Assessment of aquifer vulnerability to industrial waste water using resistivity measurements. A case study, along El-Gharbyia main drain, Nile Delta, Egypt

Journal of Applied Geophysics 75 (2011) 140–150

Contents lists available at ScienceDirect

Journal of Applied Geophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o

Assessment of aquifer vulnerability to industrial waste water using resistivity measurements. A case study, along El-Gharbyia main drain, Nile Delta, Egypt Kh.S. Gemail a,⁎, A.M. El-Shishtawy b, M. El-Alfy c, M.F. Ghoneim b, M.H. Abd El-Bary b a b c

Faculty of Science, Zagazig University, Zagazig, Egypt Faculty of Science, Tanta University, Tanta, Egypt Faculty of Science, Mansoura University, Mansoura, Egypt

a r t i c l e

i n f o

Article history: Received 11 November 2009 Accepted 27 June 2011 Available online 6 July 2011 Keywords: Nile Delta Aquifer vulnerability Groundwater pollution 2D resistivity imaging

a b s t r a c t 1D resistivity sounding and 2D resistivity imaging surveys were integrated with geological and hydrochemical data to assess the aquifer vulnerability and saltwater intrusion in the north of Nile Delta, Egypt. In the present study, the El-Gharbyia main drain was considered as a case study to map the sand bodies within the upper silt and clay aquitard. Twenty Schlumberger soundings and six 2D dipole–dipole profiles were executed along one profile close to the western side of the main drain. In addition, 14 groundwater samples and 4 surface water samples from the main drain were chemically analyzed to obtain the major and trace elements concentrations. The results from the resistivity and hydrochemical data were used to assess the protection of the groundwater aquifer and the potential risk of groundwater pollution. The inverted resistivities and thicknesses of the layers above the aquifer layer were used to estimate the integrated electrical conductivity (IEC) that can be used for quantification of aquifer vulnerability. According to the aquifer vulnerability assessment of an underlying sand aquifer, the southern part of the area is characterized by high vulnerability zone with slightly fresh to brackish groundwater and resistivity values of 11–23 Ω.m below the clay cap. The resistivity sections exhibit some sand bodies within the clay cap that lead to increase the recharging of surface waste water (650 mg/l salinity) and flushing the upper part of underlying saltwater aquifer. The region in the north has saltwater with resistivity less than 6 Ω.m and local vulnerable zones within the clay cap. The inverted 2D dipole–dipole profiles in the vulnerable zones, in combination with drilling information have allowed the identification of subsoil structure around the main drain that is highly affected by waste water. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Producing safe drinking water is the high priority challenge in the Nile Delta area that supports about 44% of the population of Egypt. The groundwater quality in the Nile Delta area is affected mainly by saltwater intrusion from the north, in addition to surface contaminants. RIGW/IWACO (1990) constructed a monitoring network throughout a group of shallow wells to study the groundwater contamination in the Nile Delta. This study showed that groundwater is contaminated with elevated levels of nitrates derived from the application of excessive fertilizers in agricultural areas, in addition to localized industrial wastes. Large open drain networks characterize the area. These drains are used as canals, receiving large amount of industrial and agricultural waste water and sometime domestic waste water. These open drains are commonly underlain by a clay cap over long distances. Where present, ⁎ Corresponding author at: University of Saskatchewan, Geology Department, Saskatoon, Canada. E-mail address: [email protected] (K.S. Gemail). 0926-9851/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.06.026

the clay cap protects the underlying sand aquifer from contamination. In some places, the waste water in the open drains infiltrates into subsoil and reaches the groundwater bearing layers down to a depth of 15 m below the surface (Hoencamp, 1997). This type of contamination is a direct threat to public health in the areas where shallow hand pumps are common place. In the area, the clay content in the upper cap is the dominating factor determining the infiltration speed in the subsoil where sand and silt formations are permeable while clayey formations are impermeable (Christiansen et al., 2005). Therefore, it is very important to fully understand the detailed distribution and the variation of the sandy areas in the clay cap in order to study the vulnerability conditions of the aquifer with inhomogeneous clay cap. Electrical methods are efficient in mapping the variations in such layers, because clay formations show low resistivities and sandy formations are related to relatively high resistivities. The El Gharbyia main drain was selected as a case study to assess the aquifer vulnerability to industrial waste water and agricultural drainage water (Fig. 1). This open drain runs in the Nile Delta from the

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

141

south to the north near El Burlulls Lake and receives about 12% of the disposal water of the Nile Delta through industrial and agricultural subdrains. DC resistivity techniques (1D and 2D surveys) were used to map the sand bodies in the clay cap and assess the aquifer vulnerability around the main drain. To assess the effectiveness of the applied techniques, groundwater samples and lithologic information were collected from the shallow hand pumping wells located close to the main drain. 2. Site description The El Gharbyia main drain runs through the central parts of the Nile Delta near El-Mahllah Al-Kubra city to the north near El Burlulls Lake, which has an open connection with the Mediterranean Sea. The southern part of this drain was selected for study starting at El Segaeia Village in the south to Kafer Dokhmays in the north (Fig. 1). In this area, a group of open sub-drains discharge into El Gharbyia main drain. These include Samatay, Seberbay and No 5 Al-Gharbi subdrains (Fig. 1). These subdrains receive about 43,000 m3/day waste water from the main textile and sugar beet factories around El Mahllah Al-Kubra region, in addition to agricultural and sewage water from the surrounding areas. These drainage systems are opened in silty clay and fine sand soils. These silty clay soils contain an average of nearly 50% clay and the rest silt and fine sands. The permeability in the upper part (1–2 m depth) is low ranging from 0.3 to 0.15 m/day according of the amount of sand and silt (El Guindy and Risseeuw, 1987). The topographic gradient of El Gharbyia main drain in the study area changes from 5 m (+MSL) in the south to ≤3 m (+MSL) in the north. In this area, the elevation at the western side of the main drain is 3 m (+ MSL) and changes to 4 m (+ MSL) on the eastern side. The study area is an agricultural area in addition to sub-drains and urban regions (Fig. 1). In the study area, the water table in the main drain is generally maintained at 2.5 m below the ground surface. The width of the open drains in the area vary from 5 (subdrains) to 30 m along the main drain with an average depth of 4.5 m. Fig. 1. Location map of the study area and the field measurements.

