A preliminary review of potential groundwater resources of the Niger Delta.
The Niger Delta has an areal extent of 75,000[km.sup.2] and is located between latitude 4030' and 50 20' N and longitude 30 and 90E (Fig.1). It is the second largest delta in the world with a coastline spanning about 450km terminating at the Imo River entrance (Awosika, 1995). The region spans over 20,000[km.sup.2] and it has been described as the largest wetland in Africa and among the three largest in the world. The Niger Delta consists mainly of freshwater swamps, mangrove swamps, beaches, bars and estuaries. This difficult terrain made it a region mostly forgotten by the rest of Nigeria, until the advent of petroleum in the area in the late fifties.
Though the Niger Delta produces over 80% of Nigeria's petroleum, it is still very much a neglected part of the country. Beside efforts made by oil prospecting companies in the process of oil exploration and production, the region has not been studied in many areas and respects. Groundwater resources development of the Niger Delta is one such area where no serious effort has been made to investigate its nature, distribution and occurrence. Geologic considerations are rarely or inadequately incorporated into the design of boreholes (Amajor, 1989). Generally, groundwater has not attained a high level of development in the Niger Delta partly as a result of difficult environmental condition, low level of general underdevelopment of the region, inadequate finance; and partly perhaps as a result of deliberate neglect of the area by successive governments
[FIGURE 1 OMITTED]
Because of the very nature of the region, groundwater would constitute the predominant, if not the only source of water supply in the area and unless a determined effort is made to understand the nature of the groundwater in the region, serious problems would be encountered in the area of water needs of the region, in future. This paper therefore presents a general appraisal of groundwater resources potential of the area and observations made in this paper would form a guideline for meaningful collection of data for quantitative and qualitative analyses of groundwater resources of the region.
Factors Controlling Groundwater Systems in the Niger Delta
In the Niger Delta, a number of factors have been observed to be intimately linked to the groundwater system and they wield far reaching influences on the system.
The relationship between the groundwater regime of the Niger Delta and the climate prevailing in the area, constitute the first important observation in investigating the influence of external factors on the groundwater system. Two main seasons--the rainy season and the dry season mark the climatic pattern in the region. The rainy season (April--Mid August and September to early November) is a period of heavy rainfall in the area. But between November and March, a dry condition prevails, marking the dry season (Etu-Efeotor, 1981; Gobo, 1988; Gobo & Abam, 2002).
During the rainy season, there is high precipitation, and everywhere, there is evidence of the addition of water into the region. Groundwater storage increases and this is shown in wells drilled in the area, which not only indicate a marked rise of water level within the boreholes, but also give an enhanced yield
As the dry season approaches during the month of November, however rainfall decreases and water input into the region decreases as well. High evapotranspiration rate induced by the dry condition further helps to increase water losses in the region (Etu-Efeotor, 1981). The overall result is that the rate of discharge of water from the groundwater system during the dry season surpasses the rate of recharge. Groundwater levels in wells therefore fall, indicating the diminished water storage in the groundwater system. This trend is further indicated by general poor performance of wells at this season. An understanding of the seasonal changes of climate in the Niger Delta thereby becomes of paramount importance in scheduling all major groundwater operations such as drilling, pump tests, exploration works etc. in the region. Where this is strictly adhered to, more reliable results are obtained.
The hydrologic conditions in a region are important in understanding the groundwater situation in the region in that they determine the availability of water input into the basin for groundwater storage, the rate of recharging the groundwater, and the movement of the water in the groundwater system, for extraction purposes. They would comprise the factors of precipitation, runoff, evapotranspiration and infiltration. The process of water movement in the ground is more related to the factor of geology.
The amount of rainfall that takes place in a basin would determine the water input into the basin. The annual rainfall in the Niger Delta is high. It varies from 500mm per annum at the coasts, to about 300mm at the northern part of the delta (Etu-Efeotor, and Odigi, 1983). Evapotranspiration is 1000mm leaving an effective rainfall of 2000mm. Of this effective rainfall, 37% or 750mm is known to recharge the subsurface aquifers while the remaining 1250mm flows directly into the streams (Akpokodje et al 1996). This recharge which is 75% of the total precipitation is on the high side of the range commonly reported for unconsolidated sediments (Vecchioli and Miller, 1973; Legeette and Graham, 1994).This therefore ensures that the region is adequately supplied with water. Besides, rain may fall at any time of the year, even during the peak of the dry season, further ensuring an all year round water input into the region. Basically, the Niger Delta water resources are drawn from the Eastern littoral hydrological and the Niger South hydrological zones. Infiltration and percolation processes from these broad recharge networks flow southwards into the underlying aquifers of the Benin Formation beneath the Continental Shelf (Ngerebara and Nwankwoala, 2008). Both the structural and stratigraphic setting of the Niger Delta favours hydraulic gradient flow towards the coast, and hence into the Continental Shelf. This forms the basis of most freshwater aquifers located within the Continental Shelf. The general rainfall pattern in the Niger Delta ensures a permanent supply of water to the region.
