EXPLORATION OF GROUNDWATER POTENTIAL ZONES USING ELECTRICAL RESISTIVITY METHOD A CASE STUDY AT BILLINYANG VILLAGE, CENTRALEQUATORIA STATE, REPUBLIC OF SOUTH SUDAN

1.1 The study area

The study was conducted in Bilinyang village of Mangalla Payam, Juba County, Central Equatoria State, Republic of South Sudan. Geographically Bilinyang is located to the East of the White Nile, along Juba-Bor Highway and falls between longitudes 4.52'30-4.46'30°E and latitudes 31.42'0-31.46'30°N. South Sudan experiences a tropical climate; the temperature is above 25°C with high at 35°C particularly during dry season. The rainy season differ with location but usually start from April to early November. Bilinyang was selected for the study because of extended urbanization with increasing population in Juba.

Figure (1) Map of the Study Area.

1.2 Objective of the study

The main objective of this study is to contribute to the identification of groundwater potential areas and reveal the available structures by delineating vertical and lateral variation in resistivity in both 1D and 2D subsurface model

1.3 Geology and Hydrology of the study area

The geological features in the study area have undulation set of mountains mostly to the east and few are seen to the west of the river Nile. However, the rock units in the area identified are the basement complex and a sedimentary cover (El Hagab, 2013). South Sudan is predominantly underlain by Precambrian rock typically of igneous and metamorphic crystalline basement rocks (Schlüter et al., 2008) and covered by the tertiary to quaternary unconsolidated superficial sediments. These unconsolidated sediments are thought to be mix deposits of weathered basement and some sediment of Um Ruwaba formation. Predominantly, Um Ruwaba formation consist of sand and clay originating from lacustrine and alluvial deposit found overlying the basement complex. It is reported that all rock units were deformed, reactivated, intruded and metamorphosed during the Proterozoic Pan-African orogeny . However, during the Paleozoic and Mesozoic there was a dramatic sheet flow erosion which obscured any sedimentary rocks formation. The basement complex which underlain the study area, is compose of superficial weathered-rock (sedimentary overburden) as a result of tectonism and chemical weathering which has influenced the circulation of groundwater (Jones et al. , 1985). As a result of natural complexity in geology; the aquifers are characterized either confined, unconfined or fractured based on their structural composition. The basement aquifers in most African countries are of tropical and sub-tropical regions in this regard there is no readily available alternative source of water supply, particularly for rural populations.

2. Methodology

In order to achieve the objectives of the study, electrical resistivity survey among other geophysical method was preferred. The resistivity of the ground is determined by introducing currents into the ground and the resulting potential differences is measured at the surface. And the general field layout as sketched in figure 3 in which four pairs of electrodes are required. Vertical Electrical Sounding (VES) is specifically applied to define the vertical variation of ground's resistance to the applied current. Basically, as the distance between the current electrodes is increased, so the depth of which current penetrates is increased. In Schlumberger configuration four electrodes are placed along a straight line in the same order AMNB. The inner electrodes are potential with a spacing (b) which is a small and outer electrode current with spacing (a). During course of survey the two current electrodes (outer A & B) are move progressively along the line and the potentials electrodes (M & N) are maintained consequently this increased depth penetration. Usually, the position of measurement is taken as the midpoint of the electrode array. For a depth sounding, measurements of the resistance are made at the shortest electrode separation and then at progressively larger spacings. At each electrode separation a value of apparent resistivity (ρa) is calculated using the measured resistance in conjunction with the appropriate geometric factor for the electrode configuration and separation being used.

Figure (2) Schlumberger Array

Figure (3) Field VES Layout

3. Results Analysis

The data were analyzed using IX1Dv2 to generated the sounding curves and IPI2win to produced pseudo-section. Qualitatively, interpretating apparent resistivity sounding curves is done by noticing the curve shape or matching the curve with the best fit from a series of data points. The curve matching method, a curve is drawn by plotting apparent resistivity against electrode separation, and interpreted by matching the field curve with master curves of differing layers for various ratios of absolute resistivity. Sounding curves are groups into simple three electrical layers forming one of four basic curve types namely; (Type-H: ρ 1 >ρ 2 <ρ 3 , Type-A: ρ 1 <ρ 2 <ρ 3 , Type-K: ρ 1 <ρ 2 >ρ 3 and Type-Q: ρ 1 >ρ 2 >ρ 3 ) representing different subsurface layered strata. Occasionally, these curve types are combined to describe more complex field curves that may have several more layers depending on the relative thicknesses (Reynold, 2011). A geoelectric unit is characterized by two basic parameters; the layer resistivity (ρ i ) and the layer thickness (h i ). However, the combination of the thickness and resistivity of the geoelectric layers into single variables; the Dar-Zarouk parameters of Transverse resistance (RT) and Longitudinal conductance () can be used as a basis for the evaluation of aquifer properties such as transmissivity and protective capacity of the overburden rock materials (Ehirim et al.,2010). For a horizontal, homogenous and isotropic layer earth, the Dar-Zarouk parameters of transverse resistance and longitudinal conductance are obtained from;

where ρ i and hi are the layer resistivity and thickness for any obtain i th layer

