Evaluation of physico chemical quality of water stored in underground reinforced concrete tanks case study of wetlands in brazzaville congo

International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
6480(Print), ISSN 0976 – 6499(Online) Volume 5, Issue 10, October (2014), pp. 34-44 © IAEME
34
EVALUATION OF PHYSICO-CHEMICAL QUALITY OF
WATER STORED IN UNDERGROUND REINFORCED
CONCRETE TANKS: CASE STUDY OF WETLANDS IN
BRAZZAVILLE (CONGO)
Malanda N.1
, Matini L.2
, Louzolo-Kimbembe P.1, 2
1
Laboratory of Civil and Environmental Engineering, ENSP,
P.Box 69, Marien Ngouabi University. Brazzaville, Congo
2
Department of Exact Sciences, Ecole Normale Supérieure,
P.Box 69, Marien Ngouabi University, Brazzaville, Congo
ABSTRACT
The water for domestic use stored in underground reinforced concrete tanks could be altered
if the walls of the tanks are not properly sealed. The aim of this study is to know the hydro-chemical
quality of the water stored in these tanks in a wetted environment. The analyses carried out on 27
sites and for which different global parameters of the water quality are determined, showed clearly a
strong mineralization of the stored water. The study also shows an acidic nature of the aforesaid
water, the concentration in nitrite exceeding the norm of 0.1 mg/l and a strong activity of dissolved
oxygen (81.48% of tanks) revealing thus a possible contamination. The potability of this water stored
in underground concrete tanks seems doubtful.
Keywords: Water Tank, Household Waste, Hydro Chemical, Reinforced Concrete, Underground,
Pollution.
1. INTRODUCTION
Water is one of the most valuable resources of nature. As a vital substance, the potability of
water intended for human consumption must be protected against the possible effects of natural and
anthropogenic contamination. In Brazzaville, capital of Congo, considerable investments were often
devoted for building underground reinforced concrete tanks to store water for a household use. This
is to compensate for the situation of chronic water scarcity prevailing in Brazzaville [Rezaei, 2011].
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These concrete tanks are often installed in wet areas to a depth of about 2.50 meters from the ground
level and near landfills of domestic wastes. These stored water may be contaminated with dissolved
chemical elements from groundwater by diffusion through the porous walls of the tanks and thus
affect the quality of the stored water [Belkhiri et al, 2011]. Indeed, it is in the aquifer layer that are
dissolved all kinds of household wastes and so it is the main source of contamination [Moukolo et al,
1989; Moukolo et al, 2001]. Thus, it is useful to control the water tank quality for human
consumption [Das, 2013].
The statistical treatment of the data obtained on the quality of water stored in these tanks is
the subject of an ascending hierarchical classification (AHC), one of the methods that are grouped in
the multivariate statistical analyzes widely used in environmental sciences [Suk et al, 1999; Rezaei,
2011; Belkhiri et al, 2011; Soro et al, 2013].
The main objective of this study is to evaluate the hydrochemistry of this water for human
consumption and to reveal water tanks whose chemical composition is identical by using the
ascending hierarchical classification.
2. STUDY AREA
The study is performed in three districts of Brazzaville, namely Moungali, Poto-Poto and
Ouenze, defined by 4.268° to 15.254°N latitude and 4.109° to 15.907° E longitude. The average
elevation in this area is 276.44 m. From a geological point of view, the soil of Brazzaville shows the
presence of a sedimentary cover starting from secondary to tertiary and rests uncomfortably on a
Precambrian basement. The gritty series of Inkissi is considered as the sandstone of the Pan-African
resulting in this soil. The climate of Brazzaville is low Congolese or quasi tropical prevailing in the
south-western Congo. It has a dry season from June to September and a rainy season from October to
May with a slowdown of precipitation from January to February [Samba, 1978].
The average temperature varies between 25 and 26 ° C. The drainage system consists of a
few Brazzaville rivers (Tsiémé, Djoué, Mfilou, Mfoa) flowing into the Congo River. Rainfall is very
moderate with an annual average of 1.370 mm.
