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Volume 12, 2018 ARTICLE

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Nature

ISSN 2319–5703 EISSN 2319–5711

Estimation of Hydraulic Parameters from Surface Geo-electrical Data: A Case Study of Orlu and Environs, Imo River Basin, South Eastern Nigeria Opara AI1, Ekwe AC2☼, Egbujuo CI3, Essien AG1, Nosiri OP1, Mbaegbu MO1, Ibeabughichi AC1 1.Federal University of Technology, Owerri, Nigeria 2.Federal University Ndufu-Alike Ikwo, Nigeria 3.School of Science and Engineering, University of Wolverhampton, United Kingdom ☼

Corresponding author:

Federal University Ndufu-Alike Ikwo, Nigeria E-mail: [email protected] Article History Received: 20 February 2018 Accepted: 17 April 2018 Published: April 2018 Citation Opara AI, Ekwe AC, Egbujuo CI, Essien AG, Nosiri OP, Mbaegbu MO, Ibeabughichi AC. Estimation of Hydraulic Parameters from Surface Geo-electrical Data: A Case Study of Orlu and Environs, Imo River Basin, South Eastern Nigeria. Discovery Nature, 2018, 12, 32-46 Publication License This work is licensed under a Creative Commons Attribution 4.0 International License. General Note

Detailed study of the aquifer system of Orlu and environs was carried out to estimate their hydraulic parameters using surface geoelectrical method. The study area is underlain by the Bende-Ameki and Benin Formations. The Oligo-Miocene Bende-Ameki Formation consists of unconsolidated sandstones with carbonaceous mudstones, sandy clays and lignite seams while the Benin © 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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ABSTRACT

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Formation contains unconsolidated, yellow and white sands, which are occasionally pebbly with lens of grey sandy clay. The unconsolidated nature of the Formations and their high susceptibility to contamination have made this study imperative, as it would assist water resource planners and developers in the area to understand the best way to plan and site boreholes in the area. Thirty (30) vertical electrical sounding (VES) data were acquired using the Schlumberger array, with a maximum current electrode spacing (AB) of 1000 meters. Four (4) parametric soundings were carried out near existing boreholes at Umuaka, Ihioma, Umudioka-ukwu and Nkwere for correlative purposes. The VES data were interpreted using the conventional partial curve matching technique to obtain initial model parameters which were later used as input for computer iterative modelling using the OFFIX software. The layer parameters thus obtained from the analysis were combined with information from borehole logs and pumping test data from existing boreholes to estimate aquifer hydraulic parameters using Dar-Zarrouk parameters. Similarly, estimated hydraulic parameters together with information from borehole and strata-logs were used to carry out vulnerability index assessment using the DRASTIC model. Results revealed that the aquifer apparent resistivity values in the study area ranges from 323 Ωm to 5200 Ωm with a mean value of 2943.1 Ωm. The depth to the water table ranges between 30-166.7m, while aquifer thicknesses range from 13-67m. Similarly, the hydraulic conductivity in the study area ranges from 2.5 m/day to 60.1 m/day with a mean value of 9.599 m/day, while 2

2

2

the transmissivity values range from 13.47 m /day – 1598.07 m /day, with an average value of 318.44 m /day. Keywords: Hydraulic parameters, Transmissivity, Hydraulic conductivity

