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HYDROCHEMICAL STUDIES OF GROUNDWATER
IN PARTS OF LAGOS, SOUTHWESTERN NIGERIA
Abstract: Groundwater samples were randomly collected from forty-five
protected dug wells and thirteen boreholes in parts of Lagos, Southwestern
Nigeria. Samples were analysed for pH, electrical conductivity and total
dissolved solids in situ. Cations (calcium, magnesium, potassium and
sodium) and anions (chloride, bicarbonate, sulphate and carbonates) were
determined in the laboratory after standard procedure. The study is aimed
at examining the extent of groundwater pollution and its possible sources
using multivariate and graphical techniques. Rockware software was used
to characterize groundwater composition while the sample locations were
mapped with ArcMap 9.3 software. The results show high mean value
in the groundwater characteristics of the protected dug wells compared
to the boreholes. A significant relationship exists among EC/ TDS and
major ions indicating the influence of sea water on the groundwater quality.
The dominant cations and anions are in the order of: Ca2+ > Mg2+ >Na+ > K+;
and Cl- >HCO3
- > SO4
2+
respectively. Factor analysis extracted two major
sources of pollution (sea water and industrial) responsible for the processes
controlling groundwater quality in the area. The Piper plot revealed seven
distinct water types including Ca-HCO3, Ca-Cl, Mg-HCO3, Na-SO4, Mg-Cl,
Na-HCO3 and Na-Cl representing 44.8%, 25%, 15.5%, 5.2%, 3.4%, 3.4%,
and 1.7% respectively. The paper recommended monitoring of groundwater
abstraction and treatment of industrial effluent before being released into the
subsurface to prevent salinization and quality deterioration in the study area.
Key words: groundwater chemistry, anthropogenic pollution, marine
influences, Nigeria, Lagos
Bulletin of Geography – Physical Geography Series
No 6/2013: 27–42 DOI:10.2478/bgeo-2013-0002
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Introduction
Groundwater serves as a major source of drinking water globally (WWAP
2009). It was estimated that about 1,000 km3 of the world’s aggregated
groundwater is abstracted annually. Approximately 67% is used for
irrigation, 22% for domestic, and 11% for industrial purposes (Siebert
et al. 2010; AQUASTAT 2011). Naturally, groundwater is usually free from
contamination but anthropogenic activities can impact significantly on the
quality such that it will require interventions prior to usage (Yidana et al.
2012). Groundwater quality depends on several factors including lithology,
chemical composition of the aquifers, climatic conditions prevailing during
formation, quantity of water available in the aquifer and its rate of circulation
(Todd and Mays 2005). The interaction between water and the host rock can
influence of groundwater suitability and its chemistry. The degree to which
rock/mineral weathering influences groundwater chemistry is a function
of several factors such as residence time of groundwater in the host rock,
the ambient temperature and pH, among others (Wen et al. 2005; Coetsiers
and Walravens 2006; Banoeng-Yakubo et al. 2009; Shankar et al. 2011).
The concept of groundwater composition also known as hydrochemical
facies is useful for the identification of the hydrochemical facies, water type,
hydrochemical processes, chemical character of the water, their similarities
and differences in any given aquifer system (Güler et al. 2002; Sadashivaiah
et al. 2008). The Knowledge of the natural groundwater quality can provide
important inferences of the natural water chemistry, recharge, mixing
and discharge of groundwater (Todd and Mays 2005). Various methods
have been developed for the visual understanding of hydrochemical data
in order to show water classification, reveal the discernible patterns, trends,
dissimilarities of water composition and quality the world over (Back 1965;
Dalton and Upchurch 1978; Sadashivaiah et al. 2008). The simplest methods
include plotting distribution diagrams, bar charts, pie charts, radial and Stiff
diagrams. Although these are easy to construct, they are not convenient for
graphical presentation of large numbers of analyses. Hence, other techniques
used include Ternary, (Schoeller 1962), (Piper 1944) and (Durov 1948)
diagrams.
In addition to these graphical techniques, multivariate statistical methods
have also been employed for the analysis of groundwater composition.
For instance, factor and cluster analyses have been widely used with
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29
conventional graphical techniques to characterize hydrochemical systems
adequately. Their uses have assisted in providing solutions to various
environmental problems and also provided a better understanding of the
groundwater flow regime (Meng and Maynard 2001; Güler et al. 2002;
Thyne et al. 2004; Yidana et al. 2008a, 2008b). In order to ensure that the
utilization of groundwater is sustained in the study area, adequate protection
from pollution and monitoring of the quality must be ensured. Thus, the
present study aimed at examining the extent of groundwater pollution and
its possible sources using multivariate and Piper diagrams.
