Research Journal of Applied Sciences, Engineering and Technology 3(7): 617-624, 2011
ISSN: 2040-7467
© Maxwell Scientific Organization, 2011
Received: April 21, 2011
Accepted: June 10, 2011
Published: July 25, 2011
Assessing Factors Controlling the Hydrochemistry and Suitability for Irrigation
Purposes of Aquifer AQ-2 in Pointe-Noire, South-West Congo-Brazzaville
1
C. Tathy, 2L. Matini, 1G. Moukandi Nkaya and 1B. Mabiala
1
Equipe de Recherche en Matériaux et Ecoulements en Milieux Poreux, ENSP,
U.M.NG., B.P 69 Brazzaville, Congo
2
Laboratoire de Chimie Minérale et Appliquée, Faculté des Sciences,
U.M.NG. B.P 69 Brazzaville, Congo
Abstract: The groundwater of AQ-2 in Pointe-Noire (south-west town of Congo-Brazzaville) was assessed for
factors controlling the hydrochemistry and its irrigation suitability. Thirty three groundwater samples collected
from 11 piezometers were analyzed for physical parameters such as: pH, temperature (T), Total Dissolved
Solids (TDS); major cations: Ca2+, Mg2+, Na+, K+ and anions ClG , SO42G , HCO3G. Factor analysis revealed four
factors associated with the mineralization, alkalinity, temperature and pH of the groundwater. Some indexes
which can influence the groundwater quality for irrigation were determined: Sodium Adsorption Ratio (SAR),
percent of sodium (%Na), Residual Sodium Carbonate (RSC), Permeability Index (PI) and Potential Salinity
(PS). These indexes for water irrigation were compared with standard limits. They have been found within the
safe limit suitable for irrigation. The total dissolved solids in the groundwater were lower than 1500 mg/L, this
denotes that irrigation using groundwater of aquifer AQ-2 in Pointe-Noire would not cause salinity hazards.
Key words: Factor analysis, groundwater, irrigation water, Pointe-Noire
for agricultural purposes is revealed from groundwater
chemistry (Mridha et al., 1996; Obiefuna and Sheriff,
2011). Irrigation water criteria take in account some
variables such Sodium Adsorption Ratio (SAR), Total
Dissolved Solids (TDS), Percentage of Sodium (%Na),
Residual Sodium Carbonate (RSC), Permeability Index
(PI) and Potential Salinity (PS).
In a precedent study (Tathy et al., 2010), we have
shown the drinking suitability of AQ-2 water. The aim of
this one is to determine the controlling factors of the
hydrochemistry of groundwater of aquifer AQ-2 in
Pointe-Noire and its suitability for irrigation purposes.
INTRODUCTION
In term of development, the quality of water is an
important factor as well in the use of groundwater like
resources. Different quality problems characterize the
groundwater (Gupta et al., 2004). Heavy metals, nitrates
and fluoride in groundwater, to quote only the substances
that can behave as hazardous pollutants and pollute it
(Sharma et al., 2005).
The quality requirement of groundwater depends
upon its different uses like drinking, industrial and
irrigation uses. The chemical quality of the groundwater
is indispensable as a factor which enables its utilization
for the different uses quoted above. Therefore chemical,
physical and bacteriological parameters related to the
quality criteria must be determined. In developing
countries, overexploitation is a serious problem for the
quality of groundwater in urban area, due to hightly dense
population. This is the case of aquifer AQ-2 in PointeNoire (South West of Congo-Brazzaville). More than
500,000 people depend upon groundwater for their
drinking water and other uses. Irrigation of agricultural
lands accounted for 70% of the water used in the
worldwide (Vineesha and Singh, 2008). The nature of
water used for irrigation has effects in the production,
quality and type of culture (Almeida et al., 2008).