The extent to which this rainfall reaches the ground to supplement the underground storage, however, would greatly depend on the rate of infiltration, runoff pattern and the rate of evapotranspiration. None of these factors had been studied quantitatively, but the following observations have been made in a more tentative way:
1. The rate of infiltration has generally been favoured by the prevailing flat nature of the Niger Delta which reduces runoff, by increasing retention time of rainwater on the land surface. Besides, the soil in most cases, except where surface clay and swamp prevail, is unconsolidated, porous and permeable, thus permitting quick infiltration of water underground.
2. The region is adequately drained by several streams and rivers. But more important is that these surface water bodies, flow through raised channels due to heavy load of sediments which settle within the channels. Consequently, the streams and rivers overlie most of the important aquifers in the area and feed them throughout the whole year. Most of the streams and rivers are therefore influent. The aquifers do not have to wait until flooding periods to be fed by surface streams and rivers. The above prevailing conditions means that there is no period of the year when groundwater storage would have to be diminished by base flow but rather is ensured of a continuous discharge wherever the boundary with adjacent streams and rivers are continuous.
3. Evapotranspiration measurements made on local scales by agricultural establishments indicate that water losses especially in areas of unconfined aquifers with water table near ground surface, may be high, due to high evapotranspiration during the dry season. This is in respect of the dense vegetation of the region and the dry condition that prevails during the dry season. But because of the huge reservoir prevailing under the Niger Delta, the more or less constant water supply in the region and the occurrence of clay lenses which often cut off aquifers from one another, the effect of evapotranspiration even where water is nearer the surface does not demand serious consideration in the Niger Delta.
Groundwater quality, well performance and aquifer characteristics have been known to be zoned, and vary from location to location. Depending on the parameter under consideration, the prevailing groundwater condition in a particular area in the Niger Delta can be related to a number of factors due to location. For example, salinity problem in coastal aquifers result from nearness to the sea, complexity in subsurface condition persist more in certain areas, and good water quality can be predicted in certain localities. There is therefore a general geographical distribution of groundwater in the Niger Delta which is related to the geographical condition prevailing underneath.
Three such zones have been recognized on a broad basis. These are the continental areas bordering the northern part of the Niger Delta, the freshwater/mangrove swamp lands, and the beaches and bars, of the coast. Very important observations as to the quality of water, aquifer characteristics, and salinity gradients have been made based on this geographical division.
The geology of the Niger Delta is well known and have been discussed by several authors (Reyment 1965; Short and Stauble 1967; Merki 1972; Weber and Daukoru 1975; Avbovbo 1978; Agagu 1979; Whiteman 1982; Doust and Omatsola 1990; Owolabi et al 1990; Koledoye et al 2000, 2003).
The influence of geology on the groundwater resources of the Niger Delta constitutes the most important factor besides that of climate in the region. Geology has been observed to be responsible for the complex groundwater distribution, extractability and quality in the Niger Delta. Unfortunately, the present knowledge of the true geological condition prevailing within the groundwater domain of the Niger Delta is small.
Three major formations comprise the modern Niger Delta which are in turn overlain by various types of Quaternary deposits (Table 1; Fig.2 & 3). These are the Akata Formation, which is predominantly shale and clay; the Agbada Formation which is generally fluviatile and fluviomarine, and the Benin Formation, constituting a continental deposit of sand and gravel (Murat, 1971). The depositional pattern which accompanied the accumulation of sediments during the formation of the delta, gave rise to structural traps (growth faults and roll-over anticlines) in the Agbada Formation. This constitutes the petroleum containing reservoirs in the Niger Delta.The Agbada Formation while suitable for petroleum accummulation, is too deep to be relevant to groundwater storage. There arises therefore, the major difference between the region where the petroleum geologist is prospecting for oil, i.e the Agbada Formation, and that, where the hydrogeologist is searching for water--the Benin Formation, in the Niger Delta.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
Huge financial investments by oil companies have revealed the geology of the Agbada Formation in detail. Understandably, investigating the Agbada Formation, petroleum geologists had deliberately ignored the upper lying Benin Formation. Hence, the present knowledge of the Benin Formation is limited, compared with that of the Agbada Formation. An evaluation of the hydrogeology of the Niger Delta for petroleum exploration may appear to be ill-conceived. However, there is strong evidence that meteoric water from gravity--induced flow has recharged deep enough into the subsurface to possibly play a role in the distribution of hydrocarbons. According to Dickey et al (1987); Amajor and Gbadebo (1992), the extremely sandy nature of the upper Benin Formation and the abundant growth faults in the underlying Akata Formation have permitted meteoric water to penetrate very deep into the subsurface.
