natural calamities such as cli­mate change as well as sea level rise. The authority of the geological survey of Egypt, [12] elucidated the subsurface stratigraphy in the area through the drilled boreholes. According to the borehole description, El-Qaa is divided into three geomorphologic units, the eastern mountainous region, the western sedimentary hills, and the central plain [13,14]. The eastern mountainous district (Pre-Cambrian crystalline basement complex) is an allotment of the Arabian-Nubian shield discriminated by high relief ranging between 300 and 2662 m amsl. The eastern mountains are comprised of igneous and metamor­phic rocks acting as a recharge boundary along the wadi outlets [15]. Notably, the western sedimentary hills encompass the Gebel Qabaliat ridge and embodied of different sedimen­tary units along with basement rocks. This unit has an average elevation of 250m amsl, with a moderate slope towards the El-Qaa plain. This unit is characterized by a dense consequent net of drainage lines. The central plain unit in which the study area is located is outlined as a peneplain developed and produced during the Quaternary period. It is distinguished by being dissected by wadi courses, terraces, playa deposits and alluvial fans in which both sand dunes and sheets are observed along with being flat [16]. Shata, et al. [5] emphasized the alluvial deposits of this unit by high-storage capacity and receive apprecia­ble amounts of recharge from the surrounding eastern mountains. Significantly, the alluvial processes play a major role in the formation and evolution of the alluvial fans. The fans store water percolated and drained by the wadi’s drainage net. Beyond that, the groundwater potential of this structural basin depends upon the features of the rock units and the recharge conditions. Based on the aridity view of El-Tor region along with El-Tor arid climate, almost all-present and future freshwater needs goanna have to be acquired from local groundwater aquifers [6]. Withal, direct rain over El-Qaa plain Quaternary aquifer is about 10 mm/y and reaches about 60 mm/y over the high surrounding basement rocks with heavy storms producing floods every few years. Although most of the precipitation occurs in wintertime, occasional torrential showers are expected in spring as well as in autumn and are associated with the monsoon winds coming from the south-east [17]. The water level of the Quaternary aquifer in El-Tor is fluctuating according to various annual recharging conditions with NE-SW flow direction [18]. Sultan, et al. [4], indicated that the aquifer discharges groundwater either to the coastal sabkha or to the Gulf of Suez. Conspicuously, numerous hydro-geophysical investiga­tions have been conducted in the area bysundry authors [4,8-10,13,18-28]. The common subsurface section of the area aggregated from the available boreholes data is illustrated in Figure 2.
Geoelectrical measurements
The methodology of Schlumberger geo-electrical resistivity sounding (VES) together with its implementation in groundwater investigation has been utilized by many authors [15,29-36]. According to Lashkarippour, et al. [37], the material’s resistivity is influenced by numerous factors such as groundwater salinity, saturation, aquifer lithology, as well as porosity. Furthermore, the relevance between aquifer characteristics with the layer electrical parameters have been studied and reviewed by many authors [17,27,32,38-43].
1D Geoelectric data inversion
Considering that sediments resistivity is one of the most variable physical properties along with the interpretation vagueness, it became vital to correlate the observed VES data with the available bore­hole data. This correlation enables assigning resistivity ranges to various lithologic units. On the other hand, the collected field data were interpreted through the following steps: (a) Matching the field curve with the standard curves of the auxil­iary method [44,45], (b) Preparing a forward model consisting of a limited number of geo-electric layers founded on available borehole data (thicknesses and cor­responding resistivities) [46], as well as (c) Entering the initial geo-electrical model into the IPI2 Win geo-electric modeling package [47,48].
Aquifer hydraulic conditions
A considerable number of investigation either measured or/and gauged were employed to identify a number of the significant freshwater aquifer hydraulic conditions in the study area. In this context, pumping tests together with a number of borehole samples analy­ses were conducted for BH-2 and BH-3. Two types of pumping tests were carried out, the first was a long-lasting pumping with a long duration at constant discharge, while the other is a step pumping test with variable discharge rates.