Apparently calculated resistance and conductivity from the above equations (2 & 3) are very crucial in determining the aquifers parameters. The transverse resistance is numerically equivalent to the transmissivity whereas longitudinal conductance gives a measure of the impermeability of a confining clay/shale layer; such layers have low hydraulic conductivity (k) and low resistivity. Thus, the properties of a thin conducting layer can be determined in terms of longitudinal conductance, and a resistive layer can be determined by transverse resistance.

3.1 Interpretation of Sounding curves.

As observed, the study area reveals different curve types such as H, K, HK, KQ, AK, however, Q, K and QH are the dominant curves type among 48 VES stations. Each curve type defines a geological stratum representing subsurface structure of the study area. The results indicated three-four (ρ1, ρ2, ρ3, and ρ4) layers of differing soil profiles/lithologies. It is evident the highest apparent resistivity value recorded, were basically in the first layer then subsequentially follow by second, third and finally the fourth layer. Most of the readings in the first layer ranges from (20-40Ω.m) in varying depths (0.5-4m) and this could be the covers of dry top soil. And in the second layer show a minimum resistivity value ranging from (5-15Ω.m) and with thickness ranging from (5-45m). Whereas in the third layer; the lowest resistivity reading ranges from (4-15 Ω.m) with the average thickness up to 40m. This varying resistivity of each layer is attributed to be the increase in saturation encountered in different lithologies.

Figure (4) VES 12 (Q-Type)

Figure (5) VES 23 (K-Type)

3.2 Interpretation of Pseudo-sections and Resistivity cross-sections.

Since pseudo-section is an inversion of the apparent resistivity that displays both lateral and vertical variation in resistivities against depth. It reflects the resistivity distribution versus electrode separation (AB/2) and shows the electrical characteristic of layers on horizontal and vertical sections. Various VES curves and pseudo-sections are correlate between layers with comparable resistivities to build up a two-dimensional picture of both the vertical and lateral variations in resistivity to present unknown local geology. Based on result from pseudo-sections, the resistivity varies in depth following the horizontal line, with the upright and lateral changes of layers. These changes characterize a diversity of geological structures in depth and along the horizontal line. Nine (9) VES were combined making up a pseudo-section which show the variation in the apparent resistivity versus electrodes separation in order to infer the subsurface geology (aquifer) at certain depth. Generally, in the pseudo-sections of the study area, high resistivity values appeared in the upper parts because of the very dry top soil basically of alluvium deposits. The values decreased downward as resulting from influence of a conductive discontinuity or presence of an aquifer. Apparently, in both depth and resistivity models the vertical section displayed the colour-coded relative to depths of the possible potential aquifers as shown in figure 7 & 8. From the legends the blue color confirmed to be a conductive zone having low resistivity values ranging between (10-13Ω.m) at the depths range from (20-9m). However, the result confirmed upper section being a high resistive zone indicating a very dry soil from 1 to 5m thick. Obviously starting from depth 10 to 50m the section revealed a gradual decrease in resistivity indicating a conductive zone typically of a clay and clayey sand.

Resistivity-sections were generated along with the pseudo-section; which displayed the subsurface lithology resistivity varying with depth. The section models shown in figure (6), was characterized by low resistivity of (7.4-30Ω.m) indicative of conducive clay, and clayey sand in the range (30-50Ω.m) where the high resistivity readings of (50-74Ω.m) is described as the top soil. Similarly, in figure (7) the lowest observed resistivities are in the range (3-30Ω.m) indicating clay soil, and from (50 - 70Ω.m) being clayey sand, the highest resistivity starts from 70Ω.m up to 300Ω.m which is described as the top soil. It can be inferred from the sections that; resistivity gradually decreases with depth, as a result of pore fluids. Thus, resistivity section concurred with the pseudo-section, showing quaternary deposit at shallower depth.