The controlling effect of geology on groundwater occurrence in the Niger Delta is no longer in doubt. The sedimentation pattern as well as stratification determines both the quality and quantity of water in the region. Its investigation is the first step towards a meaningful groundwater study of the region. The Benin Formation therefore needs detailed investigation.
Groundwater Occurrence in the Niger Delta
Aquifer Types and Conditions
The main body of groundwater in the Niger Delta is contained in mainly very thick and extensive sand and gravel aquifers. Three main zones have been differentiated. These are: a northern bordering zone consisting of shallow aquifers of predominantly continental deposit, a transition zone of intermixing marine and continental materials and a coastal zone of predominantly marine deposits (Etu-Efeotor, and Odigi, 1983; Amajor, 1989; Etu-Efeotor and Akpokodje, 1990). A distinct trend in aquifer properties have been observed following this division. Akpokodje et al (1996) have summarized the hydrostratigraphic units of the Benin Formation as four well defined aquifers in the upper 305m that vary in thickness to over 120m. The aquifers vary from unconfined conditions at the surface through semi-confined to confined conditions at depth. The aquifers are separated by highly discontinuous layers of shales, giving a picture of an interval that consists of a complex, non-uniform, discontinuous and heterogeneous aquifer system. Although, majority of groundwater supply wells abstract water from these aquifers, there is evidence that industrial and municipal groundwater supply wells produce water from deeper aquifers in the Benin Formation.
Aquifers at the northern border of the Niger Delta are more continental in character, being composed of river loads coming from the hinter land. They are also encountered at shallower depths, so that in most cases, an average depth of 60m had been all that was required to be drilled, to obtain very pure freshwater and in huge quantity. Clay materials, except a few metres found within the top soil, do not occur at depth. The sand is coarse to very coarse generally, and gravel layers are commonly encountered. The borehole performance in this section has generally been so good and the water quality so excellent, that sinking of wells at the northern borders of the Niger Delta has always been taken for granted. Very good examples of such regions are Port Harcourt, Ogoni, and Elele areas of Rivers State, eastern Niger Delta. Figure 6 shows the hydrogeologic cross-section of the distribution of fresh water aquifer across eastern Niger Delta. A well production rate of over 20,000 litres per hour is common, in these places.
Moving coastwards from the northern borders of the Niger Delta, one comes across a transitionary zone of swamp lands. Two types of swamp lands are observed the mangrove swamp lands and the freshwater swamp lands. The mangrove swamp lands are associated with tidal inlets and they are therefore more prominent in those areas where estuaries penetrated farther inland, such as the western and eastern zones flanking the prodelta. On the other hand, fresh swamp land persists more within the front of the delta where the dense network of streams and rivers combine to empty into the sea.
A common feature of the transition zone is the presence of clay embodiments within the aquifers. These clay lenses are erratically distributed laterally and vertically within the region. In several cases, strata logs of wells drilled less than 200m apart, have been known to vary, and under such prevailing circumstance, prediction of aquifer performance in the region is difficult. However, the freshwater swamp lands which constitute the front of the delta, continue to indicate many features of continental environments, until very close to the coast. The aquifers are still shallow, consisting of predominantly sand and gravel materials, but clay intercalations become more prominent, than within the northern zones. Lignitic materials are also present in the aquifer and the presence of vegetative matter strongly point to sedimentation under shallow water condition.
Within the mangrove swamp lands, very strong evidences of marine conditions are indicated. Thicker lenses of marine clay are encountered and saline conditions are still well noticed. There is no doubt that these areas are protected from the dynamic zones of the deltaic front. This makes it possible for marine conditions to penetrate further inland, creating a more complex transition zone. This is the case in the freshwater swamp lands. There is an intermixing of continental and marine sediments resulting in a very complex aquifer system. Generally, it has been necessary to drill beyond the 200m depth before a good water yielding aquifer could be obtained and saline water intrusion problem plague the region. It is here, however, that artisan conditions, due to the interbedment of sand aquifers within clay aquicludes occur. But such confined aquifers are generally too deep seated to result in flow wells.
