1.1 Background of the study
It is fact that water level in some part of Ethiope east local government area of Delta state such as Otorho, Aragba, Umutu and using Ajanomi community as a case study at Abraka are not equally distributed when compared to other Southern part of the state like Warri and it’s environ, where all the water level in these zone (Warri South-South and Warri North local government area) are sometimes on the same depth. Thus this is due to the geological structure of each environment and the nature of the hydro-geological behaviour of the region.
The hydro-geological behaviour of such a region as described by some hydro-geologist (e.g. Uliana, 2012) is the movement distribution of water and the nature of aquifer (type of soil) in that region.
In this project with the aid of geo-electrical mapping using the Schlumberger arrangement (a vertical electrical sounding) and other computerise analytical procedure had made it possible to know the hydro-geological behaviour of the above name village (Ajanomi–Abraka) in Ethiope east local government area of Delta state Nigeria and its implication.
These above characteristic of hydrogeology, not only affect the availability of the water in some part of the region but also the quality of water and access to it.
1.2 Objective of the Study
The objective of this work is to investigate the hydro-geological significance on ground water table of the study area (Ajanomi) by:
1.3 Scope of the Study
The purpose of this survey is to determine the hydro-geological behaviour of the study area (Ajanomi –Abraka) and its implication, by carrying out a geo-electric mapping, using Schlumberger arrangement of resistivity method that involve vertical electrical sounding. The measurements taken, were used to determine the true, resistivity of the sub- surface.
1.4 Regional Geology
The study area, Ajanomi lies in Niger Delta. The Niger Delta is situated in the gulf of Guinea and extends throughout Niger Delta province as defined by Klett et al, (1997). From the Eocene to the present, the Delta has prograded southward forming depobelts that represent the most active portion of the Delta of each stage of its development (Doust and Omatsola 1990). These depobelts form one of the largest regressive deltas in the world with an area of some 300,000km2 (Kulke, 1995), a sediment volume of 500,000km3 (Hospers, 1965), and a sediment thickness of over 10km in the basin depocenter (Kaplan et al 1994).
The Niger Delta province contains only one identified petroleum system (Kulke, 1995; Ekweozor and Daukoru, 1994). This system is referred to as tertiary Niger Delta (Akata-Agbada) petroleum system.
The onshore portion of the Niger Delta province is delineated by the geology of southern Nigeria and southwestern Cameroon. The northern boundary is the Benin flank, an east-northeast trending hinge line south of the West African basement massif. The northeast boundary is defined by outcrops of the cretaceous on the Abakaliki high and further East- South- East by the Calabar flank- a hinge line bordering the adjacent Precambrian. The offshore boundary of the province is defined by the Cameroon volcanic line to the east, the eastern boundary of the Dahomey basin (the eastern-most west African transform-fault passive margin) to the west, and the two-kilometer sediment thickness contour or 4000- meter bathymetric contour in areas where sediment thickness is greater than two kilometers to the south and southwest. The province covers 300,000 km2 and includes the geologic extent of the tertiary Niger Delta (Akata-Agbada) petroleum system.
1.5 The Tertiary Niger Delta (Stratigraphy)
The tertiary section of the Niger Delta is divided into three (3) Formations, representing prograding depositional faces that are distinguished mostly on the basis of sand-shale ratios. These 3 Formations includes; The Akata Formation, Agbada Formation and Benin Formation.
This formation at the basement of the Delta is of marine origin and is composed of thick shale sequences (potential source rock), turbidite sand (potential reservoirs in deep water), and minor amounts of clay and silt. Beginning in the Paleocene and through the recent, the Akata Formation formed during lowstand when terrestrial organic matter and clays were transported to deep water areas characterized by low energy conditions and oxygen deficiency (Stacher, 1995). It is estimated that the Formation is up to 7,000 meters thick (Doust and Omatsola 1990). The Formation underlies the entire Delta, and is typically over pressured. Turbidity currents likely deposited deep sea fan sands within the upper Akata Formation during development of the Delta (Burke, 1972).
(b) The Agbada- Formation
Deposition of the overlying Agbada Formation, the major petroleum bearing unit, began in the Eocene and continues into the recent. The Formation consists of paralicsiliciclastics over 3700 meters thick, and represents the actual deltaic portion of the sequence. The plastics accumulated in delta front, deltaic-top set, and fluvio-deltaic environments. In the lower Agbada Formation, shale and sandstones were deposited in equal proportions, however, the upper portion is mostly sand with only minor shale inter-beds.
