Environ Geol (2009) 56:935–951
DOI 10.1007/s00254-008-1196-1
ORIGINAL ARTICLE
Hydraulic, textural and geochemical characteristics
of the Ajali Formation, Anambra Basin, Nigeria:
implication for groundwater quality
Moshood N. Tijani Æ Matthew E. Nton
Received: 4 October 2007 / Accepted: 8 January 2008 / Published online: 1 February 2008

 Springer-Verlag 2008
Abstract This study highlights the distribution of of Al2O3 (3.50–11.60 wt.%) and Fe2O3 (1.80–3.60 wt.%),
hydraulic conductivity (K) in the regional aquiferous Ajali which are clear indications of weathering/ferruginization
Formation of SE-Nigeria on one hand and assesses the processes with attendant trace metal release/enrichment
possible influences of textural and geochemical character- (2.5 mg/l Cu, 7.5 mg/l Pb, 6.5 mg/l Zn, 3.9 mg/l Ni and
istics on the hydraulic conductivity on the other hand. The 19.6 mg/l Cr) call for concern in respect of the chemical
investigation approach involved field sampling and collec- quality of the groundwater system. The associated
tion of 12 sandstone samples from different outcrop groundwater is generally soft, slightly acidic, and with low
locations, followed by laboratory studies such as grain-size dissolved solids (EC = 14–134 ls/cm) dominated by sil-
analysis (GSA), constant head permeameter test and geo- ica; implying water from clean sandy aquifer devoid of
chemical analysis of major and trace elements using X-ray labile and weatherable minerals. Nonetheless, most of the
fluorescence method. GSA and textural studies show that metals (with exception of Si, Fe and Mn) exhibited higher
the sandstones range from fine to medium sands, consti- degree of mobility (2–12 folds), which can be attributed to
tuting about \75–99% sand fraction, with graphic mean reduction of Fe-/Mn-oxyhydroxides as sinks/hosts for trace
grain size of 0.23–0.53 mm. Other parameters such as metals. Consequently, infiltration-induced geochemical
coefficient of uniformity (Cu) range from 1.58 to 5.25 (av. reactions (redox, ferruginization and leaching processes)
2.75), while standard deviation (sorting) values of 0.56Ø– signify potential environmental impact in terms of
1.24Ø imply moderately well sorted materials. In addition, groundwater quality as well as borehole/aquifer manage-
the order of the estimated K values is Kpermeameter [ ment, especially under humid tropical environment of the
KBeyer [KHazen [ KKozeny-Carmen [ KFair-Hatch with average study area.
values of 1.4 9 10-3, 4.4 9 10-4, 3.8 9 10-4, 2.2 9 10-4
and 8.1 9 10-5 m/s, respectively. These values fall within Keywords Hydraulic conductivity 
 Permeability test 

the range of 10-5 and 10-3 m/s for fine to medium sands. Geochemical profile 
 Metal enrichment 

However, multivariate factor analysis of the data revealed Groundwater chemistry 
 Ajali Formation 

significant positive dependence of the empirically deter- Anambra Basin 
 Nigeria
mined K values on graphic mean grain size and percentage
sand content and much less dependence on sorting and total
porosity. Geochemical profiles of the fresh samples are Introduction
dominated by quartz with corresponding SiO2 content of
76.1–98.2% (av. 89.7%) while other major oxides are Sandy aquifers, either consolidated or unconsolidated are
generally below 1.0 wt.% in the fresh samples. However, known to be prolific sources of groundwater all over the
the ferruginized samples exhibited elevated concentrations world. However, long-term sustainable management of the
groundwater resources, in terms of quantity and quality, in

 such aquifer settings requires reliable knowledge of tex-M. N. Tijani (&) M. E. Nton
Department of Geology, University of Ibadan, Ibadan, Nigeria tural characteristics and hydraulic conductivity (K) with
e-mail: tmoshood@yahoo.com; mn.tijani@mail.ui.edu.ng respect to groundwater flow and storage. This is in addition
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936 Environ Geol (2009) 56:935–951
to possible geochemical interactions between the aquifer 2006). This study therefore highlights the geochemical
matrix and groundwater which may be a critical factor characterization of both fresh and ferruginized units of the
controlling the chemical composition and quality of the Ajali Formation and possible implications of the associated
water. Furthermore, to adequately understand the ground- metal enrichment and mobility vis-à-vis groundwater
water flow condition as well as the movement of dissolved quality.
contaminants in the subsurface (aquifer), also requires In summary, this study therefore highlights the distri-
reliable information on the magnitude of the hydraulic bution of hydraulic conductivity (K) in the regional
conductivity (K) of the aquifer system. aquiferous Ajali Formation SE-Nigeria through compara-
Several laboratory and field methods are in use for tive assessment of estimated hydraulic conductivity (K)
characterization of hydraulic properties of aquiferous sed- from grain size analysis (GSA) and laboratory permeability
imentary units. There have been advances in the field-scale tests on one hand. On the other hand, utilizing an integrated
methods of measurements and evaluation of hydraulic textural and geochemical approach, possible ferruginiza-
properties in different aquifer settings/conditions (Bouwer tion-induced metal enrichment, potential mobilization with
and Rice 1976; Butler 1997; Butler and Garnett 2000; respect to the groundwater composition/quality as well as
Angeroth 2002) on one hand. On the other hand, simple hydraulic characterization of the Ajali Formation are also
laboratory procedures and empirical estimations are still highlighted. Consequently, the scope and purpose of this
considered useful and cheaper means of estimating study are:
hydraulic conductivity (K) in hydrogeological studies.
(a) to present the magnitudes and distribution of hydrau-
Although in-situ determination of K could be more accu-
lic conductivity values estimated from both labora-
rate and reliable in site-specific investigations, laboratory
tory permeameter tests and GSA-based empirical
approaches are usually preferred in many instances, espe-
estimations,
cially during preliminary hydrogeological investigations, in
(b) to highlight the geochemical composition as well as
order to cut down on cost and time. Nonetheless, lack of
weathering characteristics of the Ajali Formation,
funds for field scale operation, as well as inadequate
(c) to highlight possible influence of the recharge-
experimental laboratory facilities are serious limitations in
induced ferruginization and geochemical processes
developing countries like Nigeria. In this regard, simple
on the Ajali Formation and
procedures and empirical estimation of parameters could
(d) to assess possible influence of ferruginization-induced
be seen as appropriate alternative to generate some basic
metal mobility on groundwater quality as well as
hydrogeological data. However, there is the need to assess
borehole/aquifer management.
the limitations and level of accuracy of empirical estima-
tions in respect of actual field situations.
Furthermore, geology controls much of the natural dis-
tribution of chemical constituents in groundwater system; Study area
hence the natural quality of groundwater will depend on
rock/aquifer types and there is a scope of geochemical Geologic and stratigraphic settings
interactions between the water and the aquifer matrix along
the flow paths (Hudson and Golding 1997; Toth 1999; The Ajali Formation is one of the lithostratigraphic units in
MacDonald et al. 2005; Glynn and Plummer 2005; Appelo the Anambra Basin, SE Nigeria. The Anambra Basin is a
and Postma 2005). Such chemical interactions are usually Cretaceous sedimentary basin, located in the southeast
mediated by infiltrating/recharge rainwater that dissolves portion of Nigeria and bounded by the Abakaliki anticli-
carbon dioxide to produce weak carbonic acid that can norium of the southern Benue Trough to the east and by the
remove soluble minerals from the aquifer matrix (Llyod Basement Complex of the southwestern Nigeria to the west
and Heathcote 1985). Therefore, it may be possible to infer (Fig. 1). Geologically, the origin of the Anambra Basin is
the likelihood of occurrence of certain chemical constitu- intimately related to the tectonic and sedimentary cycles
ents, not only on the basis of the underlying/associated responsible for the origin of the adjoining southern Benue
geologic unit, but also on the basis of possible geochemical Trough during the separation of African from the South
reactions/conditions. For example, studies had shown that American plate in the Mesozoic era (King 1950; Reyment
geo-pedological weathering/ferruginization reactions can 1965; Burke et al. 1972; Benkhelil 1986). The general
lead to metals’ release into the environment, while reduc- geological map of the Upper Cretaceous sedimentary suc-
ing conditions as obtained in the groundwater zone can cession within the Anambra Basin is presented in Fig. 1.