3. Geological and hydrogeological setting The Nile Delta area covers nearly 60,000 sq. kms in the northern part of Egypt. The Nile Delta basin contains a thick sequence of

Fig. 2. Geological cross section shows the stratigraphic relationship between Bilqas and Mit Ghamr Formations along the northern parts of the Nile Delta (modified after Fergany and Claudet, 2009).

142

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

Neogene–Quaternary deposits. These deposits include Sidi Salim, Qawasim, Rosetta (Miocene), Abou Madi Formations, Kafer El-Sheikh, El-Wastani, and Mit Ghamer Formations (Pliocene–Pleistocene cycle), and Bilqas Formation (Holocene cycle) at the top. Nile Delta aquifer consists of the Mit Ghamr and El Wastani Formations (Pleistocene/Pliocene), which are composed of coarse sand and gravel with silt-clay lenses. The thickness of the aquifer ranges from 100–400 m in the southern Nile Delta to 400–1000 m in the north. Mit Ghamr aquifer is a leaky aquifer overlain by a semipervious Holocene clay cap of Bilqas Formation and underlain by impermeable Pliocene shale of Kafer El Sheikh Formation (Said, 1990 and Sherif and Singh, 2002). The Holocene Bilqas Formation is a coastal lagoonal deposit made up of silts and sandy mud constituting the agricultural soil of the delta. Fig. (2) shows the stratigraphic relationship between Bilqas aquitard (clay cap) and Mit Ghamr aquifer along the northern part of the Nile Delta. Mit Ghamr aquifer and Bilqas aquitard are hydraulically connected, but each one has its own water level. The thickness of the Bilqas aquitard is only a few meters in the southern part of the study area (Zaghloul et al., 1977), while its thickness can be greater than 70 m (north east of Nile Delta). In its upper part, the Bilqas aquitard is composed mainly of montmorillonite, which is characterized by its high shrinking and swelling properties. The lower part is clayey sand with a few meters thick and greater permeability than the upper clay part (Sherif, 1999). The clay and clayey sand cap is underlain by a thick succession of unconsolidated sand and gravel of Mit Ghamr aquifer (Fig. 2). The Mit Ghamr aquifer in the area is recharged mainly from the Nile branches, canals and drainage networks, and surplus of irrigation water (infiltrate through Bilqas aquitard). Water level ranges between 6 and 12 m above sea level in southern and middle parts of the Nile Delta, while it ranges between 0 and 5 m in the northern parts. The hydraulic conductivity varies between 60 and 70 m/day in the southwestern and northern parts of the Nile Delta. It reaches its maximum values (100–180 m/day) in the middle and southeastern parts (Dahab, 1993). The vertical movement of water through the semipervious cap layer affects, to a great extent, the water balance of the aquifer system (Sherif, 1999). This movement is downward through two different steps. The first step is the downward seepage of irrigation and waste water from the open drains and canals to the water table through the unsaturated zone of the clay cap. The second is the movement of the shallow water below the water table into the groundwater in the lower aquifer through the saturated zone of the clay cap. The velocity of the groundwater in this zone is controlled mainly by the vertical hydraulic conductivity of the clay and clayey sand cap. The average value of the hydraulic conductivity for the clay cap in the Nile Delta is 2.9 10 −8 m/s (Sherif, 1999). In the northern part of Nile Delta, the salinity of groundwater shows large variation with depth. In the upper zone, salinity values range between 2500 and 77,000 ppm (Ebraheem et al., 1997). The thickness of this zone increases towards the north to reach 50 m in the area north of Kafer Shiekh. The saltwater intrusion in the Nile Delta extends to a distance of about 130 km from the Mediterranean coast (Kashef, 1983). In the northern part of the area, shallow brackish water results from mixing between fresh groundwater flowing from the south and salt groundwater coming from the north. The depth of this zone decreases towards the north of the studied area. 4. Assessment of aquifer vulnerability Aquifer vulnerability is defined as the readiness with which a given aquifer is likely to become polluted (Younger, 2007). It is defined by the characteristics of the covering layers which are called protective layers (Kirsch, 2006). In the Nile Delta area, the clay cap (Bilqas Formation) can be defined as the protective layer, protecting the underlying sand

aquifer. The waste water from the main drain may infiltrate through the protective cap layer (clay with silt and fine sand lenses) and pollute the groundwater aquifer. Therefore, the aquifer vulnerability is closely related to the texture of the inhomogeneous clay cap where the presence of sand or silt lenses may act as a vulnerable zone where the contaminants can percolate to the aquifer (Sorensen et al., 2005). The assessment of groundwater vulnerability helps to define the environmental impacts of the surface pollutants and represents a powerful tool for determining the waste water pollution on a regional scale. Several methods can be used for assessment of aquifer vulnerability based on the hydrogeologic setting of the area and the available data. The Aquifer Vulnerability Index (AVI) is a widely used method to assess aquifer vulnerability to surface contaminants (Van Stempvoort et al., 1992). This method quantifies groundwater vulnerability by hydraulic resistance to vertical flow of waste water through the unsaturated cap layers. The hydraulic resistance (C) can be obtained using the formula: n