(c) The Benin Formation
This is the third formation that overlies the Agbada Formation, it is a continental latest Eocene to recent deposit of alluvial and upper coaster plain sands that are up to 2000 meters thick (Avbovbo, 1978). This Formation contains the most productive and hence most tapped aquifer in the Niger Delta region especially in Isoko north where it is shallow, (Olabaniyi and Owoyemi ,2006).figure 1.1 show the formation of the Region
Figure 1.1: Structure of the Niger Delta (Asseez,1989)
The hydrogeology of an area is normally intimately dependent upon the nature of the parent rock, structures, weathering process, recharge mechanism and the form and frequency of precipitation.
1.7 Regional Hydrogeology
The entire region can be divided into two distinct hydrogeological zones based on the different geology. The hydrogeological properties of these formations are different.
The following are the two hydrogeological classes in the region:
Akatar-Formation and Agbada-Formation. The above formation has been explained above in details.
Alluvium refers to sediments deposited by running water. These include gravel in a stream channel, sand deposited on a river bank, and the mud that washes into people’s homes after a flood. Alluvial systems are often excellent aquifers. The natural sorting of sediments by flowing water often results in continuous deposits of coarse sediments that have high permeability and porosity. There are three basic types of alluvial deposits that we will consider here – alluvial fans, deltas, and floodplain/river valley deposits.
1.9 Alluvia Fan
Sediment transport in a stream is controlled by the energy of the stream – highly energetic streams in mountainous areas can carry lots of large grains, while slower streams with flatter stream gradients carry finer sediments. Whenever the gradient (and therefore, the energy) of a stream suddenly decreases, much of the sediments carried by the stream will suddenly drop out of the water and settle out. This occurs when a stream flows out of a mountainous area into a flat plain, leaving behind a big pile of sediment called an alluvial fan in figure 1.2
Figure 1.2 Crosse section of an alluvia fan developing at the foot of a fault –block mountain range (Uliana 2001,2012.)
Figure 1.3: Aerial view of a single alluvial fan and abajada or Alluvial apron (image from Galloway and Hobday, 1996)
Streams flowing out onto alluvial fans tend to carry a wide variety of sediments, ranging from mud and silt to coarse gravel. The changes in stream energy across the fan results in sorting of sediments so that the coarser sediments are near the proximal end (i.e., near the mountain) and the finer sediments are near the distal end. Because of this, the most productive wells will generally be found up near the break in slope at the foot of the mountains. Sediments are generally unconsolidated, although in arid regions, extensive evaporation can cause mineral precipitation that partially cements the sediments. Also, evaporation can leave behind caliche – a layer of fairly hard calcium carbonate that inhibits infiltration of water into the fan.
Sometimes a series of alluvial fans will coalesce into a large package of sediments called a bajada, or alluvial apron (Figure 1.3). In fault-block mountain areas, like New Mexico and Nevada, entire valleys will be filled in with alluvium to depths of as much as 1000 meters.
Groundwater systems in alluvial fans are usually unconfined and receive recharge from infiltration of precipitation. Sometimes, interbedded sands and clays can result in locally-confined units. Groundwater flow is usually controlled by the topography; streams near the proximal end are usually losing flow to the aquifers, while the distal end of the fans often contain springs and gaining streams. These systems usually have very little fine material in them (as it gets carried away by streams), so permeability’s are often very high. Alluvial-filled fault-block valleys often contain significant regional-scale groundwater flow systems.
Figure 1.4: Cross section of alluvial-filled fault-block valley showing typical flow system Uliana (2001)
In arid regions, most of the alluvium will be covered with a layer of caliche that inhibits infiltration of precipitation. Also, high evapotranspiration rates will cause most precipitation in the valleys to disappear before it gets to the water table. Recharge is usually through fractures in the bedrock exposed in the highlands; these fractures convey water to the unconsolidated alluvium. The flow system is towards the middle, with eventual discharge to shallow lakes in the centre of the valley called playa lakes. In some places, groundwater discharges directly into the lake, and then flows out of the valley on the surface. In most arid and semi-arid regions, the lakes are usually dry and the primary mode of discharge is through evaporation directly from the water table. Alluvial fans are often very important sources of water in arid/semi-arid regions because
Deltas are like alluvial fans, but they form where a stream enters a larger body of water, such as a lake, inland sea, gulf, or ocean. As the stream enters the larger body of water, its velocity suddenly decreases drastically, and it drops almost all of its sediment. A big difference between alluvial fans and deltas is that deltas contain a lot more fine material – in alluvial fans, the water energy is so high that most of the fine material gets washed away, but in the typical delta, fluid energies are low, and coarse-grained sediments are only deposited in localized packets of sediment. Deltas can be classified based on their overall shape and the forces that control their geometry. Deltas can be fluvially-dominated, wave-dominated, or tidally-dominated.