With an area of 270 square kilometers, the aquifer of Brazzaville is part of the
hydrogeological set Plateaux Batéké, real Congo water tower which has produced the great rivers of
the Congo (Djoué, Léfini, Niari). The limits of groundwater are defined by conditions of imposed
potentials [Mayima, 2007] which are: the Djoué River to the north, Djouari River to the west and the
Congo River to the east, and the variable flow condition along tributaries of sandstone Inkisi in
south.
The location of water tanks is shown in Appendix-A.
3. MATERIAL AND METHODS
Twenty seven (27) water tanks (WT) samples were collected (period March-May 2013) in
polyethylene bottles of 0.5 l, previously washed with hydrochloric acid, then with distilled water. In
situ, bottles were rinsed three times with water to be analyzed and filled to the brim then screw with
a plastic cap to prevent exchange with the environment. The samples were brought in a cooler at 4°C
to the laboratory for analysis. Water samples were tested for field parameters such as temperature,
pH, electrical conductivity (E.C), total dissolved solids (TDS), redox potential (ORP), dissolved
oxygen (D.O) by using a portable multi-parameter CONSORT type C933; turbidity was determined
with an electronic turbidity-meter type HACH 2100P. Calcium, magnesium, total alkalinity, total
hardness (TH), bicarbonate and sulfate contents were determined by titrimetric methods in the
laboratory. Two forms of inorganic nitrogen were analyzed by spectrophotometric methods: nitrates

36
(NO3
-
) and nitrites (NO2
-
); fluoride by SPADNS. Total iron content was analyzed by
spectrophotometric method with phenantroline after reducing of Fe3+
to Fe2+
by hydroxylamine. All
the selected parameters were determined using standard methods [APHA, 1989].
The statistical procedure used for the classification of the water tanks is the ascending
hierarchical classification (AHC) whose objective is to sort water tanks into clusters or groups. The
association is strong between members of the same group and weak between members of different
groups. The method provides intuitive similarity relationships between any one sample and the entire
dataset, and is illustrated by a dendrogram. The Euclidean distance was used as similarity distance
and Ward’s method as an analysis of variance approach for the evaluation of the distances between
clusters.
4. RESULTS AND DISCUSSION
4.1 PHYSICAL AND CHEMICAL PARAMETERS
The parameters analyzed in the water tanks samples have been compared with the drinking
water standards prescribed by WHO [WHO-a, 2004]. All the parameters are expressed in mg/l,
except pH, T (°C) and E.C (µS/cm). The details of results are presented in Appendixes-B1 and B2.
The descriptive statistics of the selected parameters are shown in Table 1.
Table (1): Descriptive statistics of selected parameters of water tank samples
Parameters Min Max Mean S.D Median Variance Skewness
pH 4.55 8.6 6.45 0.93 6.48 0.87 -0.35
T 20 20 20 0 20 0
Turb. 0.41 10.44 2.62 1.97 2.15 3.88 2.54
ORP -80.99 151.1 32.01 55.24 27.5 3050.95 0.61
E.C 22.28 1319.72 124.34 245.1 54.77 60073.35 4.8
TDS 11.98 852.4 70.22 158.81 28.8 25221.27 4.94
D.O 0.84 7.05 4.06 1.4 4.18 1.95 0.08
TA 1.7 152.79 16.97 27.94 9.93 780.64 4.74
TH 7.52 527.68 48.89 98.31 21.4 9664.93 477
T.S.S 0 5.33 1.18 1.06 1 1.12 2.38
Ca2+
2.23 170.07 15.97 31.74 6.79 1007.38 4.73
Mg2+
0.18 53.7 8.52 14.83 1.58 220.05 2.3
HCO3
-
2.08 186.16 19.58 34.35 9.98 1179.9 4.7
SO4
2-
0.11 13.42 1.97 3.12 0.85 9.74 2.96
NO3
-
0.59 258.32 19.82 49.22 5.92 2422.13 4.71
NO2
-
0.01 4.41 0.48 1.02 0.14 1.04 3.02
F-
0 1.04 0.15 0.23 0.05 0.05 2.55
Fetot 1.14 4.58 2.71 0.84 2.77 0.71 0.36
- Temperature: The temperature value of water samples of all the tanks was uniform as 20°C. This
indicates that the temperature was homogeneously distributed in the water tanks.