1. INTRODUCTION The sedimentary sequences of southeastern Nigeria, including those of the Imo River basin are known to contain several aquifer units (Uma 1989). The Orlu area which is part of the Imo River Basin is underlain by the sedimentary rocks of southeastern Nigeria. The study area is drained by the Mamu and Orashi rivers and some tributaries of Imo River from the eastern part of the study area. Sedimentary basins worldwide have been shown to generally possess enormous hydrological and hydrogeological potentials due to their good porosity, permeability and hydraulic conductivity (Ugada et al., 2013b; Freeze and Cherry 1979). Previous investigators delineated aquifers and estimated their hydraulic parameters using surface geophysical methods in different parts of the world (Majumdar and Das 2011; Rai et al. 2013). Studies performed by Ugada et al. 2013a; Leite and Barker 1978, have helped to improve the optimization and proper management of the hydrogeological potentials of such basins in order to enhance safe utilization of the groundwater resources and for appropriately safeguarding their quality status. Nevertheless, the characteristics of these aquifers such as their transmissivity, hydraulic conductivity and storage potentials are not fully understood. The determination of aquifer hydraulic characteristics (hydraulic conductivity, transmissivity, and storage potentials) is best made on the basis of data obtained from well pumping test data (Ugada et al. 2013a). These properties are important in determining the natural flow of water through an aquifer and its response to fluid extraction. However, in the case of paucity of pumping test data, these characteristics may be estimated using the Dar-Zarrouk parameters from geophysical sounding. Estimation of aquifer hydraulic parameters using DarZarrouk parameters is well known and has been extensively discussed by previous investigators (Henriet 1977; Niwas and Singhal., 1981). Similarly, several authors have over the years successfully estimated aquifer hydraulic characteristics from Dar-Zarrouk parameters in many parts of south eastern Nigeria from surface electrical resistivity sounding data (Ekwe and Opara 2012, Ugada et al 2013a, 2013b). The present study summarizes the hydro-geophysical assessment of the aquifer system of Orlu area in the Imo River basin. It assesses the nature of the aquifers, their distribution, characteristics and thus, provides data for the assessment of the productivity of the aquifers. Background of study Study Site: Location, Climate, Vegetation, Structural Evolution, Geology, and Hydrogeology o

o

The study area is in the Orlu and environs, southeastern Nigeria. It lies between latitudes 05 44N and 06 00N and longitudes o

o



2



06 55 E and 07 10 E, with mean elevation profile of 173 m above mean sea level and covers an area of approximately 225 km . It is

a humid tropical climate with high temperature and seasonal rainfall. Two seasons are prominent in the area - dry and rainy seasons. o

The mean annual rainfall is between 2,000 and 2,250mm, while mean daily temperature ranges from 20 C during the rainy season to o

o

o

about 33 C in the dry season. The mean annual temperature is between 26.5 C and 27.5 C while the relative humidity lies between © 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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important towns within the study area include Ideato, Uruatta, Nkwere and Njaba. The area is situated in a tropical rain forest. It has

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bounded to the east by Okigwe, to the south by Mbano and Mbaitoli, and to the west by parts of Anambra State (Fig. 1). Some

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65 and 75%. Evapo-transpiration in the area is above 1,450 mm/yr (Nigerian meteorological Agency (NIMET), 2012). The study area is drained by the Mamu and Orashi rivers, which empty into the Imo river. The Imo River rises at the Umuchu hills in Anambra State at the flank of the Awka – Uga-Akoka – Orlu uplands and flows through Anambra, Imo, Abia and River states with numerous tributaries into the Atlantic Ocean. 6.00 deg N N

6

0

0 E

W

7 0 0

U t u

0 3 0 S

10 0 0

Akpuru

A k o k wa 0 2 0

2 0 0

I h i t e n a n s a

Osonachu 2 0 0

sh Or a

U r u a l l a

i 2 0

I h i o m a

A m a i f e k e 6 0 0

2

0

A w o - I d e m i l i 5 0 0

ORLU

0

0

I s i e k e N d i u c h e

LEGEND U m u n a Dikenafia

contours

6

Rivers

0

Nkwere

0

Umu di o k a Roads Towns

A m a i g b o

5.68 deg N 6.94 deg E

7.12 deg E

5 km S C A L E

Figure 1 Topographic map of the study area showing drainage system The study area falls within the Imo River basin, where the bedrock consists of a sequence of sedimentary rocks with a mean thickness of 5480m (Uma, 1989).These sediment pile range in age from upper Cretaceous to Recent. A striking feature in the geologic map is the similarity in the pattern of surface outcrops of the Formations. Almost all the Formations in the study area occur along NW – SE bands that were grossly parallel to the regional strike. The rock units also get younger south-westward, a direction that is parallel to the regional dip of the Formations. The Bende-Ameki and Benin Formations are the major Formations within the study area (Fig. 2). The Bende-Ameki Formation (Eocene) consists of a series of highly fossiliferous greyish-green sandy-clay with calcareous concretions and white clayey sandstones. It has two lithological groups recognized in parts: the lower, with fine to coarse sandstones and intercalations of shelly limestone and the upper with coarse, cross-bedded sandstone, bands of fine, grey-green sandstone and sandy clay (Reyment, 1965). The Benin Formation is the youngest Formation (Miocene to Recent) in the Imo River

foreset beds alternate between coarse and fine grained sands. The generalized stratigraphy of the Imo River Basin is as shown in Table 1. © 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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contains pods and lenses of fine grained sands, sandy clays and clays (Uma, 1989). The Formation is in part cross-stratified and the