Aquifer electrical conditions
Electric anisotropy ( λ )
The quantitative interpretation of the geo-electric sounding data is meaningful if a horizontally stratified layers characterized by a certain resistivity and thickness for each layer is present in conjunction with electric anisotropy [49-53]. Dar Zarrouk Parameters (DZ) were first introduced by Maillet [54], and it deals with the explanation and identification of electric anisotropy (λ) from the VES. Unless the column is completely uniform, Majumdar and Das, [55] implies that the average resistivity gauged for a current flow vertically through such a column will differ from that flowing horizontally. Not only has this dependence of resistivity on direction of current flow comprised an effective anisotropy, caused by the layering and not by the inherent anisotropy within the layers [56]. But also, it can be fundamentally attributed to the fact that the longitudinal resistivity (ρ l ) is less than the transverse resistivity (ρ t ), meaning that the current flow density along the bedding planes is greater than that normal to the bedding planes.
Dar Zarrouk parameters can be reckoned as follow;
1) Longitudinal unit conductance:Si = hi/ρi
2) The total longitudinal conductance: St = ∑ni = 1(hi/ρi)
3) Longitudinal resistivity: ρl = H/S = ∑ni = 1hi/∑ni = 1(hi/ρi)
4) Transverse unit resistance: Tt = hi∗ρi
5) The total transverse resistance: Tt = ∑ni = 1(hi∗ρi)
6) Transverse resistivity: ρt = T/H = ∑ni = 1(hi∗ρi)/∑ni = 1hi
7) Electric anisotropy: λ = (ρt/ρl)1/2>1
8) Root mean square resistivity: ρm = (ρt∗ρl)1/2 = λ∗ρl = (1/λ)∗ρt
Where; ρi
is the true resistivity and hi
is the thickness for each layer (i).
Electric conductivity (EC)
Electrical conductivity (EC) stands for the measurement of the dissolved mate­rial in an aqueous solution coupled with the ability of the mate­rial to conduct electrical current through it. The higher the dis­solved material in water or soil, the higher the EC will be in that material. Electrical conductivity (EC) can be expressed as [33]:
EC = 1ρ
A scrutiny field survey was aforethought in the study area to allocate the VES taking into consideration the availability of boreholes in order to correlate the resistivity data. In this respect, 20 VES were carried out in the study area using the Schlumberger configuration (Figure 1), with a maximum current electrode half-spacing (AB/2) of 1000m. Moreover, the digital signal enhancement resistivity meter (ABEM-TERRAMETER, SAS 4000) was used to conduct VES. Noteworthy, to ensure the interpretation accuracy, the resistivity data were measured every sixth of a logarithmic decade. On the other hand, a number of aquifer tests along with water quality sampling were employed to recognize some of the hydrau­lic parameters and/or hydro-chemical composition of the fresh to brackish water aquifer.
In the study area, each VES was subjected to one-dimension (1D) inverse modelling, in which the iterative procedure of Zohdy [57] was applied. Iterations were carried out to reach the best fit between the smoothed field curve and the calculated one. Nevertheless, the maximum root mean square (RMS) errors of the resulting models were 5%. Noteworthy, Figure 3 displays the correlation between the interpreted resistivity layers of VES 15 and its corresponding borehole data in the area. The inter­preted VES stations were used to produce two geoelectric cross-sections (profiles 1 & 2) in the study area (Figure 4). Moreover, geological subsurface data along with borehole data in the study area were integrated in this geoelectric cross-section as well. Notable, a total of four geoelectrical layers were recognized in the cross-sec­tions (Figure 4), from top to bottom as follows:
A thin surficial layer characterized by true resistivity values range from 283 to 682 ohm.m. These values correspond to wadi deposits with miscellaneous nature of alluvial dry Quaternary sediments (surficial deposits of silt, sand, and gravel igneous boulders). Because of the high variation in the grain size distribution across the area (in terms of vertical and horizontal distributions), along with the spatial variation in the ratio of gravel/sand/silt, high fluctuation of the resistivity values was imputed. The thickness of this layer range between 1 and 15 m.
The second layer characterized by true resistivity range from 172 to 287 Ohm.m, coincide with alluvium dry Quaternary sediments of fluctuating size of silt, sand, gravel, and fine rock fragments. The thickness of this layer varies between 15 and 55 m.