promote the mobilization of metals that are of environ- Studies of Murat (1972), Nwachukwu (1972) Hoque and
mental and health significance into the groundwater Ezepue (1977) and Ladipo (1986, 1988) among others,
(Bruand 2002; Smedley and Kinniburgh 2002; Tijani et al. have shown that, subsequent to the uplift of the Benue-
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Environ Geol (2009) 56:935–951 937
7oE 8oE LEGEND short-lived Maastrichtian trans-regression with sedi-
R. Benue 9 Ameki Formation ments derived from westerly areas of the Abakaliki
N Lokoja 
Ayangba 8 Imo Shale anticlinorium and the granitic basement units of
A 7 Nsukka Formation Adamawa-Oban massifs to the eastern side of the
Oturkpo 6 Ajali Sandstone basin (Benkhelil 1986; Amajor 1987; Ladipo 1986,
Auchi 5 5 Mamu Formation 1988; Adediran 1991). Further details in respect of the
4 Enugu/Nkporo Shale
7oN R. Niger 7 Ajali Formation, as the focus of this study, is3 3 Awgu Shale 
2 2 Eze-Aku Shale presented below under the section on hydrogeologic9 8 6 4
1 Asu-River Group setting.
Enugu B Anambra Basin axis (d) Nsukka Formation: This is a Late Maastrichtian unit,
AFRICA 1 Abakaliki Anticline lying conformably on the Ajali Formation and
Study Area 2 Afikpo Syncline consists of alternating succession of sandstone, dark
NIGERIA
0 50 100km shales and sandy shales with thin coal seams at
various horizons, hence termed the ‘‘Upper Coal
Measures’’. The sedimentary successions indicate a
Fig. 1 Geological map of Anambra Basin showing the different
sedimentary units. (A–B line of cross-section in Fig. 3) paralic environment.
Abakaliki fold belt during the Santonian, the geological
history of the Anambra Basin is linked to the post-Santo-
Physiography and drainage
nian subsidence of the then Anambra platform. This was
followed by a series of trans-regression cycles leading to
Within the Anambra Basin, the topography is generally
sedimentation of about 6 km thick of Cretaceous and
lowly undulating, with elevations of about 150–250 m
Tertiary sediments (Obaje et al. 1999) within the basin to
above sea level. As shown in the cross-sectional view of
the west of the Abakaliki uplift during the Campanian––
the Ajali Formation alongside other lithostratigraphic units
Paleocene. A summary of the stratigraphic relationship of
(Fig. 3), the most prominent relief feature in the study area
the main sedimentary units within the basin is presented in
is the N–S trending Udi-Enugu escarpment at the eastern
Fig. 2 and brief descriptions presented below:
edge of the basin, which rises to an average elevation of
(a) Enugu-Nkporo Shales Group: These early Campanian about 400 m above sea level. This slopes to the west at less
units underlie the eastern plain of the Udi-Enugu than 30 towards the main area of the basin with lowly
escarpment and consists of dark grey fissile, soft shale undulating topography characterized by elevation of about
and mudstone with maximum thickness of about
1,000 m and characterized by interbedded sandy units AGE (Ma) LITHOLOGY FORMATION ENVIRONS 
and sulfur coated marl. A shallow marine environ-
ment was predicted due to the presence of EOCENE Bende-Ameki Grp. / Deltaic / 
Nanka Sand Continental
foraminifera and ammonites (Reyment 1965; Agagu 54
et al. 1985). Imo Shale Grp. / Shallow
PALEOCENE
(b) Mamu Formation: This is a Late Campanian sedi- Umuna Sst. Marine Shelf 
65
mentary unit, also known as the ‘‘Lower Coal Nsukka Formation
Measures’’. It consists of mainly dark blue to grey MAASTRI- 
CHTIAN Fluvio-deltaic / 
shales/mudstones units with alternating sandy units Ajali Sandstone Marginal
Marine
and coal seam horizons to form a characteristically
stripped rock unit. The Mamu Formation was depos- Mamu Formation
ited as shallow water of the paralic facies of a deltaic CAMPANIAN 
complex (Cratchley and Jones 1965). Nkporo/Enugu Marine / Shelf 84 Shales
(c) Ajali Formation: This is a Maastrichtian sandy unit Santonian Folding Unconformity 
overlying the Mamu Formation and consists of white,
CONIACIAN Anambra Platform Unit (Awgu Shale)
thick friable, poorly sorted cross-bedded sands with
thin beds of white mudstone near the base. It is Sand units Coal measures Cross-bedded Sst. 
characterized by large scale cross bedding with dip Shale/Claystone Shales/Siltstone
angle as high as 20. Studies have suggested that the
Ajali Formation is a continental/fluvio-deltaic Fig. 2 Stratigraphic profiles and depositional environment of the
sequence, characterized by a regressive phase of a sedimentary units within the Anambra Basin
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UPPER CRETACEOUS TERTIARY ADAN LI
Anambra Basin BRARY
938 Environ Geol (2009) 56:935–951
Okene Anambra Basin of about 1.3 to about 3.8 m) and shallow hand-pumpAbakaliki
Enugu-Udi Escarpment Anticlinorium boreholes (usually less than 50 m deep) with average
R. Anambra 3
A B capacity of about 2.0 m /h. However, deep boreholes with
7 motorized pumps are characterized by intermittent yield
6 2 (Q \ 5.0 m
3/h), indicating limited capacity of the uncon-
3
fined shallow aquifer east of Udi-Enugu escarpment.
5
1 However, the second hydrogeologic group is characterized
4 by the occurrence of a deep and thick confined and semi-
Basement Complex confined Ajali Formation aquifer, which is the main focus
7 Nsukka Formation 3 Awgu Shale of this study.
6 Ajali Sandstone 2 Eze-Aku Shale Hydrogeologically, the cross-bedded Ajali Formation
5 Mamu Formation 1 Asu-River Group is an extensive regional aquiferous stratigraphic unit
4 Enugu/Nkporo Shale Faults / Fracture (Reyment 1965) and composed of Maastrichtian sand unit,
0 50km Dyke / Intrusion that serves as an important source of water resource within
the Cretaceous Anambra Basin. It conformably overlies the
Fig. 3 A cross-sectional view of the sedimentary units within the Mamu Formation, and is partly overlain by the Late Ma-
Anambra Basin (line of section A–B in Fig. 1) astrichtian Nsukka Formation, that is characterized by
alternating sandy and shaly units. The Ajali Formation
150–250 m above sea level while the scarp slope towards outcrops and extends from Fugar/Agenebode area in the
the east dips at high angles of about 60–70 (Uma and west and extends eastward along the Enugu-Udi escarpment
Onuoha 1988; Okogbue 1988). Generally, this relief fea- where groundwater is recharged (Okagbue 1988) and
ture is dissected by numerous gullies resulting in a narrows southwards towards Okigwe area forming a char-
characteristic rugged terrain. acteristic ‘‘question mark’’ shape (see Fig. 1). The thickness
In terms of the drainage setting, the crestal zone of Udi- ranges from over 350–450 m in places and thins southward
Enugu escarpment serves as water divide; the western side to few tens of meters around Okigwe area (Hoque and
is characterized by hummocky terrain of the Adada-Ag- Ezepue 1977). Field study has shown that the upper portion
hawinili River Basin with headwaters such as Adada, of the Ajali Formation is ferruginized in places; this
Mwuyi, Ajali, Mamu, Karawa and Oji Rivers as tributaries alongside with the clay/shale unit of the overlying Nsukka
of the River Niger. The slopy eastern side of the escarp- Formation and the basal Mamu Formation favours the
ment is drained and dissected by the gullying drainage development of confined/semi-confined aquifer.