Important information on the suitability of groundwater
MATERIALS AND METHODS
Study area: This study was conducted in July and August
2009 at Pointe-Noire town. The area of Pointe-Noire
which is chosen for this study is located on the Atlantic
coast of central Africa at the south-western extremity of
Congo-Brazzaville and its geographical coordinates lie
between the meridian lines 11º30 and 12º East and
parallels 4º30 and 5º South (Fig. 1). Its surface which is
approximately about 15,660 ha spread out within a radius
of 15 km. Presenting a dominating geographical position
to which it owes its most characteristic features, the
studied area is an ideal zone of the contact oceancontinent where the effects of the marine currents and
Corresponding Author: C. Tathy, Equipe De recherche En Matériaux et Ecoulements En Milieux Poreux, ENSP, U.M.NG., B.P
69 Brazzaville, Congo
617
Res. J. Appl. Sci. Eng. Technol., 3(7): 617-624, 2011
Fig. 1: Localization of the piezometers in the study area
mass of Atlantic maritime air were announced. Annual
precipitations are relatively moderate on average 1200
mm compared to the whole of the country. The average
temperatures range between 22.2 and 28ºC with weak
thermal variations. Belonging to the coastal sedimentary
basin of cretaceous and tertiary age, the area of PointeNoire is covered by formations with age plio-pleistocene
formations other than the series of circuses (Fig. 2), made
of very permeable sands including multiple resistant
horizons. The soils of the region of Pointe-Noire are
classified in the Ferralic Arenosols group (Mareschal
et al., 2011) with a sandy texture (80-90%) on >1 m
depth. The aquifer tank of Pointe-Noire is of deep sands
type belonging to the category of the permeable layers to
semi permeable which endows it with the potentiality in
drinking water provisioning. The present study concerns
one of the two deeper aquifers, aquifer AQ-2, and the
most exploited of the region (Fig. 3).
Sampling and analysis: To characterize the chemical
composition of aquifer AQ-2, 11 piezometers of the
National Company of Water Distribution (N.C.W.D)
whose localization is shown in Fig. 1 were sampled.
Sampling was carried out during the dry season (July and
August, 2009). The hydrochemistry of groundwater was
essentially based on the determination of the basic
parameters which are pH, Temperature, Total Dissolved
Solids (TDS), Calcium (Ca2+), Magnesium (Mg2+),
Sodium (Na+), Potassium (K+), Chloride (ClG), sulphate
(SO42G), Bicarbonate (HCO3G), Total Hardness (TH) and
Total Alkalinity (TA). Determinations were performed
using procedures recommended in the Standard Methods
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Res. J. Appl. Sci. Eng. Technol., 3(7): 617-624, 2011
Fig. 2 : Cross section of the lithostratigraphic column of Pointe-Noire
for Examination of Water and Wastewater (APHA, 1995).
Calcium and magnesium (Ca2+ and Mg2+) cations were
analyzed by complexation volumetry with EDTA. Sodium
and potassium (Na+ and K+) were analyzed using flame
photometry. Bicarbonate ions (HCO3-) were analyzed by
acid-base volumetry using sulfuric acid (H2SO4) and the
chloride ions (ClG) were analyzed by argentometric
volumetry using silver nitrate (AgNO3). The
determination of sulphate was carried out by the
turbidimetric method. TDS and pH were measured using
a portable multiparameter Consort C933. Total Hardness
(TH) was calculated by the following equation (Todd,
1980):
TH = 2.497 Ca2+ + 4.115 Mg2+ (mg/L CaCO3)
(1)
Sodium Adsorption Ratio (SAR) was calculated by
the following equation given by Richard (1954) as:
SAR =
619
Na +
Ca 2+ + Mg 2 +
2
(2)
Res. J. Appl. Sci. Eng. Technol., 3(7): 617-624, 2011
Fig. 3: Vertical cross section of the multi-layer aquiferous system of Pointe-Noire
(Usunoff and Guzman-Guzman, 1989; Suk and Lee,
1999; Sing et al., 2005; Venugopal et al., 2009).
FA reduces the analytical data of each sample, which
are intercorrelated to a smaller set of factors that are then
interpretable (Ramesh Kumar and Riyazuddin, 2008).
