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Advances in Social Sciences Research Journal – Vol. 11, No. 2
Publication Date: February 25, 2024
DOI:10.14738/assrj.112.16198.
Hebie, A., Kafando, S., & Nakolendousse, S. (2024). Hydrochemical Characterisation of Groundwaters in a Crystalline Basement
Environment: The Case of Fractured Aquifers in the Poni Watershed in the South-West Region of Burkina Faso, West Africa.
Advances in Social Sciences Research Journal, 11(2). 582-597.
Services for Science and Education – United Kingdom
Hydrochemical Characterisation of Groundwaters in a Crystalline
Basement Environment: The Case of Fractured Aquifers in the
Poni Watershed in the South-West Region of Burkina Faso, West
Africa
Hebie, Adama
Kafando, Sayoba
Nakolendousse, Samuel
ABSTRACT
The Poni watershed is located in the south-west of Burkina Faso. Its geology is made
up of fractured crystalline and crystallophyllian basement, where fractured
aquifers are the most exploited for water needs. The aim of this study is to
determine the physico-chemical characteristics of the groundwater in the fractured
aquifers of the Poni watershed in order to gain a better understanding of the
mineralisation processes and hydrochemical properties of the groundwater in
these aquifers. To do this, approaches based on the determination of Calcite
Saturation Indices (CSI) and Dolomite Saturation Indices (DSI), Principal
Component Analysis (PCA) and hydrochemical properties show that the majority of
groundwater in the basin is undersaturated with respect to carbonates (79.37%)
and divided into three families in relation to circulation speed: very slow
circulation water (20.63%), slow circulation water (55.56%) and fast circulation
water (23.81%). The approaches also show that the mineralisation of groundwater
in the basin is governed by the acid hydrolysis of minerals in the surrounding rocks,
residence time and surface inputs. They are characterised by calcic and magnesian
bicarbonate facies (85.7%), calcic bicarbonate facies (6.35%), sodium and
potassium bicarbonate facies (1.59%) and calcic and magnesian sulphate chlorides
(6.35%).
Keywords: Fractured aquifer, Hydrochemistry, Hydrolysis of minerals, Surface input,
Poni watershed
INTRODUCTION
Groundwater resources play a key role in Burkina Faso's drinking water supply policy.
Groundwater resources account for 85% of the country's water supply (Derouane et al., 2006).
This groundwater supply rate can be explained, on one hand, by the lack of sustainability of
surface water and, on the other hand, by the high cost of mobilising and processing surface
water. Groundwater is under ever increasing threat in terms of both quality and quantity, due
to demographic pressures and certain agricultural, livestock farming and gold mining practices.
Natural and man-made pollution seems to be hampering the supply of drinking water in certain
localities. Several studies (Yaméogo, 2008; Lasm et al, 2008; Barry, 2016; Hébié, 2019) have
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Hebie, A., Kafando, S., & Nakolendousse, S. (2024). Hydrochemical Characterisation of Groundwaters in a Crystalline Basement Environment: The
Case of Fractured Aquifers in the Poni Watershed in the South-West Region of Burkina Faso, West Africa. Advances in Social Sciences Research
Journal, 11(2). 582-597.
URL: http://dx.doi.org/10.14738/assrj.112.16198
highlighted the problem of groundwater pollution by organic and mineral substances.
Understanding the hydrochemical characteristics of groundwater remains a necessity for
drinking water supply and is essential for good integrated management of these resources. In
the Poni watershed, groundwater is the main source of drinking water. This source of drinking
water is under increasingly threat from the use of chemical fertilisers and pesticides in
agriculture, the increase in gold mining practices and the demographic weight of the
population, all of which are potential sources of deterioration in its quality. In this context, it is
necessary to study the hydrochemical characterisation of the groundwater in the fissured
aquifers of the Poni catchment area. The aim of this study is to determine the physico-chemical
characteristics of the groundwater in the fractured aquifers of the Poni watershed, based on
the processing of the results of analyses of physico-chemical parameters, in order to provide a
better understanding of the mineralisation processes and hydrochemical properties.