system such as Iyakwa, Atafo, Iyoko, Awra, Eme and Iddo The major hydrogeologic feature is the occurrence of a
Rivers. deep and thick confined and semi-confined aquifer, espe-
cially in areas directly overlain by the Nsukka Formation,
while unconfined conditions exist only in the outcrop areas
Hydrogeologic settings of the Ajali Formation. In addition, occurrence of localized
perched aquifer systems are said to be common in areas
The general geology and physiography of the Anambra where the lateritized Nsukka Formation occurs as outliers
Basin controls the occurrence of groundwater in the study on the Ajali Formation (Uma and Onuoha 1988). Most of
area. This falls into two hydrogeologic groups: the boreholes tapping this deep aquifer have depths of
about 120–200 m and saturated thickness in the range of
(a) First hydrogeologic group: Areas underlain by the 42–150 m, while the yields vary from 10 to 100 m3/h
predominantly shaly formations (i.e., Enugu-Nkporo (Okagbue 1988). In addition, transimssivity values of
Shale, Mamu Formation) to the east of the Udi-Enugu 1.0 9 10–2 to 1.7 9 10-2 m2/s and storativity of about
escarpment. 0.02 (Egboka and Uma 1986) are indications of the prolific
(b) Second hydrogeologic group: Areas underlain by nature of the Ajali Formation aquifers. The potentials of
predominantly sandy horizons (Ajali and Nsukka the aquifer, however, appear to decrease westwards due to
Formations) to the west and northwest of the Udi- decrease in thickness as also noted through hydrogeo-
Enugu escarpment. physical investigations (Uma and Onuoha 1988; Okagbue
The first hydrogeologic group is characterized by uncon- 1988). Nonetheless, it should be pointed out that the friable
fined shallow aquifer of the upper weathered horizon in and permeable characters of the Ajali Formation is con-
continuity with the fractured shales and intercalated sandy sequential to environmental land degradation in form of
horizons. Uma and Onuoha (1988), noted that aquifers in gully erosions in some areas as highlighted by Nwajide
this group support only dug wells (with saturated thickness (1979); Nwajide and Hoque (1979) and Uma and Onuoha
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Environ Geol (2009) 56:935–951 939
(1988). This also implies problems in terms of shallow analyses of dissolved metal concentrations. Although,
groundwater occurrence in some area, due to its relatively details lithologic logs of the sampled boreholes were not
high permeability that allows complete drainage of water to available, the field information shows that the wells in
the deeper section of the formation. sample locations 1, 3, 4, 5, 6. 9 and 10 are deep boreholes
(100–120 m), apparently tapping the fresh aquifer unit
while locations 2, 7 and 8 are characterized by shallower
Methodology boreholes (20–30 m) tapping the ferruginized upper section
of the Ajali Formation, Subsequent to the field sampling
Field sampling, laboratory tests and analyses operations, laboratory grain size analysis (GSA), perme-
ameter tests, geochemical analyses of major and trace
A hydrogeological reconnaissance field survey of the elements of both fresh and ferruginized outcrop samples of
Anambra Basin was undertaken. The field study trip cov- the Ajali Formation as well as the hydrochemical analyses
ered most of the outcrop areas of the Ajali Formation and of the groundwater samples were carried out.
entailed collection of representative samples from 12 dif- For the GSA, the collected samples were air-dried and
ferent locations as shown in Fig. 4. It should be noted that since the samples were friable, disaggregation was done
due to the friable nature of the Ajali Formation, it was with fingers. This was followed by sieve analyses using
difficult to collect undisturbed samples. Hence, samples standard laboratory procedure (ASTM D-422). Cummula-
were collected from outcrop sections (using grab/scooping tive curves were plotted from the GSA data and relevant
auger) in deep sand quarry pits and road cuts. Details of statistical and textural parameters were also deduced.
lithologic logs of some of the sample locations are pre- In addition, laboratory determinations of hydraulic con-
sented in Fig. 5. Furthermore, the selection of the 12 ductivity were carried out using the constant head
representative samples across the outcrop stretches of the permeameter tests following standard procedure (ASTM
Ajali Formation (see Fig. 4) was considered adequate in D-2434). The set-up of the permeameter tests was based on
view of the seemly homogenous and similar characteristics the principle of Darcy’s experiment, which established the
of the Ajali Formation. relationship between the flux of water through a porous
In addition, ten representative groundwater samples medium and the hydraulic head difference at both ends of
from different water wells (boreholes) tapping the aquifer the medium. For the porosity determination, the weight of
(see Fig. 4) were also collected in sterilized plastic bottles, dry sample of the Ajali Formation filled into the perme-
appropriately acidified and preserved prior to chemical ability tube (cylinder) of known weight was recorded
before addition of water for saturation. The saturated
R. Benue sample and the cylinder were weighed, while the volumeo
0 50km 7 E of the saturated sample was recorded. The difference
between the volume of saturated sample and dry sample
Lokoja noted gives the pore volume and the porosity was calcu-
7
Okene 23   4 4 Okaba lated accordingly.
3
8 Following appropriate sample preparation/filtration, the 5/6 
R. Niger acidified groundwater samples from the Ajali aquifer were
1 subjected to hydrochemical analyses of the major and trace
Auchi 1 2 Otukpa metals concentrations using ICP-OES technique. For the
5 analyses of major and trace elements composition, samples
Nsukka 
6 were subjected to appropriate treatment by homogenization
9 of 2 g of each sample with 4 g of analytical spectroflux
8 powder and 0.6 g LiNO3 salt in agate mortar. The mixture
10
N was then subjected to heat fusion and glass beads prepa-7 Enugu ration in platinum plate (3.5 cm diameter) using bead and
fuse sampling machine (model TK-410, Rigaku-Tokyo).
The respective glass beads were then used for the deter-
Okigwe 9 11 mination of major and trace elements using automated
2 Formation Sample location 6oN X-ray fluorescence (XRF) machine (model Rigaku ZSX).
2 Groundwater Sample location 10 The GSA, permeability and porosity tests were per-12
formed at the Department of Geology, University of
Fig. 4 Map of the Ajali Sandstone Formation (ASF) showing both Ibadan, Nigeria. The geochemical XRF analyses were
the rock unit sand groundwater sample locations carried out at the Department of Earth and Planetary
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940 Environ Geol (2009) 56:935–951
Fig. 5 Representative  Ankpa   Okigwe   Enugu   Fugar 
lithologic profiles of some
of the sample locations
5m
Sample point Laterite crust 
Fresh Ajali Fm. Ferruginized Ajali Fm. 
0m
Science Systems, Hiroshima University, Japan, while the parameters such as sorting, skewness, kurtosis, etc. were
hydrochemical analyses of the associated groundwater undertaken using following the relations of Folks and Ward
samples were carried out at the Activation Laboratory, (1957):
Ontario in Canada.
Graphic mean: M ¼ ð[16 þ[50 þ[84Þ=3
Inclusive graphic skewness:
Data evaluation ¼ ð[84 þ[16  2ð[50Þ þ ð[95 þ[5  2ð[50ÞS
2ð[84 [16Þ 2ð[95 [5Þ
In line with the above laboratory studies, data evaluation Inclusive graphic standard deviation:
and estimations of some indices were subsequently
D ¼ ½ð[84 [16Þ=4 þ ½ð[95 [5Þ=6:6
undertaken. Using the laboratory data from the permeam-
eter tests, the respective hydraulic conductivity values (K) ð[ [5ÞGraphic kurtosis: M = 95
were calculated using the following Darcy (1856) relation: 2.44([75 [25)
K ¼ Q=A  dl=dh Uniformity coefficient: Cu ¼ D60=D10
Coefficient of curvature: Cc ¼ ðD 230Þ =ðD10  D60Þ
where
2 Notation: The subscript in the phi terms (/ ) refers toA cross-sectional area of sample (m ) x
the grain size at which x% of the sample is coarser than the
dl L = length of sample chamber (m)
specified size, or also, the size at which x% of the sample is
Q volumetric flow rate = volume of water released/time
3 retained on a specified sieve size and any coarser screened(m /s)
sieves above that particular sieve. Also Dx terms represent
dh (hf-h0) = difference between initial head and final
the respective grain diameter in mm, for which x% of the
head (m).
sample is finer than.