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Basin. It is made up of very friable sands with minor intercalations of clays. It is mostly coarse-grained, pebbly, poorly-sorted and

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© 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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Figure 2 Geological Map of the study area

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Table 1 Generalized Stratigraphy of the Imo River Basin (after Uma, 1989) Formation

Age

name

Maximum approximated thickness (m)

Characteristics Unconsolidated, yellow and white

Miocene recent

Benin

2000m

sandstones occasionally pebbly with lenses of grey sand clay.

Ogwash/Asa

Oligocene-

ba

Miocene

Unconsolidated, sandstones with 500m

Formation Ameki

Eocene

Formation

carbonaceous mudstones, sandy clays and lignite seams. Sandstones, grey argillaceous

1460m

sandstones, shales and thin limestone units. Blue to dark grey shale and

Imo

Paleocene

Formation

subordinate, sandstones. It 1200m

includes two sandstone members the Umuna and Ebenebe sandstones. White to grey, coarse-medium

Upper

Nsukka

Maastrrichtian

Formation

350m

grained sandstones: carbonaceous shales, sandy shales subordinate coals, and thin limestones. Medium- to–coarse grained cross-

Lower

Ajali

Maastrichtian

sandstone

3504m

bedded sandstones, poorly consolidated, with subordinate white and pelagic shales.

2. MATERIALS AND METHODS The research method adopted for this study involved a detailed review of the literature of previous work carried out within the study area, and subsequent acquisition of vertical electrical sounding (VES) data, electric log and pumping test data. Since resistivity is a fundamental electrical property of rock materials that is closely related to lithology, the determination of the subsurface distribution of resistivity from measurements on the surface yields useful information on the structure and composition of buried formations (Ugada et al. 2013a). VES using Schlumberger array is based on this fundamental theory. The resistivity of the subsurface material observed is a function of the magnitude of the current, the recorded potential difference and the geometry of the electrode array used. Similarly, the depth of penetration is a function of the geometry of the Schlumberger array. Resistivity techniques generally used in hydrogeophysics require the measurement of apparent resistivity,a which is obtained from the four electrode array, using the relation below:

 v  ..............................................................................................................................................................................(1)  I 

 a  2G 

where G=Geometric factor of the electrode configuration, V=Potential difference and I= current In the present study, 30 VES, with a maximum current electrode separation (AB) of 1000 m, were acquired using the Schlumberger array (Fig.3.0). The ABEM Terrameter SAS 4000 which is a digital meter that gives a direct readout of resistance (V/I)

earth (Ekwe, et al. 2006; Opara etal.2012). Four of these VES data were parametric since they were carried out near existing boreholes within the study area where pumping test data were available.


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thicknesses and resistivities. For this reason, the method is invaluable for investigations on horizontally or near horizontal stratified

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was used for data collection. With VES, we can delineate the vertical sequence of different conducting zones and their individual

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Figure 3 Geological map of the study area showing sounding locations and interpretative cross sections The intervals between the potential and current electrodes were increased at appropriate steps in order to obtain potential differences large enough to be measured with satisfactory precision. Readings were taken at different electrode spreads and the resultant values for each distance was recorded. The observed field data were converted to apparent resistivity values by multiplying with the Schlumberger geometric factor:

 a2  b 

G  



b ..............................................................................................................................................................(2) 4

layers on horizons. A useful approximation is that the depth of the interface is equal to 2/3 of the electrode spacing at which the point of inflection occurs (Opara et al. 2012).The sounding curve for each point was obtained by plotting the apparent resistivity © 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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The electrode spacing at which inflection occurs on the graph provides an idea of the depth to the interface between geologic al

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where a is the current electrode separation (AB) and b is the potential electrode separation

