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

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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.

Lafia Sandstone

Ezeaku FormationKeana/Awe FormationAwgu Formation

Lafia

Obi         Keana                   Daudu            Yandev

Asu River GroupAnticlinal exposure of older formations as a result of erosion

Maastrichtian

Lafia Sandstone

Continental

Santonian

keana / Awe Formation

Ezeaku Formation

Asu River Group

Awgu Formation

Marine

Continental

Marine

Marine

Albian

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.

LAFIA

OBI Giza

Keana

9 0

RIBI

Kanje

Azara

Akin

Awe

Tse-Tavaan

80

Kertyo

River BenueAkwana

MAKURDI

Arufu

Geological Map of the

Middle Benue Trough

0                                  50km

LEGEND QUATERNARY                   CRETACEOUS

Alluvium

TERTIARY

Tertiary to

Recent Volcanics

Anticlinal axis

Synclinal axis

Fracture

Nkporo Shale

Group

Awgu-Ndeaboh

Shale Group

Eze-aku

Shale Group

Asu River Group

PRECAMBRAIN

Basement

Complex

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).

AGE GEOLOGIC FORMATIOMS ROCK UNIT AQUIFERS Santonian- Campanian Maastricchtian Volcanics       Lafia Formation         Fine  to  coarse  grained,       friable                        and   feldsparticsanstone,           Coniacian         Awgu Formation brownish    at    top    and whitish at depth Greybedded   shale   with occasional sandstone bed       Late         Turonian-     Ezeaku Formation and limestone.   Thick  calcareous  shales,   Early Turonian           Late Cenomanian             Keana Formation micaceous   and   fine   to medium                grained sandstones Crystalline   fine,   coarse and pebbly sandstone             Aquifer   Early Cenomanian   Awe Formation   Flaggy,  whitish, medium   Aquifer    

EPOCH

Cretaceous

Early

Cretaceous

to        coarse        grained feldspartic      sandstones, calcareous      sandstones, limestone       interbedded with carbonaceous shale.

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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