From the GSA results and the respective cumulative Due to the importance of K in the fields of hydrogeol-
curves, estimation of some textural indices (uniformity ogy, petroleum geology, and water and waste-water
coefficient and coefficient of curvature) and statistical engineering, a number of studies had empirically related K
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Environ Geol (2009) 56:935–951 941
to the particle size distribution of unconsolidated materials
(e.g., Hazen 1892, 1930; Krumbein and Monk 1942; Fair
and Hatch 1933; Beyer 1964; Shepard 1989; Alyamani and
Sen 1993) among others. Detailed review of some of these
methods, their applications and limitations can be found in
Egboka and Uma (1986); Uma et al. (1989) and Lee
(1998). A common aspect of those studies is the determi-
nation of an empirical relationship between the hydraulic
conductivity and some statistical textural parameters such
as the geometric mean, mode, standard deviation (disper-
sion), or effective diameter etc. of the aquifer materials.
The applicability of these empirical formulae in this study
is based on the fact that these empirical relations are based
on granular sandy materials as also exemplified by Ajali Fig. 6 Representative grain size distribution curves of the ASF
Formation aquifer. Consequently, as part of data evalua-
tion, four of such existing empirical relations were different outcrops within the Anambra Basin are presented
employed as presented below: in Fig. 6, while the results of the estimated textural and
2 statistical indices are presented in Table 1. As shown inHazen (1930): KHazen = C(D10)
2 Table 1 and also presented in Fig. 6, the sandstone underBeyer (1964): KBeyer = [g/t]Cb 9 D10 study falls predominantly within uniform medium sand
Kozeny-Carmen (in Carmen 1939): KK-Carmen = (qg/t)
3 2 2 range. This is clearly supported by the graphic mean (Gm)[n /(1-n) ] (Dm /180)
3 of 0.98–1.98 u; with the exception of sample from Dekina,Fair and Hatch (1939): KF-Hatch = (qg /t) [n /
2 having graphic mean of 2.26 u, which implies fine sand(1-n) ] 9 1/m(h/100 9 R Pi/Di) and apparently attributable to weathering influence. How-
ever, the estimated inclusive graphic standard deviation (D)
Notations ranges from 0.55 to 1.29 u, suggesting a wider range from
poorly sorted to moderately well-sorted material. Also
K hydraulic conductivity (in cm/s or m/s or m/day). varied values of inclusive graphic skewness from -0.24 to
C a dimensionless coefficient based on grain size and +0.20 (av. 0.02) imply nearly symmetrical to coarse
sorting character. skewed arrangement while graphic kurtosis (KG) ranges
D10 grain size for which 10% of grains are finer from 0.94 to 1.51 u (av. 1.20) and suggest mesokurtic
(effective grain size); through leptokurtic with majority of the samples having
D50 grain size for which 50% of grains are finer; leptokurtic population.
Cb a coefficient defined as 6.0 9 10
-4 9 log10(500/Cu); For the textural indices, the uniformity coefficient (Cu),
t viscousity of the fluid water (0.01 g/cm s); which provides a measure of uniformity of the grain sizes
g acceleration (980 cm/s2); ranges from 1.58 to 5.25 while the coefficient of curvature
Cu uniformity coefficient; (Cc) ranges between 0.89 and 1.57. However, the samples
q density of the fluid; from Okigwe-Uturu-1 have relatively higher values of 5.25
Dm representative grain size; and 1.57 for Cu and Cc respectively (see Table 1). This
n porosity; alongside with 25% fines imply relatively poorer hydraulic
m packing factor, experimentally determined as 5; characteristic at the Okigwe-Uturu-1 area compared to
h shape factor (6 for spherical grains and as 7.7 for other sample locations. Budhu (2000) pointed out that Cu
angular grains); value\4 implies uniform grading of grains with enhanced
Pi % by weight of grains held between sieves; drainage/permeability, and values [4, indicate a wider
Di geometric mean size of grains held between the sieves. assortment of grain sizes. In this study, Cc values between 1
and 3, coupled with uniformity coefficient greater than zero
(Cu [ 0), indicates a well-graded granular material. Fur-
Results, interpretations and discussions thermore, the % fines of 0.3–6.0, porosity of 25.5–32.8%; %
sand of [94% are clearly consistent with the range of val-
Textural and statistical indices ues for medium sands (Table 1). Nonetheless, the % fine of
24.9% and a lower porosity of 18.0% for the Okigwe-Uturu-
The plots of cumulative curves of representative samples of 1 sample (Table 1) is a pointer to the relatively higher
the grain size distribution of the Ajali Formation from values of Cu and Cc compared to other samples. By and
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942 Environ Geol (2009) 56:935–951
Table 1 Results and summary
Location Gmean (u) IGStd. (u) IGSkw. Gkurt. Cu Cc n (%) % Fine % Sand
of statistical and textural indices
based on GSA data (sample Fugar-Agenebo-I 1.48 0.55 0.20 1.51 1.58 0.92 27.9 0.6 99.4
code same as given in Table 2)
Fugar-Agenebo-II 1.96 0.61 0.18 0.94 1.75 0.89 nd 1.6 98.4
Ojuwo 1.41 0.94 0.01 1.22 2.53 1.15 25.5 3.0 97.0
Ayangba-Ia 1.63 0.70 -0.24 1.38 2.24 1.21 32.8 3.1 96.9
Ayangba-Ib 1.43 0.74 0.12 1.02 2.21 1.05 nd 1.4 98.6
Iyale 1.36 1.28 0.04 0.99 3.43 0.93 nd 3.8 96.2
Ankpa 1.45 1.07 -0.01 1.38 3.07 1.22 27.9 0.3 99.7
Igbo-Etiti-I 0.98 1.29 -0.05 1.23 3.28 1.09 nd 4.3 95.7
Enugu-Km8-Ia 1.21 1.01 -0.10 0.96 3.00 1.11 29.4 2.8 97.2
Okigwe-Uturu-I 1.69 0.64 nd nd 5.25 1.57 18.0 24.9 75.1
Gmean graphic mean (u)
Okigwe-Uturu-IIa 1.10 0.87 -0.04 1.34 2.08 0.94 32.6 3.4 96.6
IGStd. inclusive standard
Dekina-I 2.26 0.82 0.07 1.40 2.30 1.26 nd 6.0 94.0
deviation (u), IGSkew. inclusive
graphic skewness, Gkurt. Minimum 0.98 0.55 -0.24 0.94 1.58 0.89 18.0 0.3 75.1
graphic kurtosis, Cu uniformity Maximum 2.26 1.29 0.20 1.51 5.25 1.57 32.8 24.9 99.7
coefficient, Cc coefficient of Mean 1.50 0.88 0.02 1.20 2.73 1.11 27.4 4.6 95.4
curvature, n porosity (%)
large, from the textural and statistical indices, the Ajali and 7.6 9 10-5 m/s, respectively (see Table 2). Nonethe-
Formation can be interpreted as uniformly and moderately less, it can be seen that the values from the laboratory
graded medium sand materials, with high hydraulic poten- permeameter tests and those from the empirical estimations
tials in terms of groundwater recharge. for the Ajali Formation fall within a range of 10-4–10-6 m/s
(10-1 to 10-3 m/day) for clean sands; interpreted to be
permeable to highly permeable (Freeze and Cherry 1979).
Empirical K-values estimations and permeameter tests Furthermore, the estimated K-values from both laboratory
permeameter tests and empirical estimations are compa-
Results of hydraulic conductivity (m/s) estimations using rable to similar aquiferous materials such as Nanka Sands
different empirical (GSA-based) relations and that from the with *8.1 9 10-5–1.4 9 10-4 m/s (*7–75 m/day),
laboratory permeameter tests are presented in Table 2. The Benin Formation with *6.9 9 10-5–9.3 9 10-4 m/s
estimated GSA-based K-values revealed a minimum range (*6–80 m/day) and Lokoja Sandstone with *3.5 9 10-5
of 2.9 9 10-5–1.1 9 10-4 m/s (Fair-Hatch method) and a –8.1 9 10-4 m/s (*3–70 m/day) (Okagbue 1988; Vrbka
maximum range of 7.4 9 10-5–8.8 9 10-4 m/s (Beyer et al. 1999) as also graphically presented in Fig. 7. It can be
method). These are in slight contrast to the range of concluded, however, that properly conducted laboratory
3.11 9 10-4–3.0 9 10-3 m/s, obtained from the labora- determination and empirical estimations of K-values can be
tory constant head permeameter tests (Table 2). The taken as cheaper means of generating reliable hydraulic
relatively higher values for the permeameter tests may be parameter data in absence of field or in-situ measuring
attributed to the possible effects of sample repacking and facilities as obtained in most developing countries like
loss/removal of fines with drainage water during the tests. Nigeria.