The third layer utilized by true resistivity range between 71 and 110 Ohm.m, correspond to fresh to brackish water bearing formation of Quaternary and Pre-Quaternary alluvial sediments of silt and gravel. The thickness of the layer varies between 55 and 90 m. Conspicuously, the shape of the aquifer was greatly influenced by the surface topography of the study area.
The bottom (fourth) layer characterized by very low true resistivity range from 3 to 5 Ohm.m, coincide with salt water bearing formation of Quaternary and pre-Quaternary sediments of silt, sand, and gravel intercalated with some clay pocket with limited spatial extension. Worth mentioning, the fresh water of the third layer floats over the denser, more saline deeper water of the fourth layer (Quaternary and pre-Quaternary).
Newsworthy, the fresh/saline water transition zone is effectively displayed. The highest thickness of the fresh aquifer is recognized in the eastern part of the area, especially at VES 15 (90 m) and a gradual decrease toward the western part can be observed with the lowest thickness at VES 2 (14.2 m) (Figure 5). Figure 6 illustrates a depth contour map to the top of the fresh aquifer ranged from 5 to 90 m. The minimum depth is illustrated at the southwestern part of the area (3-18 m) with gradual increase toward the eastern zone. Moreover, the true resistivity of the aquifer ranges from 10 to 130 Ohm.m (Figure 7). Since, the geometries of the fresh aquifer indicate its sensitivity to the variation in the pumping regime, therefore, the safe yield of the fresh aquifer is controlled by its thickness.
Porosity (φ), tortuosity (T), hydraulic conductivity (K), transmissivity ( τ ), formation resistivity factor (F) as well as storativity ( S ) were investigated for the freshwater aquifer in the study area. Collins [58], pointed out, as an overarching principle the smaller the grains size, the greater the porosity (φ) in any natural unconsolidated material of uniform grain size [59]. In the area, porosities of the fresh water layer were obtained from the available well logs [60]. Nemours authors [61-64] stated that Tortuosity ( T ) increases with the increase of the grain size, whereas, an inverse relationship between porosity and grain size occurs. In the area, Maxwell’s equation [65], was applied to calculate tortuosity from the porosity (T = (3-φ)/2). Abdu, et al. [66] pointed out the direct relationship between the grain size and permeability (K). Straface, et al. [67] demonstrated that various attempts have been made to establish empirical relationships between Transmissivity ( τ ) and different geo-electric parameters. Schopper, and Batte, et al. [68,69] emphasized that the formation resistivity factor ( F ) describing porous media is dependent on the intricate geometry of the pore channels, therefore it describes the manner in which the grains are arranged. Notable, Paterson [70], aforementioned that F = T2/Φ
. In water investigations, storativity (S) has several applications such as groundwater numerical modelling, prediction of well performance, assessing the transport of contaminants, along with selection of the potential areas [71]. In the area, S were obtained from the available well logs.
In the area, the western zone is characterized by high porosity (φ) (42-44%) reflecting coarser grains, whereas, porosity decreases gradually toward the eastern part (37-40%) reflecting finer sediments (Table 1). For the fresh aquifer, the tortuosity ranges between 1.31 and 1.33, corresponded to an average range of porosity between 37 and 40% (Table 1). A direct relation between porosity and tortuosity can be recognized. Whereas, an inverse relation can be observed between tortuosity and grain size [72]. The pumping test of the fresh aquifer of BH-2 in the eastern zone is characterized by the highest permeability (67 m/day) (Table 1), which reflects an increase in the grain size. In the area, transmissivity values range between 1,756 and 5,432 meter 2 /day (Table 1). The highest transmissivity is observed at BH-2, which indicates high yielding potentiality of the aquifer in this sector. Paterson [70], equation is used to calculate the formation resistivity factor which range between 3.72 and 4.67 (Table 1). BH-2is characterized by the highest formation factor (4.67) which reflect higher aquifer potentiality and better characteristics [73]. Storativity (S) of the Quaternary aquifer is deduced from the pumping test and range from 0.224 (BH-3) to 0.303 (BH-2) (Table 1). Should be acknowledge, these results are in good agreement with the typical value corresponding to uncon­fined aquifer with shallow depths [74].
In the area, electric anisotropy (λ) along with electric conductivity (EC) were analyzed for the fresh aquifer.