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against the half current electrode spacing on a bi-logarithmic paper. Parameters such as apparent resistivity and thickness obtained from both partial curve matching were further used as input data for computer iterative modelling (Zohdy, 1976; Choudbry et al. 2001). Hence, the OFFIX software was used to fully process and model the acquired resistivity data. Estimation of aquifer hydraulic parameters from geoelectrical data It is a standard practice to estimate aquifer hydraulic parameters using Dar-Zarrouk parameters. Niwas and Singhal (1981) established an analytical relationship between aquifer transmissivity and transverse resistance on one hand, and between transmissivity and longitudinal conductance on the other hand. From Darcy's law, the fluid discharge Q is given by the relationship:

Q  KIA  .................................................................................................................(3) Where K = hydraulic conductivity, I = hydraulic gradient, A = cross sectional area perpendicular to the direction of flow, and from Ohm's law,

J  E ..................................................................................................................( 4) Where J = current density, E= electric field intensity, σ = electrical conductivity (inverse of resistivity). Taking into account a prism of aquifer material having a unit cross-sectional area and thickness h, Niwas and Singhal (1981) combined Equations (2) and (3) to get the relationship given as:

T  KR  KS  ...................................................................................................(5) T = Aquifer transmissivity from borehole, R = Transverse resistance of the aquifer, and S = Longitudinal conductance. S and T are often referred to as the Dar-Zarrouk parameters.

3. RESULTS The generated resistivity curves from computer iterative modelling using OFFIX Software were studied in details to estimate aquifer layer parameters. Both quantitative and qualitative interpretation was carried out using the interpreted curves. Curve types identified ranges from simple HK curve types to complex KHK curve types, which is indicative of lithological variations in the area (Mbaegbu 2013). Fig. 4 interpreted from VES station 9 at Ihitenansa Orlu shows a typical curve type in the study area. The shape of the curve for each sounding point gave an insight into the character of the layers between the surface and the maximum depth of penetration. This is because the shape of a VES curve depends on the number of layers in the subsurface, the thickness of each layer, and the ratio of the resistivity of the layers (Vineesha and Khare 2008). The general signature of the curves suggests an alternate sequence of resistive-conductive layers. Aquifer resistivity, depth and thickness in the study area The aquifer apparent resistivity values across the study area were evaluated from VES curves. Aquifer apparent resistivity ranges from 323 ohm meters (Ωm) at Nkwere Local Government Headquarters (VES 2) to 5,200Ωm at Umudara Umutanze (VES 18) with a mean value of 2943.1 Ωm. Similarly, the depth to the water table was deduced from the VES sounding results and it indicates that the water table is shallow at Amanachi Orlu with a depth of 30 m and much deeper at Ihitte Owerre with a depth of over 166.7 m, with an average value of 115.97 m. The thicknesses of the aquifers of the study area are highly variable with the thinnest area being in the vicinity of VES 19. The thicknesses range from 13 m at VES 19 around Amaifeke Orlu to 67 m at VES 11 within the vicinity of Umueleke Amaifeke (VES 11) with a mean value of 40.91 m. Comparison of geoelectrical section with litho-log of borehole

prominent geoelectric layers were identified in the south-eastern part of the study area. For the purpose of correlation, 5 1

1

1

1

1

interpretative profiles which include A-A , B-B , C-C , D-D and E-E were taken as shown in Fig. 3 above. Two of these cross sections covering the two distinct hydrogeologic zones are presented in Fig. 5 below and were used to infer the regional hydrostratigraphy © 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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Based on the generated geoelectrical sections, which correlated well with strata-logs from boreholes in the study area, 5 to 6

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The apparent electrical resistivity values derived from this study were used to generate geoelectrical cross sections in the study area.

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of the study area. Information extracted from geoelectrical cross sections, litho-logs and electric logs in the study area revealed that the aquifers within the Benin Formation consists of medium to coarse grained, poorly consolidated sands and gravels with clay lenses while those within the Ameki Formation have sandy clay at very shallow depth with very thick consistent layer of shale/clay extending to deeper levels (Mbaegbu 2013). Furthermore, geoelectric sections were correlated with electric logs/ litho- logs at some of the points of parametric shootings for the purpose of correlating surface geo-sounding data with sub-surface geophysical data. The correlation of geoelectric section with litho-log is presented for Njaba as shown in Fig. 6 below. The comparison of the litho-log and VES results of the Njaba borehole revealed a section of reddish lateritic topsoil, fine to medium grained silty sand, mixture of silty clay and fine sand, a thin layer of lignite overlain with a thick clay layer, and a medium to coarse grained sandy layer which serves as the aquifer and underlain by clay. The aquifer material is confined by clay aquitards. There is a fairly good correlation between the geoelectric and geologic sections in the study area.

