ABSTRACT
An investigation was carried out on the groundwater potentials and aquifer protective capacity of Ishiagu area, southeastern Nigeria. The study area lies within the lower Benue trough of Nigeria and is underlain by the Albian Asu River Group and the Turonian Ezeaku Shales.The methodologies employed in the study include measurements of static water levels of 15 hand dug wells, pumping tests carried out on 4 boreholes and vertical electrical sounding (VES) of 20 stations. Groundwater potentials of the area was thoroughly characterizedusing aquifer parameters of hydraulic conductivity and transmissivity within unit cells.These parameters were supplemented with those determined from empirical relationships.The hydraulic conductivity and transmissivity determined from the pumping test data range from 0.6m/day to 3.04m/day and 4.86m2/day to 34.93m2/day respectively while those from empirical relationships range from 0.04m/day to 4.34m/day and 0.07m2/day to 61.69m2/day respectively.Three groundwater potential ratings were defined based on the aquifers’ hydraulic conductivity and transmissivity data; poor (45%), fairly good (35%) and good (20%). The hydraulic head map reveals two divergence and two convergence zones. The computed hydraulic heads range from 58.2m to 84.5m.Vertical electrical sounding reveals that the area is characterized by 4-, 5- and 6- subsurface geo-electric layers with the 6-layer type being the dominant type. Longitudinal unit conductance of the 20 VES stations was estimated from the layers’ resistivity and thickness data.Longitudinal unit conductance of the overburden units ranged from 0.07mhos to 2.22mhos. Based on the estimated longitudinal unit conductance, three aquifer protective capacity types were defined namely, weak (25%), moderate (40%) and good (35%). It was observed that areas of good groundwater potentials also have good aquifer protective capacity. Groundwater development should therefore be concentrated more in areas of good groundwater potential for continuous/steady supply of potable water.
CHAPTER ONE
INTRODUCTION
1.1 BACKGROUND INFORMATION
The Ishiagu area
lies within the lower Benue trough of Nigeria and is underlain by the Albian
Asu River Groupand the Turonian Ezeaku Shales. The area is also intruded in
many places by Santonian intrusive rocks and mineralized in some parts by
hydrothermal Pb –Zn ores. (Kogbe, 1976)
The geology of
the area is not favourable for groundwater availability. With population
explosion in Ishiagu town occasioned by the presence of several mining and
quarrying companies such as Crushed Rock Industries Ltd., Setraco Nig. Ltd.
etc, a greater need for potable water has become obvious. More so, the need to
assess the protective capacity of the aquifer in the area cannot be overemphasized
as there is the danger of its contamination from surface industrial effluents
and pollutants emanating from mining and quarrying activities in the area.
As exploitable
quantity of groundwater is expected to be available only in fractured or
weathered zones of the area, surface geoelectrical method becomes very necessary
to locate such areas of reasonable groundwater potential. Furthermore,
knowledge of values of aquifer parameters such as hydraulic conductivity and
transmissivity in the area is useful in evaluating the groundwater potential of
the area.
Although the conventional way of determining the
mentioned aquifer parameters has been the use of pumping test method, it has
proved to be expensive, tedious and time consuming. Also various formulas available for
calculating the aquifer characteristics from pumping test data analysis are
valid only if various assumptions about aquifer continuity, thickness,
homogeneity, isotropy, well storage and nature of fluid flow are valid under
field conditions (Freeze and Cherry 1979),hence the need for a reasonable
alternative.Surface geoelectric methods have proved to be a handy alternative
to pumping test method in the determination of aquifer parameters (Guérin,
2005). The method is cost and time effective and is used to predict the
parameters for even those areas where there are no existing wells (Ekwe et al
2006).
1.2 OBJECTIVESAND SCOPE OF THE STUDY.
The objectives of this research work are to:
(1) Determine the lithological succession
and the prospective water bearing horizons in terms of resistivity, thickness
and depth across the study area using vertical electrical sounding method
(2) Evaluate hydraulic conductivity and transmissivity of aquifers
in the study area using Da-Zarouk parameters from vertical electrical sounding
(3) Compare the
value of aquifer hydraulic conductivity obtained from this study with that
obtained by pumping test method using statistical correlation tool.
(4) Assess the overall groundwater potential
across the study area
(5) Assess the level of protection of
aquifers in the Ishiagu area from surface pollutants(aquifer protective
capacity) using Da-Zarouk parameters from vertical electrical sounding
(6) Correlate geoelectric sections in the
study area with existing borehole logs.
(7) Determine groundwater flow direction
using existing well data and elevation values.
1.3 LOCATION
AND ACCESSIBILITY
The Ishiagu area is located between latitude 5.54o N and 6.0o N and longitude 7o 30’ E to 7o35’E; and is in Ivo local Government Area of Ebonyi State, South Eastern Nigeria (Fig.1).
The area is
easily accessible through road networks that link it to neighboring communities
and cities (Fig. 1). The area is
generally gently undulating with occasional hills traceable to intrusive
bodies.
1.4
DRAINAGE, CLIMATE AND VEGETATION
The Ishiagu area
is drained by several rivers such as the Ivo, Ngada and Iyi Odu rivers (Fig.1).
