Assessment of groundwater potentiality of northwest Area, Central Elsayed Zeinelabdein1, K.A., Elsheikh2, A.E.M., Abdalla, N.H.3

1Faculty of Petroleum and minerals – Al Neelain University – – Sudan 2 Faculty of Petroleum and minerals – Al Neelain University – Khartoum – Sudan, [email protected], 3Faculty of Petroleum and minerals – Al Neelain University – Khartoum – Sudan, [email protected]

Abstract

Butana plain is located 150 Km east of Khartoum, it is the most important area for livestock breeding in Sudan. Nevertheless, the area suffers from acute shortage in water supply, especially in dry seasons due to climatic degradation. Considerable efforts were made to solve this problem, but little success was attained. Hills, hillocks, ridges and low lands are the most conspicuous topographic features in the studied area. Geologically, it is covered by Cenozoic sediments and sandstone of Cretaceous age unconformably overlying the Precambrian basement rocks. The objective of the present study is to assess the availability of groundwater resources using remote sensing, geophysical survey and well inventory methods. Different digital image processing techniques were applied to enhance the geological and structural details of the study area, using Landsat (ETM +7) images. Geo-electrical survey was conducted using Vertical Electrical Sounding (VES) technique with Schlumberger array. Resistivity measurements were conducted along profiles perpendicular to the main fracture systems in the area. The present study confirms the existence of two groundwater aquifers. An upper aquifer composed mainly of alluvial sediments and shallow sandstone is found at depths ranging between 20- 30 m, while the lower aquifer is predominantly Cretaceous sandstone found at depths below 50 m. The seasonal streams represent the main source of groundwater recharge in the study area. Groundwater movement within the Cretaceous Sandstone aquifer shows different flow directions, among which the south and northeast are the dominant. The aquifers are characterized by average hydraulic conductivity of 8.0x10-3 m/min and the transmissivity (T) of 3.85x10-2 m²/min. Groundwater samples obtained from each of the aquifers are totally different in terms of hydrochemical properties. The groundwater in the upper aquifer is characterized by TDS of 150-1800 ppm. This water is mainly dominated by alkaline earth with calcium and magnesium increasing with the flow direction. The groundwater in the sandstone aquifer is predominantly of the chloride- calcium type that reflects deficiency of recharge.

Key words: resistivity measurements; remote sensing; water scarcity; hydrogeology; water chemistry; Butana area; Sudan.

1. INTRODUCTION

In arid and semi-arid regions, rainfall distribution is not uniform in all seasons. In such areas, groundwater plays an important role and provides the water demands for various purposes such as domestic, agricultural, and industrial usage (Khodaei and Nassery, 2011).

The Butana plain is located in central Sudan, about 150 Km east of Khartoum, occupying the area between the River and River and bounded by latitudes 15° 45' - 16° 00' N and longitudes 33°30'-34°00'E (Fig. 1). It represents an important pastoral area in the country, from which great amount of the livestock of Sudan comes. Nevertheless, the area suffers from acute shortage of water, especially in dry seasons. Considerable studies were made to solve this problem (e.g. Saad, 2001; Kheiralla and Ibrahim, 2004), but little success was attained. Thus, the objective of the present study is to assess and evaluate the groundwater resources in the area in terms of quantity and quality.

Low lands are the most conspicuous topographic features in the study area, with scattered hills, hillocks and ridges. Geologically, the area is a transitional zone between the sedimentary formations to the west and northwest and the basement terrain to the east. It is covered by Cenozoic sediments (Butana clays) and sandstone of Cretaceous age, unconformably overlying the Precambrian basement rocks (Ahmed, 1968).

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Figure 1: Location map of the study area.

The area is located within the northern part of savanna belt, in which the rainy season extends from July to September with average annual rainfall ranges from 100– 250 mm. In the recent years, considerable degradation in the precipitation rates was noticed. This climatic degradation strongly influenced the agricultural and livestock activities. The average annual temperature in summer is around 40Cº (March – October) and 25Cº in winter (November-February). Vegetation includes short grass, bushes and short shrub trees of acacia type. The region is characterized by dendritic drainage pattern which appears to be controlled by geological structures. The seasonal streams are dry almost all of the year, but they carry considerable runoff water during the short rainy season.

2. DATA TYPES

The following materials were made available for the present study: - Landsat 7 ETM+ image acquired on 24-12-2000 (path 173 and row 49) in digital format, obtained from the Global Land Cover Facility, University of Maryland, USA. - Resistivity survey data including 30 Vertical Electrical Sounding VESs. - Well inventory data.

3. METHODS OF INVESTIGATION

The methodology adopted in the present study involves synthesis of various Landsat 7 ETM+ images through a number of digital processing techniques such as different combinations of color composites and spatial filtering aiming at enhancing the geological and structural details of the study area. Geo- electrical survey was conducted using Vertical Electrical Sounding (VES) technique with Schlumberger array. The measurements were conducted along six profiles perpendicular to the main Nile Basin Water Science & Engineering Journal, Vol.5, Issue 2, 2012 49 Assessment of groundwater potentiality of northwest Butana Area, Central Sudan

fracture systems in the area. The resistivity data was processed and interpreted using IP2win software. Well inventory was performed including measurements of water level, pumping test and water quality sampling and analysis.