- pH: The value of pH characterizes the acidity (pH<7) or the basicity (pH>7) of a solution. It
represents an important indication with regard to the aggressiveness of water [Mohamed et al,
2013; Ghazalid et al, 2013]. Water tanks are characterized by pH values ranged from 4.55 to 8.6
with a mean of 6.45±0.93. Sixteen (16) samples of water tanks (59.26 % of samples) are within
the WHO limits [WHO-b, 2008] allowed for drinking water. On the other hand, eleven (11)

37
water samples (40.74 % of samples) exhibited pH values below 6.5 including 4 water samples of
which the pH was between 4.6 and 5. These 11 water samples were slightly acidic in nature.
- Electrical conductivity (EC) and total dissolved solids (TDS): EC is function of the water
temperature, it is more important when the temperature increases. It is used to appreciate the
quantity of salts dissolved in water [Pescod, 1985; Rodier, 1996]. The ability of an aqueous
solution to conduct electric current is characterized by EC which is highly correlated with TDS.
EC ranged from 22.28 to 1319.72 µS/cm with a mean of 124.34 µS/cm. The variance shows a
wide variation in the values of EC in water tanks. One sample (WT 22) has a very high value of
the electrical conductivity corresponding to the maximum value (Appendix-B1). TDS of water
tank samples ranged from 11.98 to 852.40 mg/l with a mean of 70.22 mg/l. The water tank
sample with the maximum value of EC corresponds also to the maximum of TDS. The maximum
observed in EC and TDS values can be explained by poor condition of the tank that is cracked or
high porosity of the walls and causing a sewage infiltration. On the whole the water tanks are
slightly mineralized (EC < 250 µS/cm and TDS < 125 mg/l), except the water tank WT22 which
has the maximum value of EC and TDS (Appendix-B1). This indicates the presence of high
amount of dissolved inorganic substance in ionized form [Bhattacharya et al, 2012] in water tank
WT22.
- ORP (Redox potential): The ORP value characterizes water that can be either in oxidizing
conditions or under reducing conditions. ORP varied from -80.99 to 151.10 mV with a mean of
32.01 mV. This parameter exhibit a great variance in the study area (Table 1). The variation of
ORP with water tank sample is shown in Figure 1. Some water tanks were under reducing
conditions (WT4, WT11, WT15, WT18 and WT23). The remaining water samples were under
oxidizing conditions. It is possible that the redox conditions in wetlands where water tanks are
implanted influence the redox potential of the water.
-100
-50
0
50
100
150
200
WT1
WT2
WT3
WT4
WT5
WT6
WT7
WT8
WT9
WT10
WT11
WT12
WT13
WT14
WT15
WT16
WT17
WT18
WT19
WT20
WT21
WT22
WT23
WT24
WT25
WT26
WT27
ORP (mV)
Water tanksample
ORP
Figure (1): Distribution of ORP in the water tank samples

38
- Dissolved oxygen (D.O): In the water tanks dissolved oxygen ranged from 0.84 to 7.05 mg/l with
a mean of 4.06 mg/l. Among the 27 water tanks only 18.5% had a dissolved oxygen
concentration greater than the permissible limit of 5 mg /l. This would indicate the presence of
organic matter in high concentration in the other water tanks.
- Turbidity and total suspended solids (T.S.S): The turbidity of water is mainly due to colloidal
particles and sometimes very fine dispersions [Sawyer et al, 1993]. The turbidity of water tank
samples which is correlated with suspended solids ranged from 0.41 to 10.44 NTU with a mean
of 2.62±1.97 NTU. On the whole, water tank samples have turbidity lower to 5 NTU, except the
sample WT5 with the maximum value of turbidity in the study area. Suspended solids in the
water samples varied from 0.00 to 5.33 mg/l with a mean of 1.18 mg/l. From an aesthetic point of
view, cloudy water is not accepted by consumers as drinking water.