PRÉSENTATION OF THE STUDY AREA
The Poni watershed is a sub-basin of the Mouhoun watershed, located between longitudes
3°45' and 2°55' West and latitudes 9°50' and 10°36' North. It is drained by the Poni river over
an area of 5,453 km2. The sudan-type climate (900 to 1200 mm) is characterised by two distinct
seasons: a rainy season lasting 5 months and a dry season lasting 7 months, with temperatures
fluctuating between 16.32°C in December and 38.08°C in March. Topographically, the
watershed has a very uneven relief, with altitudes ranging from 221m to 580m, marked by a
succession of hills and topographical depressions, with a geomorphology dominated by a
functional pediment crowned at the southern end by low-lying areas. It is mainly made up of
plateaux with an average altitude of around 350m (Figure 1).
Figure 1: location map of the Poni watershed area (source data: image SRTM 2019 and BNDT
2015).
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Advances in Social Sciences Research Journal (ASSRJ) Vol. 11, Issue 2, February-2024
Services for Science and Education – United Kingdom
Geological and Hydrogeological Context
From a lithological point of view, three series of formations (volcanic, volcano-sedimentary and
sedimentary) cover the Poni watershed. The volcanic and volcano-sedimentary series are
essentially made up of various types of basalt and andesite intruded by microgranites,
microdiorites, rhyolites and dacites. The sedimentary series is composed of cherts, greywackes
and interlayered graphitic rocks. All of these series are intruded successively by granitoids,
microgabbros and microdiorites. (Ouiya, 2020)
Structurally, three types of structures, oriented regionally NE-SW and locally NW-SW, affect the
formations in the Gaoua region. (Naba, 2010 and Baratoux et al., 2011). There are also shear
zones with mylonitic deformation (Ouédraogo, 1986) and fractures of various orientations
intersect the regional structures and shear zones (Ouiya, 2020). The watershed is rich in mining
potential, with gold, diamond and copper showings. According to a map of gold panning sites
drawn up by the Mouhoun Water Agency in 2018 (AEM, 2018), seventy-five (75) sites have
been identified in the Poni watershed (Figure 2).
Figure 2: Geological and mining map of the Poni watershed (Data source: BUMIGEB)
The hydrogeology of the Poni watershed is characterised by two types of aquifers: alterite
aquifers and fracture aquifers. These two types of aquifers are often superimposed and closely
linked by drainage (Savadogo, 1984). The fractured aquifers are the ones most used by the local
population. In terms of hydrochemistry, the catchment area has never been studied, apart from
the occasional analysis of water during drilling. This study is therefore a godsend in terms of
gaining a better understanding of the hydrochemical characteristics of the groundwater in the
fissured aquifers in this watershed.
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Advances in Social Sciences Research Journal (ASSRJ) Vol. 11, Issue 2, February-2024
Services for Science and Education – United Kingdom
Determination of Calcite Saturation Index (CSI) and Dolomite Saturation Index (DSI) in
Groundwater
The calcite saturation index (CSI) and the dolomite saturation index (DSI) are used to
determine the relative residence time of groundwater. The CSI and DSI indices can be calculated
using the equations:
ISC=pkca -pk2+ log [Ca2+] + log [HCO3-] + real pH or ISC= real pH - calcite equilibrium pH
ISD= (pkdol-2pk2+2 log [HCO3-] + log [Ca2+] + log [Mg2+]) /2 + real pH or
ISD= actual pH- dolomite equilibrium pH
According to (Lasm et al; 2011), the character of water is determined as a function of the
variation between real pH and equilibrium pH, which corresponds to either the calcite or
dolomite saturation index, and three situations can arise: when ∆pH= real pH - equilibrium pH
= 0, the water is in equilibrium; ∆pH > 0, waters are said to be supersaturated with respect to
calcite and ∆pH < 0, waters are undersaturated with respect to calcite.