C = ∑ hi = ki

ð1Þ

i=1

Where ki and hi are, respectively, the hydraulic conductivity and the thickness of the layers above the aquifer zone. The k values for sandy material (10 −5–10−1 m/s) are several magnitudes higher than those of clayey layers (Kirsch, 2006). According to Van Stempvoort et al. (1992), the hydraulic resistance (C) is based mainly on the depth to the aquifer and the types of geological materials above it; and it can be used as a rough estimate of the vertical travel time of water in the above unsaturated layers. The vertical travel time of water in the upper unsaturated layers can be related to the resistivity properties, which are based on the geological materials (Kalinski et al., 1993). Rottger et al. (2005) and Kirsch (2006) have discussed this issue in detail. The hydraulic resistance of the protective layer is based on the effective porosity for sandy materials and on the clay content for clay materials. Both parameters control the electrical resistivity of the materials (Kirsch, 2006). In this case, the hydraulic conductivity (Ki) can be replaced by the electrical conductivity (σi) or the resistivity (ρi) to calculate the hydraulic resistance (C) which is called Integrated Electrical Conductivity, IEC (Rottger et al., 2005) or a Geophysical Based Protection Index, GPI (Casas et al., 2008). The Integrated Electrical Conductivity (IEC) can be used to assess the aquifer vulnerability by: n

IEC = ∑ hi  σi or

ð2Þ

i=1 n

IEC = ∑ hi  1 = ρi i=1

where:

σi = 1 = ρi

ð3Þ

The resistivity (ρi) and thickness (hi) of each layer above the aquifer are obtained from the inversion of resistivity sounding or resistivity tomography data. The well known principle of equivalence problem is common in the quantitative interpretation of resistivity data. To overcome this problem, Casas et al. (2008) used the longitudinal conductance (S) which is based on the ratio between resistivity and thickness of the conductive layer where: n

S = ∑ hi = ρi i=1

ð4Þ

In this case, IEC value will have a maximum value when the thickness of low resistivity cap above the aquifer is greater, giving the highest protection level to the groundwater against the continuation from the surface (Casas et al., 2008). The estimated IEC unit is ohm −1 (Ω −1) or Siemens (S).

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

5. Resistivity measurements Initially, DC resistivity surveys were performed by measuring 20 sounding points along a profile close to the western side of the main drain (Fig. 1). This survey was conducted to define clay cap conditions and the extension of saltwater intrusion along the main drain and the northern part of El Gharbyia Governorate. A Schlumberger array was selected for this survey because it is sensitive for mapping of shallow variations. The applied current electrode separations (AB) were ranged logarithmically from 1 m to 200 m. This spread was considered long enough to evaluate the upper clay soil and penetrate the sand aquifer, to a depth of about 35 m. This profile was extended to 10 km and sounding spacing was about 500 m. Some of these soundings were conducted adjacent to available lithologic information obtained from shallow wells used for calibration process. After the Schlumberger sounding survey, three villages (Nimra El Basal, El-Banawan, and Kafer Dukhmays) were selected for more detailed investigations using 2D resistivity imaging survey (Fig. 1). These sites are located close to the main drain and are characterized by known problematic conditions in the groundwater, in addition to the anomalous results of the chemical analysis and resistivity soundings. The apparent resistivity data were collected along six profiles perpendicular to the main drain. These profiles were executed on both sides of the main drain to map the variations in the clay cap with short distance and to assess the aquifer vulnerability and waste water

143

leakage from the open drain. The dipole–dipole array was used in this survey with minimum electrode spacing of 7.5 m. In this array, the two current electrodes were separated from the potential electrodes and usually earthed close to the main drain. The dipole–dipole array has relatively higher model sensitivity values near the edges (Loke, 2003) and represents one of the most common arrays for monitoring near surface resistivity variations (Szalai and Szarka, 2000). Before the field measurements, different electrode spacings (3, 5 and 7.5 m) were tested using a synthetic forward model obtained from the available geological information and the former Schlumberger sounding survey. According to this comparison, the applied electrode spacing (7.5 m) was selected with maximum dipole spacing of 52.5 m and dipole separations (n) 1 to 7. These field parameters were considered long enough to penetrate the clay cap and reach the sand aquifer (N15 m depth) with a satisfying resolution. The resistivity measurements were preformed using terrameter SAS 300C system from ABEM instruments. The applied current was selected manually with a maximum of 20 mA without booster, and the instrument adjusts the output voltage to reach the selected current. Four cycles position was selected during resistance measurements. For each electrode set-up a measuring cycle was repeated 4 times. The four resistance readings were averaged if the error is not higher than an acceptable value of 1%. If the error is too high, the measurements are repeated until the error reaches to an acceptable limit. Data quality was assessed during the field work by plotting the

Fig. 3. Correlation between the inverted resistivity models and lithologic data from the nearby boreholes.