Figure 4 Typical geoelectric curve from the study area showing HK type

Aquifer hydraulic parameters in the study area The empirical relation of Niswas and Singhal (1981) was used to estimate various aquifer parameters of all the sounding locations, including areas where no boreholes exist. This was done by extracting K values from pumping tests at sites with boreholes and using those values to estimate K for other areas without pumping test data from boreholes. The sites with pumping test data from boreholes include Umuaka Njaba, Nkwere, Ihioma and Akokwa. The hydraulic parameters (transverse resistance (R), Hydraulic conductivity (K) and transmissivity (T) as deduced from the sounding interpretations are shown in Table 2. The hydraulic conductivity (K) refers to the ability of a rock material to conduct fluids under a unit hydraulic gradient, and is measured in m/day. K is generally estimated from the product of the aquifer apparent resistivity and the diagnostic constant (diagnostic in the sense that it is the only model parameter that is directly related to the subsurface).The diagnostic constant is the product of the measured hydraulic conductivity and aquifer conductivity. For this study, four measured K values were available, two for the areas covered by the Benin Formation (at Umuaka Njaba and Nkwerre Local Government Headquarters respectively) and another two for the areas covered by the Bende- Ameki Formation (Ihioma Orlu and Akwu Akokwa respectively). For the estimation of aquifer hydraulic parameters, mean diagnostic constants (Kσ) used were 0.0118765 and 0.001246 for the areas covered by the Benin Formation and the Bende- Ameki Formation respectively. Thus the mean diagnostic constant of 0.0118765 was used for the estimation of hydraulic parameters at VES locations 1, 2, 3, 4,5,10, 11 and 19 while 0.001246 was used for all the other locations. The hydraulic conductivity in the study area varies from 0.96 m/day at Amanachi Orlu (VES 20) to 60.1 m/day at Amaifeke Orlu, with a mean value of 9.599 m/day for the study area. However mean values of 4.053 and 24.85 m/day were estimated for Bende- Ameki and Benin Formations respectively. Aquifer transmisivity defined as the product of hydraulic conductivity (or permeability) and 2

2

2

thickness of the aquiferous unit and is measured in m /day ranges from 13.47 m /day at Amanachi Orlu to 1598.87 m /day at 2

Umuobom Amaifeke, with a regional mean value of the study area as 318.44 m /day. The average transmissivity values for the 2

2

Page

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Bende- Ameki and Benin Formations are 171.378 m /day and 722.84 m /day respectively.


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(a)

1


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1

Figure 5 Geoelectric sections interpreted across AA (a) and DD (b) cross sections for correlation purposes

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(b)

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Figure 6 Comparison of litho log with the VES obtained near Njaba borehole (VES 1).

Formation

10.1

349.29

BENIN

2

Nkwere L.G HQ

323

3.10 x 10-3

50

16150

0.155

5.81

0.0166

3.84

191.81

BENIN

625

1.60 x 10-3

60

37500

0.096

0.0086

7.42

445.37

BENIN

3

Umudioka ukwu


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41

Transmissivity (m2/day)

0.0063

Page

K

4.9

(K) (m/day)

K (from Borehole)

0.041

Hydraulic Conductivity

conductance

29410

Aquifer longitudinal

Aquifer transverse

34.6

resistance

Aquifer Thickness (m)

1.18 x 10-3

(Siemens)

850

Aquifer conductivity

Umuaka Njaba

(m)

Location

1

Apparent resistivity

VES No.

