A HYDROCHEMICAL STUDY OF THE BRINE FIELDS OF  AWE KEANA AND GIZA AREAS MIDDLE BENUE TROUGH NIGERIA

ABSTRACT

This research work presents findings of the extent and distributions of saline zone and the hydrochemical characteristics of the groundwater of Awe, Keana and Giza areas in the Middle Benue Trough of Nigeria.Staticwater levels(swl) of hand dug wells were measured andthe results used to generate hydraulic head data and hydraulichead map. The static water levels range from 1.0m to 13.5m at Awe, 1.0m to 4.5m at Keana and 2.2m to 9.2m at Giza. The hydraulic head maps for the three areas show different patterns/directions of groundwater flow for each area. Geophysical investigations, mainly vertical electrical sounding (VES) using Schlumberger electrode configuration array were performed at 47 locations spread across the entire study areas. Results of the VES revealed that the studyareas are characterized by3, 4 and 5geo-electrical layers with 9 different curve type signatures. Also, results of the VES show that Awe area has the highest occurrence of saline zone, followed by Keana and Giza respectively. Multi-layer saline zones were observed only at Awe. Thirty (30) groundwater samples (10 each from the 3 areas) were analyzed for physiochemical parameters (rare earth elements, heavy metals, major cations and major anions).Results of hydrochemical analysis revealed five (5) hydrochemical facies namely,Ca-HCO₃, NaCl, NaHCO₃, Ca-Na-HCO₃ and Ca-Mg-Cl facies. Principal component analysis of the hydrochemical data revealed that the groundwater chemistry of the areas is controlled by the mineralization of the host rocks, weathering of galena, uranite, felsdspathic minerals and dissolution of limestone and dolomite. The spatial distribution pattern of the rare earth elements of groundwater of the study areas showed a strong positive correlation exists between brine and the elements. Uranium, however, showed a district distribution pattern which may indicate a distinct source from the other elements. Of the three areas studied, Keana has the most potable water supply.  Awe and Giza groundwater have chloride (Cl), nitrate (NO₃) and lead (Pb) contaminations.It was observed that the groundwater quality of the study areas is not only salinity dependent, but also influenced by anthropogenic activities such agricultural wastes and domestic waste released into the environment.Based on the irrigation water quality indexes employed, groundwater of the areasrange from unsuitable to suitable for irrigation purposes.

CHAPTER ONE: INTRODUCTION

1.0       INTRODUCTION

1.1       BACKGOUND OF STUDY

The Benue Trough of Nigeria is one of the most prominent geologic features in West Africa. It extends over a length of 800km trending NNE-SSW from the Niger Delta to the south-west of Lake Chad basin and ranges in width from 130 to 250 km (Figure 1.1).  Due to the large regional extent, studies in the Trough are often divided geographically (though arbitrarily) into upper, middle and lower regions (the approximate boundaries of these regions are given in Figure 1.1. No concrete line of subdivision can be drawn to demarcate the individual regions, but major localities (towns/settlements) that constitute the depocenters of the different regions have been well documented (Obaje et al., 1999). The depocenters of the Lower Benue Trough comprise mainly the areas around Nkalagu and Abakaliki, while those of Middle Benue Trough comprise the areas around Makurdi through Yandev, Lafia, Obi, Awe, Keana, Giza, Jangwa to Wukari. In the Upper Benue Trough, the depocenters comprise Pindiga, Gombe, Nafada, Ashaka (in the Gongolaarm) and Bambam, Tula, Jessu, Lakun and Numan in the Yola arm.  

The origin of the Trough (which is still controversial in details) has long been associated with the breakup of Gondwana – the separation of Africa from South America and the opening of the South Atlantic Ocean. 

Occurrences of saline groundwaters as springs, ponds or in dug wells and boreholes are common in parts of the Trough. Prominent outcrops (which commonly support local salt industries) are found in the Lower Benue Trough and Middle Benue Trough (Figure 1.2). Brine is saline or salty water, particularly a highly concentrated solution of common salt (sodium chloride). It occursnaturallyas an underground salt lake and is one of the commercially important sources of common salt in the world (Leford and Jacoby, 1983).  Thesaline groundwaters in the Lower and Middle Benue Trough are frequently associated with tectonic elements such as intrusive and mineralized veins (Uma, 1998).