Prior to analysis, the data are standardized by z-scale
transformation (Pejman et al., 2009). The principal
components extraction method has been used in this
procedure. The factors retained are those with an
eigenvalue > 1 (Kaiser, 1958). These factors will be easily
interpretable in terms of particular process (Lee et al.,
2001; Reghunath et al., 2002).
The Percentage of Sodium (%Na) was computed by the
equation (Todd, 1995):
% Na =
( Na
+
)
+ K + × 100
Ca 2 + + Mg 2 + + Na + + K +
(3)
The Residual Sodium Carbonate (RSC) was
determined by the formula:
RSC = HCO3G - (Ca2+ + Mg2+)
(4)
The Permeability Index (PI) was calculated according
to Doneen (1964). The following equation was employed:
PI =
Na + +
HCO3−
Ca 2 + + Mg 2 + + Na + + K +
× 100
RESULTS AND DISCUSSION
The values of the hydrochemical data of the study
area are shown in Table 1. Table 2 shows the descriptive
statistics of the hydrochemical data. The pH values of
groundwater in the aquifer AQ-2 ranged from 5.9 to 7.2.
At the sight of the variance, which is 0.14, no distinct
groupings of piezometers were observed. Total Dissolved
Solids (TDS) of groundwater varied between 41.16 to
229.10 mg/L with a variance of 5334.37. This denotes a
non-homogenous composition of the groundwater in the
aquifer AQ-2. Temperature values of the groundwater do
not exhibit significant variation and its distribution can be
considered as normal. The concentrations of cations in the
groundwater were small, 4.67-49, 0.70-60.48, 0.94-159.00
and 2.20-7.10 mg/L for Ca2+, Mg2+, Na+ and K+,
respectively. Calcium, magnesium and sodium have a
high variance, except potassium (Table 2). About the
concentrations of anions ClG, SO42G and HCO3G, these
ranged between 0.80-7.30, 2.47-35 and 15.13-61 mg/L,
(5)
The Potential Salinity (PS) was calculated according to
Doneen (1964) by the formula:
PS = Cl- + ½ SO42G
(6)
In the irrigation indexes, the concentrations of ions
are expressed in meq/L.
Statistical analysis: The factors controlling the
hydrochemistry of aquifer AQ-2 were determined from
factor analysis (Adams et al., 2001; Hajalilou and
Khaleghi, 2009). Factor Analysis (FA) is one of the
multivariate statistical analyses which differentiate the
water samples on the basis of their composition and origin
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Res. J. Appl. Sci. Eng. Technol., 3(7): 617-624, 2011
Table 1: Physico-Chemical parameters of groundwater in AQ-2 (Pointe-Noire, Congo-Brazzaville)
Piezometer pH
TDS
T
Ca2+ Mg2+
Na+
K+
ClSO42G
HCO4G
P1
6.8
41.16
26
6 .0
3.6
0.94 2.20 2.5
8.00
28.06
P2
6.7
43.12
26.2 6.2
3.48
1.45 2.41 1.9
8.61
30.02
P4
5.9
54.38
26.9 4.67 0.7
