ABSTRACT
This
paper assesses and evaluates geologically related problems and issues associated
with five ‘abortive’ boreholes drilled within the Abakaliki metropolis,
southeastern Nigeria. Results of the Vertical Electrical geophysical soundings
(VES), carried out in these borehole locations, show that the underlying Abakaliki
Shale Formation in those areas has six layered model. While the uppermost layer has an apparent resistivity value of range 240 to 1120 ohm-m and a thickness of range 1.0 to 1.2 m, and is interpreted as lateritic overburden, the sixth layer records an apparent resistivity value of 320 to 1025 ohm-m and undetermined thickness.
Shale Formation in those areas has six layered model. While the uppermost layer has an apparent resistivity value of range 240 to 1120 ohm-m and a thickness of range 1.0 to 1.2 m, and is interpreted as lateritic overburden, the sixth layer records an apparent resistivity value of 320 to 1025 ohm-m and undetermined thickness.
The range of the apparent resistivity
values and a correlation with local geological data reveal that the formation,
although layered, is predominantly shale, which become fresh, unweathered and
‘unfractured’ at greater depth. The depths of studied boreholes ranged from 45
to 80 m, hence, they most certainly tap the sixth layer fresh, unweathered shale;
a geological material notoriously known for its low permeability, high water
retention capacity and very low specific yield of water to boreholes.
Keywords: Abakaliki Shale; Abortive boreholes; Aquiclude; Vertical electrical
sounding.
INTRODUCTION
In the
Abakaliki metropolis (Fig 1), the Ebonyi State Water Company Limited, which is
mandated to provide potable water for the inhabitants of city and urban areas,
is currently unable to supply adequate quantities due to the ever-increasing
population accentuated by inability to expand the infrastructure to cater for
the requirement of potable water. Most places do not have pipelines and those
who have do not have water flowing through their taps constantly. This has led
to the people resorting to alternative means of getting water, such as hand-dug
wells, surface water bodies and drilled boreholes, for their daily water needs.
The surface water sources are usually associated with diseases and sicknesses. It
has been estimated that lack of clean drinking water and sanitation services leads
to water-related diseases globally and between five to ten million deaths occur
annually, primarily of small children (Snyder and Merson, 1982). This leaves
the groundwater are the best means of constant and safe water for most
inhabitants.
The
availability and exploitation of the groundwater, on the other hand, is also
becoming problematic. While some of the borehole drilled within the Abakaliki
metropolis yield water at economic quantity all round the year (i.e.,
productive boreholes), some are only productive only in the peak of the rainy
season, and the rest are completely unproductive, and are locally referred to
‘abortive’ boreholes. This work, therefore, attempts to evaluate, by means of geophysical
investigation, why five boreholes despite being drilled in the proximity of (i.e.,
few metres away from) productive ones failed to yield water at economic
quantities. Highlights on the causes of the failure of the boreholes are made
strictly made on the basis on the results of the geophysical surveys conducted;
other probable causes are not much emphasized in this work but can be found
elsewhere.
DESCRIPTION
OF THE STUDY AREA
Physiography
and climate
Abakaliki,
the capital territory of the present day Ebonyi State of Nigeria, is generally
undulating and no location exceeds 400 m above-sea-level (Fig 1). A major
relief structure is hills formed by the pyroclastic bodies. No trend has been
established by previous research (Ofoegbu and Amajor, 1987) of these conical
shaped hills and other residual hills that spread sporadically within the area.
The predominant shale has favoured the low erodability of the lithology,
resulting in absence or near absence of deep cut valleys and erosion channels.
Stunted trees and pockets of derelict woodland exist where the lithology has
undergone high degree of laterisation. Elsewhere, typical characteristics of
the tropical rain forest are displayed; multitude of evergreen trees, climbing
plants, parasitic plants that live on the other plants, and creepers.
Two
main seasons exist in the Abakaliki area, the dry season which spans from
November to March and the rainy season which begins in April and ends in October
with a short period of reduced rains in August commonly referred to as “August
break”. Most hand dug wells and boreholes in this area are recharged during the
peak of the rainy season. Temperature in the dry season ranges from 20°C to
38°C, and results in high evapotranspiration, while during the rainy season
temperature ranges from 16°C to 28°C, with generally lower evapotranspiration.