However, among the empirical estimations, only Fair-
Hatch approach yielded values comparable to the field
pumping/aquifer test data compiled from Okagbue (1988) Statistical reduction of textural parameters
and presented in Fig. 7 alongside other K-estimations. This and hydraulic conductivity
can be attributed to the fact that the Fair-Hatch relation
takes into account the different grain size fractions and the In this study, multivariate factor analysis was employed for
shape factor, which seems to be a better approximation of assessment of interrelationship between the textural-sta-
the in-situ packing conditions under the influence of the tistical indices and the hydraulic conductivity. The
overburden pressure. Generally, the observed trend of dif- application of multivariate statistical methods for classifi-
ferent estimations is Kpermeameter [ KBeyer [ KHazen [ cation and interpretation of large hydrological data set,
KKozeny-Carmen [ KFair-Hatch, with corresponding average allows for the reduction of the dimensionality of the data
values of 1.4 9 10-3; 4.1 9 10-4; 3.6 9 10-4; 2.1 9 10-4 and extraction of few component factors for simplification
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Table 2 Results of hydraulic
Code No. Location K-lab K-Beyer K-Hazen K-KC K-FH k-Darcy
conductivity (m/day)
estimations using different AJSt-01 Fugar-Agenebo-I 142.5 73.1 59.7 16.0 8.0 45.6
empirical relations and
permeameter tests AJSt-02 Fugar-Agenebo-II 62.8 31.9 26.5 6.9 4.3 17.9
AJSt-03 Ojuwo 258.8 33.7 30.0 18.0 7.3 30.8
AJSt-05a Ayangba-Ia 81.5 34.5 30.0 12.6 5.8 29.2
AJSt-06a Ayangba-Ib 147.8 43.2 37.4 17.5 8.1 38.5
AJSt-07 Iyale 55.3 21.5 20.3 19.6 5.8 21.4
AJSt-08 Ankpa 143.1 25.3 23.3 16.9 7.7 24.3
AJSt-09a Igbo-Etiti-I 182.8 35.9 33.6 36.4 9.0 39.1
AJSt-10a Enugu-Km8-Ia 99.1 32.6 30.0 25.2 8.2 39.4
AJSt-12 Okigwe-Uturu-I 67.3 6.4 6.6 11.3 2.6 28.6
KLab K from laboratory AJSt-13a Okigwe-Uturu-IIa 157.5 75.6 64.8 29.9 9.7 55.5
permeameter test, K-KC K from AJSt-04a Dekina-I 26.9 11.9 10.4 3.7 2.5 7.2
Kozeny-Carmen method, K-FH
K from Fair-Hatch method, Minimum 26.9 6.4 6.6 3.7 2.5 7.2
k-darcy intrinsic permeability Maximum 258.8 75.6 64.8 36.4 9.7 55.5
(a function/property of aquifer Mean 118.8 35.5 31.1 17.8 6.6 31.5
medium only)
As presented in Table 3, the PCA extracted four main
principal component factors with eigenvalues[1 summing
up to 85% of the total variance in the dataset. Furthermore,
only variables with factor score greater than 0.4 are con-
sidered significant and as such included in the controlling
variables. The first factor, which accounts for 25.4% of the
total variance, was loaded in favour of all the empirical-
based K estimations and graphic mean (Gm). This factor,
tagged ‘‘empirical K-index’’ revealed the common funda-
mental basis of the different empirical methods in relation
to the average grain size characteristic (Gm) of the
unconsolidated sands, in this case, Ajali Formation.
The second factor, which weighed positively in favour of
the textural parameters such as Cu, Cc, % sand and nega-
tively against % fine could be termed as ‘‘textural control’’
and signifies the textural influence in terms of grain sizes,
Fig. 7 Permeability evaluation chart showing the range of hydraulic grading, sorting and packing on the hydraulic conductivity.
conductivity (K) and intrinsic permeability (k) from the different Nonetheless, the observed negative loading of -0.95 with
estimation approach respect to % fine is a clear indication of inverse relationship
and negative influence on the hydraulic conductivity as
well as other components of factor-2. The third factor,
of interpretation of the overall data trend (Massart and termed ‘‘size-porosity control’’, was loaded in respect of
Kaufman 1983; Simeonov et al. 2003). Principal compo- porosity, skewness as well as D10 and D50 grain sizes. This
nent analysis (PCA) is a valuable pattern recognition clearly underlies the significant association between the
technique that attempts to explain the variance of a large grain-size distribution and the porosity of the materials,
data set of inter-correlated variables with a smaller set of which in turn will influence the hydraulic conductivity of
independent variables, i.e., principal components. Hence R- the granular Ajali Formation materials. The fourth factor
mode principal component analysis (PCA) was employed was loaded with respect to the permeameter-based K val-
in this study to characterize the relationship between tex- ues, standard deviation (sorting) and kurtosis. This clearly
tural-statistical indices and the different hydraulic shows the significant control of kurtosis and sorting on the
conductivity estimates in order to identify possible con- results of the permeability tests, which can be attributed to
trolling factors. the influence of possible resorting during repacking of the
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944 Environ Geol (2009) 56:935–951
Table 3 Results of factor analysis showing the PC grouping and dominance of sub-angular to rounded polycrystalline
scores for hydraulic and statistical textural data of the Ajali Formation quartz, while the presence of dispersed whitish fines sug-
Variables Extracted factor components gests possible clay cementing material. As expected, the
results of the major element geochemistry, as summarized
1 2 3 4
in Table 4, show that the analyzed fresh samples are
K-Kozeny-Carmen 0.867 0.432 dominated by quartz with corresponding high proportion of
K-Fair-Hatch 0.844 SiO2 (94.8–99.0 wt.%), while other major oxides are gen-
K-Hazen 0.822 -0.445 erally below 1.0 wt.%. The results generally revealed
G-Mean 0.806 0.522 similar elemental composition for all samples reflecting the
K-Beyer 0.794 -0.455 homogeneity of the Ajali Formation as well as a uniform
% Sand 0.953 depositional or sedimentary environment.
% Fine -0.953 However, the ferruginized samples, though with similar
Uniformity coeff. C 0.883 chemical trend, exhibit elevated concentrations of Al2Ou 3
Coeff. of curvature C -0.529 0.708 (3.50–11.60 wt.%) and Fe2O3 (1.80–3.60 wt.%), which arec
D : Grain size 0.898 clear indications of weathering/ferruginization processes.10
D : Grain size 0.761 This is clearly supported by the low Si/Al ratio of 6.6–25.850
Porosity 0.747 -0.464 in the weathered samples compared to the relatively higher
St. deviation 0.910 values of 37.3–300 for the fresh samples. Furthermore, the
Skewness 0.520 -0.606 plot of the mineralogical and chemical composition of the
samples using Silica–Sesquioxides-Alkalis ternary plot as
Kurtosis 0.805
presented in Fig. 8a, shows that all samples lie towards the
K-Permeameter 0.658
silica (Q) corner, indicating severe depletion of the Ses-
Eigenvalue 5.88 3.20 2.33 1.54
quioxide and the Alkali. The Al O –CaO + Na O–K O
% Cum. variance 25.4 49.3 68.8 85.1 2 3 2 2
ternary plot (Fig. 8b), shows the distribution of the studied
samples, alongside the primary mineralogy of the source
granular unconsolidated Ajali Formation samples in the Basement Complex rocks. As indicated, the Ajali Forma-
permeameter tube. tion samples plot at the Al2O3 corner, implying intense
The overall evaluation as presented in Table 3, shows primary weathering process and removal of feldspars and
varied inter-dependence of various K-estimations with the other mobile elements that characterized the primary
textural-statistical characteristics of the Ajali Formation. In source rock materials. However, cross plots of Fe + Mg
addition, the textural and hydraulic data set revealed sig- and K/Na ratio (Fig. 9a), as well as chemical weathering
nificant positive correlations among the empirical K- index (CIA) against %SiO2 (Fig. 9b), demonstrate the
estimations (r = 0.53–0.99). Also the relatively significant secondary weathering process and enrichment of Fe and
positive dependence of the empirically determined K val- Mg in the ferruginized samples compared to the fresh
ues on graphic mean grain size (0.36–0.90), percentage samples. Therefore, it can be concluded that the depletion
sand content (0.30–0.61) and porosity (0.26–0.56) is a of all other major and some trace elements in the Ajali
confirmation of the textural controls on the hydraulic Formation are related to the removal of ferromagnesian
characteristics of the granular Ajali Formation. However, minerals and feldspars through reworking and transporta-
the low correlations (r = 0.01–0.20) of the empirical K- tion of the source materials during sedimentary process.