Within the sand bars and beaches of the coastal lands, boreholes still need to go deeper to reach quality good water aquifers. Beneath the coastal sands that form the surface deposits of this last zone, marine conditions predominate, until at depth where deep seated aquifers empty into the sea. Aquifers within the Niger Delta generally produce and perform better during the rainy season. They dwindle in yield during the dry season. At the coastal lands, rains feed and maintain phreatic aquifers during the rainy season. But with the incoming of the dry season, such aquifers dry up, and wells sunk into them commonly go without water at that season.
Groundwater Flow Condition
Though no groundwater flow measurement exercise is known to have been carried out in the region, it is generally deduced that groundwater flow direction is towards the sea, following the common configuration of the delta. Thus, the great quantities of water entering the region from rainfall and streams into the shallow aquifers in the north, discharge in depth beyond the coast. The greater part of the sediment is therefore flushed and groundwater is rarely stagnant except in those places where saline, presumably connate water entrapped in aquicludes, persist in depth. Such a condition is speculated at Borokiri- Port Harcourt, Buguma, Bille, Kulama, Bonny and Akasa in Rivers State, eastern Niger Delta (Oteri, 1988). Figure 5 shows areas affected by saline water intrusion.
[FIGURE 5 OMITTED]
Salinity problems are encountered in the Niger Delta (Fig.7). But this is a case prevalent within the mangrove swamp lands and the coastal aquifers. Because of the importance of saline problem in the groundwater development of the Niger Delta, it is necessary to know the main types of saline pollution and where they would be expected. Generally, two types of saline gradient are noticed. There is a vertical gradient which changes with respect to distance from the sea.
Vertical salinity gradient develops within the estuarine areas. They arise from the penetration of saline water inland through creeks and estuaries. They are therefore saline conditions that originate from the infiltration of salt water from creeks into underlying sediments. Such saline pollution however, applies only where the aquifer is uninterrupted in depth. A thick or extensive aquiclude can and does exclude further penetration of salt water under such circumstances.
Human activities at times enhance such saline intrusion in the area. This was the case at Isaka near Port Harcourt where the dredging of the Port Harcourt harbor admitted saline water into the aquifer in the area. Also, influence of land reclamation in Borokiri area of Port Harcourt induced saline water. It requires further deep drilling beyond a thick lens of clay before an uncontaminated aquifer will be encountered. Within sands on islands and beach ridges at the coast, a unique condition of salinity prevails in which a cone of freshwater overlies a saline layer. Saline pollution becomes a problem, if such pool of freshwater is not intelligently extracted.
So far, only relatively few coastal aquifers have been developed in the Niger Delta and water demand has been comparatively low for the needs of local communities. There had therefore been no established cases of overdraft within coastal aquifers to induce salt water intrusion. Where saline contaminations have so far been observed within coastal aquifers of the Niger Delta, they are still being debated, whether of connate origin, island condition, or true intrusion.
Groundwater Quality in the Niger Delta
The quality of groundwater in the Niger Delta closely follows the sedimentation pattern. As a result, three distinct zones are recognized. The continental deposits of the Northern border produce the best quality water in the region - fresh, pure and commonly uncontaminated groundwater. Within the transition zones, however, the complex sedimentary environment greatly influences the water quality. Most remarkable are the freshwater swamp lands where quality degradation associated with the breakdown of organic matter derived from vegetation buried in the sediments, are encountered. These generally take the form of high carbonate acidity and introduced hydrogen sulphide, commonly identified by the bad smells of some water samples from the area.
Iron contamination is also another feature of groundwater quality within the transition zones. The intensity of its occurrence has been observed to be higher within the freshwater swamp zones (Etu-Efeotor, 1981; Etu-Efeotor and Odigi, 1983). But in the mangrove swamp land areas, notably Buguma, Bonny and Abonnema in Rivers State, cases of iron contamination have been encountered. They are commonly in the form of ferrous iron which generally remains in solution when water samples are freshly collected. On standing the samples, the ferrous iron comes in contact with oxygen of the air and is oxidized into its ferric equivalent which is generally brownish in colour.