A number of hand dug wells are completely dried up in the peak of this season.
The average monthly rainfall ranges from 31mm in January to 270 mm in July,
with the dry season experiencing much reduced volume of rainfall unlike the
rainy season, which has high volume of rainfall. Average annual rainfall varies
from 1,500 mm to 1,650 mm. These climatic conditions are responsible for the
development of thick lateritic soils in the Abakaliki area and its environs
(Ezeh and Anike, 2009).
Geology
and Hydrogeology
The Abakaliki metropolis is,
geologically, underlain by the Abakaliki Shale Formation of the Asu River Group
(Reyment, 1965). The Asu River Group sediments are predominantly shales, and
localized occurrences of sandstone, siltstone and limestone intercalations
(Ofoegbu and Amajor, 1987). It was generally believed to have started depositing
in the mid-Albian period and was deposited within the lower (or southern) Benue
Trough, southeastern Nigeria. The geology of the Abakaliki metropolis is shown
in Fig 1. Emplaced in these Asu River Group sediments are intermediates to
basic intrusive, extrusives and pyroclastics (Murat, 1972; Nwachukwu, 1972; Ofoegbu
and Amajor, 1987; Tijani et al., 1996). The group has average thickness of
about 2000m and rests unconformably on the Precambrian Basement (Benkhelil et
al., 1989). The Abakaliki Shale Formation, which has an average thickness of
about 500 m, is dominantly shale, dark grey in colour, blocky, and
non-micaceous in most locations. It is calcareous (calcite-cemented) and deeply
weathered to brownish clay in the greater part of the formation (Okogbue and
Aghamelu, 2010).
Aghamelu
et al. (2011) note that the major part of the Abakaliki metropolis is underlain
by aquiclude; except in locations or zones where secondary aquiferous conditions
were made possible by syn- and post depositional circumstances. The syn-depositional
circumstance is the occurrence of lenses of sandstone or siltstone beds, while
the post depositional circumstances include weathering, fracturing or shearing,
and volcanic intrusions. The zones are recharged mostly in the peak of rainy
season and by surface waters in the area. The major river that drains the area
is the Ebonyi River and its tributaries; Udene and Iyiokwu Rivers. Both
tributaries are perennial and usually overflow their banks at the peak of the
rains.
METHODOLOGY
Geophysical survey
The Vertical Electrical
Sounding (VES) using the Schlumberger electrode configuration was carried out within
the locations of the studied ‘abortive boreholes’. The VES stations are shown in Fig 1. The VES survey
following guidelines specified in Telford et al. (1990). The
procedure involves driving current into the ground using a pair of electrodes
and the resulting distribution of the potential in the ground measured using
another pair of electrodes connected to a sensitive voltmeter (a recording
instrument). The difference in the potential in ohms is converted to apparent
resistivity by using a factor that depends on the electrode configuration. The recording instrument used was OMEGA 5 (Signal
Averaging System, SAS1000). The instrument automatically calculated the average of five cycles
of the ratio of the potential difference across the layer and the current
passing through the layer (recorded in ohms-metre, Ωm).
Some precautions were taken during the field surveys
in order to generate an accurate or near accurate data. For example, the electrodes were hammered
firmly into the ground to at least a depth of 35 cm in order to avoid error
reading and achieve good electrical contacts with the ground. Water was poured
at the point of electrode contact with the ground to possibly enhance ionic
flow of current; survey was carried in the dry season. Also, positions of the electrodes were kept
far away from power-lines to avoid altering the value of the potential
difference.
Data analysis and Interpretation
The
processing of the data generated during the survey involved automatic plotting
of apparent resistivity values of the formation surveyed against half the
electrode spread with the aid of a computer software, known as OFFIX. This
software ensured breaking of the subsurface into distinct layers of differing
resistivity in relation to thickness and depth. The shape of the resulting
curve depends on the resistivity contrast between the layers. The computer
system also used a family of curves, given by Telford et al (1990), to automatically
match the curves and determine the layer resistivity.