144

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

pseudosection for each profile, which gives an intuitive view of the stability of data. The effect of topography in the measurements is simple where the ground surface along measured profiles was flat. The inversion process of 1D and 2D resistivity measurements was aimed to obtain the vertical and lateral layer distributions above the aquifer layer. The smoothed field curves of the measured soundings were inverted by IPI2WIN software, which was based on Newton algorithm to solve the inverse problem (Bobachev et al., 2003). This inversion program provided the thickness, depth and inverted resistivity of each geoelectric layer. An important advantage in this program is that the concept of the interpreting a profile where all soundings along the profile were treated as a unity representing the geological structure of the surveyed profile as a whole, rather than a set of independent objects dealt with separately. To overcome the ambiguity problem in the inversion of resistivity curves, the lithologic information of six nearby boreholes were used to build the input models of the inversion process. These drilled boreholes are distributed along the sounding profile and closed to some sounding points (Fig. 1). Some model parameters were fixed during the inversion process according to the available geological data. Fig. (3) shows some examples of sounding curves and the lithologic description from the adjacent boreholes. The apparent resistivity profiles of dipole–dipole array were inverted using the RES2DINV software (GEOTOMO, 2006) to obtain a 2D image of the subsurface resistivity distributions. This inversion process is based on an iterative smoothness-constrained least-squares inversion algorithm (DeGroot-Hedlin and Constable, 1990 and Loke and Barker, 1996). During the inversion procedure, the apparent resistivities along the profile were estimated and compared with the measured values. The differences between the observed and calculated blocks were minimized to obtain an acceptable agreement of the fitting process (Loke and Barker, 1996). A measure of this difference is given by the root-mean-square error (RMS%). However, the lowest

RMS error may show large and unrealistic variations in the model resistivity values and might not be the best from a geological concept. To check the accuracy of the inversion process and its efficiency to locate the lateral variation within the clay cap, a synthetic model was created for each selected site using the same field parameters. At each site, the available boreholes (15–20 m depth) were used to construct a geological model. This model simulates some sand pockets with different dimensions in clay background over the lower sand aquifer. The resistivity ranges of the synthetic model were inferred from the former 1D sounding survey and the parametric measurements near the boreholes at that site. The synthetic modeling process was started by calculating the apparent resistivities along the suggested geological model using the 2D forward modeling program, RES2DMOD (Loke, 2002). The calculated apparent resistivities from the forward modeling were exported as data file with the addition of 5% random noise then inverted using RES2DINV program to display the resistivity signatures resulting from the suggested geological model. During the inversion process, the inversion parameters were adjusted to image the suggested geological model at each site. The final inversion parameters were saved and applied before the inversion process of the measured profiles. During the inversion process at each site some parameters were changed such as removing bad data point, damping factor, setting of vertical and horizontal filter ration and smoothing of model resistivity. These inversion parameters are based mainly on the quality of the measured data and signal to noise ratio. The obtained resistivity patterns from both synthetic and measured profiles were compared to define the permeable features (high resistivities) which may act as windows for the surface waste water from the main drain. 6. Results and discussions The geoelectric horizons, which are produced from the inversion process are listed in Table (1) and correlated with the lithologic data

Table 1 Interpreted resistivity results of the measured sounding points and the closed water samples. VES No.

Water sample no.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

3 4 5 6 7 8 9

10 12

14

Resistivity (Ωm) ρ⁎1 3 3.09 5.77 4.29 3.84 3.1 7.7 3.36 5.04 4.4 2.2 10.1 2.47 2.83 8.3 2.56 16.6 4.03 5.16

Thicknesses (m)

ρ2

ρ3

ρ4

11.2 10.8 6.8 15

1.75 1.02 1.49 1.9 1.43 3.2 2.1 2.97

13.1 12.2 22.8 14.2 18.5 13.4 18.4 1 6.1 5.35 6.4 6.2 6.2 5.3 5.2 5 6.2 6.2 6.1 3.6

4.59 7.6 5.9 4.36 4.68 7.41

1.87 1.9 2.8 1 1 1.35 1.7 1.37 1.2 1.5 1.91

⁎Geological descriptions of the interpreted geoelectrical layers. Bilqas Formation (aquitard) ρ1 Surface clay with silt (agricultural soil). ρ2 Fine to medium sand and silt ρ3 Soft clays Mit Ghamr Formation (aquifer) ρ4 Saturated medium to coarse sands ρ5 Saltwater saturated sands

ρ5

4 4.4 2.84 4.9 2.76 4.2 1.6 2.07 1.83 1.9 1.61 1.8 1.88 1.54 1.77 2.28 1.2

h1 1.2 2.8 3.1 1.2 1.3 2.4 0.5 13.1 0.9 2.5 1.8 0.5 0.5 1.9 1.1 1.4 0.5 1.0 2.4

h2

h3

2.2 1.6 1.4 1

4.6 3 5.9 2.3 2.9 6.8 3.8 6.2

1.0 2.1 3.1 2 5.4 1.7

5.8 9.2 7.1 5.8 4.6 3.9 3.3 7.6 4.2 3.7 7.8

h4

17.4 18.3 18.5 14.9 16.7 7.2 18.3 21.1 18.9 15.9 18.4 19.0 19.0 10.9 11.9 19.0 21.8

TDS (mg/l)

Ec (ms/cm)

Rw (Ωm)

1740 2290 2270

3470 4590 4550

2.88 2.18 2.2

2420 2760

4840 5510

2.07 1.81

3840 3850

7680 7720

1.32 1.3

6020 5680

12,060 11,330

0.83 0.88

7570

15,150

0.66

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

and TDS of the groundwater from the nearby boreholes to define the subsurface conditions. The inverted resistivity of all Schlumberger soundings (Table 1) were combined and plotted with depths as one section to show the resistivity distributions along the western side of the main drain (Fig. 4a). The inverted resistivities of each sounding were sampled with depths. The sampling process was started with a small rate and increased with the depth to detect the vertical variations of resistivity at shallow depths. These vertical data distributions are horizontally interpolated to create a grid using Kriging gridding method. The interpolated resistivities were contoured along the measured profiles. Along this profile, a high resistivity anomaly could be mapped in the southern part at the depth of aquifer layer as indicated from the nearby boreholes. These high resistivity values were extended to sounding 9 to reflect the low salinity of the groundwater (Fig. 4a). Towards the north, a conductive zone (2–6 Ω.m) was differentiated at the aquifer level indicating the saltwater intrusion from the northern side. Some high resistivity anomalies can be noticed in the upper conductive cap to reflect the

