PAP_048

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An Appraisal of Subsurface Geology and Groundwater Resources of Owerri and

Environs based on Electrical Resistivity Survey and Borehole data Evaluation.

K.M. Ibe1 , Sr. and S.C. Uzoukwu

2

Abstract

The research was aimed at determining the depth to the water table, aquifer thickness and

subsurface geology of the study area thus revealing its groundwater distribution as well as its

potential as a substitute to the surface water resources. Vertical electrical soundings were carried

out in the study area with maximum electrode spread. The schlumberger electrode configuration

technique was adopted. VES data were processed using Schlumberger analysis package. Lithologic

logs of already existing boreholes in the study area were collected, evaluated and comparison

carried out. The results reveal alternating layers of sands, sandstones, gravel and clay. The

Lithologic logs revealed that the study area is underlain by coastal sands (Benin formation). The

water table varies from 10 – 64 m and thickness of the aquifer ranges from 20 – 80 m. Results showthat the study area is underlain by a thick extensive aquifer that has transmissivity 2.8 x 10 -2 m2 /s to

3.3 x 10-1 m2 /s and storativity 1.44 x 10-4 to 1.68 x 10-3 m/s values. The specific yield is about 0.31.

The sandy component of the study area forms more than 90 % of the sequence, therefore

permeability, transmissivity and storage coefficient are high with an excellent source of

groundwater resources.

Key Words: Groundwater distribution, Electrical resistivity survey, Boreholes lithologic logs,

Aquifer thickness.

1. Introduction

The Owerri urban has experienced immense population, agricultural and industrial activities which have

caused and will continue to cause great stresses on the Oramiriukwa and Otamiri Rivers water-resources in

the study area. The water needs of the study area is expected to rise to about 11.4 million m 3/day by the

year 2000 (Ibe et al., 1992) and this volume of water would be supplied by the surface and groundwater

systems conjunctively. Presently, the major sources of the surface water supply to Owerri urban are the

Oramiriukwa and Otamiri rivers. The increase of population and industrial activity has resulted in the

generation of substantial domestic wastes which are disposed off in uncontrolled manner within the

Oramiriukwa and Otamiri Rivers watersheds. Such disposal method can lead in the long term to a

widerspread pollution/contamination of the surface water resources of the watersheds.

The research was aimed at determining the depth to the water table, aquifer thickness and subsurface

geology of the study area thus revealing its groundwater distribution as well as its potential as a substitute to

the surface water resources.

1. Assistant Professor Department of Geology, Federal University of Technology,

P.M.B 1526, Owerri, Nigeria.

2. Graduate Student, Department of Geology, Federal University of Technology,

P.M.B 1526 Owerri, Nigeria .

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2. Geology

The study area is located within southeastern Nigeria sedimentary basin. The study area consists of Owerri

metropolis and environs. It is bounded by latitutdes 50 20 / and 50 40 / N and longitude 70 00 / and 7010 /

E (Fig. 1). It is underlaid by the sedimentary sequence of the Benin formation (Miocene-Recent), and the

underlying Ogwashi-Asaba formation (Oligocene). The Benin formation is made up of friable sands with

minor intercalations of clay. The sand units are mostly coarse-grained (Short and Stauble, 1967;

Onyeagocha, 1980). The formation starts as a thin edge at its contact with Ogwashi-Asaba formation in

the north of the area and thickens southwards (seawards). The average thickness of the formation at the

study area is 800 m (Avbovbo, 1978).

The terrain of the study area is characterised by two types of land forms; highly undulating ridges and

nearly flat topography. The flat southern part is underlaid by thick unconsolidated sand with the minorlenses and stringers. Borehole lithologic logs reveal that the undulating hills and ridges are underlain by a

succession of thick unconsolidated sandstones and relatively thin clay units belonging to the Benin formation.

The ridges trend in the north-south direction and have an average elevation of about 122 m (Uma, 1984).

In between those ridges are valleys of the Otamiri River, Oramiriukwa River and their tributaries.