estimations with the laboratory permeameter-based K val- Investigations of siliciclastic sedimentary rocks in sev-
ues can be attributed to the marked differences in the eral regions of the world show that their chemical
textural controls on the respective K-values. composition is largely dependent on the composition of the
weathering conditions at the source area (Nesbitt and
Young 1982, 1989; Nesbitt et al. 1996). Hence, based on
Geochemical profiles and weathering characteristics the profiles of the geochemical data presented above,
assessment of the degree of weathering and provenance
The summary of the results of the geochemical analyses of setting were undertaken. As demonstrated by Nesbitt and
the major and trace element concentrations of both fresh Young (1982), a measure of the degree of chemical
and ferruginized samples of Ajali Formation are presented weathering/alteration of the sediments’ source rocks can be
in Table 4, alongside with ratios between some selected constrained by calculating the chemical index of alteration
elements and estimated weathering indices. Initial miner- (CIA), [where CIA = molar Al2O3/(Al2O3 + CaO* +
alogical examination under binocular microscope of Na2O + K2O)] while CaO* represents the amount of CaO
selected samples of Ajali Formation revealed the in silicate minerals only [i.e., excluding those of carbonates
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Table 4 Summary of
Parameters Fresh samples (N = 12) Ferruginized samples (N = 4)
geochemical analyses results of
the major oxides including some Min. Max. Mean Median St. Dev. Min. Max. Mean Median St. Dev.
selected elemental ratios and
weathering indices for the Ajali SiO2% 94.8 98.9 97.2 97.3 1.20 76. 91.9 83.8 80.2 7.24
Formation samples Al2O3% 0.33 2.56 1.16 0.97 0.61 3.56 11.6 7.93 10.2 3.92
Fe2O3% 0.03 0.59 0.23 0.19 0.17 1.80 3.57 2.70 2.96 0.79
TiO2% 0.04 0.84 0.24 0.16 0.24 0.32 1.03 0.74 0.90 0.34
MnO% bdl 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.00
MgO% 0.01 0.05 0.02 0.01 0.01 0.02 0.04 0.03 0.03 0.01
CaO% 0.01 0.06 0.02 0.01 0.02 0.01 0.02 0.01 0.01 0.00
Na2O% 0.01 0.04 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.00l
K2O% bdl 0.08 0.03 0.04 0.02 0.01 0.03 0.03 0.03 0.01
P2O5% bdl 0.02 0.01 0.01 0.00 0.01 0.06 0.04 0.06 0.02
ZrO2% bdl 0.13 0.04 0.03 0.03 0.02 0.11 0.07 0.06 0.04
LOI 0.06 2.34 1.07 1.06 0.63 2.36 6.88 4.69 5.61 2.17
Rb (ppm) 0.67 123.2 34.5 1.56 47.2 3.37 4.08 3.67 3.56 0.37
Sr (ppm) 7.14 13.3 9.69 8.96 2.08 17.2 75.0 54.5 71. 32.4
Ba (ppm) 7.32 25.1 19.8 23.1 6.43 25.9 58.1 46.9 57.1 18.3
Si/Al 37.3 299.9 113.5 100.6 74.2 6.64 25.8 14.4 7.88 9.68
Na/K 0.17 5.00 1.42 0.88 1.64 0.33 1.00 0.53 0.33 0.30
Bdl below detection limit, LOI
Fe/Mg 3.0 59.0 18.1 11.8 15. 81.3 98.7 91.2 90.0 6.96
loss of ignition, CIA chemical
index of alteration after Nesbitt Zr/Ti 0.06 0.73 0.22 0.17 0.17 0.08 0.18 0.12 0.13 0.04
and Young 1982, CIW chemical % CIA 86.8 98.6 93.7 93.4 3.60 98.3 99.6 99.2 99.4 0.51
index of weathering after
% CIW 93.5 99.1 96.5 96.9 1.84 99.2 99.8 99.6 99.7 0.28
Harnois 1988
weathering (CIW) of Harnois 1988, is similar to the CIA
with the exception of exclusion of K2O in the equation
[CIW = molar Al2O3/(Al2O3 + CaO* + Na2O + K2O)].
Usually, the CIA or CIW are interpreted in similar way
with values of about 50, for unweathered (fresh) upper
crust material and about 100, for highly weathered residual
soils.
In this study, the estimated CIA and CIW (see Table 4)
yielded values in the range of 86.8–99.6 for the fresh
samples and 98.3–99.6 for the ferruginized samples. The
values [98 for the ferruginized samples are obviously
expected, while Fe/Mg values of 81.3–98.7, compared to
3.0–59.0 for fresh samples clearly confirm the weathering-
ferruginization process. However, relatively high values of
CIA and CIW (86.8–99.6) for the fresh samples is an
indication of the fact that the primary source material(s)
must have been subjected to substantially high degree of
weathering and reworking that had resulted in the removal
Fig. 8 Average mineralogical and chemical composition of the of the ferromagnesian minerals and feldspars. Therefore, it
Basement Complex rocks alongside with ASF samples plotted on: can be deduced that the CIA and CIW values for the
(A) Silica–Sesquioxides-Alkalis and (B) Al2O3–CaO + Na2O–K2O
bold arrow ferruginized samples reflect the recent (secondary) weath-ternary diagrams (Note: indicates trend of plagioclase and
biotite removal during weathering and transportation processes) ering-ferruginization process, while that of the fresh
samples are reflections of primary weathering of the source
which are incidentally not presented in the fluvio-conti- materials.
nental Ajali Formation as indicated by very low content of To assess the possible impact of the above secondary
CaO (0.1–0.04%)]. Also the proposed chemical index of weathering-ferruginization process in terms of metal
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946 Environ Geol (2009) 56:935–951
Fig. 9 Correlation plot of (a)
(Fe + Mg) against K/Na ratio
and (b) chemical index of
alteration (CIA) against silica
contents
enrichment, a summary of the metal concentrations in both Fe2þ þ 1=2O2 þ H2O ¼ FeðOHÞ2
fresh and ferruginized units are presented in Table 5. As
4Fe2þ þ O þ 10H O ¼ 4FeðOHÞ þ Hþ
shown in Table 5, there are only slight differences in the 2 2 3
concentrations of the major elements between the fresh and b) Under reducing condition (i.e., saturated zone of the
ferruginized units of Ajali Formation. However, the trace aquifer)
metals (Ba, Sr, Cu, Pb, Zn, Cr, Co, and Ni) exhibited about
3Fe2þ þ 4H2O ¼ Fe3O þ 4H2–5 folds increase in the ferruginized units compared to the 4 2
concentrations in the fresh Ajali Formation. Such trace Taylor and Eggleton (2001) noted that the formation of Fe-
metals profile of the weathered samples are consistent with oxides and oxyhydroxide as shown above dominates
trace metals association (Cr, Co, Cu, As, Pb, Zn, Mn, and weathering process while the ferric hydroxide precipitate
Mo) in ferruginous regolith as outlined by Roquin et al. is said to be highly reactive due to the large surface area.