145

inhomogeneity of the constituents or the salinity of soil water within the clay cap. Fig. (4b) is a geoelectric section showing the subsurface layer distributions along the western margin of the drain. The topmost layer has an average resistivity of 2 Ω.m and represents the clay cap (the protective layer for the aquifer in the area). Some resistivity variations (5–11 Ω.m) exist within this layer as indicated from high values with limited thickness and extension (soundings 3, 4, 6, 7, 8, 13, 14, 16, 18, and 19). These variations were interpreted as fine and silty sand lenses with relatively high hydraulic conductivity compared with the clay background. The high resistivity values in some top parts of this layer may also reflect the leakage of surface water from the drain. With respect to the salinity of the collected water samples, waste water (water samples 15, 16, 17 and 18, Table 2) in the drain has low salinity (about 650 mg/l) compared with the groundwater in the boreholes (2000–7000 mg/l) specially in the north (soundings 14, 15, 18, and 19 in Fig. 4b). The presence of these sand and silt lenses close to and underneath the main drain may provide preferential pathways

Fig. 4. Resistivity distributions and IEC along the western side of the main drain. a) inverted resistivity section integrated with TDS of the groundwater, and b) geoelectric section shows the subsurface layer distributions, and c) the estimated Integrated Electrical Conductivity (IEC) from Schlumberger soundings.

146

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

Table 2 Concentration of major and trace elements in the collected water samples. S. no.

Name

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Azbit Sakes Azbit Gamela Elsgaea Azbit Elshok Azbit Eslah Abo Elmakarema Nemrat Albasla Mazrait Koheeiaa El Banawana Mazrait Abdelatya Azbit Elsharkaa Azbit Elsharkaa Dokhmeysa Ka-Dokhmeysa SW1 SW2 SW3 SW4 WHO (2006)b

a b

Depth (m)

PH

20 22 27 23 25 21 18 25 15 30 24 16 15 40

7.1 7.3 7.85 7.43 7.26 9 9.3 9.26 9.07 8.92 7.78 7.34 7.3 7.1 6.97 6.97 8.95 7.17 7.5

TDS (mg/l)

Major elements (mg/l) K

Na

Mg

Ca

Cl

NO3

SO4

HCO3

Al

Trace elements (mg/l) B

Cu

Fe

Mn

Pb

Zn

1760 1240 1740 2290 2270 2420 2760 3840 3850 6020 4830 5680 5910 7570 600 620 690 650 500

4.6 9.9 8.8 10.2 12.3 21 13.5 13 15.5 15.8 17 21.4 20.3 23 3.8 5.6 8.7 8.7

461 477 720 812 897 500 854 975 1427 1798 2164 2225 2200 2748 142 113 165 156

31 20 17 51 24 123 86 86 138 140 99 88 128 222 22 24 24 18

216 44 22 84 79 318 114 110 211 190 128 140. 119 286 66 49 70 75

934.4 564 800.7 1115 1081 1470 1268 1402 2169 2736 3003 3070 3083 4538 120.1 113.5 177 150

0.88 5.72 4.10 4.40 3.80 3.08 6.60 2.30 8.80 2.10 8.80 17.2 3.52 5.72 6.60 14.1 16.3 15.4

44 105 104 106 110 63 202 210 210 394 368 328 350 512 160 72 152 122

305 396.5 427 457.5 573.9 303 366 394.5 390 305 381.25 457.5 484.22 533.75 228.75 259.25 366 396.5

b0.01 b0.01 b0.01 0.03 0.016

0.19 0.43 0.31 0.58 0.13

b 0.003 b 0.003 b 0.003 0.01 b 0.003

b 0.004 b 0.004 b 0.004 0.581 b 0.004

0.22 b 0.001 b 0.001 0.229 0.33

0.004 0.005 0.006 0.005 0.006

0.6 0.01 0.002 2.8 0.05

b0.01 b0.01 0.02

0.45 0.43 0.69

b 0.003 b 0.003 b 0.003

b 0.004 b 0.004 0.04

0.59 0.9 0.79

0.005 0.005 0.006

0.09 0.25 0.93

0.023 b0.01 0.03

0.82 0.61 0.73

b 0.003 b 0.003 b 0.003

1.59 b 0.004 0.74

0.54 0.33 0.9

0.014 0.006 0.014

3.7 0.01 1.95

0.3 0.22 0.46 b0.01 0.2

0.04 0.06 0.05 0.19 0.5

0.06 0.03 0.058 b 0.003 2.0

0.007 0.102 b 0.004 b 0.004 0.30

0.222 0.19 0.22 0.22 0.4

0.003 0.002 0.005 0.004 0.01

0.1 0.04 0.07 0.6 3.0

High aquifer vulnerability according to the resistivity measurements. Guideline standard of the trace elements maximum concentration in the drinking water after WHO (2006).