The DSI = f(CSI) diagram is used to classify waters into three families according to their
circulation speed: very slowly circulating waters (CSI> 0), slowly circulating waters (-1.5< CSI<
0) and rapidly circulating waters (CSI <-1.5). Each family is characterised by well-defined
properties (Lasm et al; 2011).
Statistical Data Processing
The statistical processing consisted of classical statistics and Principal Component Analysis
(PCA). PCA is a multidimensional descriptive method used as a tool for interpreting a matrix of
data. It is used to highlight relationships between variables (chemical parameters) in order to
group together those with similar behaviour (Travi and Mudry, 1997).
Several tools were used to process the results of the physico-chemical analyses and produce
the maps, the main ones being mapping software such as Arc Gis 10.5 and QGIS 3.16.16,
statistical software such as IBM SPSS Statistics 20 and EXCEL 2016 spreadsheet software, and
Diagramme 6.77 hydrochemistry software.
Following these processing operations, sixty-three (63) water samples were selected for the
study. The spatial representation of the water samples analysed is shown in the figure below.
(Figure 3)
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Hebie, A., Kafando, S., & Nakolendousse, S. (2024). Hydrochemical Characterisation of Groundwaters in a Crystalline Basement Environment: The
Case of Fractured Aquifers in the Poni Watershed in the South-West Region of Burkina Faso, West Africa. Advances in Social Sciences Research
Journal, 11(2). 582-597.
URL: http://dx.doi.org/10.14738/assrj.112.16198
Figure 3: Distribution map of boreholes studied (source: BUMIGEB and DGRE
RESULTS
Ionic Balance
The results of the ionic balance of the water samples analysed show that out of the 102 samples,
63 had an ionic balance of between -10% and +10%, meaning that 61.76% of the water samples
collected had reliable analysis results.
Analysis of Physical Parameters
The results of the physical parameter analyses are shown in Table 1.
Table 1: Descriptive statistics for physical parameters
Variables Number of
samples
Minimum Maximum Mean Standard
deviation
VC (%)
PH 63 5.76 7.95 6.77 0.49 7.21
TH (°F) 63 0.80 72.56 22.88 19.77 86.42
Conductivity (μS/cm) 63 6.22 732.00 288.51 191.87 66.50
The pH values ranged from 5.76 to 7.95, with an average of 6.77. Analysis shows that 66.66%
of water samples have an acid pH and 33.33% a basic pH.
The conductivity of groundwater in the Poni watershed varies between 6.22 and 732 μS/cm,
with an average of 288.51 μS/cm and a standard deviation of 191.87 μS/cm.
The hardness of groundwater in the Poni watershed fluctuates between 0.8 °F and 72.56 °F
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Advances in Social Sciences Research Journal (ASSRJ) Vol. 11, Issue 2, February-2024
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with an average of 22.88 °F. 22.22% of the water has low hardness (TH ˂ 10°F), 53.97% has
normal hardness (10°F ˂ TH ˂ 30°F) and 23% is hard (TH ˃ 30°F).
The alkalinity of groundwater in fractured aquifers in the Poni watershed ranges from 0.88 to
41.2°F, with an average of 17.25°F (Figure 4).
Figure 4: distribution of physical parameters analysed
Calcite Saturation Index (CSI) and Dolomite Saturation Index (DSI) in Groundwater
CSI values for groundwater in the Poni catchment fluctuate between -3.71 and 0.67 and DSI
values between -3.64 and 0.58. The CSI and DSI values of 79.37% of the waters in the basin are
negative, and 20.63% of the waters have positive CSI and DSI.
The diagram of DSI versus CSI shows that 20.63% of the waters in the basin have very slow
flow rate, 55.56% have slow flow rate and 23.81% have very slow flow rate. (Figure 5.)
Figure 5: graph DSI = f(CSI)