The presence of brine constitutes a serious hydrogeological problem on groundwater and as noted by Adeoti et al. (2010), saltwater intrusion into aquifers has become a major concern in most of the areas around brine fields as it constitutes the commonest of all the pollutants in freshwater.Therefore, understanding the point of saline intrusion is essential for the management of groundwater in such areas.Awe, Keana and Giza brine fields inAweand Keana Local Government Areas (LGAs) of Nasarawa State are some of the brine fields

Figure 1.1: Geological Map of Nigeria showing location of Benue Trough                                             (Obaje, 2006).

Figure 1.2: Map of Benue Trough showing location of salt ponds                                                          (Modified from Offodile, 2002).

located within the Middle Benue Trough.

            According to Offodile (1983), the saline ponds in the areas appear to originate from underground brine issuing from the western flanks of Keana anticline and perhaps also from the interbedded shale of fractured sandstone of Awe Formation and feeding the Awe and the other brine fields in the Middle Benue Trough.

1.2       STATEMENT OF THE PROBLEM                                                                                 Thepresence of salt ponds in Awe, Keana and Giza areas is suspected to have led to an inflow of saltwater into the fresh aquifers of the areas, thereby resulting in serious groundwater contamination. Thus there is increasing demand for potable water supply in these areas.         

1.3       OJECTIVES OF THE STUDY

The primary objective of this work is to investigate the saline water intrusion into the freshwater aquifers of the areas. The outcome of the investigation is considered important for management of water resources of the areas.

            The secondary objectives/scope of this study is:

1. To investigate the extent and distribution of brine in each of the study areas.
2.  To delineate saline/brackish and freshwater zones of each of the study areas; this would be useful for groundwater management with minimum risk of saline contamination.
3. To investigate the geochemical properties of the waters in the various areas and determine the suitability of the waters for various purposes.

1.4       STUDY AREAS DESCRIPTION                                                                           1.4.1   Location                                                                                                                                    Theareas, Awe, Keana and Giza are politically located in Awe and Keana local government areas(LGAs) in the south-eastern part of Nasarawa State. They are located between Awe lies between latitude 8° 06’ – 8° 12’N and longitude 8° 38’ – 9° 09’E. The major localities are in Awe and Keana local government areas(LGAs) of Nasarawa State (figure 1.3). 

Figure 1.3: Google earth map of the study areas showing the locations of the salt ponds and   saline artisan wells

1.4.2    Climate

            The study areas (Awe, Keana and Giza) have similar climate which is made up of two (2) major and distinct seasons: a wet season which usually lasts from March to October and a dry season which lasts from November to February. Occasionally, there is rainfall in the first two months of the year. The annual average rainfall ranges between 1000mm and 1500mm while the mean annual humidity is 70% and relative humidity 60% to 80% (Binbol, 2006). The annual average temperature is 28.5⁰C with annual average sunshine hour of 6.7 per day. A high temperature of 33⁰C – 36⁰C is experienced in the areas during the dry season (Iloeje, 1981). Vegetation of the areas is of the guinea savannah type, with dense (gallery) forests fringing some of the rivers.

1.4.3    Geomorphology        

            Awe, Keana and Giza have low to moderate relief with few scattered laterite capped hills of elevation ranging from 115m – 165m above mean sea level. They are drained by minor tributaries of the River Benue such as Rivers Tunga, Giza, Keana, Okpoya, Owunobi. The drainage system shows a dendritic pattern, where relatively homogeneous rocks have the same resistance to erosion.

1.4.4    Geology

1.4.4.1 Regional Geology

            In the Middle Benue Trough, six Upper Cretaceous lithogenic formations (Asu River Group,Ezeaku Formation,Keana Formation,Awe Formation,Awgu Formation and Lafia sandstone) comprise the stratigraphic succession (Figure 1.4). The Asu River Group consists of AlbianArufu, Uombaand Gboko Formations (Offodile, 1976; Nwajide, 1990). These are overlain by the Cenomanian-TuronianEzeaku Formation which shares common boundary with the Konshisha River Group and the Wadata Limestone in theMakurdi area and followed by the Keana and Awe Formations.The Late Turonian-Early Santonian coal-bearing Awgu Formation lies conformably on the Awe Formation. In the Makurdi area, the Makurdi Sandstone interfingers with the Awgu Formation. The mid-Santonian was a period of folding throughout the Benue Trough. The post-folding Campano-MaastrichtianLafia Formation ended the sedimentation in the Middle Benue Trough, after which widespread volcanic activities took over in the Tertiary.      