- Total Alkalinity (TA) and total hardness (TH): In water the main ions responsible of TA are
bicarbonates (HCO3
-
) and carbonates (CO3
2-
). The ability of water to neutralize acids is
represented by total alkalinity. TA values in the water tanks samples ranged from 1.70 to 152.79
mg/l CaCO3 with a mean value of 16.97 mg/l CaCO3. On the whole the values of TA are within
the permissible level for alkalinity [WHO-a, 2004; WHO-c, 2004]. In water the ions which
mainly contribute to hardness are calcium and magnesium. Other metallic ions such as iron,
manganese, strontium and barium contribute also to total hardness [Pescod, 1985]. Total
hardness of the water tank samples ranged from 7.52 to 527.68 mg/l CaCO3 with a mean of 48.89
mg/l CaCO3. These water tanks can be classified [Sawyer et al, 1993] as: soft for 88.88% of the
samples (TH < 60 mg/l), slightly hard for WT10 and WT15 (60 – 67 mg/l) and very hard for the
sample WT22 which has the maximum value of TH (527.68 mg/l CaCO3). Excessive
mineralization of WT22 degrades water quality which is also characterized by a very high
hardness. Water in the tank WT22 may lead to deposition of scales if used in boilers and the
consumption of such a water may cause heart and kidney problems [Bhattacharya et al, 2012].
- Ca2+
, Mg2+
, HCO3
-
and SO4
2-
: The content was in the range 2.23 – 170.07, 0.18 – 53.70, 2.08 –
186.16 and 0.11 – 13.42 mg/l with mean of 15.97. 8.52, 19.58 and 1.97 mg/l, respectively. These
contents are in the permissible limit for drinking water [WHO-a, 2004].
- Nitrate (NO3
-
) and nitrite (NO2
-
): The two inorganic nitrogen ions recorded a range as follows
0.58 – 258.32 and 0.01 – 4.41mg/l with a mean of 19.82 mg/l and 0.48 mg/l, respectively. On the
whole nitrate content was in the permissible limit of 50 mg/l for drinking water, except the water
tank sample WT22 with the maximum value of 258.32 mg/l. Fourteen (14) samples as 51.85 %
of the samples presented a content of nitrite higher than the WHO permissible limit of 0.1mg/l
for drinking water. Nitrite concentration higher than the permissible limit for drinking water can
cause the methemoglobinemy to infants which is characterized by a lack of oxygen.
- Fluoride (F-
): The primary source of fluorine in water is the dissolution of fluoride bearing
minerals [Subba et al, 2003]. Its presence in water with concentration higher than the acceptable
limit of 1.5 mg/l can cause harmful effects on the human health, such as the dental and skeletal
fluorosis, adverse effects on the kidney [Mushini et al, 2012]. F-
content ranged from 0.00 to 1.04
mg/l. Although the content was lower than the WHO permissible limit of 1.5 mg/l for drinking
water. Twenty six 26 samples (96.30% of the samples) have an F-
content low than 0.6 mg/l
which can cause dental caries and poor development of bones [Pillai et al, 2002].

39
- Total iron (Fetot): Usually iron in water does not present a health problem. Iron allows the blood
to carry oxygen. The iron concentration varied from 1.14 to 4.58 mg/l. On the whole the
concentration was higher than the permit limit of 0.3 mg/l for drinking water. Higher
concentration of iron in water can cause yellow, red, or brown stains on laundry and aesthetic
problem to consumers [WHO-d, 2004].
4.2. ASCENDING HIERARCHICAL CLASSIFICATION (AHC)
The AHC on the physico-chemical parameters considering water tanks as objects is presented
in Figure 2. AHC allows identifying natural grouping in the water tanks. The dendrogram shows
three clusters or groups. Group I contains the water tanks WT1 – WT3, WT5 – WT10, WT16, WT17
and WT24 – WT27 as n = 14. Group II (n = 12) contains WT4, WT11 – WT15, WT18 – WT21,
WT23 and WT27. Group III contains the water tank WT22 which has the higher values of the
parameters, except pH, T, ORP, Turb, D.O, Fetot and NO2
-
.
Figure (2): Dendrogram showing the clustering of water tanks based on physico-chemical
parameters
The discriminant parameters between group I and group II are highlighted in Figure 3 which
compares the median value of the parameters. These parameters are: EC, TDS, TH, Alk, Ca2+
, Mg2+
,
HCO3
-
, SO4
2-
, F-
and NO3
-
. Thus the content of the parameters increases as: Group I < Group II <
Group III.