for seepage of waste water to the aquifer. The percolation of low salinity waste water from the main drain through these fine sand lenses leads to flushing and decreasing the salinity of the upper parts of the saltwater aquifer as seen as relatively high resistivity anomalies (Fig. 4a). The thickness of the protective clay layer varies from 7 to 12 m. Below the protective clay layer, the sand aquifer has variations in the resistivity distributions, reflecting saltwater intrusion in the area. The relatively high resistivity values (11–23 Ω.m) in the southern margin is interpreted as a saturated slightly freshwater wedge followed by transition zone with resistivities between 4 and 6 Ω.m (Fig. 4). In the northern part of the profile, the lower aquifer boundary is formed by saturated saltwater sands as indicated by the very low resistivities (b2 Ω.m). Based on the resistivity distributions and TDS of groundwater (Fig. 4a), the aquifer can be divided into two parts with different water qualities. The southern part extends to near AlBanawan site and represents the minimum salinity values in the area (1240–2700 mg/l, Table 2). The north part is characterized by poor water quality where the aquifer is totally saturated with saltwater (3800–7500 mg/l). In addition to the saltwater intrusion in the area, the aquifer vulnerability to waste water represents the other challenge for the sand aquifer along the main drain. The inverted resistivities and thicknesses of the geoelectrical layers above the aquifer layer were used to estimate the integrated electrical conductivity (protection index) using Eq. (4) and presented as a profile along the main drain (Fig. 4c). As a practical example of estimation of this parameter, if the clay cap above the aquifer layer is 10 m thick, the integrated conductivity will vary from 2.5 (s) in the case of pure clay with low resistivity of 4 Ω.m, to 1.5 (s) when the clay cap mixed with 5 m of fine sands with a resistivity of 15 Ω. m. In the case of fine sand cap (15 Ω.m and 10 m thick), the integrated conductivity reaches to its minimum value of 0.66 (s). According to the obtained protection index, IEC (Fig. 4c), the degree of protection in the southern part (the area between soundings 3 to 10) displays low values of 1.5 (s) with a thin clay cap and fine sand inclusions as indicated from the resistivity distributions section (Fig. 4b). The fine sand inclusions in this area may act as windows for surface waste water from the drain as well as fertilizer and leads to increase the nitrate concentration in the groundwater (Thorling et al., 1997). This area includes Nimra El Basal and El Banawan villages which

are characterized by high polluted groundwater and anomalous increasing in the pH values (water samples from 6 to 11, Table 2). The nitrate concentration reaches to its maximum values of 17.2 mg/l at Azbit Elsharka (sample 16) near El Banawan area. At that place, the nitrate concentration of the surface water is 16.3 mg/1 with pH of 8.95 as indicated from water sample 17 (Table 2). Towards the north of the area, the protection index increases except for some local anomalies around Dukhmays (between soundings 14 and 16) and Kafer Dukhmays villages (Fig. 4b). These low protection anomalies (high vulnerability) are closely related to the local silt and fine sand lenses as well as the thin clay cap. The upper parts of the aquifer below these relatively high vulnerability zones are characterized by relatively high resistivity anomalies as indicated from Fig. (4a). The high salinity of the groundwater in the northern part of the area may reduce the importance of protection function in that area where the salinity varies between 4000 and 7000 mg/l and therefore, the groundwater cannot be used as domestic water. In the northern part of the surveyed area, sand formations saturated with saltwater may show resistivities as low as the upper clay cap. This makes it impossible to distinguish saltwater sands from clay layer by using only resistivity measurements without borehole information. In this case, the estimated longitudinal conductance is related the accuracy of the interpretation process of resistivity data and cannot always predict the pollution distribution in the area to a satisfying degree. 7. Resistivity imaging Three study sites were chosen along the main drain for resistivity imaging survey. These sites include Nimra El Basal, El Banawan and Kafer Dukhmays (Fig. 1). Nimra El Basal site lies in the southern part of the main drain. The area around the main drain in that site is almost flat and the salinity of surface water in the drain is 620 mg/l and the depth of the drain is 4.62 m from the ground surface. The closed shallow well is located away from the drain with 50 m and the salinity of groundwater in that well is 2420 mg/l. Fig. (5) shows the dipole–dipole resistivity imaging profiles in the Nimra El Basal field site. Along the eastern profile (P1), the clay cap (protection layer) displays some lateral variations as indicated from the relatively positive resistivity anomalies in low resistivity background at the area close to the drain and extends to the middle part of

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

147

Fig. 5. Dipole–dipole inverted sections and soil sequence at Nimra El Basal village.

the profile. These features are compared with the soil sequence at that site to reveal fine sand and silt bodies partially saturated with percolated water from irrigation and the main drain. The thickness of the protection layer varies between 7 m in the middle part and 10 m in the left margin of the profile. The presence of the relatively high resistivity values towards the main drain may be attributed to the infiltration of the partially treated waste water (low salinity) from the drain and depends on the hydraulic conductivity of the materials near the drain. In the western profile (P2), the nearest side to the drain is characterized by relatively high resistivity values compared with low resistivity values of the clay cap. The positive resistivity anomalies in both sides of the drain in Nimra El Basal site reflect the high aquifer vulnerability and can act as paths of the polluted water to the shallow sand aquifer. The low resistivity values of the protection layer along this profile (P2) reflect the high clay contents of this layer away from the main drain. El Banawan site lies north to the Nimra El Basal site and the groundwater in that site is characterized by high salinity compared with the former site. Fig. (6) shows the resistivity distributions along both sides of the main drain at El Banawan village. On the eastern side (P3), the area closed to the main drain shows relatively high resistivities compared to the western edge of the profile and interferes with the lower sands aquifer that has the same resistivity ranges in some places. This fact reflects the low clay contents and high vulnerability conditions