4. RESULTS AND DISCUSSIONS

4.1 Geophysical Data Interpretation

The electrical resistivity method is one of the most popular geophysical techniques that are used in groundwater exploration. Geophysical studies have attempted not only to relate resistivity to the hydraulic properties of the aquifer (e.g. Mazac et al., 1990; Cassiani and Medina, 1997), but also as a means of assessing water chemistry (e.g. Mhamdi et al., 2006; Al-ahmadi and El-Fiky 2009). In the present study, Schlumberger array was used in which the electric current was introduced into the ground by means of the current electrodes and the potential difference between two potential electrodes was measured. Then the resistance was calculated and converted to the apparent resistivity.

As mentioned above, the VESs were measured along 6 profiles crossing the main structural elements detected from the interpretation of satellite images (Fig. 2).

Figure 2: Vertical Electrical Sounding location map

Vertical Electrical Sounding resistivity data was converted into a three dimensional (3D) diagram for the area under investigation Figure (3). The blue color represents the relatively low resistivity values ranging between 1 and 100 Ohm-m caused by the top clayey soil occurring in the eastern and southeastern parts of the study area. The dominated green color in the model represents the relatively medium resistivity values that range between 100 to 600 Ohm-m and refer to the sandstone aquifer. The yellow color is interpreted as weathered basement horizon. The red color represents the basement rocks at different depths. In the southern and southeastern parts of the area, the basement crop out at the surface, or occurs at shallow depths with increasing depth towards the north and northwest by the effects of faulting.

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Figure 3: 3D model obtained from resistivity values.

The lateral variations at four different depths were extracted (Fig. 4). Accordingly, the alluvial deposits are encountered at depth ranging between 20 to 30 meters from the ground surface, underlain directly by the Cretaceous Sandstone. The alluvial deposits representing shallow groundwater aquifers are confined to the seasonal streams. The sandstone aquifer starts with a limited thickness near Abu Deleig village and increases towards the north and northwest, where the existence of the sandstone in the area is structurally controlled.

Figure 4: Resistivity distribution at four different depths.

4.2 Hydrogeological Investigations

4.2.1 Groundwater occurrence

Considerable quantities of groundwater are known to occur in alluvial deposits which are confined along and around the main seasonal streams. The seasonal streams represent the main recharging source of the Dug wells, in which the water table ranges from 15 - 20 m below the ground surface

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(b.g.s). The Cretaceous sandstone covers about 50% of the total surface area under investigation, and represents the deeper aquifer. This aquifer is composed of sandstone layers of sandy clay, clayey sand and gravels which overly the basement. Geophysical survey and well data showed that thickness of the Cretaceous sandstone aquifer is increasing towards the northwest direction and linked according to isotopic investigations with Shendi Basin (ACSAD, 1987).

4.2.2 Groundwater movement

Groundwater in the study area shows diverse direction of flows as depicted in Figure (5). East of Khartoum, the groundwater aquifer is recharged from the (Saeed, 1976). However, in the study area the, significant sources of recharge are only the direct precipitation and seasonal streams (ACSAD, 1987). Groundwater elevation varies from 267 to 500 meter above sea level (a.s.l.). South of Abu Deleig village, the main groundwater flow direction is to the south, whereas, in the northern parts, the groundwater flows to the northeast, with average hydraulic gradient of 0.0008.

Figure 5: Groundwater flow map

4.2.3 Aquifer parameters

In order to determine the hydraulic conductivity (K), the transmissivity (T) and the storativity (S), pumping tests were carried out at 8 wells distributed in the study area. Table (1) shows the calculated values from pumping test data for estimating the hydraulic conductivity and transmissivity. The non equilibrium formula based on Theis, Jacob and Hantush principles were used according to the type of aquifer. Nile Basin Water Science & Engineering Journal, Vol.5, Issue 2, 2012 52 Assessment of groundwater potentiality of northwest Butana Area, Central Sudan

Table 1: Shows the calculated aquifer parameters from pumping test data.

Well Average Transmissivity (m²/min) No. Hydraulic Conductivity (m/min) Theis Coopr & Jacob Hantush average

BH06 0.00148 0.01330 0.01220 0.01330 0.01293

BH09 0.00197 0.01000 0.01020 0.01000 0.04000

BH23 0.00045 0.00270 0.00270 0.00270 0.00270

BH24 0.00460 0.02630 0.05390 0.00260 0.02760

BH20 0.02450 0.14600 0.14900 0.01460 0.14700

BH27 0.001830 0.01550 0.01570 0.00170 0.01100

BH02 0.008350 0.03310 0.04650 0.02950 0.03637

Mean 0.006030 0.03913 0.04360 0.02990 0.03853

The hydraulic conductivity ranges between 4.60x10-4 and 2.45x10-2 m/min. The transmissivity varies from 2.70x10⎯³ to 1.47x10⎯¹ m²/min in the sandstone aquifer. Table (1) shows wide variations in these parameters across the aquifer due to the lateral and vertical lithological variations as well as the compartment of the aquifer as a result of the structural effect.