A) Electric anisotropy ( λ )
In the study area, the above men­tioned Dar-Zarouk geo-electric parameters were reckoned to the top of the fourth layer of the saline water bearing formation. The total longitudinal conductance (S t ) range between 0.21 and 3.58 1/Ohm. The maximum total conductance (S t ) is revealed at VES-15 and the minimum at VES-14 (Figure 8a). Oteri [75], pointed out that the remarkable increase in S t may concur with an average increase in the clay content. The total transverse resist­ance (T t ) range from 727 to 18266 Ohm.m 2 . Noteworthy, the maximum T t value is recorded at VES-7 and the minimum at VES-20 (Figure 8b). The average longitudinal resistivity ( ρ l ) range between 9.1 and 158 Ohm.m. The minimum ρ l is observed at VES-2 while the maximum is displayed at VES-14 (Figure 8c). The average transverse resistivity ( ρ t ) range between 24 and 507 Ohm.m. The minimum ρ t is observed at VES-15, whereas the maximum is recorded at VES-7 (Figure 8d). Noteworthy, ρ t is generally greater than ρ l which indicate that the average hydraulic con­ductivities and current flow along the bedding planes are greater than those nor­mal to the bedding planes [76]. According to Keller [77], ρ l is governed by the more conductive layers, whereas ρ t rapidly increase even if a small fraction of resistive layers were present. The prior clarification introduced the concept of electric anisot­ropy (λ) in the area which range from 1 to 2.1. The minimum (λ) is observed at VES-15 and the maximum is revealed at VES-10 (Figure 8e). On the other hand, the isotropic layers alternation can be transmuted into an equivalent heterogeneous and anisotropic structure. Pursuant to Rucker, et al. [78] this anisotropy may interpret as a result of alternating layers of clay and fine sands and/or the intercalation of different grain sizes in the same layer. However, in the area, anisotropy can be due to both of these two conditions. Noteworthy, the eastern sector of the area is characterized by low values of anisotropy which corresponded to high aquifer potential zones [79]. The mean resistivity (ρ m ) ranges from 17 to 244 Ohm.m. The minimum ρ m is observed at VES-5 and the maximum is at VES-7 (Figure 8f). The change of the resistivity is dependent upon the direction of flow and the influence of lithology variation when there is a difference between the longitu­dinal, transverse, and mean resistivities.
B) Electrical conductivity (EC)
EC was measured for layers 3 and 4 corresponded to the fresh and saline aquifer, respectively (Table 2). Solinst TLC conductivity meter (Model 107) was used for EC measurements. The smart conductivity probe demonstrates conduc­tivity that has been standardized to 25 °C, i.e., specific conduct­ance, providing standardized repeatable comparable measure­ments. Furthermore, the Solinst TLC meter provides simple and proper calibration either by single- or double-point calibration. The measurements were carried out downward with a 1 m interval. EC measurements in the area range from 0.017 to 0.048 mS/m for the fresh aquifer, while range from 0.193 to 0.562 mS/m for the saline aquifer.
Well test was conducted to predict the maximum rate at which water can be pumped from a well along with water fall level (drawdown) for a given pumping rate. Furthermore, total drawdown in a well (St) can be manifested using Meier, et al. [80] equation, were:
St = Sa + Sw
Sa: Drawdown in the aquifer at the radius of the pumping well (aquifer loss).
Sw: Drawdown inside the well (well loss).
Due to the aquifer laminar flow resistance, aquifer loss (m) is instigated and is usually independent of the pumping rate. On the other hand, the specific capacity (m 2 /h) illustrates the well yield per unit of drawdown [81]. Of the most significant consideration in well design is well efficiency (E) as well as in well construction and development. Jacob [82], defined well efficiency (E) as; E = (Sa/St) × 100. Worth mentioning, the well yield depends on the aquifer characteristics, well parameters and the pump efficiency [83]. In the study area, well loss (m), aquifer loss (m), specific capacity (m 2 /h), and efficiency (%) were carried out for BH-2 and BH-3 with different pumping rates (m 3 /day). Ground Water for Windows package (GWW) was used to evaluate the acquired data of the step-drawdown test [84]. Figure 9 displays graphically the analyses results. To be noticed, an increase in the pumping rate was accompanied by an increase in both well and aquifer losses along with a decrease in well specific capacity and well efficiency. Therefore, for each development well, the pumping rate must be optimized to obtain the highest well efficiency and well specific capacity with less well and aquifer losses. Consequently, an adequate well design and appro­priate pumping rate are essential to maintain the aquifer, decrease well loss, and ensure a long operating life.