Study area
The study area is located approximately between latitudes 6o23’30’N and
6o 34’15N and longitudes 3o28’0E and 3o38’45E. It is bounded on the East
by Ibeju-Lekki, in the North by the Lagos Lagoon and in the South by the
Atlantic Ocean and parts of the Lagos metropolis in the West (Fig. 1).
Fig.1. Sampling locations
The climate is tropical, hot and wet and the area is characterized by coastal
wetlands, sandy barrier islands, beaches, low-lying tidal flats and estuaries (Adepelumi et
al. 2009). The average temperature is about 27
0
C with an annual average rainfall of about
1,532 mm (Adepelumi et al. 2009). The major seasons are wet and dry seasons. The wet
season lasts for 8 months (April to November) and the dry season covers a period of 4
months (December to March (Adepelumi et al. 2009). The dominant vegetation consists of
tropical swamp forest (fresh water and mangrove swamp forests and dry lowland rain
forest).
The area is drained by Lagos Lagoon (Emmanuel and Chukwu 2010). The geology is
underlain by the Benin Formation and is made up of unconsolidated sands and gravels (Oteri
and Atolagbe 2003). The groundwater flow direction shows a general North to South
direction with two small cones of depression in Apapa and Ikeja because of intense
groundwater output (Coode et al. 1997; Oteri and Atolagbe 2003).
The Hydrogeology is
Fig.1. Sampling locations
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The climate is tropical, hot and wet. The area is characterized by
coastal wetlands, sandy barrier islands, beaches, low-lying tidal flats and
estuaries (Adepelumi et al. 2009). The average temperature is about 270C
with an annual average rainfall of about 1,532 mm (Adepelumi et al.
2009). The major seasons are wet and dry seasons. The wet season lasts
for 8 months (April to November) and the dry season covers a period
of 4 months (December to March) (Adepelumi et al. 2009). The dominant
vegetation consists of tropical swamp forest (fresh water and mangrove
swamp forests and dry lowland rain forest).
The area is drained by Lagos Lagoon (Emmanuel and Chukwu
2010). The geology is underlain by the Benin Formation and is made
up of unconsolidated sands and gravels (Oteri and Atolagbe 2003).
The groundwater flow direction shows a North to South direction with two
small cones of depression in Apapa and Ikeja due to intense groundwater
abstraction (Coode et al. 1997; Oteri and Atolagbe 2003). The hydrogeology
is characterized by sand and clay from the underlying aquifer formation
(Longe 2011). The major aquifer formations consist of coastal plain sand
aquifer (CPS) and recent sediments (alluvium). CPS is categorized into four
types namely the recent sediments/alluvium, the upper and lower CPS and
the Abeokuta formation (Longe 2011). Groundwater occurrence in the CPS
is essentially semi-confined to unconfined aquifers (Adelana et al. 2008).
Variation in the thickness between the first and third CPS aquifer ranged
between 200 m and 250 m respectively (Adelana et al. 2008); while the mean
estimate of groundwater storage of the first CPS aquifer is about 2.87×103 m3.
The water table ranged between 0.4–21 m with an annual fluctuation of less
than 5 m (Asiwaju-Bello and Oladeji 2001). The upper coastal plain sand
aquifer (UCPSA) is a water table aquifer with a relatively annual fluctuation
below 5 m (Asiwaju-Bello and Oladeji 2001). The UCPSA is tapped by
a hand dug well and is usually prone to pollution because it is near to the
ground surface. Unlike the lower coastel plain sand aquifer (LCPSA), it is
tapped by a borehole and is not vulnerable to pollution. The CPS aquifer
is the most productive and exploited aquifer in Lagos state. More than
30% of groundwater supply in Lagos and its environs tap from this aquifer
(Longe 2011).
Water supply in Lagos state is mainly through the mini water works across
twenty-three towns with a total design capacity of about 53.2 million gallon
per day (Mgd) while the micro water works covered about seventeen towns
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across the state with a total design capacity of about 16.3Mgd (LWC 2012).
Other sources include protected dug wells sunk by various households or
private individuals who have the ability to construct one. According to LWC
(2012) the mini and micro water works account for about 2,797.38Mgd
and 1,208.77Mgd water supply respectively from the LCPSA (Longe 2011).
The major settlements of the study area are located within Eti-Osa local
government area (LGA). The major challenge is the rate of population
growth, urbanization, sand mining/dredging, and ocean surge, among
others. In addition, the LGA is also noted for high commercial and industrial
establishments and tourist centres. These factors have resulted in increased
waste generation with significant impact on the subsurface that serves as the
major source of water supply in the area. Thus, for sustainable groundwater
utilization in the area, the alarming rate of groundwater abstraction and
waste generation into the subsurface must be curtailed.