Table 2 Aquifer hydraulic parameters for the study area

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5 6

Umuobom Amaifeke Umuduru, Okporo Umuoba Orlu

3750

2.67 x 10-4

35.9

134625

0.01

0.0014

44.54

1598.87

BENIN

2040

4.90 x 10-4

22

44880

0.011

0.0026

24.23

553.02

BENIN

4010

2.49 x 10-4

31

124310

0.008

0.0011

5

154.89

7

Umuozu Atta

5000

2.0 x 10-4

45

225000

0.009

0.0009

6.23

280.35

8

Ekwe Orlu

2030

4.93 x 10-4

33.3

67599

0.016

0.0022

2.53

84.23

-4

BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDE-

9

Ihitenansa Orlu

3760

2.66 x 10

53

199280

0.014

0.0012

4.68

248.3

10

Ezioha okporo

2720

3.68 x 10-4

23

62560

0.008

0.0020

32.3

772.994

BENIN

1370

7.30 x 10-4

67

91790

0.049

0.0039

16.27

1090.14

BENIN

0.0014

3.86

146.78

0.0014

3.88

111.6

0.0022

2.5

142.75

11

Umucleke Amaifeke

12

Ndiowere, Orlu

3100

3.23 x 10-4

38

117800

0.012

13

Ihioma Orlu

3110

3.22 x 10-4

28.8

89568

0.009

2010

4.98 x 10-4

57

114570

0.028

14 15 16 17 18

Ndiokwu Owere ebeiri Umuzike Umuna Ihite-owerre Ojike Mem Sec Sch Orlu Umudara Umutanze

4.72

3560

2.81 x 10-4

31

110360

0.009

0.0012

4.44

137.51

3846

2.60 x 10-4

40.7

156532

0.01

0.0011

4.79

195.04

3870

2.58 x 10-4

60

232200

0.016

0.0011

4.82

289.32

5200

1.92 x 10-4

20.3

105560

0.004

0.0008

6.48

131.53

-4

19

Amaifeke Orlu

5060

1.98 x 10

13

65780

0.003

0.0011

60.1

781.24

20

Amanachi Orlu

772

1.30 x 10-3

14

10808

0.018

0.0057

0.96

13.47

21

Umuegbe Orlu

2550

3.92 x 10-4

49.2

125460

0.019

0.0017

3.18

156.32

22

owere-ebeiri Orlu

4130

2.42 x 10-4

49.1

202783

0.012

0.0011

5.15

252.67

23

umueze amike

3226

3.10 x 10-4

21

67746

0.007

0.0014

4.02

84.41

24

amike Orlu

1530

6.54 x 10-4

55

84150

0.036

0.0029

1.91

104.85

4167

2.40 x 10-4

50

208350

0.012

0.0011

5.19

259.6

2400

4.17 x 10-4

52

124800

0.022

0.0018

2.99

155.5

4167

2.40 x 10-4

50

208350

0.012

0.0011

5.19

259.6

4167

2.40 x 10-4

50

208350

0.012

0.0011

5.19

259.6

4167

2.40 x 10-4

50

208350

0.012

0.0011

5.19

259.6

25 26 27 28 29 30

Umudimoha Amarie Umuauji Ogberuru Uruala Ideato L.G.A Uruala Ideato L.G.A Akwu Akokwa Ideato PTF Police Awo-Idimmiri

784

1.28 x 10-3

43.4

34025.6

0.055

4.06

0.0006


0.98 OPEN ACCESS

42.4

AMEKI

BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENIN BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI BENDEAMEKI

42

4

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Figure 7 Kσ variation within the study area VES 29 5.90 VES 17 VES 9 8 VES 16

5.88 5.86

VES 28 27

BENDE AMEKI FORMATION VES 7

5.84 VES 30 5.82

Seimens/day

VES VES VES13 12 14 15 18 21 22 23 24 25 26 4 5 VES10 6 VES

5.80

0.016

19 20 VES 11

0.015

Latitude

0.014

VES 3 VES 2

5.78

0.013 0.012 0.011

5.76

0.010 0.009 0.008

5.74

0.007 0.006

5.72

0.005

BENIN FORMATION

0.004 0.003

5.70

6.94

6.96

6.98

7.00

7.02

0.000

7.04

7.06

7.08

7.10

Longitude

Figure 8 Kσ variation as a diagnostic tool to delineate lithology within the study area © 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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0.001

VES 1

5.68

43

0.002

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Kσ variation as a diagnostic tool for mapping lithology within the study area A histogram showing the variation of Kσ (lithology diagnostic parameter) values within the study area is shown in Fig. 7. On the basis of this parameter, Umuaka Njaba, Nkwerre, Umudioka and Amaifeke Orlu, with an average Kσ value of 0.0086 are believed to be hydrologically homogeneous and likely represent the Benin Formation. Similarly, Umuoba-Umuzike - Ihitenasa-Amanachi-Uruala areas, with average Kσ value of 0.0010, are also fairly hydrologically homogeneous and probably represent the Bende-Ameki Formation. Thus, the diagnostic Kσ parameter has been applied in the study area to delineate different lithologies (Fig. 8). The 3-D model showing the Kσ variation of the study area is shown in Fig. 9.