12.27 2.90 3.9
12.00
32.94
P6
6.6
47.13
26.6 19.4 0.96
16.06 5.00 1.2
9.52
15.13
P7
7.0
126.5
26.3 35.6 16.31
38.16 3.02 3.9
32.00
47.64
P8
6.6
53.21
26
11.9 0.96
2.67 3.60 1.7
6.87
28.06
P10
7.2
229.1
26.8 49.0 60.48
159
7.10 1.4
16.90
22.31
P11
7.2
140
26.8 38.0 17
35.52 3.70 4.9
35.00
61.00
P13
6.6
49.1
26.8 20.08 0.89
12.73 2.70 0.8
2.47
31.72
P14
7.0
140.8
26.8 43.0 20
65.34 6.10 7.3
8.00
46.36
P15
6.4
228.9
26
42.0 60
135.9 5.60 5.7
18.00
48.8
Table 2: Descriptive statistics of the hydrochemical data
Parameter
Minimum
Maximum
pH
5.90
7.20
TDS
41.16
229.10
T
26.00
26.90
Ca2+
4.67
49.00
2+
Mg
0.70
60.48
Na+
0.94
159.00
+
K
2.20
7.10
ClG
0.80
7.30
2
SO4 G
2.47
35.00
HCO3G
15.13
61.00
TA
12.40
50.00
TH
14.54
371.41
Mean
6.73
104.85
26.47
25.08
16.76
43.64
4.03
3.20
14.31
35.64
29.21
131.64
TA
23.00
24.61
27.00
12.40
39.05
23.00
18.29
50.00
26.00
38.00
40.00
TH
29.81
29.81
14.54
52.4
156.06
33.67
371.41
164.89
53.8
189.73
351.95
Median
6.70
54.38
26.60
20.08
3.60
16.06
3.60
2.50
9.52
31.72
26.00
53.80
Table 3: Correlation matrix among 13 water quality parameters of groundwater in aquifer AQ-2
pH
TDS
T
Ca2+
Mg2+
Na+
K+
pH
1.00
TDS
0.40
1.00
T
0.09
0.09
1.00
2+
Ca
0.61
0.90
0.28
1.00
2+
Mg
0.30
0.96
- 0.02
0.80
1.00
Na+
0.29
0.96
0.11
0.83
0.98
1.00
K+
0.32
0.77
0.28
0.78
0.76
0.84
1.00
ClG
0.08
0.47
0.08
0.48
0.32
0.30
0.29
SO42G
0.44
0.50
0.09
0.52
0.35
0.28
0.08
HCO3G
0.26
0.43
0.06
0.48
0.24
0.17
- 0.02
TA
0.26
0.43
0.06
0.48
0.24
0.17
- 0.02
TH
0.42
0.99
0.08
0.90
0.98
0.98
0.80
SAR
0.17
0.92
0.30
0.82
0.92
0.96
0.85
SAR
0.07
0.12
1.40
0.96
1.32
0.20
3.57
1.20
0.75
2.06
3.14
%Na
0.14
0.17
0.68
0.44
0.36
0.24
0.49
0.33
0.37
0.44
0.46
RSC
- 0.14
- 0.11
0.25
- 0.80
- 2.36
- 0.22
- 7.12
- 2.32
- 0.56
- 3.06
- 6.30
PI
112.21
115.31
156.67
68.42
52.99
100.41
52.2
52.34
78.12
55.77
52.29
S.D
0.38
73.04
0.37
16.81
22.69
55.11
1.66
2.11
10.48
13.57
11.13
129.42
PS
0.15
0.14
0.23
0.13
0.44
0.12
0.22
0.50
0.05
0.29
0.35
Variance
0.14
5334.37
0.14
282.52
515.05
3036.87
2.74
4.44
109.73
184.27
123.80
16748.98
ClG
SO42G
HCO3G
TA
TH
SAR
1.00
0.39
0.79
0.79
0.38
0.39
1.00
0.68
0.68
0.42
0.30
1.00
1.00
0.33
0.21
1.00
0.33
0.21
1.00
0.93
1.00
between various parameters of the groundwater of AQ-2
have been done and are presented in Table 3. Calcium
values are moderately correlated to pH, TDS is strongly
correlated to Na+, Mg2+ and Ca2+ (r = 0.90-0.96). The
major exchangeable ions Na and Ca; Na and Mg are
correlated positively. Among the anions, HCO3- shows
moderate correlation with ClG and SO42G (r = 0.79 and r
= 0.68). TH is strongly correlated with TDS (r = 0.99),
consequently with the major cations (r = 0.80-0.98).