Materials and method
Fifty-eight samples including 45 protected dug wells (PDW1–45) and 13
boreholes (BH46–58) were randomly selected for the hydrochemical analysis
of groundwater composition in the study area. Samples were collected
in clean 150ml polyethylene bottles and preserved in ice chests for onward
delivery to the chemistry department, University of Lagos, Akoka. A
pocket pH-102 meter (RoHS) was used to determine the pH. The pH meter
was calibrated with a standard buffer solution prior to sampling. EC was
measured in situ by EC DiST-3 meter (HANNA, HI 98303). TDS was
measured using the TDS-3 meter, TDS/TEMP (HM Digital). The cations
(Na+ K+, Ca2+ Mg2+) and anions (Cl-, HCO3, CO3 and SO4
2- were determined
in the laboratory using standard methods as suggested by the American
Public Health Association (APHA 1998).
Calcium was determined using 0.05 N EDTA titration, and chloride
by a standard solution of 0.005 N AgNO3 -argentometry titration using
potassium chromate as an indicator. Bicarbonate and carbonates were
determined by titration against standard H2SO4 solution (0.0392 N).
Magnesium, potassium and sodium were determined by atomic absorption
spectroscopy (AAS) (Hanna, HI 98180). Each metal was analysed at its
specific wavelength. Sulphate was determined by direct reading using
a spectrophotometer (HACH, DR/2000). Rockware (2006) software was
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used for the graphical characterization of groundwater composition of the
study area using a Piper diagram. The data were analysed using descriptive,
bivariate and multivariate techniques using SPSS software 17.0 version.
Co-ordinates of the sample locations were recorded by global positioning
system (GPS) (Garmin map, 76CSX model) and thereafter were exported
into ArcMap 9.3 software to produce map of the sampling locations.
A reliability test was conducted for the major ions. A reliability
test is a method of identifying apparent inconsistencies in an analysis.
Inconsistency only indicates that an analysis is unusual, not necessarily
wrong (Hounslow 1995). The concentrations were computed using the
charge balance error (CBE) formula (Appelo and Postma 2005) equation 1.
Ʃ cations – Ʃ anions /
Ʃcations + Ʃ anions X 100% (1)
where the concentrations were expressed in meq/L. The calculated CBE
shows that about 31% of the samples are less than or equal to ± 10.4%.
The %CBE ranged between -61.7 and 77.1 with a mean value of 9.6.
Results and discussion
Groundwater characteristics
The descriptive statistics of the groundwater characteristics of the protected
dug wells and boreholes are presented in Table 1.
The correlation analysis shows that a significant relationship exists
among EC/ TDS and major ions in groundwater of the study area (Table 2).
The strong positive correlations between TDS and Na+, Cl-, Mg2+, and SO4
2-
indicate the influence of sea water on the groundwater quality of the study
area (Aiman and Mohamed 2010; Lu et al. 2011; Rao et al. 2012).
Factor analysis
The result of the factor analysis (FA) indicates two factors that can be used
to explain the various processes controlling groundwater composition in the
study area. The rotated factor matrix shows that the two factors extracted
explain 82.54 % of the total variance (Table 3). According to Liu et al. (2003)
factor loadings can be classified as strong, medium and weak corresponding
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to values of >0.75, 0.75 – 0.50, <0.50 respectively. A factor loading of 0.75
was chosen as the parameter that influences that factor in this study (Kathy
and Niranjali 2010). Factor I explains 68.62% of the total variance. It has
a strong positive loading on Cl-, Na+, EC, Mg2+, TDS, Ca2+, SO4
2- and K+.
The loading on pH, HCO3
- and CO3
- is very low. These parameters (Mg2+,
Cl−, Na+, K+, and SO4
2-) represent the dominant components of salinity (Lu
et al., 2011). Factor II accounts for about 12.92% of the total variance
which is characterized by medium positive loading of HCO3
- and pH
while carbonate is characterize by medium negative loading. The rest
of the variables show weak loadings. The negative loading of CO3
- and
SO4
2- on factor II confirms that the concentration of CO3
- and SO4
2- in the
groundwater of the study area does not contribute significantly to HCO3
-
value. The application of FA to reduce the number of parameters needed
to explain the groundwater data in this study is medium. Thus, 10 out of the
examined parameters were needed to explain 82.54% of the variance in the
data set across the two factors comprising 11 parameters. However, FA
successfully identified the most significant sources/processes controlling
the groundwater of the study area.