Total dissolved solids (TDS), major cations (K, Na, Mg, and Ca), and major anions (Cl, SO 4 and HCO 3 ) was sampled and chemically analyzed for fresh aquifer at BH-1 to BH-3 (Table 3). To be noticed, BH-2isutilized by the lowest TDS concentrations of 496 ppm indicating freshwater along with NaCl-CaSO 4 ionic type and Na 2 SO 4 and MgCl 2 genetic types. Moreover, BH-2 is located in the middle of the El-Qaa plain and mainly recharged by wadis of the eastern mountainous of the Pre-Cambrian crystalline basement. Accordingly, it receives higher amounts of recharge. Furthermore, BH-2 is characterized by high hydraulic conductivity of 52 m/day (Table 3) causing higher flow veloc­ity and good conditions for water desalination. BH-3 is utilized by TDS concentrations of 1,679 ppm indicated brackish water. BH-3 has NaCl-CaSO 4 ionic type and Na 2 SO 4 and MgCl 2 genetic types. Based on the hydro-chemical parameters (Table 3), the best water quality can be recognized in the eastern part at BH-2 and the worst toward the western part at BH-3. The good correlation between SO 4 and CI pointed to the mixing processes between different water geneses. Davis and De Wiest [85], pointed out that the good correlation between ionic concentrations of HCO 3 and K referred to the release of K cations from rock material by the percolated meteoric water. Therefore, a network of monitoring wells is highly recom­mended to observe the changes in the groundwater quality and protect the aquifer from deterioration.
Scouting of the freshwater qualitative and quantitative conditions in the El-TOR coastal aquifer was carried out through integrating 1D Schlumberger geoelectric resistivity inversion, along with geohydrologic analysis, borehole data, pumping tests and hydro-chemical analysis. On top of that, a number of electric and hydraulic conditions were investigated. A total of 20 VES with a maximum AB/2 of 1000 m were conducted in the area. The results demarcate effectively the fresh aquifer with thickness and true resistivity range from 55 to 90 m and 71 to 110 Ohm.m, respec­tively (Figure 5 and Figure 7). Furthermore, a notable potential of the freshwater aquifer is recognized toward the eastern zone of the area with a maximum aquifer thickness of 90m. Praiseworthy, freshwater aquifer is characterized by porosity of 37%, tortuosity of 1.31, and permeability of 56 m/day. In addition to, transmissivity, formation resistivity factor, and Storativity of 3,565 m 2 /day, 4.67 and 0.224, respectively (Table 1). Not only, the eastern sectoris characterized by increased values of electrical conductivity (0.048 to 0.356), but also with low values of TDS (496 ppm) (Table 2 and Table 3) demonstrating coarser grains size along with high groundwater yielding potentiality. The average transverse resistivity (ρt
) is generally greater than the average longitudinal resistivity (ρl
) which indicated that the average hydraulic conductivities and current flow along the bedding planes were greater than those nor­mal to the bedding planes [86]. Moreover, ρl
was governed by the more conductive layers, whereas ρt
rapidly increased even if a small fraction of resistive layers were present. In the area, electric anisot­ropy (λ) range from 1 to 2.5 (Figure 8), whereas, the mean resistivity ranges from 17 to 255 Ohm.m. Electrical conductivity for the fresh aquifer (third layer) range from 0.015 to 0.045 mS/m, while range from 0.208 to 0.769 mS/m for the saline aquifer (fourth layer). Figure 9 illustrate the well loss (m), aquifer loss (m), well specific capacity (m 2 /h), and well efficiency (%) employed for BH-1 and BH-3 with different pumping rates (m 3 /day). Noteworthy, an increase in the pumping rate is accompanied by an increase in both well and aquifer losses, as well as decreasing in the well specific capacity and well efficiency (Figure 9). Consequently, an adequate well design and appro­priate pumping rate are essential to maintain the aquifer, decrease well loss, and ensure a long operating life.