The study area is within rain forest belt of Nigeria. The annual rainfall is 2200 mm and is spread over most

of the year with peaks during the month of June, July and September, and low rains in December and

January. The soil type belong to ferralic. The soil profile is remarkably uniform throughout the area, deeply

weathered and intensely leached.

3. Method of Study

Two basic measurements were carried out in the quantitative assessment of groundwater resources in the

study area thus: existing Boreholes and VES survey. An aquifer can be detected through detailed

hydrogeologic survey using surface and subsurface geological methods and subsurface geophysical

methods. A total of thirty-eight (38) lithologic logs within Owerri and environs were obtained from

UNICEF and analysed (Fig. 2). The lithologic logs are prepared by on the spot monitoring of the changes

in lithology during borehole drilling. Vertical Electrical Sounding (VES) were carried out using

Schlumberger array resistivity survey method with one current electrode profile at a maximum spread of500 m on either side of the pole. The instrument used was the ABEM SAS 300 B which gave a direct

readout of resistance. The interval between the potential and current electrodes was increased only a few

times and in relatively small steps in order to obtain potential differences large enough to be measured with

satisfactory precision. Five types of sounding curves have been identified from the study area. Although

there are some striking similarities between some of these curve types with respect to the probable

geoelectrical and geological succession. They have been classified as follows:

Class 1: A – type

Class 2: K – type

Class 3: KH and HK – type

Class 4: KHK – type

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3.1 Curves and Types Description

A = Type curve is characterised by a continuous increase in resistivity from the topsoil, with increasing

electrode separation (Fig. 3). In this curve type, the overburden can be subdivided into 2 layers, with

aquiferous zone underlying it.

Description of VES 2 curve: They survey intercepted 5 Lithologic units as follows:

Lithologic Unit Depth (m) Description

1 0 - 3.8 Top soil

2 3.9 - 18.7 Sandstone

3 18.8 - 34.3 Sand

4 34.4 – 78.1 Consolidated Sandstone

(saturated zone)

5 78.2 – Below SandstoneThe VES 2 curve (Fig.3) is a typical example of A-type curve. This type of sounding curve is

characterised by a continous increase in resistivity from the top soil, with increasing electrode separation.

In this curve type the overburden can be sub-divided into 2 layers with aquiferous zone underlying it.

The steady decrease in resistivity with increase in electrode separation could be attributed to the difference

in porosity between the upper sand unit and the lower consolidated sandstone unit. The Geo-electric

section of VES 2 is the general interpretation of the geology indicated by the curve (Fig. 4a).

K – Type curve shows maximum graph plot (Fig. 3a & b). The sounding curves of this class are the most

common in the study area. In this curve type, the third layer consisting of clay or clayey sand has a lowerresistivity than the overlying layer of higher resistivity. This intermediate layer, however consists of sand and

gravelly sand and is the shallow aquifer overlain by the weathered top soil that shows a relatively, low

resistivity. According to Uma and Egboka, (1986) these thin bands of clay are used to subdivide the

regional aquifer.

Description of VES 4 curve: The survey also intercepted 5 lithologic units as follows:

Lithologic Unit Depth (m) Description

1 0 - 0.4 Top soil (sand)

2 0.5 - 2.5 Gravel3 2.6- 41.5 Sandstone

4 41.6 - 95.5 Sand (saturated zone)

5 95.6 - Below Consolidated sandstone

The VES 4 curve belongs to K 4-type curve. The sounding curve shows both maximum, and minimum

graph plot.

The maximum graph plot could be attributed to the presence of gravel and sandstone with high resistivity in

between two sand units of lower resistivity. The lithologic unit below the saturated sand has a high

resistivity, can only be consolidated sandstone. The Geo-electric section for the illustration of the geology

of the area is shown in figure 4 b.