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Table 1. Descriptive statistics of groundwater characteristics
TDS
Na+
+
Ca2+
2+
N
Mean
TDS
Na+
+
Ca2+
2+
N
Mean
UCPSA – upper coastal plain sand aquifer, LCPSA – lower coastal plain sand aquifer
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Table 2. Relationship of groundwater parameters
TDS Na+ Ca2+ 2+
1
1
TDS 1
Na+ 1
+ 1
Ca2+ 1
2+ 1
1
1
1
1
** Correlation is significant at p> 0.01
* Correlation is signicant at p> 0.05
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Table 3. R-mode factor matrix of groundwater parameters
% of Variance 68.616 12.924
Cumulative % of Variance 68.616 81.540
Characterization of groundwater composition
The results of the dominant cation are in the order of Ca2+ > Mg2+ > Na+ > K+
while the anions indicate HCO3
- > Cl- > SO4
2- (Table 4).
The characterization of the groundwater composition of the study area
is presented in Figure 2. The result shows seven distinct hydrochemical
water types including Ca-HCO3, Ca-Cl, Mg-HCO3, Na-SO4, Mg-Cl, Na-
HCO3 and Na-Cl .The Ca-HCO3 is dominated by alkaline earths and weak
acids (Karanth 1987). This water type occupies the diamond-shape section
designated as II and accounts for 44.8% of groundwater in the study area.
It comprised 44.4% and 46.1% of the contribution from PDW and boreholes
respectively. Another water type in this section is Mg-HCO3 representing
15.5% of groundwater and 20% of the contribution from PDW. Also
included in this section is Na-HCO3 water type representing 3.4% of the
groundwater and 15.4% of the contribution from boreholes. The source
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of these water types comprises either dolomite or calcite in the aquifer and
is characterized by temporary hardness (Tay and Kortatsi 2007). The Na-
SO4 and Na-Cl water types have saline characteristics. These water types
occupy the diamond-shape section designated as V and account for 5.2% and
1.7% respectively of groundwater in the area with contributions from the
boreholes only. Their sources may originate from the sea or rain water (Tay
and Kortatsi 2007). A minor water type, Ca-Cl water representing 25.1%
of the groundwater is delineated in the diamond-shape field I. It comprised
28.9% and 15.4% of the contribution from PDW and boreholes respectively.
The source aquifers contain gypsum or anhydrite that may have undergone
dissolution and, therefore, are characterized by permanent hardness (Tay
and Kortatsi 2007). The Mg-Cl water type occupies the section designated
as III and forms 3.4% of groundwater with a 4.4 % contribution from PDW.
Table 4. Concentration of ions according to water type in the study area
Ca2+
2+
Na+
+
Min
Mean
Min
Mean
Min
Mean
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Ca2+
2+
Na+
+
Min
Mean
Min
Mean
Min
Mean
Min
Mean
Min – minimum, Max – maximum, Std – standard
Table 4. contd.
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Fig. 2. Piper plot of groundwater composition
CONCLUSION
The study reveals a relatively higher mean value of the protected dug wells compared to the
boreholes. A significant relationship exists at p>0.05 level of significance among EC/ TDS
and major ions indicating the influence of sea water on the groundwater quality in the study
area. The dominant ions are in the order of Ca
2+
> Mg
2+
>Na
+
> K
+
; Cl
-
>HCO
3-
> SO
42+
.
Factor analysis indicates sea water and industrial wastes as the major sources of pollution
responsible for the processes controlling groundwater quality in the area. The Piper plot
characterized seven distinct water types including Ca-HCO
3
, Ca-Cl, Mg-HCO
3
, Na-SO
4
, Mg-
Cl, Na-HCO
3
and Na-Cl representing 44.8%, 25. %, 15.5%, 5.2%, 3.4%, 3.4%, and 1.7%
respectively. The paper recommended monitoring of groundwater abstraction and treatment of
industrial effluent before being released into the subsurface to prevent salinization and quality
deterioration in the study area.
Fig. 2. Piper plot of groundwater composition
Conclusions
the study reveals a relatively higher mean value of the protected dug wells
compared to the boreholes. A significant relationship exists at p>0.05 level
of significance among EC/ TDS and major ions indicating the influence
of sea water on the groundwater quality in the study area. The dominant
ions are in the order of Ca2+ > Mg2+ >Na+ > K
+ ; Cl- >HCO3
- > SO4
2+.
Factor analysis indicates sea water and industrial wastes as the major
sources of pollution responsible for the processes controlling groundwater
quality in the area. The Piper plot characterized seven distinct water types
including Ca-HCO3, Ca-Cl, Mg-HCO3, Na-SO4, Mg-Cl, Na-HCO3 and
Na-Cl representing 44.8%, 25. %, 15.5%, 5.2%, 3.4%, 3.4%, and 1.7%
respectively. The paper recommended monitoring of groundwater abstraction
and treatment of industrial effluent before being released into the subsurface
to prevent salinization and quality deterioration in the study area.
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Acknowledgements
The author is grateful to Prof. A.E. Edet and also an anonymous reviewer
for their valuable comments and suggestions in improving the quality of this
paper.
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