The climate is typical of the tropics. The average annual temperature range is
between 25o C and 31oC while an average annual rainfall
of about 1500mm to 2400mm is prevalent (Aroh et al 2006, Akintola, 1986,). The
area is characterized by two distinct climatic seasons, namely, the rainy
season which usually starts and ends around April and October respectively and
the dry season that prevails between November and March. A period of extreme
coldness and dryness called the harmathan dots the dry season from around the
months of December to January (Iloeje,
1981).
In terms of
vegetation, the study area falls within the vegetation zone of Eastern Nigeria,
which type is forest/derived savannah, dominantly made up of grasses, shrubs
and trees.
1.5LITERATURE REVIEW
Since the late
1960s, several studies have been carried out to investigate the interconnection
between aquifer geoelectric properties and their hydraulic parameters such as
hydraulic conductivity and transmissivity as well as aquifer protective
capacity. Such studies include those of Archie (1942), Zohdy (1976), Kelly (1977),Niwas
and Singhal (1985), Salem (1999), Purvance, and Andricevic (2000). Although many
of these studies were area specific, the approaches can be applied in different
geological conditions. Three types of relationships have been derived between
geoelectric and hydraulic properties:
i. Relation
between Formation factor (F) and hydraulic properties
ii. Relation
between Formation resistivity (ρ) and hydraulic conductivity
iii. Relation
between transverse resistance (R) and transmissivity
A relation
between the aquifer intrinsic permeability and Formation factor to estimate
transmissivity from borehole resistivity measurements was developed by Croft
(1971). Kosinski and Kelly (1981) did a work on geoelectric soundings for
predicting aquifer properties. 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. Niwas and Singhal (1985) noted that it is better to use a
“modified transverse resistivity” instead of transverse resistance where there
is remarkable variation in the groundwater quality. Frohlich, and Kelly, (1985)
studied the relation between hydraulic transmissivity and transverse resistance
in a complicated aquifer of glacial outwash deposits.Mazac et al. (1985) worked
on the correlation between aquifer and geoelectrical parameters in both the
saturated and unsaturated zones of the aquifers.Ako and Osondu (1986) carried out groundwater investigations
at Darazo on the Kerri-Kerri Formation; they concluded that Dar-Zarouk
parameters are related to borehole characteristics and pointed out that the
highest traverse resistance (T) corresponds to the zone with the highest
borehole yield. Mbonu, et al (1991) determined aquifer characteristics
in parts of the Umuahia area of Nigeria using geoelectric sounding. McDonald et
al (1999) used geoelectric method to estimate the transmissivity of aquifers in
the Thames gravels in the basement complex area of Jos, Plateau State. De Lima
and Niwas, (2000) estimated hydraulic parameters of shaly sandstone aquifers
from geoelectrical measurements. Amadi (2006) carried out a hydrogeophysical
survey of Ebonyi South area of Southeastern Nigeria, including Ishiagu and
Ohaozara areas. Casmir (2006) used geoelectric investigation involving
twenty-six vertical electrical soundings, to investigate the aquifer potential
of the underlying bedrock as well as the transmissivity of aquifers in the Jos
Plateau area.Yehia, (2004) used geoelectric resistivities sounding for ground
water evaluation in the New Qena city and El-Khatatba area of Egypt. Ekwe, et
al. (2006) used the concept of Dar-Zarrouk parameters (transverse unit
resistance, R and longitudinal conductance, S) in porous media to determine
aquifer hydraulic characteristics within the middle Imo river basin Nigeria.
Most of the previous studies where the concept of Da- Zarouk parameters was
applied were carried out in sandstone aquifers. However, Ekwe et al (2010) used
thesame method to estimate aquifer hydraulic characteristics of low
permeability (shaly) formation in Oduma town, Enugu State where they determined
the aquifer hydraulic conductivity to fall between 0.624m day-1 and
5.5091m day-1 and transmissivity to range from 14.17 m2 day-1
to 174.89m2day-1. The geology and hydrogeology of the
Oduma area can be said to be similar to thoseof the present study area, where
shale isthe major lithology underlying the area.
Oladapo and
Akintorinwa (2007) and Abiola et al (2009) related directly the longitudinal
unit conductance, Sof overburden units above an aquifer to the protective
capacity (insulation from pollution) of the aquifer.
Utom et al,
(2012), used geoelectric methods to delineate groundwater parameters in parts
of Enugu, South eastern Nigeria and successfully demarcated areas with good
groundwater potential.
Amos-Uhegbu,
(2013), established a graphical linear relationship between a set of pumping
test data and vertical electrical sounding data and subsequently applied the
relationship in determining aquifer parameters in areas where pumping test data
were not available in some parts of Umuahia, South Eastern Nigeria.
Batayneh, (2013) applied the Da-zarouk Parameters in exploration of aquifer systems in the Gulf of Aqaba and demonstrated the behavior of the Dar-Zarrouk parameters and its patterns in space over large areas with respect to the occurrence of water aquifer systems in deltaic coastal area.
DETERMINATION OF AQUIFER PARAMETERS IN THE ISHIAGU EBONYI STATE USING GEOELECTRIC METHOD