40
Figure (3): Distribution of the median values of the parameters in groups I and II
5. CONCLUSION
The physico-chemical study of the water stored in the concrete tanks has showed an acidic
nature in 37% of the water tanks. The nitrite concentrations exceed the acceptable limit of 0.1 mg/l
for the drinking water in all the analyzed water samples, which is a risk to the infants whom can
consume this water. The low oxygen content in the majority of the WT (81.48 % of samples)
indicates a strong bacterial activity in water. The high concentrations of iron, higher than the
acceptable limit can pose an aesthetic problem. So this water appears strongly mineral-bearing, their
potability is made doubtful in the near total of the analyzed samples, which lets predict a danger to
the health of the consumers. The seepage waters of the domestic discharges in the ground water
would be thus a source of contamination of water stored in the tanks. The technological aspect of the
waterproofness of the walls in the concrete tanks under these conditions has weaknesses and lack of
reliability. The potability of this water becomes doubtful. It would be thus dangerous to use this
water for drinking water.
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Appendix-B1: Values of physico-chemical parameters of water stored in the underground
tanks and collected from different sites
Average arithmetic mean of the values of the parameters
Code pH T (°C)
ORP
(mV)
TDS
(mg/l)
EC
(µS/cm)
TA (mg/l) TH (mg/l)
TSS
(mg/l)
WT1 6.554 20 20.06 24.12 37.62 15.048 15.526 1.2
WT2 5.97 20 57.338 19.42 36.32 7.64 11 2.07
WT3 6.686 20 14.198 19.58 35.66 11.876 10.362 1.2
WT4 8.604 20 -80.992 38.48 72.58 18.764 17.326 0.2
WT5 6.298 20 36.94 13.96 26.42 7.91 10.896 0.8
WT6 6.468 20 28.32 11.98 22.28 8.446 8.852 0.8
WT7 6.388 20 32.76 25.98 49.3 9.664 17.48 1.6
WT8 6.83 20 27.47 17.73 33.43 7.30 10.84 5.33
WT9 6.98 20 2.164 23.59 44.74 20.432 34.276 1.55
WT10 6.304 20 34.76 14.62 27.58 7.508 9.378 0.80
WT11 7.386 20 -35.04 99.64 187.76 21.112 67.302 0.550
WT12 4.72 20 139.272 52.518 98.96 4.332 32.276 1.650
WT13 5.912 20 59.180 55.940 105.42 13.452 32.584 1
WT14 4.9825 20 113.325 71.2 133.45 4.265 63.7725 0.625
WT15 7.294 20 -12.36 77.26 146.42 23.748 60.548 2.2
WT16 6.453 20 27.50 28.80 54.767 14.960 21.10 1.523
WT17 6.974 20 -2.1 28.542 53.92 18.996 21.4 0.6
WT18 7.28 20 -21.13 68.30 128.37 11.91 50.80 1.33
WT19 4.55 20 148.83 40.28 76.33 1.70 30.90 0
WT20 6.9 20 19.7 41.58 78.904 18.926 32.96 2.412
WT21 6.302 20 35.92 36.532 144.48 5.386 56.48 0.5
WT22 6.486 20 25.54 852.4 1319.72 152.792 527.68 0.65
WT23 7.826 20 -50.536 53.31 101.164 25.344 40.08 0
WT24 6.592 20 17.818 28.4 53.92 9.934 21.12 1.6
WT25 6.294 20 42.742 12.68 24.504 8.054 7.522 1.1
WT26 6.476 20 31.492 14.532 27.144 6.554 10.96 0.55
WT27 4.57 20 151.10 124.50 236 2.25 97.10 0
Caption :
pH : Potential hydrogen
T : Temperature
ORP: Redox potential
TDS: Total dissolved solids
EC: Electrical conductivity
TA: Total alkalinity
TH: Total hardness
TSS: Total suspended solids

44
Appendix-B2: Values of physico-chemical parameters of the water stored in the underground
tanks and collected at different sites
Average arithmetic mean of the values of the parameters
Code
Mn2+
mg/l
Ca2+
mg/l
Mg2+
mg/l
NO3
-
mg/l
SO4
2-
mg/l
F-
mg/l
D.O.
