ABSTRACT
Investigations were carried out for groundwater potentials and hydro-geochemical characteristics of the Nanka-Oko area and environs, southeastern, Nigeria. The areas are underlain by the Nanka Sands Formation. Data from 14 vertical electrical soundings were interpreted using computer aided techniques (IPI2win software). A total of 16 borehole groundwater samples, were also analyzed for their physiochemical and biological properties with the aim of assessing their characteristics and quality for domestic and agricultural purposes. The results of the interpretation of the geophysical data show that the area is characterized by variable subsurface layering ranging from six layers to eight layers.Lithologic logs, geoelectric sections and static water levels data reveal that the area is characterized by two aquifer horizons (upper and lower) and two aquifer types (confined and unconfined).Based on the estimated hydraulic conductivity, transmissivity and specific yield, two groundwater potentials zones (low and high) were defined. Results also show that the order of magnitude of the ionic species of groundwater of the area is > Na+> Ca2+>> Cl–> Mg2+>> Fe2-/3-. Only among the major ions (, Cl–,,Ca2+,Mg2+ and Na+) shows a pronounced difference in concentrations from the two aquifer types. The chemical composition of the groundwater is controlled mainly by weathering processes with little contribution from dilution processes. Three hydro-geochemical facies were deduced from the Piper diagram namely, CaHCO3, mixed CaMgCl and CaCl2. The geochemical data also reveal that the groundwater from the area is potable as drinking water and suitable for irrigation purposes.
CHAPTER ONE
INTRODUCTION
1.1 General
Information
The Nanka
Sandstone has proven to be a prolific aquifer with intriguing hydraulic and
challenging geotechnical properties. The characteristic nature of the sandstone
continues to attract the interest of researchers in the fields of hydrogeology,
geotechnical engineering, and environmental geology among others for obvious
reasons. Dozens of works have been done on the Nanka Sandstone in recent times,
but none so far has holistically treated all the challenges posed by this litho
unit as each work has concentrated on a particular/peculiar challenge. In
general, increased urbanization with its attendant increase in demand for
potable water for domestic and agricultural purposes has increased interest in
the study area. With the presence of Federal Polytechnic at Oko, and a
perennial expanding Ekwuluobia Market for example, more groundwater exploration
has become necessary.
Thisresearch
project reports the groundwater potentialsof the Nanka Sands using vertical
electrical soundings (VES), geologic logs of boreholesand hydrochemical data of
the groundwater.
1.2
Objectives
The objectives of this study were to
determine the groundwater potentials as well as the hydrochemical
characteristics of the aquifers in the study area.
1.3 Location of the study area
The study area
(Nanka-Oko and environs) lies within latitudes 6000’ and 6008’North,
and longitude 7000’ and 7008’ East. Some of the towns
include Nanka, Ekwuluobia, Oko, Obeledu and Agulu (Fig 1).
- Climate
And Physiography
The climate of
the area is tropical with an average yearly rainfall of 1478mm, daily minimum
and maximum temperatures of 220 and 330 C, respectively.
Relativehumidity ranges from 60% (at dry seasons) to 90 % (at rainy seasons).
Two seasons exist namely, wet season (March to October) and dry season
(November to February).
Figure 1: Location and Geologic map of the study area
The dry season
is characterized by dry air mass, causing high evapo-transpiration rate and
lowering of water levels while the rainy season is a period of heavy downpours,
ground infiltration and elevation of ground water table (Egboka and Okpoko,
1984). Undulating landforms characterize the topography of the area. The
undulation is caused by serious gullying that is less intense towards the
southeastern parts of the area.
1.5 Geology and Hydrogeology
The study area
is part of the Anambra River Basin of southern Nigeria. It is drained by the
Agulu Lake and several rivers namely, the Idemili River, the Akponkwu River,
the Mama River, the Obibia River, the River Odo, the Utalu River and the Uchu
River. These rivers originate either from the lake or from the groundwater
reservoirs beneath the complex gully through effluent seepages.
The dominant
geological Formation is the Eocene Nanka Sands which is a sequence of
unconsolidated or poorly consolidated sands, 305m thick, underlain by the Imo
Shale Formation of Palaeocence age and overlain by the lignite clay seams of
the Oligocence Ogwashi Asaba Formation. The Nanka Sands are predominantly sandy
with thin claystone and siltstone bands, lenses and laminations. The sand is
poorly sorted, cross bedded and medium to coarse grained. The deposits also
exhibit well developed patterns of alternating crossbedded sands and layers of
dark grey shales. The aquifer-aquitard units form a multi-aquifer system.