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H – Type curve shows minimum graph plot. The intermediate layer in this H type curve is commonly water

saturated and is often characterised by low resistivity, high porosity and low permeability. This type curve

is usually obtained when the sounding station is close to a river. This is also responsible for the

shallowness of the aquiferous zone. However this type curve was not obtained in this

survey.

Other type curves include curves obtained from combination of the type curves listed above thus: HK,

HKH, KHK, AK. The KH- type curve is also common in the study area (Fig. 4). The HK- and KHK-

type curves were encountered in the study area.

The class 1 and 3 type curves were common in the study area. Only four of such curves were encountered.

The sounding curves of class 2 are most common and the class 3 and 4 types curves are characteristic of

sedimentary basins and they were soundings done along or close to the bank of a river.

4. Result and Discussions

The subsurface geology was evaluated from three geologic cross-sections taken along A – A1, B – B1, C –

C1, (Fig. 2); geoelectric sections; as well as from correlation studies of borehole litho-logs outside the lines

of section.

It is evident from the cross-section (Fig. 5) that the northern part of the area is predominantly, underlain by

sands. The sand facies grades into a gravelly sequence towards the southwest. The south eastern part is

distinguished by a thick clayey facies that thins out towards the Owerri metropolis. The observed facies

changes were consistent with the characteristics of the coastal plain sand formation as established by

Reyment (1965).

Figure 5d, shows the general groundwater flow pattern in the study area. It shows that the pattern of

groundwater flow has been influenced by the topography. Flow is southwest the study area which serves

the head water of the Otamiri River and the Otamiri River valleys. Figure 5e illustrates the hydraulic

relationship between the groundwater flow and surface water in the study area.

Based on results, the area was subdivided into three cells of distinct lithostratigraphic unit: sandy area (Cell

1), sand-gravelly sequence (Cell II) and clay-sand unit (Cell III) (Fig. 6).

Based on the thickness of the unsaturated zone deduced from the depth of water table data, an overburdenthickness map of the study area (Fig. 7) was constructed. The properties of the unsaturated zone materials

show that the northern end of the area (Cell I) consist of a lateritic layer with an average thickness of 8 m.

The laterite is thicker northwestwards (Fig. 5a). The laterite is underlain by a thin layer of reddish, fine to

medium sand which grades into a thick sequence of medium to coarse whitish sand. The cumulative

thickness of the unsaturated zone is between 30 m and 60 m, being progressively thicker northwards. In

Cell II the unsaturated zone is composed of a reddish brown lateritic unit of about 8 m – 12 m thick. This

unit is directly underlain by medium to coarse sand with lenses of gravels occurring northwards (Okuku and

Owerri metropolis). The gravelly sequence becomes more predominant southwards (Obinze and Ihiagwa)

with intercalations of fine to medium sand (Fig. 5 b & c). The cumulative thickness of the unsaturated zone

ranges from 10 m – 30 m.

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The southeastern end of the study area (Cell III) is underlain by a relatively thin layer of laterite (3 – 10 m)

which grades into a bed of clay with sandy intercalations. The clayey bed thickens eastwards of Owerri

metropolis, towards Azaraegbelu (Fig. 5c). Southeastwards of Emekuku area (Agbala and Ulakwo), the

clayey bed is located a greater depth being overlain by fine to medium sand. The cumulative thickness of

the clayey unit is between 10 m – 30 m.

Results of static water level measurements (Table 1) show that depth to water table decreases

progressively southwards from about 65 m around Atta in the north to 19m in Obinze to the south.

Aquifer units in the study area consists mainly of unconsolidated medium to coarse sands and occasional

gravels. The lithology of the aquifer materials is fairly uniform with minor-facies changes. Groundwater

occurs in unconfined conditions in most places. However, the occurrence of an impermeable clayey unit

within the saturated zone in Cell III confers a semi-confined condition on the aquifer unit in this sub-area.

The study area is underlain by a thick extensive aquifer that has transmissivity (2.8 10-2 to 3.3 x 10-1 m2/s)

and storativity (1.44 x 10-4 to 1.68 x 10-3 m/s) values. The specific yield is about 0.31 (Uma and Egboka,

1986).