Figure 1.4: Stratigraphic succession in the Middle Benue Trough (Obaje, 2009).

The Asu River Group (with an average thickness estimated to be about 1,800m) outcrops mainly in the Keana Anticline east of Keana town and south of Azara, and in the area around Gboko with a typical section in the Quarry of the Benue Cement Company near Yandev. The lithologic composition of the Asu River Group comprises limestones, shales, micacous siltstones, mudstones and clays (Offodile, 1976; Obaje et. al., 1994). The deposition of the Ezeaku Formation marked the beginning of marine transgression in the Late Cenomanian whichtook place in a presumably shallow marine coastal environment. The sediments are made up mainly of calcareous shales, micaceous fine to medium friable sandstones and beds of limestones which are in places shelly.  Outcrops of the Ezeaku Formation include those at Ortese, about 4 km east of Jangerigeri, where the sediments are composed mainly of shelly limestones (almost entirely of oyster shell). In the bank of River Tokura, about 20 km east of Keana Town, on the Chikinye- Awe Road, a typical section of the Ezeaku Formation occurs, consisting mainly of intercalations of shelly limestones and black shales, with brownish fine to coarse grained feldspartic sandstones at the top.                                            The Keana Formation resulted from the Cenomanian regression which deposited fluviodeltaic sediments. The Formation consists of cross-bedded, coarse grained feldsparthic sandstones, occasional conglomerates, and bands of shales and limestones towards the top. Massive outcrops occur at Keana, Noku, Chikinye, Jangerigeri, Azara, and Daudu.The Awe Formation was deposited as passage (transitional) beds during the Late Albian Early Cenomanism regression. Its typical sections occur around the town of Awe, where Offodile (1976) estimated the thickness to be about 100 m. The formation consists of flagy, whitish, medium to coarse grained calcareous sandstones, carbonaceous shales and clays.                                                                                                  The deposition of the Awgu Formation marked the end of marine sedimentation in this part of the Benue Trough. The formation is made up of bluish-grey to dark-black carbonaceous shales, calcareous shales, shaleylimestones, limestones, sandstones, siltstones, and coal seams. The major outcrop of the coal-bearing Awgu Formation is at the bank of River Dep in Shankodi, 7 km to the west of the village of Jangwa. Along the bank of this river, the coal seams can be traced laterally for about 500 m. The borehole cores of the Steel RawMaterials Exploration Agency (formerly National Steel Council) stock-piled at the Obi camp contain coal seams and coal bands at various depths within the Awgu Formation.                                                                          

The Lafia Formation is the youngest formation in this area. The Formation was deposited under continental condition (fluviatile) in the Maastrichtian and lies uncomfortably on the Awgu Formation. It is lithologically characterized by ferruginized sandstones, red, loose sands, flaggy mudstones, clays and claystones. Outcrops and sections of the Lafia Formation occur in and around the town of Lafia, and along the bank of River Amba on the Lafia-Doma Road.

1.4.4.2 Local Geology                                                                                                                       A good understanding of the geology of the study areas is necessary for thorough assessment of the characteristics of the sub-surface rocks and formation fluid. Available information indicates that, Awe, Keana and Giza brine fields which fall within the Middle Benue Trough are underlain by the following geological sequence; Asu River Group, Ezeaku, Keana, Awe and AwguFormations and finally the Lafia sandstone. The sedimentary formations listed above are underlained by the Basement complex of Precambrian age (Figure 1.5).                                                                                                                                             The EzeakuFormation is the oldest formation deposited during the early Cenomanian age. It is composed of calcareous shale, micaceous fine to medium grain friable sandstone and some highly fossiliferous limestone beds. Keana Formation overlies the Late CenomanianEzeaku Formation andconsists of mainly cross bedded, coarse-grained feldspartic sandstone. The sandstone is generally poorly sorted and occasionally contains conglomarates and bands of shales and limestone. Keana Formation is generally non fossiliferous. The Awe Formation (Early Cenomanian) directly overlies the Keana Formationand is the salt bearing rock sandwich between Keana Formation and oldestAsu River Group (Offodile, 1975, 2002; and Obaje et al., 2006). It consists of flaggy, whitish, medium to coarse grained, sometimes calcareous sandstone, some limestone and interbedded shale or clays from which brine issue. The rock formation underlying Awe Formation is AsuRiver Group. This sedimentary rock unit according to Offodile (2002) and Obaje etal. (2006)was deposited during the Mid-Albian marine transgression of the south Atlantic-Gulf of Guinea. The lithologic composition of the group comprises mainly limestone, shale and calcareous shales, micaceous siltstones, mudstone and clay.                                                                                 Structurally, the study areas occupy the northwest limb of the Keana anticlinorium of the Middle Benue Trough. The flank of the anticlinorium is marked by a complimentary synclinorium, described sometimes as the Giza synclinorium.