The source of the iron contamination is not quite known but it is suggested to have been emplaced by iron fixing bacteria associated with sedimentary environments of decaying vegetative matter. According to Allen (1965) and Oomkens (1974), the Quaternary glaciation was accompanied by eustatic lowering of the sea level such that the paleo- strandline was at the present edge of the continental shelf. This geologic event would have exposed the sediments and created paleo-soils rich in iron oxides. The subsequent rise in sea level would have incorporated the paleo-soils into the geologic record.
Prospects of Groundwater Development and Management in the Niger Delta
Groundwater deserves serious attention in the Niger Delta. This is because there are no alternative sources of water supply to that of the ground sources, in many parts of the Niger Delta. This is particularly the case in those regions where estuaries penetrate very far inland, leaving many areas surrounded by saline creeks and inlets.
Moreso, the network of deltaic tributaries create problem of isolation in the region. Such a condition has not favoured the growth of towns and cities. Little riverine communities scatter all over the delta and there is no likelihood that this situation would change in the very near future. The need therefore for the establishment of huge water development schemes would not arise for a very long time in the area. What would be needed would be water development projects tailored to expansion through increase in boreholes.
Furthermore, with particular reference to irrigation, waterlog problem prevalent in the Niger Delta, could be further aggravated through pumpage from the surface streams. Irrigation schemes, if planned in the area, would therefore be better served by boreholes through which groundwater levels are lowered, rather than raised, in order to improve the waterlog condition in the area.
Whether for domestic, industrial or irrigational purposes, it appears evident that groundwater constitutes the most economical, practical and sensible source of water supply in the Niger Delta. It would have to be harnessed in order to meet the water needs of the several development projects now planned for the region. Important steps need to be taken in order to utilize the groundwater resources of the region more efficiently and usefully in future. Among the steps are:
1. a deliberate effort to promote more understanding in the profession of hydrogeology. Several practicing Engineers today in Nigeria's water industry dismiss hydrogeology as irrelevant in the process of supplying the nation with more water. Consequently, wells are drilled haphazardly and are pumped without regard to the characteristics of the producing aquifer. While such a state of affair may constitute no problem under our present level of development, there is no doubt however that things might not continue in a similar way for long.
2. a National Water Resources research body should specifically be created to coordinate studies on both ground and surface water resources of the nation. It is only such a national body that can absorb the demand in money, material and men that such a complex region as the Niger Delta demands.
3. there should be national and state laws governing the abstraction of groundwater in the country. Today, no laws prohibiting individuals from drilling water wells anyhow, anywhere and at anytime exists. Over-pumpage in coastal aquifers can be dangerous where no regulation exists.
The groundwater condition in the Niger Delta is greatly influenced by local climate, hydrology, geology and geography as observed in this paper. It is this cause and effect relationship between these local factors and the groundwater condition in the Niger Delta that constitute the basis of any observed unique groundwater characteristics in the region.
The catalogue of problems associated with groundwater development in the Niger Delta as outlined in this paper is a long one. The development and management of groundwater and solutions to the problems of the area are not easy. However, this paper advocates that a detailed study to understand all aspects of the problems will be a step in the right direction.
Though there are few and scattered hydrogeological studies embarked upon by some independent researchers and some government agencies, but such studies are by no means holistic. They are too simplistic and do not take into account all the necessary variables that affect the reliability of such studies. More importantly, greater coordination, management and concerted efforts to study and to understand all aspects of the water resources problems should be made. It is a careful and systematic study of this relationship that would also yield a meaningful result to any programme of groundwater management in the region. Governmental intervention, control and appropriate policy is therefore advocated so as to achieve sustainable groundwater development and management in the region.
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H.O. Nwankwoala and G.J. Udom
Department of Geology, University of Port Harcourt, PMB 5323, Choba, Port Harcourt, Nigeria
Table 1: Geological units of the Niger Delta (After Short and Stauble, 1967) Age Geological Unit Quaternary Alluvium (general) fresh water back swamp meander belt. Mangrove and salt water. Back Swamps Active/Abandoned Beach ridges Sombreiro Warri Delataic plain. Miocene Benin Formation (coastal plain sand) Eocene Agbada Formation Age Lithology Quaternary Gravel, sand, clay, silt, sand, clay, some silt, gravel. Medium-fine sands, Clay and some silt. Miocene Coarse to medium grain sand with subordinate silt and clay lenses. Fluviatile marine Eocene Mixture of sand, clay and silt, fluviatile marine.
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|Author:||Nwankwoala, H.O.; Udom, G.J.|
|Publication:||International Journal of Applied Environmental Sciences|
|Date:||Feb 1, 2011|
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