(1990) and Butt and Smith (1992). The potential for Consequently, it is this characteristic that makes Fe-oxides
enrichment of such trace/heavy metals could be related to and oxyhydroxide alongside with Mn-oxides and Al-
the formation of Fe oxides/oxyhydroxide as shown by the hydroxyl complex to serve as sinks capable of adsorbing
following relation: wide range of trace/heavy metals as observed for the
a) Under oxidizing condition (i.e., unsaturated/phreatic ferruginized samples of Ajali Formation. Further
zone of the aquifer) confirmation of this is clearly reflected in the estimated
Table 5 Summary of
Parameters Fresh formation (N = 12) Ferruginized formation (N = 4) Enrichment factor (EF)
concentration profiles of major
and selected trace metals (mg/ Min. Max. Mean Min. Max. Mean Min. Max. Mean
kg) alongside estimated
enrichment factor (EF) Ca 71.5 429 143 71.5 143 71.5 0.17 0.27 0.21
Mg 60.3 301.5 120.6 120.6 241.2 180.9 0.33 1.09 0.60
Na 74.2 296.8 148.4 74.2 148.4 74.2 0.17 0.27 0.21
K bdl 664 249 83 249 249 0.57 4.08 1.83
Si 443758 463086 454943 359377 429905 392184 0.16 0.22 0.18
Fe 209.7 4124.1 1607.7 12582 24954 18873 0.06 10.78 5.34
Mn bdl 77.5 77.5 77.5 77.5 77.5 0.06 1.67 1.02
Ba 7.32 25.1 19.8 25.9 58.1 47.0 0.26 0.87 0.50
Sr 7.14 13.3 9.7 17.2 75.0 54.5 0.40 1.81 0.91
Cu 0.01 1.16 0.27 3.07 3.73 3.5 0.59 98.95 50.24
Pb 2.05 4.43 3.05 8.01 15.6 12.3 0.62 1.08 0.92
Zn 2.66 6.78 4.37 7.2 15.2 11.6 0.45 0.50 0.48
Co 0.01 0.66 0.22 0.78 2.79 1.87 6.43 53.55 24.33
Cr 5.09 12.8 9.42 17.9 58.5 42.9 0.59 1.41 1.06
Ni 1.52 3.66 2.53 5.41 12.4 9.3 0.42 1.19 0.83
Bdl below detection limit
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enrichment factors (EF) normalized with respect to between the groundwater and the aquifer materials of the
immobile Ti based on the following relation: ASF is presented.
EF ¼ ½ðMe/TiÞ =ðMe/TiÞ  A general look at the chemical profiles of the ground-weatheredunit freshunit
water samples from 10 different locations within the Ajali
whereby; Me is the respective trace metal of interest and Ti Formation as presented in Table 6, indicates that both major
is the reference immobile element in weathered unit on one and trace metal concentrations are within the limits of WHO
hand and fresh bedrock on the other hand. EF value of (1993) and Standard Organization of Nigeria, SON (2007)
approximately 1 for any element/metal imply concentration standards for drinking water quality. The respective low
representing the geogenic input, while EF [1 and EF \1 concentrations of the metals, alongside with the low EC
indicate enrichment and depletion respectively of the values of 14–268 ls/cm, imply low mineralized water from
affected metal. clean sandy aquifer devoid of labile and weatherable min-
Using the above relation, the estimated normalized erals. However, a closer look at the chemical data revealed a
enrichment factor (EF) presented in Table 5, revealed that clear distinction between the deep boreholes (110–120 m)
only K, Fe, Mn, Co and Cr exhibited enrichment in the tapping the fresh aquifer unit and the shallow boreholes
ferruginised units (EF of 1.01–50.24). Such enrichment can (20–30 m) tapping the ferruginized unit. The pH of the
be attributed to the highly reactive (large surface area) water samples from the fresh aquifer unit range between 6.9
oxides/oxyhydroxides of Fe and Mn responsible for the and 8.7 and characterized by low EC values of 14–34 ls/
common reddish-brownish coloration of the ferruginized cm, However, water samples from the ferruginized unit
units. In addition, studies had shown that such oxides/ exhibit slightly higher acidity and dissolved solid with pH
oxyhydroxides of Fe and Mn and associated clay minerals values of 5.2–6.5 and EC values of 115–268 ls/cm, Also
do serve as sinks/hosts for a wide range of trace/heavy the water samples from the ferruginized unit exhibit rela-
metals such as Cu, Co, and Cr as observed for the fer- tively higher concentrations of the major elements (Ca, Mg.
ruginized Ajali Formation (Sharma and Rajamani 2000; Na. K including Ba and Sr) compared to those from the
Taylor and Eggleton 2001). Hence, the observed ferrugi- fresh aquifer unit (see Table 6).
nization enrichment of trace metals, in this study, is an The observed distinctions may be attributed to the
indication of possible/potential threats to groundwater possible impact of the rapid and intense ferruginization of
quality. This arises from the fact that infiltration/recharge the Ajali Formation, as observed in the exposed sections in
induced geochemical redox process can further enhance the the field, Hence, the observed average composition of 7.93
dissolution and redistribution of such trace/heavy metals and 2.7 wt.% for Al2O3 and Fe2O3, respectively in the
into the groundwater (Bruand 2002). ferruginized units compared to values of 1.2 and 0.2 wt.%,
respectively in the fresh units can be attributable to infil-
trating/recharge water-mediated weathering processes.
Metals mobility and groundwater quality Consequently, environmental implication in terms of pos-
sible ferruginization-induced release of metals through
From the overall assessment as presented so far, there is no such weathering-induced geochemical processes is further
doubt as to the high aquiferous potentials of the ASF in terms highlighted by estimation of potential metal mobilization
of storage capacity and yield. This is adequately supported factor using the following relation modified after Minarik
by the estimated average porosity of about 27% as well as by et al. (1998):
the estimated and laboratory hydraulic conductivity (K) cMeðwaterÞ cAlðrockÞ
values in the range of 1.2 9 10-5–1.2 9 10-3 m/s. Such MF ¼ 
cMeðrockÞ cAlðwaterÞ
prolific hydraulic characteristics alone are not enough in
terms of comprehensive management of groundwater and whereby;
aquifer system, which should also take the quality aspect of
cMe average conc. of respective metal in the
the groundwater into consideration. Consequently, it should (water)
groundwater system (mg/l);
be pointed out that weathering-ferruginization processes
cMe average conc. of respective metal in the
constitute a significant source of trace metals release into the (rock)
aquifer horizon in mg/kg;
environment and possible enrichment of the mobilized
cAl average conc. of Al in the groundwater
metals in soils (aquifer), surface and groundwater systems. (water)
system (mg/l);
The dynamics of such ferruginization-weathering products
cAl average conc. of Al in the aquifer horizon in
are said to be partly controlled by percolating/recharging (rock)
mg/kg.
water and surface erosion (Taylor and Eggleton 2001).
Therefore, in this section, brief highlight of the possible Using the above relation, the average concentrations of the
influence of weathering-induced geochemical interactions respective metals in the analyzed groundwater samples
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948 Environ Geol (2009) 56:935–951
Table 6 Concentration profiles of major (mg/l) and trace metals (lg/l) in the analyzed groundwater samples from the Ajali Formation aquifer
Parameters Fresh aquifer unit Ferruginized unit WHO/SON
L1 L3 L4 L5 L6 L9 L10 L2 L7 L8
Temp. oC 23.6 27.4 28.4 27.4 27 28.1 27.8 29.4 28 29.2 Variable
pH 8.7 7.4 7.5 6.7 7.2 6.9 6.9 5.24 6.5 6.5 6.5–9.5
EC (lS/cm) 14 19 17.5 21 31 23 34 268 133 115 400–1480
Ca (mg/l) 0.9 0.8 1.2 0.7 1.0 0.3 0.4 7.0 10.7 10.6 75–200
Mg (mg/l) 0.3 0.3 0.3 0.2 0.7 0.3 0.4 6.4 2.8 2.8 50–150
Na (mg/l) 1.4 2.3 1.2 1.6 2.2 0.7 1.1 22.3 14.5 14.4 20–200
K (mg/l) 0.2 0.3 0.4 0.4 1.9 0.3 0.3 13.5 3.1 3.1 10–12
Si (mg/l) 5.2 5.4 6.2 5.5 5.3 5.6 5.7 32.6 5.6 5.7
Fe (mg/l) 0.35 0.04 1.12 0.03 1.24 0.02 0.03 0.14 0.19 0.2 0.3–1.0
Mn (mg/l) 0.04 0.01 0.03 0.02 0.06 0.03 0.03 0.27 0.12 0.11 0.2
Ba (lg/l) 20 20 20 20 20 20 20 250 50 50 700
Sr (lg/l) 10 10 10 10 10 10 10 100 40 40
Cu (lg/l) 2 2 21 94 85 17 83 3 2 2 200
Pb (lg/l) 10 10 10 10 60 10 10 10 10 10 10
Zn (lg/l) 43 137 47 25 463 22 140 92 14 17 300
Co (lg/l) 2 2 2 2 2 2 3 14 2 2
Cr (lg/l) 20 20 20 20 20 20 20 20 20 20 50
Ni (lg/l) 5 5 5 5 5 5 5 14 5 5 20
WHO (1993)––Guidelines for Drinking Water Quality, SON (2007)––Standard Organization of Nigeria: Standards for Drinking Water Quality
L1 Abocho, L2 Dekina, L3 Anyangba, L4 Iyale, L5 Nsukka, L6 Igbo-Etiti, L7 9th-Mile I, L8 9th Mile II, L9 Okigwe I, L10 Okigwe II (see
location in Fig. 4)
from the fresh and ferruginized units were used for the elements including Ba and Sr (with exception of Fe) is
estimation of respective metal mobilization factor (MF) in about 2–36 folds higher under ferruginized setting (see
respect of the rock/aquifer unit (Table 7). A general look at Table 7). The relatively higher degree of mobilization for
the estimated MF indicates that the potentials for mobili- Fe and trace elements in the fresh aquifer unit compared to
zation of the major cations (Ca, Mg, Na, K) and the the ferruginized unit is an indication of possible geo-
selected trace metals (Mn, Fe, Ba, Sr, Cu, Pb, Zn Co, Cr chemical water-rock interactions (especially reduction of
and Ni) from the rock/aquifer are higher. However, the Fe- and Mn-oxyhydroxides), leading to higher metal
ferruginized unit exhibited higher MF in respect of the mobility under reducing condition found in deep aquifer.