in the area near the drain and may represent a source point of contaminated water into the ground. The protection zone along this profile has a thickness range between 7 and 9 m as established from the 2D resistivity profile and 7.5 m as indicated from soil sequence (Fig. 6). In the western profile (P4), the aquifer boundaries appear to be well defined with the upper clay cap (7 to 9.5 m thick). The low formation resistivities in this site are appeared in aquifer layer as result of high salinity compared with the former site where the groundwater salinity increases gradually to the north. Fig. (7) shows the inverted sections at Kafer Dukhmays site which is located in the northern part of the area. Normally, the resistivity distributions along these sections represent the minimum values (0.5 to 7 Ωm) due to the high salinity of the groundwater in this site (more than 7000 mg/l). Both sections show relatively high resistivity values in the top surface layer compared with the underlying very low resistivity zone. These resistivity variations extend to depth of 3 m from the ground surface and reflect the gradation of the water salinity and saturation in the clay cap. In this site, the surface water in the main drain is used for irrigation of the areas around the drain where it has low salinity (650 mg/l) compared with the groundwater. In this case, this type of irrigation water mixed with the used fertilizers in the agricultural activities and may reach to the aquifer layer through the high permeable zones in the upper clay cap. The protective cap thickness varies between 9 and 13 m. The random pumping from the

148

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

Fig. 6. Dipole–dipole inverted sections and soil sequence at El Banawan village.

shallow wells may increase the water flow towards the aquifer from the high contaminated water in the upper layer. 8. Summary and Conclusions This study was conducted to investigate the applicability of 1D and 2D resistivity techniques for assessment of aquifer vulnerability to surface waste water and saltwater. The field site was selected around El Gharbyia main drain, Nile Delta (Egypt). This drain receives a huge amount of industrial waste water, agricultural drainage water and, on occasion, sewage water from the surrounding urban areas. The resistivity measurements were integrated with hydrological and hydrochemistry data obtained from the nearest shallow wells around the main drain. The interpreted resistivities and thicknesses were used to calculate the vulnerability index (integrated conductivity) for all layers above the Quaternary aquifer. The results obtained from Schlumberger sounding and 2D resistivity imaging techniques with shallow boreholes lithologies

provide an improved picture of the lithology above the aquifer layer that can control the percolation of surface contaminants around El Gharbyia main drain. The resistivity distributions in the area indicated that a conductive clay layer (protective cap) overlies a relatively more resistive sand layer (aquifer). In some locations, the inverted models showed resistive anomalies within the protective layer to indicate permeable sand and silt lenses. The presence of these lenses above the aquifer layer may provide preferential pathways for waste water from the main drain and the agricultural activities to the shallow aquifer. On the other hand, the sand aquifer layer showed a sharp resistivity dropping towards the north direction indicating saltwater intrusion to represent other environmental problems in the area. Fig. (8) is a schematic representation of the protective layer and the environmental problems in the area. The area between Nimra El Basal and El Banawan villages is characterized by shallow low salinity aquifer above the saltwater but it has a low a protective index as a result of the inhomogeneities of the protective layer. These soil conditions lead to flushing the upper parts of

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

149

Fig. 7. Dipole–dipole inverted sections and soil sequence at Kafer Dukhmays village.

the aquifer by waste water from the drainage networks. Random and excessive groundwater pumping in this area may affect groundwater quality by upward flowing of salt water or laterally and downward flowing of the waste water from the drain through the sand interbeds in the protective layer (Fig. 8). Thus, the area needs an achievable pumping regime and continuously monitoring system to protect the aquifer from the pollutants. The detailed investigation conducted in this area showed the value of 2D resistivity imaging for identifying variations within the protective clay cap over short distance and assessing the potential for the recharge of waste water especially, the areas proximal to the main drain. The limitation of resistivity imaging technique was largely in the areas with low resistivity contrast because of the presence of saltwater and clays. Moreover, from the obtained models it was possible to correlate between the ranges of resistivity and subsurface lithologic information available from shallow boreholes in each site.

Vulnerability mapping based on resistivity data provides a good evidence to assess the protection and management of the groundwater resources. It can be used as a tool for raising awareness about groundwater risk. However, this kind of vulnerability mapping describes the relative vulnerability of the aquifer based on the available resistivity data of different levels of noise quality and resolution. Resolution depends on the number and proximity of the measured resistivity points. Integration of resistivity data with geological and hydrogeological data may be enhanced by the accuracy of these maps, but is not sufficient to determine site specific vulnerability.

Acknowledgment The authors are thankful to Jim Merriam, professor of Geophysics, and Jim Hendry, professor of Environmental Hydrogeology, University

150

K.S. Gemail et al. / Journal of Applied Geophysics 75 (2011) 140–150

Fig. 8. Schematic block diagram shows the relationships between the protective layer and underlying aquifer with the surface water infiltration and flow paths within the aquifer due to pumping.

