GROUNDWATER POTENTIALS OF THE NANKA SANDS AROUND NANKA-OKO AND ITS ENVIRONS, ANAMBRA STATE

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    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).

  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