RESULTS AND DISCUSSION
A set of resistivity
curves obtained by software interpretation of data generated by means of VES
surveys in this study is presented in Figs 2. It can be deduced from the curves
that the surveyed portions of the geological formation coincidentally have five
to six distinguishable layers. Each of the layers however differs significantly
with each other with respect to their resistivity to electric current. Despite the
variation, however, all the layers recorded relatively high apparent
resistivity values (ranging between 50 and 1120 ohm.m).
A
correlation of these resistivity values of the geology of the Abakaliki area
give indications that the layers are shales, most probably, with slightly differed
water bearing capacities due to heterogeneity in their physical properties.
Grain size distribution, sorting, cementation, porosity, permeability and mineralogy
(Pettijohn et al. 1973; Prothero
and Schwab, 1996), amongst other syn- and post
depositional factors, could result in the observed the resistivity contrasts in
the model layers. Lithologic models developed from the resistivity curves are
presented in Fig 3.
The
depths of the studied ‘abortive’ boreholes ranged from 45 to 80 m (see Table 1).
It is therefore most likely, as shown in Fig 3, that they tap the hard shale
layer (bottom) and an indeterminate depth and apparent resistivity values that ranged from 320 to 1025 ohm-m.
Although the Abakaliki Shale is known to be highly fractured and sheared (Ezeh
and Anike, 2009), limited knowledge exist of the depth and general
distributions of the fracture system. It is possible that the fracturing is a
near surface phenomenon, hence could not be found at greater depth. Shale, as a
geological material, is notoriously known for its low permeability, very low
specific yield and high water retention capacity. This notoriety become more
pronounced if it is fresh and unweathered, thus, most be a major reason, among
others, why the boreholes failed.
Aghamelu
et al. (2011) have highlighted the fact that most hand dug wells, and even some
boreholes, in the Abakaliki area dry up in the middle of dry season leaving the
town with acute shortage of portable water. They, however, suggest that
boreholes that tap the fractured and weathered zones, sandstone and limestone
layers or members of the Abakaliki Shale, as well as the bedrock interfaces
with the shale formation are likely to be productive. According to Ismael
(1990), the fractured and weathered zone shale-aquifer systems have specific
yields values as follows; range 1.26 × 10-3 to 1.60 × 10-3
litre per second (l/s) and 1.22 × 10-3 to 1.81 × 10-3
l/s, with average values of 1.48 × 10-3 l/s and 1.59 × 10-3
l/s, respectively. Fresh and unweathered types, however, have specific yield
values much lower than these given (Ismael, 1990).
CONCLUSIONS
This
study has provided insights which indicated that the existence of secondary
aquifer systems; via weathering, fracturing and lenses of siltstone or
sandstone beds, is likely not a continuous (both laterally and vertically)
phenomenon in the Abakaliki Shale Formation, especially within the Abakaliki
metropolis. Consequently, the concept of a ground water table with regional flow
is very likely to be unrealistic. This fact may also be supported by the
principle of ‘heterogeneity of geological material’, which emphasizes that
geological materials have tendencies to differ, laterally and/or vertically, in
properties especially distances (even few metres) apart.
The
economic wastage, material lost and frustrations associated with ‘abortive’
boreholes necessitate the need for a thorough geophysical investigation prior
to citing of boreholes within some localities in the Abakaliki area. This is despite
whether the site is in the proximity or distances away from a productive
borehole or hand-dug wells.
The
implementation of a water resources management strategy by the state water
corporation is imperative to ensure the availability and sustainable provision of
potable water to the inhabitants of the Abakaliki metropolis. This becomes very
necessary to reduce the health risk the inhabitant would be exposed to in an attempt
to source water from alternative means especially when they are faced with the
‘abortive borehole’ phenomenon.
Acknowledgements
Dr. B.I. Odoh of the
Department of Geological Sciences, Nnamdi Azikiwe University, Awka, and Mr. E.
O. Ezim of the Department of Geology, University of Ibadan, Ibadan, are
gratefully acknowledged for their invaluable contribution towards this work.
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ARTICLE SOURCE
Department of Geology and
Exploration Geophysics