Figure (6) the representation of pseudo and resistivity cross-section for VES (1-9)

Figure (7) the representation of pseudo and resistivity cross-section for VES (20-28)

Table 1 Aquifer’s Parameters (Transmissivity and Hydraulic Conductivity)

VES No:

Layer Resistivities

Layer Thicknesses

Transverse Resistance

Longitudinal Conductance

1

ρ 1 =54.1, ρ 2 =20.6, ρ 3 =11.3

h 1 =1.6, h 2 =5.11

576.389

0.07174

2

ρ 1 =35, ρ 2 =27, ρ 3 =10, ρ 4 =16

h 1 =0.9, h 2 =5.9, h 3 =40.4

4153.6

0.53182

3

ρ 1 =27.2, ρ 2 =18.13, ρ 3 =10.4

h 1 =2.7, h 2 =5.3

445.84

0.14355

4

ρ 1 =21.0, ρ 2 =24.1, ρ 3 =11.2

h 1 =0.47, h 2 =3.58

228.015

0.07194

5

ρ 1 =26, ρ 2 =18, ρ 3 =11, ρ 4 =35

h 1 =2.02, h 2 =7.2, h 3 =38.5

4294.8

0.53022

6

ρ 1 =28.9, ρ 2 =13.2, ρ 3 =10.54

h 1 =2.6, h 2 =10.2

673.792

0.24316

7

ρ 1 =22, ρ 2 =23, ρ 3 =11, ρ 4 =18

h 1 =1.7, h 2 =5.21, h 3 =34.1

3034.74

0.55419

8

ρ 1 =22.8, ρ 2 =25.4, ρ 3 =10.2

h 1 =0.45, h 2 =5.7

359.16

0.10531

9

ρ 1 =12.64, ρ 2 =23.22, ρ 3 =9.97

h 1 =3.37, h 2 =37.60

1877.66

0.89396

10

ρ 1 =21, ρ 2 =15, ρ 3 =6.8, ρ 4 =26

h 1 =2.6, h 2 =8.8, h 3 =38.6

3440

0.72674

11

ρ 1 =27.1, ρ 2 =44.4, ρ 3 =18.47

h 1 =6.03, h 2 =10.54

1488.104

0.18384

12

ρ 1 =40, ρ 2 =25.9, ρ 3 =11, ρ 4 =20

h 1 =1.5, h 2 =4.4, h 3 =24.9

2984.52

0.31785

13

ρ 1 =21, ρ 2 =32, ρ 3 =13, ρ 4 =7.8

h 1 =0.98, h 2 =6.8, h 3 =33.0

3009.564

0.55257

14

ρ 1 =8.7, ρ 2 =32, ρ 3 =92, ρ 4 =12

h 1 =1.4, h 2 =2.1, h 3 =5.0

1229.95

0.00587

15

ρ 1 =33.9, ρ 2 =18.5, ρ 3 =10.5

h 1 =1.4, h 2 =5.08

405.194

0.10363

16

ρ 1 =28, ρ 2 =19.33, ρ 3 =10.13

h 1 =2.2, h 2 =3.8

344.46

0.10442

17

ρ 1 =21.63, ρ 2 =24.31, ρ 3 =11.02

h 1 =0.4, h 2 =6.0

364.544

0.11236

18

ρ 1 =20.34, ρ 2 =6.7, ρ 3 =12.59

h 1 =7.2, h 2 =37.1

1755.609

1.11784

19

ρ 1 =21.54, ρ 2 =17.34, ρ 3 =10.52

h 1 =3.9, h 2 =10.8

726.18

0.29757

20

ρ 1 =23.41, ρ 2 =15.05, ρ 3 =8.80

h 1 =3.33, h 2 =10.72

664.003

0.29729

21

ρ 1 =22.15, ρ 2 =18.24, ρ 3 =10.99

h 1 =2.13, h 2 =6.39

437.7576

0.16582

22

ρ 1 =19.79, ρ 2 =21.23, ρ 3 =11.07

h 1 =0.93, h 2 =5.31

325.0416

0.11979

23

ρ 1 =23.16, ρ 2 =19.03, ρ 3 =10.17

h 1 =1.06, h 2 =6.03

371.2324

0.13541

24

ρ 1 =39.43, ρ 2 =29.82, ρ 3 =11.18

h 1 =1.87, h 2 =5.64

524.2225

0.10931

25

ρ 1 =29, ρ 2 =23, ρ 3 =10.8, ρ 4 =52

h 1 =1.06, h 2 =5.0, h 3 =59.3

7503.328

0.56934

26

ρ 1 =26.17, ρ 2 =17.41, ρ 3 =10.78

h 1 =2.71, h 2 =5.20

429.9876

0.14551

27

ρ 1 =21.4, ρ 2 =21.9, ρ 3 =10.93

h 1 =0.53, h 2 =6.04

229.7529

0.18788

28

ρ 1 =19.05, ρ 2 =18.70, ρ 3 =10.98

h 1 =3.13, h 2 =4.52

372.7845

0.15698

29

ρ 1 =21.53, ρ 2 =18.22, ρ 3 =11.25

h 1 =3.24, h 2 =4.72

405.96

0.15607

4. Discussion of the Results

Among the objectives was to identify the groundwater potential zone. The results from sounding curves and resistivity cross-section have displayed three to four layered formations. As observed from the acquired data most of the first layers show apparent resistivity in the range 20 - 50Ω.m this is mainly of clayey and loose sand (typical dry layer showing high resistivity readings). And the aquifers are predominantly found in the second- and third-layers having resistivity with range from 5 - 20Ωm. The lithologies comprised of clayey sand and fine sand formation with thicknesses in the range of 5 - 10 m. Noticeably, the results matched with Adepelumi et al., (2006) statement that, the groundwater occurs under water-table conditions in the clayey sand/sand aquifer as well as under semi-confined to confined conditions in the weathered zone. The thickness of layers varies from 0.5 - 40 m while some ranges of 5 - 80 m. Based on observed result from pseudo section there are parts which show low resistivity corresponding to the saturated zone. The 2D pseudo section model shows that aquifer potential being widespread and evenly distributed in the study area. The aquifers are mostly unconfined with relative thicknesses and generally, found near the land surface. The aquifer potential indicated in the subsurface by the form of clay and sand which shows that the study area is a medium productive