of Saskatchewan for their reviewing the manuscript, which have improved the accuracy of the text. References Bobachev, A., Modin, I., Shevinin, V., 2003. IPI2Win V2.0: User's Guide. Geological Faculty, Dept of Geophysics, Moscow State University. Casas, A., Himi, M., Diaz, Y., Pinto, V., Font, X., Tapias, J.C., 2008. Assessing aquifer vulnerability to pollutants by electrical resistivity tomography (ERT) at a nitrate vulnerable zone in NE Spain. Environmental Geology 54, 515–520. Christiansen, A.V., Auken, E., Sorensen, N., 2005. From resistivity to geophysical clay thickness. Near surface. . Palermo, Italy. Dahab, K.A., 1993. Hydrogeological evolution of the Nile Delta after the high dam construction. Ph.D dissertation, Geology Dept., Faculty of Science, Menofia University, Shibin El-Kom, Egypt. 256 pp. DeGroot-Hedlin, C., Constable, S.C., 1990. Occam's inversion to generate smoothdimensional models from magnetotelluric data. Geophysics 55, 1613–1624. Ebraheem, A.M., Senosy, M.M., Dahab, K.A., 1997. Geoelectrical and hydrogeochemical studies for delineating ground-water contamination due to salt-water intrusion in the northern part of the Nile Dela, Egypt. Ground Water 35, 216–222. El Guindy, S., Risseeuw, I.A., 1987. Research on water management of rice fields in the Nile Delta, Egypt. International Institute for Land Reclamation and Improvement/ ILRI, Wageningen, Netherlands. publication 41, 20 pp. ISBN 90 70759 088. Fergany, E.A., Claudet, S.B., 2009. Microtremor measurements in the Nile Delta basin: response of the topmost sedimentary layers. Seismological Research Letters 80 (4) 591–598 pp. GEOTOMO, 2006. RES2DINV Software-Manual. www.geoelectrical.com2006. Hoencamp, T.E., 1997. Monitoring of groundwater quality: experiences from the Netherlands and Egypt. ILRI Workshop: Groundwater Management: Sharing Responsibility for an Open Access Resource, Proceedings of the Wageningen Water Workshop. PDF copy in http://www2.alterra.wur.nl/Internet/webdocs/ilripublicaties/special_reports/Srep9/Srep9-h8.pdf. Kalinski, R.J., Kelly, W.E., Bogardi, I., Pesti, G., 1993. Electrical resistivity measurements to estimate travel time through unsaturated groundwater protective layers. Journal of Applied Geophysics 30, 161–173. Kashef, A., 1983. Salt water intrusion in the Nile Delta. Groundwater 21 (2) 160–167 pp. Kirsch, Reinhard, 2006. Groundwater Geophysics—A Tool for Hydrology. Springer Berlin Heidelberg. 459–471 pp. Loke, M.H., 2002. RES2DMOD version 3.01—Rapid 2D resistivity forward modeling using the finite-difference and finite element methodsaccessed at URL http://www. geoelectrical.com/download.html2002.

Loke, M.H., 2003. Tutorial: 2-D and 3-D electrical imaging surveys. Penang, Malaysia, University Sains Malaysia, Unpublished course notes, 129 pp. Loke, M.H., Barker, R.D., 1996. Rapid least-squares inversion of apparent resistivity pseudosections by a quasi- Newton method. Geophysical Prospecting 44, 131–152. RIGW/IWACO, 1990. Development and management of groundwater resources in the Nile valley and delta project: assessment of groundwater pollution from domestic, agricultural and industrial activities. Internal project report RIWG/IWACO, Cairo. Rottger, B., Kirsch, R., Scheer, W., Thomsen, S., Friborg, R., Voss, W., 2005. Multifrequency airborne EM surveys — a tool for aquifer vulnerability mapping. In: Butler, D.K. (Ed.), Near Surface Geophysics. : Investigations in Geophysics, no. 13. Society of Exploration Geophysicists, Tulsa, pp. 643–651. Said, R., 1990. The Geology of Egypt. A. A. Balkema Publishers, Rotterdam, Netherlands, p. 734. Sherif, M.M., 1999. The Nile Delta aquifer, Chapter 17 in, Seawater intrusion in coastal aquifers: concepts, methods, and practices. In: Bear, et al. (Ed.), Book Series, Theory and Application of Transport in Porous Media, vol. 14. Kluwer Academic Publishers, Netherland, pp. 559–590. Sherif, M.M., Singh, V.P., 2002. Effect of groundwater pumping on seawater intrusion in coastal aquifers. Agricultural Sciences 7 (2), 61–67. Sorensen, Kurt I., Auken, E., Christensen, N., Pellerin, L., 2005. An integrated approach for hydrogeophysical investigations: new technologies and a case history. In: Bulter, Dwain K. (Ed.), Near-surface Geophysics. Society of Exploration Geophysicists, Tulsa, USA, pp. 585–597. Szalai, S., Szarka, L., 2000. An approximate analytical approach to compute geoelectric dipole–dipole responses due to a small buried cube. Geophysical Prospecting 48 (5), 871–885. Thorling, L., Sorensen, K.I., Ernstsen, V., 1997. Effect of “windows” mapped by geophysical methods on groundwater quality. Proceedings Environmental and Engineering Geophysical Society European Section EEGS-ES, pp. 9–12. Van Stempvoort, D., Ewert, L., Wassenaar, L., 1992. Aquifer vulnerability index: a GIScompatible method for groundwater vulnerability mapping. Canadian Water Resources Journal 18, 25–37. WHO, 2006. Guidelines for drinking-water quality incorporating the first addendum to third edition, 3rd ed. Recommendations, volume 1. World Health Organisation, Genevahttp://www.who.int/water_sanitation_health/dwq/gdwq3rev/en/index. html. Younger, Paul L., 2007. Groundwater in the Environment: An Introduction. Blackwell publishing, Oxford OX4 2DQ, Uk. Chapter 9. Zaghloul, Z.M., Taha, A.A., Hegab, O., El Fawal, F., 1977. The Neogene–Quaternary sedimentary basins of the Nile Delta. Egyptian Journal of Geology 21 (1), 1–19.