major elements, while the fresh unit exhibited MF with This is consistent with the study of Quantin et al. (2002),
respect to the trace elements including Fe. The higher MF which revealed that under reducing condition, typical of
with respect to the ferruginized units for most of the major groundwater environment, chemical and bacterial pro-
elements including Mn, Ba and Sr is a clear indication of cesses do lead to reduction of Mn-oxide and hence the
weathering-induced release, while the higher MF for the release of Co and Ni associated with the Mn-oxide lattice.
trace elements including Fe with respect to the fresh Therefore, it can be inferred that the relatively higher MF
aquifer unit may be attributed to higher mobility potentials of the trace metals (Pb, Zn, Cu, Co, Cr and Ni) with respect
of metals under reducing conditions in deep aquifer com- to the fresh Ajali units may be related to similar reduction
pared to possible fixation in metal-oxides/hydroxide under of Fe-/Mn-oxyhydroxides, which serve as sinks/hosts for
possible oxic condition in shallow aquifer unit. such trace metals.
In addition, the generally lower mobilization of Si, Fe Consequently, possible remobilization through infiltra-
and Mn under both fresh and ferruginized units (\50), tion-induced leaching processes is a clear indication of the
compared to other major elements (56–8,356), may be potential environmental impact in terms of groundwater
partly due to the low solubility of Si and possible immo- quality as well as borehole/aquifer management, especially
bilization of Fe and Mn in form of oxides and under humid tropical environment of the study area. Apart
oxyhydroxides within the aquifer matrix. Again, a com- from the fact that formation of Al-, Fe- and Mn-oxides are
parison of the metal mobility, in form of mobilization ratio associated with contaminant trace metals as mentioned
(MR), revealed that the degree of mobility for the major earlier, their formation can lead to reduction in the
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Environ Geol (2009) 56:935–951 949
Table 7 Results of the
Parameters Fresh formation Ferruginized formation M-ratio
estimated metal mobilization
factors with respect to fresh and Av. GW Av. AU MF Av. GW Av. AU MF
ferruginized Ajali Formation (mg/l) (mg/kg) (mg/l) (mg/kg)
aquifer N = 7 N = 12 N = 3 N = 4
Ca 0.8 143 136.9 9.4 71.5 4896 35.8
Mg 0.4 120.6 76.6 4.0 180.9 820.6 10.7
Na 1.5 148.4 261.3 17.1 74.2 8536 32.7
K 0.5 249 56.4 6.6 249 978.7 17.4
Si 5.6 454943 0.3 14.6 392184 1.4 4.4
Fe 0.40 1607.7 6.5 0.18 18873 0.3 0.1
Mn 0.03 77.5 10.5 0.17 77.5 49.8 4.7
Ba 0.02 19.78 26.1 0.12 46.97 92.4 3.5
Sr 0.01 9.69 26.7 0.06 54.51 40.9 1.5
MF mobilization factor, M-ratio Cu 0.04 0.27 4117 0.00 3.48 24.5 0.0
mobilization ratio w.r.t. Pb 0.02 3.05 144.1 0.01 12.31 30.2 0.2
ferruginized unit, Av. GW
Zn 0.13 4.37 739.9 0.04 11.62 130.9 0.2
average composition of the
respective groundwater system, Co 0.00 0.22 232.0 0.01 1.85 120.6 0.5
Av. AU average composition of Cr 0.02 9.42 54.9 0.06 42.92 51.9 0.9
the respective rock (aquifer) Ni 0.01 2.53 51.1 0.01 9.33 31.8 0.6
unit
permeability of the filter/gravel layers around the intake inter-dependence of various K-estimations with the textural-
portion of the boreholes and thus reduce the efficiency or statistical characteristics of the Ajali Formation. In addition,
yield on one hand. On the other hand, reduction-oxidation the multivariate factor analysis of the textural and hydraulic
processes involving such Fe-Mn-Al-oxyhydroxides can data set revealed the significant positive correlations among
lead to chemical reactions characterized by precipitation, the empirical K-estimations (r = 0.53–0.99). The relatively
encrustation and corrosion of the well (borehole) materials. significant positive dependence of the empirically deter-
Detail assessment of such flow-induced geochemical pro- mined K values on graphic mean grain size (0.36–0.90),
cesses related to iron-oxidation, corrosion and encrustation percentage sand content (0.30–0.61) and porosity (0.26–
in water wells are presented elsewhere in Tijani 1996. 0.56) are consistent with the PCA component factors as
Finally, it should be pointed out that the foregoing dis- outlined in Table 3 and also underly the reliability of the
cussion in respect of the geochemical interactions and empirical estimation employed in this study.
attendant groundwater quality problems is by no means Furthermore, the assessment of the degree of weathering
exhaustive; hence a detailed geochemical modeling of the revealed that the CIA and CIW values for the ferruginized
overall groundwater-aquifer interactions involving leach- samples reflect the recent (secondary) weathering-ferrugi-
ing tests at different pH and redox conditions can be seen nization process, while that of the fresh samples are
as the focus of future study. reflections of the primary weathering of the source mate-
rials of the Ajali Formation. However, while the textural
characteristics exert positive impacts on groundwater
Summary and conclusion occurrence and recharge, the geochemical components of
the ferruginized Ajali Formation in respect of the weath-
In this study, assessments of the textural, hydraulic and ering-induced enrichment of metals signify possible
geochemical characteristics of Ajali Formation were negative impacts on groundwater quality. The groundwater
undertaken. The results indicate the Ajali Formation to be is generally soft, slightly acidic, and low in dissolved sol-
well sorted fine to medium grained sands, with minor ids; silica makes up a large part of the dissolved
amounts of silt. The evaluated textural parameters constituents, hence, the major processes affecting ground-
(Cu = 1.5–3.4, n = 18–32% and D10 = 0.1–0.25 mm) in water composition are the dissolution process mediated by
addition to statistical parameters (such as skewness, kurto- CO2-charged infiltrating rainwater and the associated
sis, graphic mean etc.) are indications of high aquiferous weathering/ferruginization process. Assessment of the
potentials of the Ajali Formation in terms of the ground- potential metal mobilization revealed higher MF with
water occurrence. For the comparative assessment of the respect to the ferruginized aquifer unit for most of the
estimated K-values, the overall evaluation shows varied major elements including Mn, Ba and Sr, which is a clear
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950 Environ Geol (2009) 56:935–951
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Prof. K. Jinno (Institute of Environmental Systems, Kyushu Univer- Darcy H (1856) Les Fontaines Publiques de la Ville de Dijon,
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