Alkalinity exhibits high positive correlation with
bicarbonates. Sodium adsorption ratio (SAR) is strongly
correlated to TDS (r = 0.92), TH (r = 0.93) and
consequently with the major cations (r = 0.82-0.96). We
can therefore postulate that the concurrent
increase/decrease in the composition of ions in the
groundwater of aquifer AQ-2 could be due predominantly
to the result of dissolution/precipitation reaction and
concentration effects.
respectively. Sulfate and bicarbonate ions have each one
a high variance (109.73 and 184.27, respectively),
compared to chloride ions with a variance equal to 4.4.
Total Alkalinity (TA) varied from 12.4 to 50 mg/L CaCO3
with a high variance (Table 2). Groundwater in the aquifer
AQ-2 presents a low alkalinity. Total Hardness (TH)
values ranged from 14.54 to 371.41 mg/L CaCO3. TH
exhibited a high variance. This denotes a great variability
in the lithostratigraphy of the study area (Fig. 2).
Following the water hardness, Environmental Canada
(1979) classified water having 0-30 mg/L CaCO3 as very
soft, 31-60 mg/L CaCO3 as soft, 61-120 mg/L CaCO3 as
moderately soft, 120-180 mg/L as hard and > 180 mg/L
CaCO3 as very hard. In the aquifer AQ-2, water in the
piezometers P7, P10, P11, P14 and P15 are classified as
hard while water in the other piezometers are soft to
moderately soft based on TH values.
Correlation analysis: Correlation coefficient is
commonly used to establish the relation between variables
(Nair et al., 2005). In this study, correlation analyses
Factor analysis: Table 4 summarizes the sorted FA
results, including the variable loadings, eigenvalues,
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Res. J. Appl. Sci. Eng. Technol., 3(7): 617-624, 2011
Table 4: Varimax rotated factor loading matrix for groundwater chemistry data in
aquifer AQ-2
Parameter
Factor 1
Factor 2
Factor 3
Factor 4
pH
0.22
0.09
0.06
0.91
TDS
0.92
0.33
- 0.03
0.20
T
0.09
0.04
0.98
0.05
0.79
0.35
0.20
0.41
Ca2+
0.96
0.14
- 0.15
0.12
Mg2+
Na+
0.99
0.07
0.00
0.09
0.89
- 0.09
0.26
0.07
K+
ClG
0.29
0.84
0.09
- 0.19
0.18
0.64
- 0.02
0.54
SO42G
0.08
0.97
0.00
0.17
HCO3G
TA
0.08
0.97
0.00
0.17
TH
0.95
0.22
- 0.04
0.22
SAR
0.96
0.15
0.20
- 0.03
Eigenvalue
7.28
2.69
1.1
1.07
Total variance (%) 55.99
20.67
8.49
8.23
Cumulative (%)
55.99
76.66
85.15
93.38
of variance
than 10 as "excellent". With these SAR values, all
groundwater samples fall in the low sodium class (S1).
This implies that groundwater of aquifer AQ-2 is suitable
for irrigation.
Percentage of sodium (% Na): The %Na values varied
from 0.14 to 0.68. According to Wilcox classification of
groundwater on the basis of %Na (Wilcox, 1955),
groundwater of two piezometers (P1 and P2) are in the
water class "excellent" (%Na < 20); groundwater of four
piezometers (P7, P8, P11 and P13) are in the water class
"good" (20 # %Na #40); and groundwater of four
piezometers (P6, P10, P14 and P15) are in the class
"permissible" and groundwater of one piezometer (P4)
fall in water class "doubtful" in the aquifer AQ-2. The
particularity of the piezometer P4 is that groundwater
collected has the lowest concentration of Ca2+ and Mg2+.
percentage of explained variance and cumulative
percentage of variance associated with each factor.
Factor 1 account for 55.99% of the total variance and has
a high loading value of TDS, Na+ , Mg2+ , K+ , Ca2+, TH
and SAR. Factor 1 is related to the mineralization of
groundwater of which Total Hardness (TH) and Sodium
Adsorption Ratio (SAR) proceed. Factor 2 explains
20.67% of the total variance and includes HCO3G, TA and
ClG. This factor was related to alkalinity of groundwater
due to bicarbonate ions. Factor 2 shows also the
competition between anions of same valence in the
groundwater (HCO3G and ClG). Factor 3 which explains
8.49 % of the total variance includes the parameter T.