Analysis of data was done in two stages. The first involved the critical evaluation of strata and geophysical

logs to determine actual thicknesses of the aquifers penetrated, evaluate the extent of penetration of the

boreholes into the aquifers and assess the general hydro-geology of test sites with a view to understanding

hydraulic boundary conditions that may have affected the time drawdown data from the pumping test

activities.

4.1 Methods for fully – penetrating wells

The draw-down (s) in a well fully penetrating a confined aquifer is (Theis, 1935):

)(44

uW T

Q

y

dye

T

Q s

u

y

ππ

α

== ∫ −

whereT

S r u

π4

.2

= 2

4

r

TtuS =

s = The drawdown in meters measured in a piezometer

at a distance in meters from the pumped well.

S = the dimensionless coefficient of storage or

storativity .

Q = the constant well discharge in m3/hr.

T = Kb = the transmissivity of the aquifer

t = time in hours since pumping started

⋅⋅⋅+−+−+−−=!4.4!3.3!2.2

uuIn5.0)(432 uuu

uW

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The Theis aquation is routinely solved using the graphical method of Theis (1935), Jacob (1940), and

Chow (1952). Any of these methods gives results that are generally accepted as being reliable. In this

study, two methods, the Theis and Jacob’ s were employed for comparative purposes.

5. Conclusion

The VES surveys conducted and borehole logs analysed revealed that the study area is underlain by

the Cenozoic formation of the Niger Delta complex. The Miocene-Pleistocene – Benin formation is

frequently exposed along the river channels, banks and road cuts along the Owerri – Onitsha road. The

Benin formation consists of gravels (conglomerates) and very coarse to medium grained sands and

sandstone. The Benin formation acts as multilayer aquifer of medium to high yields. The results revealed

that the geology of the study area is satisfactory for the exploitation of the groundwater resource. The study

area is underlain by a thick extensive aquifer that has transmissivity (2.8 x 10-2 to 3.3 x 10-1 m2/s) and

storativity (1.44 x 10

-4

to 1.68 x 10

-3

m/s) values. The specific yield is about 0.31. These results indicatehigh groundwater resources potentials.

References

Avbovbo, A. 1978. Tertiary Lithostratigraphy of Niger Delta. Bull. Amer. Ass. Petr, Geol. 62, 295 –

305.

Chow, V.T., 1952. On the determination of transmissiblity and storage coefficient from pumping – test

data. Trans. Am. Geophys. Union, Vol.33, 397 – 404.

Ibe, K.M., A.H.O Sowa and O.C. Osondu, 1992. Environmental Contamination and otheranthropogene impacts on Otamiri and Nwaorie Rivers, Owerri, Nigeria. Journal of Min. and

Geol. Vol. 28 No.1, 87 - 91

Jacob, C.E., 1940. On the flow of water in an elastic aquifer. Trans. Am. Geophys. Union Vol. 72, No.

II, 574 – 586.

Onyeagocha, A.C. 1980. Petrography and Depositional Environment of the Benin formation. Nig. Journ.

Min. Geol. 17, 147 – 151.

Reyment, R.A. 1965. Aspect of the Geology of Nigeria. Ibadan University Press.

Short, K. and Stauble, J. 1967. Outline of the Geology of the Niger Delta. Bull.

Amer. Ass. Petr. Geol. 51, 661-779.

Theis, C.V., 1995. The relation between the lowering of the piezometricsurface and the rate and duration of discharge of well using groundwater storage. Trans. Am.

Geophys. Union, Vol. 16, 519 - 524

Uma, K.O. and Egboka, B.C.E., 1986. Water resources of Owerri and its Environs, Imo State, Nig.

Journ. Min. Geol. 22, 57 – 64.

Uma, K.O., 1984. Water Resources Potential of Owerri area and the environs. Unpub. M.Sc. Thesis,

University of Nigeria, Nsukka.

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