aquifer. Consequently, this aquifer is reliable and can yield potable water for the residents of the area. Similarly, the unconsolidated nature of the aquifer with clay materials which exhibit low permeability and high porosity which associated with aquifers of relatively low resistivity, fine to medium grained sands and high-water content. Decisively, the alluvial or till covering the bedrock and obscuring its topography which control the flow of groundwater. Accordingly, the water-bearing weathered basement can be found at depths ranging from 20 up to 80 m. Inasmuch as the fractures are not commonly observed and thus fresh basement is deep. The Dar-Zarrouk parameters describing the aquifer properties shown in Table (1) above. For the transverse resistance describes the unit areas with high recharge capability and regarded as the best ground exploration site of the area whereas longitudinal conductance which defined groundwater potential zone based on unit resistivity over a known thickness. The transverse resistance in the study area show that some VES point with low reading ranging between (200-500Ωm 2 ) being less resistive zone which implied zone of high rechargeability (transmissivity). Moreover, the highest (RT) values ranges (1000-7000Ωm 2 ) signifying a resistive media with low permeability or transmissivity only shown in table 1 especially for VES (02, 07, 12, 13, 20,23 & 25).

5.1 Conclusions

The Study conducted at Bilinyang village; was to map the groundwater potential zones, in which forty-eight (48) soundings were collected in order to defined lateral and vertical variation of electrical resistivity/conductivity. The varying curve types are Q, H, K, QH, KH and AK however, the dominant types were Q, K and QH. These curves indicate inhomogeneity nature of the subsurface formation. Comparably, the upper portion of pseudo-section show a high resistive zone an indicative of a very dry soil at depth of 1 - 5 m. Nevertheless, at 10 - 30 m the section revealed gradual decreases in readings which demonstrate a conductive media within the ranges of saturated clay. However, with increasing depth, starting at 30 -100 m encountered low resistivity (5-12Ω.m) which is a conductive zone literally of wet clay sometimes interbed with clayey stand. Thus, the findings reveal the basement to be deep and overburden are deposits of Um Ruwaba formation comprising of three layers; very dry top soil, statured clayey, and weathered beds. What incised this clay-dominated explained that; these sediments were either of the streams/river bed, alluvial, erosion or paleochannel chiefly of Quaternary deposits. Since most of the VES points show high conductivity hence it has been concluded that, the study area poses a potential zone relatively of shallow aquifers at depths around 30-90 m. The aquifers described above are of thick, unconfined with low permeability. In conclusion the overall observed findings ranging from interpretations of the sounding curve, pseudo-section and resistivity cross-section; they display the presence of an alluvial aquifer mainly of clay and clayey sand considered to be good potential aquifers.

5.2 Recommendations

Since the arrays used is limited to shallow investigation it is therefore recommended to opt for more detail method such as integrated electrical with remote sensing and GIS, induced polarization, electrical tomography and in order to realize the subsurface lithologies seismic reflection is in preference. From the results of this study; it was apparent that the approximated water table/saturated zone starts from 30m and for the potential aquifer which contains adequate volume of fresh groundwater was within this range of 50-100m; this zone can be exploited by installing borewells for consumption. In the study area the fresh basement rock is very deep and to recognize it, requires borehole data.

The authors would like to express their appreciation to the reviewers for their useful comments and suggestions which helps in the improvement of the research paper, and much thanks to the South Sudan Mining Journal for the support to publish this research.