Fetot
mg/l
CN-
mg/l
HCO3
-
mg/l
NO2
-
mg/l
GM
mg/l
WT1 0.0122 5.724 13.052 2.424 0.584 0.004 2.54 1.382 0.0002 20.706 0.2008 44.874
WT2 0.0376 3.076 0.214 1.474 3.218 0 2.49 1.902 0.0022 6.74 0.0162 39.868
WT3 0.0236 3.392 0.184 2.062 0.332 0.004 2.746 1.139 0.0164 9.976 0.2074 42.26
WT4 0.0346 4.244 14.496 3.102 1.9360 0.0562 2.546 1.992 0.0008 19.02 0.2122 55.494
WT5 0.0198 3.492 16.854 0.588 2.038 0.2 0.840 1.53 0.0002 5.456 0.0112 29.13
WT6 0.008 2.616 12.994 1.4614 10.912 0.002 2.480 1.85 0.0002 7.764 0.0052 24.926
WT7 0.0436 5.256 35.762 2.488 3.064 0.006 2.538 1.944 0.0002 7.212 0.2228 45.782
WT8 0.00 3.70 0.30 1.38 0.90 0.01 7.050 4.52 0.00 8.90 0.3400 44.81
WT9 0.0426 12.438 0.792 9.56 0.8766 0.19 4.176 2.772 0.0038 21.532 0.0240 48.438
WT10 0.0166 3.076 0.42 1.1522 0.532 0.00 4.136 2.774 0 9.156 0.1286 62.486
WT11 0.104 23.100 1.462 28.458 2.794 0.440 3.482 2.428 0.016 25.752 0.358 153.33
WT12 0.0856 11.012 50.698 17.432 0.460 0.038 3.678 2.560 0.0028 3.336 2.436 97.15
WT13 0.0314 11.642 0.868 10.1916 0.852 0.0752 4.730 3.504 0.004 16.408 0.0966 92.532
WT14 0.0545 23.4125 1.3075 31.93 2.535 0.17775 4.3475 2.8175 0.00575 5.2 0.275 110.09
WT15 0.0854 19.4 2.28 19.674 3.160 0.2948 4.398 2.828 0.0118 28.970 0.190 121.61
WT16 0.117 6.777 1.033 3.250 0.847 0.009 5.673 3.760 0.001 18.247 0.031 37.260
WT17 0.0214 6.214 0.658 1.316 0.41 0.0098 6.924 4.58 0 23.17 0.0202 54.628
WT18 0.06 16.01 2.70 20.63 1.02 0.16 3.37 2.26 0.01 14.48 4.410 120.66
WT19 0.04 9.74 1.64 15.47 0.40 0.16 4.52 2.86 0.01 2.08 0.14 71.76
WT20 0.03 10.676 7.844 7.434 0.323 0.0478 4.802 2.944 0.0066 23.084 0.0812 71.856
WT21 0.176 18.414 2.6768 27.074 0.667 0.368 4.328 3.422 0.014 6.57 0.268 135.812
WT22 0.62 170.072 53.698 258.324 13.422 1.0364 3.628 2.506 0.0506 186.16 2.624 1134.864
WT23 0.0892 13.496 1.582 7.8076 0.392 0.1596 4.74 2.742 0.006 30.912 0.0752 95.09
WT24 0.0314 6.786 1.038 5.924 0.258 0.047 5.218 3.148 0.004 12.12 0.0588 57.008
WT25 0 2.23 0.484 1.8354 0.1096 0 4.102 2.508 0 4.964 0.0166 33.324
WT26 0 3.476 0.554 2.336 0.136 0 5.484 3.424 0 7.992 0.0204 36.914
WT27 0.30 31.79 4.42 50.27 1.11 0.56 4.76 2.96 0.03 2.73 0.53 191.69
Caption :
Mn2+
: Manganese
Ca2+
: Calcium
Mg2+
: Magnésium
NO2
-
: Nitrite
NO3
-
: Nitrate
SO4
2-
: Sulfate
F-
: Fluorure
D.O. : Dissolved oxygen
Fetot : Total iron
CN-
: Cyanid
HCO3
-
: Bicarbonate
GM : General mineralisation

Evaluation of physico chemical quality of water stored in underground reinforced concrete tanks case study of wetlands in brazzaville congo

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