Figure 9 3D model showing the geologic Formations within the study area, as deduced from Kσ values

4. DISCUSSION The aquifer apparent resistivity across the study area ranges from 323Ωm at Nkwere Local Government Headquarters (VES 2) to 5,200Ωm at Umudara Umutanze (VES 18) with a mean value of 2943.1 Ωm. Similarly, the depth to the water table indicates that the water table is shallow at Amanachi Orlu with a depth of 30m and much deeper at Ihitte Owerre with a depth of over 166.7 m giving a regional mean of 115.97m. The thicknesses range from 13m at Amaifeke Orlu to 67 m at Umueleke Amaifeke (VES 11) with a mean value of 40.91m. The hydraulic conductivity across the study area varies from 0.96 m/day at Amanachi Orlu (VES 20) to 60.1 m/day at Amaifeke Orlu, with a mean value of 9.599 m/day for the study area. Hydraulic conductivity values for the areas underlain by the Benin Formation ranges from 3.84 m/day to 60.1 m/day with a mean value of 24.85 m/day. On the other hand, for the areas underlain by the Bende- Ameki Formation, the hydraulic conductivity varies between 0.96 m/day to 6.48 m/day with a mean value of 2

2

4.053 m/day. Similarly, aquifer transmissivity values ranges from 13.47 m /day at Amanachi Orlu to 1598.87 m /day at Umuobom 2

Amaifeke, with a regional mean value of the study area as 318.44 m /day. The transmissivity values for the Benin Formation varies 2

2

2

from 191.81 m /day to 1598.87 m /day with a mean value of 722.84 m /day, while for the Bende-Ameki Formation, the transmissivity 2

2

2

varies between 13.47 m /day to 289.32 m /day with a mean value of 171.38 m /day. These findings have led to the delineation of the aquifer zones in the study area into two distinct zones. This is in conformity with the geology of the study area which revealed two geological zones of Benin and Ameki Formations respectively with the area covered by the Benin Formation having a more prolific aquifer. These findings are in agreement with the results of previous studies within and around the study area (Mbonu et al.1991; Igbokwe et al. 2006).The diagnostic constant, Kσ (which have previously been discussed in details by Keller and Frischnechk 1979; Opara et al. 2012), has also proved very useful in this study. Because it clearly delineated two hydrological zones with a distinct groundwater divide. This variation between the two areas may possibly be traced to the variation in geology and topography. It is therefore hoped that results of this study will be invaluable to planning of future

5. CONCLUSION The close agreement between results from pumping tests and those obtained from VES interpretation is an indication of the reliability of the present work. This shows that electrical resistivity method is very useful for understanding the aquifer systems within © 2018 Discovery Publication. All Rights Reserved. www.discoveryjournals.org

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water supply schemes within the area.

ARTICLE

RESEARCH

the study area. The diagnostic constant, Kσ, which have previously been discussed in details by Niwas and Singhal (1981) and Ekwe and Opara (2012) has proved very useful in this study. It was used effectively to delineate two distinct lithostratigraphic units (Benin and Bende-Ameki Formations) within the study area. The Kσ parameter was also used to estimate the hydraulic conductivity and transmissivity for all the sounding locations across the study area, including areas without boreholes. The average hydraulic conductivity value within the Bende-Ameki and Benin Formations are 4.053 m/day and 24.85 m/day respectively while average 2

2

transmissivity values varies between 171.38m /day and 722.84 m /day for Bende-Ameki and Benin Formations respectively.

ACKNOWLEDGEMENTS We appreciate our field guides for their invaluable assistance during field data acquisition. We thank the Golden software company for granting us unlimited access to their

TM

Surfer software.

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