Around Nanka,
Idemili and Oko, the water table is generally deep to very deep ranging from
30m to 300m in depth (Offodile, 2002). Thus the aquifers in these areas can be
tapped only by deep boreholes. Shallower water tables are obtained in boreholes
located in lowland areas or valleys usually interspersing the predominantly
hilly areas. Springs issue profusely at
outcrop points, where the water table is intersected by deep erosional valleys
forming scenic lakes at the foot of the hill.
1.6
Literature Review
Several works have been done in the areas underlain
by Nanka Sands. Most of these works however center on the gully erosion which
characterizes the area. Very few works have been done on the groundwater
characteristics. Murat (1970) had proposed the Nanka Sands as a unit of the
Ameki Formation. Kogbe (1976),however, recognized the Nanka Sands as a distinct
Formation and a lateral equivalent of the Ameki Formation. Nwajide (1980) agreed
with Kogbe 1976 and described the Nanka Formation as a sequence of unconsolidated,
poorly sorted and poorly cemented sandstone with inter-layers of distant units
of shale, siltstone and mudstone. Egboka and Okpoko (1984) noted that heavy
rainfall causes a rise in the water table of the major
aquifers and aquitards systems of the Nanka Sands, a rise that causes increase
in hydraulic head and in turn, produces rapid flow rates that enhance gullying
processes. Egboka and Okpoko also attributed the loose structure of the
Nanka Sands to the decomposition of the binding cement by slight acidity of
groundwater of the Nanka Sands. Okagbue and Ezechi (1988) observed that high
permeability, high porosity and high void ratio that increase with depth favour
internal erosion which is responsible for intense gullying at the Nanka Sands.
They found out that the cementing materials include limonite, calcite and iron
oxide. Ofomata and Ezeigbo
(1997) reported that groundwater of the Nanka Sands consists predominantly of
low concentrations of major ions and microbial content and consequently suitable
for domestic purposes. Nfor et al (2007) observed that the extent
and distribution of groundwater in the parts of Anambra basin underlain by the
Ogwashi Asaba Formation, Nanka Sands Formation and the Imo Shale Formation,
with varying water storage and yielding capacities are controlled predominantly
by lithology and other secondary factors including topography and nearness to
source of recharge. They also reported that the Ameki/Nanka Sands is a
prolific water producer. Four aquifer horizons were identified within the
formation, designated: shallow, upper, middle and deep aquifers. The most
exploited are the upper and middle aquifers, while the least, but most prolific
is the deep aquifer with average yield of 5l/s.
Okoro et al (2010) in their study of the groundwater
potentials in parts of the escarpment areas underlain by Nanka Sands observed
that the depth to water table in aquifer varies between 10 to170m during the
rainy season and 80 to 260m in the dry season. The geomorphic characteristics
of the escarpment are major controlling factors in groundwater recharge
/discharge and availability. Although favourable values of the aquifer
parameters of hydraulic conductivity and the transmissivity measured in the
area range from 2.08×10-1
cm/s to 8.46×10-1 cm/s and 150×10-3 m2/hr to 481×10-3 m2/hr
respectively, variation in water table and loss of surface water still occur
exacerbating water scarcity, public health and food security. The principal
flow direction of the surface and groundwater sources in the area is in the NE
and NW direction following the influence of the escarpment and consequently
creating both surface and subsurface water divide.
Okoro et al 2010 evaluated aquifer parameters of
areas underlain by Nanka Sands in southeastern Nigeria using vertical
electrical sounding (VES). The transmissivity and hydraulic conductivity
calculated from interpreted 40 VES results varied from 0.48 to 19.5m2/day
and 3.06 to 3.75 m/day respectively. This coupling has proved useful and most
valuable in areas of scarce hydrogeologic data such as the study area.
Okolo et al 2010 studied the groundwater quality and supply situation in Akwa southeastern Nigeria an area underlain by Nanka Sands and Imo Shale. Their work revealed two aquifers types in the area, semi-confined and confined aquifer with water table of 125m to 140m and 73m to 216m respectively. The results of the water quality analysis show relatively high iron concentrations (0.3 to 1.0mg/l), relatively high nitrate concentrations (21 to 24mg/l), chloride concentrations (4.1 to 57mg/l) and high coliform counts (11 to 213ml/100ml). high coliform count is indicative of biological pollution which is linked to the increasing incidences of water borne diseases (predominantly diarrhea and typhoid) in the area.
GROUNDWATER POTENTIALS OF THE NANKA SANDS AROUND NANKA-OKO AND ITS ENVIRONS, ANAMBRA STATE