Fluvially-dominated deltas (figure 1.5) is a deltas whose shape is controlled by the input of sediment. These form in relatively low-energy environments; usually some sort of inland sea or gulf. These are often called “birds-foot” deltas because the streams tend to form linear piles of sediment that extend out into the bay or ocean. The Mississippi delta is an example of a fluvially-dominated delta.
Figure 1.5: Basic delta types (from Galloway and Hobday, 1996)
In most deltas, the coarse-grained deposits are generally in abandoned channels. These channels are usually surrounded by mud and are isolated from each other. Therefore, the success of wells drilled into delta sediments depends upon hitting one of these channel deposits. In wave or tidally-dominated deltas, the delta fronts will be very sandy and will contain lots of water, but that water is usually non-potable sea water. In general, deltas are usually not very good sources of potable water. Permeable units are too few and far between, and the water that is there is usually too salty to drink or contains too much organic material from swamps and tidal flat areas. Exceptions do occur; the Nile Delta in Egypt is generally very permeable, and water is clean enough to drink. However, high concentrations of manganese and iron causes their steel well screens to clog with rust and manganese oxides; usually their wells only last a few years before they have to drill a new one.
The suitability of the formations in the investigated area, which are basically Agbada and Benin in origin, are comprises of metamorphic rock (mud) and sedimentary rock(sand stone) respectively , as aquifers depend largely on the development of secondary structures mainly faults, their subsurface extent and interconnection to other similar structure on regional scale. Another major factor is the degree of weathering of these rocks and their porosity. The major aquifers in the area are however confined to weathered metamorphic rock and Old Land Surfaces between contemporaneous mud rock successions.
The investigated area is located in a hydrogeological zone of mostly Deltas and a few Alluvial, which is characterized by medium of very poor groundwater potential. The aquifers in the area occur in the deltas deposits, weathered and fractured metamorphic rocks overlying the Basement System rocks at much greater depths. A significant groundwater discharge occurs in the faulted, fractured and weathered zones of the Ethiope River. Shallow aquifers occur at the sub-surface zone.
1.11.1 Groundwater Flow
It can be deduced that groundwater flows from the land surface and Basement aquifer to the Deltas aquifer in the Ethiope River. During high rainfall, water percolates from the surface down to the groundwater aquifer storage. The groundwater level rises and the aquifer expands both laterally and vertically.
During periods of moderate rainfall, subsurface outflow from the area occurs through base flow along ephemeral drainage channels and groundwater flow from the aquifer into the deltas and a few Alluvia plains. During low rainfall periods no surface outflow is observed. The aquifer discharges water only through slow groundwater flow, and evapotranspiration. During the dry season no recharge is experienced and the aquifer maintains its poor state through hydrodynamic balance in such a way that the aquifer shrinks in size laterally and there is vertical decline in water levels.
Ground water recharge
When rainfall, runoff soil moisture changes and evapotranspiration data is known, the amount of water which is yearly added to the permanent ground can be estimated (recharge). For an accurate water balance calculations very precise and extensive hydrological data of the concerned area is required, which is rarely available. The present water balance study can only be regarded as an estimation.
One of the recharge areas for the aquifers in this area is formed from the source of River Ethiope at Umutu area. Here water percolates directly into the faults and cracks within the Pleistocene rock through which deeper and adjacent units are recharged over time.
Fig.1.6 Recharge and Discharge view of the Area (Uliana, 2012.)
Discharge paths in the investigated area are reflected in geology, land use and cover type, and fall into one of the following categories:
Interception, transpiration and evaporation of rainfall; this covers all losses from rainwater before it leaves the rainfall part of the hydrological cycle and becomes either runoff or percolation water. Transpiration losses (vegetal metabolism) in the area have also a great effect.
An aquifer is a geologic unit that can store and transmit a sufficient amount of water to supply a well. The factors that determine if a geologic unit is an aquifer include the following:
Fig.1.7 Basic type of Aquifer.
1.12 Location and Geology of the Study Area
Ajanomi is a nearby village Abraka town Orhuovie (King street) at old Abraka Sapele road, located to the South-west of river Ethiope in Ethiope east local government area of Delta state, southern Nigeria. It is situated (Latitude 06004N and Longitude 06004E and 06005 E) on a hill and its highest elevation is 100ft (Akpor, 2014).
While its lowest elevation is about 16ft above the sea level. The Southern- Easterly flow of the river are said to be perennial and its water head is located at Umutu in Ethiope-east local government Delta state. The figure 1.8 and 1.9 show the map of the Region and the Area of study (Ajanomi at Abraka).
Map of Delta State Nigerian.
Fig.1.8 Map of Ajanomi Abraka location of Delta state
Fig.1.9: Map location of Ajanomi Abraka first sand dredging road