   Fig 1.5: Geological Map of the Study Areas (modified from Offodile, 1983)

This synclinorium runs roughly northeast – southwest parallel to the Keana anticlinorium (Offodile, 1976). The Giza synclinorium on a regional sense represents an extensive hydrogeological basin while a smaller basin described as the Lafia sub-basin marks the northwestern extension. The Giza synclinorium forms the geological basin in which the younger formations (Keana Formations, Awe Formations and Lafia sandstone) are preserved.

2.5       HYDROGEOLOGY OF THE STUDY AREAS

Awe, Keana and Gizabrine fieldsare some of the numerous brine fields found withinthe Benue Trough of Nigeria. Thebrine fields of the study areas are known to have very strange and difficult hydrogeological situations. These conditions arise from the fact that most of the potential aquifers are either limited in extent, thinly developed with consistent clay and shale interbeddings or even highly indurated that only the development of secondary voids created by fractures, joints and solutions channels can attract hydrogeological interest. The stratigraphic sequence (Table 1.1)shows that the study areas are made up of alternate shale and sandstone horizons which are suspected to correspond to the sources of the saline and freshwater respectively. Offodile, (1992)studied the Awe area and grouped the aquifers encountered in the boreholes into three, namely,Ezeaku, Keana and Awe Formation aquifers. These formations range from Early to Late Cretaceous age.                                                                    The uppermost aquifer is the sandstone member of the EzeakuFormationwhich is composed of series of shale-limestone and sandstone beds. It is however, one of the productive aquifer units in the New Awe Area (SabonGari) area. Its presence, gives the area most favourable hydrogeological characteristic as water fromthewells are fresh.                                  Keana Formation is composed of more heterogeneous, massive and predominantly fine, coarse and pebbly sandstone beds. Keana Formation is a good aquifer but its limit in extent renders it unproductive for groundwater exploration. The sandstone near the core of the Keana anticline is hard and less permeable than the one in the synclinal area. However Keana together with Ezeaku Formations form a very thick productive aquifer when encountered in a borehole.                                                                                                     Awe Formation aquifer is the lowest aquifer as it is below the Keana Formation aquifer. It is composed of series of shale and porous sandstone beds and ishighly productive.However, the presence ofsalt in it renders it unfavourable for groundwater exploration as the water from wells tapping the aquifer around Old Awe Town (TsohonGari) show high saline

Table 1.1: Hydrostratigraphic units of Rocks in the study areas (Modified from                     Offodile, 2002).

EPOCH	AGE	GEOLOGIC FORMATIOMS	ROCK UNIT	AQUIFERS
Cretaceous	Santonian- Campanian	Volcanics
	Maastricchtian	Lafia Formation	Fine to coarse grained, friable and feldsparticsanstone, brownish at top and whitish at depth
	Coniacian	Awgu Formation	Greybedded shale with occasional sandstone bed and limestone.
	Late Turonian-Early Turonian	Ezeaku Formation	Thick calcareous shales, micaceous and fine to medium grained sandstones
	Late Cenomanian	Keana Formation	Crystalline fine, coarse and pebbly sandstone	Aquifer
	Early Cenomanian	Awe Formation	Flaggy, whitish, medium to coarse grained feldspartic sandstones, calcareous sandstones, limestone interbedded with carbonaceous shale.	Aquifer
Early Cretaceous	Mid-Late Albian	Asu River Group	Marine shales, clays siltstones and mudstones
Pre Cambrian		Basement Complex and Meta-sediments	Crystalline rock

concentration because of the out-cropping brine-bearing Awe Formation (Offodile, 2002). Beneath the Awe Formation, is the shale aquitardof the Asu River Group which underlies the Giza area and whose hydrogeologic characteristics are worthless for further discussion.

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