4.3 Hydrochemistry

The quality parameters of groundwater are a matter of serious concern today, which represents a critical point to be evaluated. These parameters aim at human consumption and irrigation or industrial use (Sghaier et al., 2011). The aim of this part of the study is to evaluate the hydrochemical significance of groundwater to assess its suitability for domestic and agricultural uses. 13 water samples were analyzed for this purpose. The chemical analyses include the most common ions such as K, Na, Mg, Ca, Cl, SO4, CO3 and HCO3 (Table 2).

Table 2: Hydrochemical parameters recognized from water samples analysis

Calcium Sodium Sulfate Chloride HCO3+ Well Magnesium Potassium Conductivity Ca Na SO4 Cl CO3 pH No. Mg (mg/L) K (mg/L) (µmoh/cm) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L)

BH02 20.040 33.600 21.840 37.100 40.100 75 187.860 - 7.300 BH06 33.600 18.240 65.480 0.500 60.300 35 158.784 760 7.000 BH09 31.260 11.660 48.960 44.000 40.000 85 125.956 710 7.300 BH19 10.400 9.600 42.000 0.460 35.700 75 65.360 670 7.200 BH20 20.040 9.965 92.200 36.700 30.000 35 90.956 2670 6.900 BH21 14.230 18.350 25.750 47.300 40.000 30 110.810 640 6.600 BH23 23.250 14.340 26.700 35.600 30.000 130 116.919 230 7.200 BH24 24.850 20.910 85.060 48.800 30.000 45 147.856 530 6.800 BH27 60.320 4.985 4.370 42.600 30.000 40 171.238 1430 6.700 BH34 20.040 10.120 21.840 37.100 40.000 60 91.592 330 7.100 DH11 26.250 11.430 11.720 3.128 40.000 45 112.488 1430 7.600 DH30 25.250 13.490 27.800 4.690 172.000 40 118.434 694 6.600 DH33 11.020 20.180 46.210 39.500 30.000 45 110.288 510 6.700

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The pH ranges between 6.6 and 7.7 with gradual increase to the east and southeast directions due to the influence of the basement rocks. The total dissolved solids (TDS) vary from 150 to 1800 ppm. Figure (6) shows the spatial distribution of TDS in the study area. Six variables (Na+K), Ca, Mg, CL, (HCO3+ CO3) and SO4 were used to construct Piper diagram to detect the groundwater facies (Fig. 7). The +2 +2 + + -2 Waters are relatively dominated by alkaline earth (Ca Mg ) (Na + K ), HCO3 type with a tendency of increasing SO4 and Cl at the expense of HCO3. According to the international acceptable limits for the World Health Organization (WHO), groundwater in the studied area is fit for human and animal consumption with some exceptions at certain sites.

Figure 6: Total Dissolved Solids map.

Figure 7: Piper diagram used to detect the water chemistry facies

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In order to evaluate the groundwater for agricultural purposes, Wellcox diagram was used to classify the irrigation waters. The hydrochemical elements were plotted (Fig. 8). The figure displays three classes of water. The dominant water class is characterized by C2-S1 (medium salinity, low sodium adsorption ratio) which is suitable for irrigation.

Figure 8: Wellcox diagram used in determining the suitability of groundwater for agricultural purposes.

5. CONCLUSIONS

The study area is located at the western periphery of the Butana plain. Two type of aquifers have been recognized namely; the alluvial aquifer and the sandstone aquifer. The alluvial aquifers are confined to the seasonal streams; therefore, they are of limited storage capacity and small aerial extent.

The sandstone aquifers occur as small sub-basins formed by faults. The thicknesses of these aquifers increase towards the northwest direction. They depend on the seasonal rainfall, so far, no evidence of regional recharge from adjacent sedimentary basins. The groundwater potentialities in the study area are limited and depend on the climatic conditions. Water management system is needed for conservation of the water resources.

The hydraulic conductivity ranges between 4.60x10-4 and 2.45x10-2 m/min. The transmissivity varies from 2.70x10⎯³ to 1.47x10⎯¹ m²/min in the sand stones aquifer. The analyzed water samples are relatively dominated by alkaline earth (Ca + Mg) (Na + K), HCO3 type with a tendency of increasing SO4 and Cl values at the expense of HCO3. The dominant water class in the study area is C2-S1 of medium salinity, low sodium adsorption ratio which is suitable for irrigation.

According to the international acceptable limits for the World Health Organization (WHO), groundwater in the studied area is fit for human and animal consumption with some exceptions at certain sites.

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6. ACKNOWLEDGEMENT

Al Neelain University is acknowledged for financial and logistic support during the field work and the chemical analysis. The support and comments from the internal reviewers from our university: Dr. Adil Balla Magboul, Department of Hydrogeology and Dr. Ali Eisawi from the Department of Geology, are gratefully acknowledged.

7. REFERENCES

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