Factor 3 could explains the importance of temperature in
the solubility of some minerals which control the
chemical composition of groundwater. Factor 4 explains
8.23 % of the total variance and includes pH. Ca2+ shows
a medium correlation with SO42G (r = 0.54). Factor 4
characterized by pH could be a controlling factor of gypse
solubility in the aquifer.
Residual Sodium Carbonate (RSC): When water has
high concentration of bicarbonate ions, there is a tendency
for Ca2+ and Mg2+ to precipitate as carbonates. RSC index
characterizes this effect (Eaton, 1950). If the RSC exceeds
2.5 meq/L, water is unsuitable for irrigation. For RSC
between 1.25 to 2.5 meq/L, water is of marginal quality;
on the other hand, when the values are lower than 1.25
meq/L or negative, water is suitable for irrigation. All the
groundwater samples collected in the 11 piezometers
showed negative value of RSC, except water in the
piezometer P4 with a RSC value of 0.25 (Table 1). This
indicates that dissolved calcium and magnesium contents
were higher than bicarbonate content about the
piezometer P4. The RSC values are negative and less than
1.25 meq/L, then groundwater in aquifer AQ-2 fall in
excellent category.
Permeability Index (PI) and Potential Salinity (PS): A
long term use of irrigation water affects the soil
permeability; the parameters which are influenced are soil
type, Total Dissolved Solids (TDS), sodium as Na+ and
bicarbonate as HCO3G. The groundwater PI values ranged
from 52 to 154. According the classification of Doneen
(1964), groundwater of 2 piezometers fall in the class I, 5
piezometers in class II and 4 piezometers in class II. PS
values ranged from 0.05 to 0.50 meq/L, which are lower
than 3 meq/L, then groundwater fall in the class I
(Doneen, 1964). The results of PI and PS suggest that
groundwater is suitable for irrigation.
Groundwater quality for irrigation purposes:
Total dissolved Solids (TDS): The concentration and
composition of dissolved constituents in water determine
its quality for irrigation use. Chemical constituents i.e.,
TDS and the relative proportion of sodium to calcium and
magnesium affect water suitability for irrigation. TDS and
SAR are some important parameters for the determination
of suitability of irrigation water. The TDS contents of the
groundwater in all the groundwater samples are less than
1500 mg/L. Waters are fresh in aquifer AQ-2, therefore
TDS content is considered satisfactory. All the
groundwater samples collected in the piezometers are
considered suitable for irrigation uses according TDS
values.
CONCLUSION
Factor analysis performed on the hydrochemical data
highlighted the mineralization as a process controlling the
composition of groundwater; alkalinity, temperature and
pH are also some controlling factors. Total dissolved
solids in groundwater of aquifer AQ-2 is less than 1500
Sodium Adsorption Ratio (SAR): SAR (sodium
adsorption ratio) expresses the sodium hazard of irrigation
water. SAR varied from 0.07 to 3.57 (Table 1). Todd
(1980) classified irrigation water with SAR values lower
622
Res. J. Appl. Sci. Eng. Technol., 3(7): 617-624, 2011
mg/L and SAR values are to be less than 10. RSC values
on the whole are less than 1.25 meq/L. Percentage of
sodium and permeability index are also in acceptable
range for irrigation. Hence, one can conclude that
groundwater of aquifer AQ-2 in Pointe-Noire is suitable
for irrigation.
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ACKNOWLEDGMENT
We would like to thank the National Company of
Water Distribution (NCWD) of Congo-Brazzaville for the
authorization and the assistance that it granted to us to
work on its piezometers, as well as the International
Foundation of Sciences (IFS) for the support which it
gave to Mr. Guy Moukandi NKaya uer grant W/45071.
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