Groundwater Vulnerability in the Kou Sub-Basin, Burkina Faso: A Critical Review of Hydrogeological Knowledge

Abstract

Groundwater resources in the Kou sub-basin of southwestern Burkina Faso play a critical role in supporting domestic water supply, agriculture, and industry in and around Bobo-Dioulasso, the second-largest city in Burkina Faso. This study synthesizes over three decades of research on groundwater vulnerability, recharge mechanisms, hydrochemistry, and residence time across the region’s sedimentary aquifers. The Kou basin hosts a complex stratified system of confined and unconfined aquifers, where hydrochemical analyses reveal predominantly Ca–Mg–HCO₃ facies, alongside local nitrate (0–860 mg/L), iron (0–2 mg/L) and potassium (<6.5 mg/L–190 mg/L) contamination. Vulnerability assessments—using parametric (DRASTIC, GOD, APSU) and numerical (MODFLOW/MT3D) models—consistently indicate moderate to high vulnerability, especially in alluvial and urban/peri-urban areas. Isotopic results show a deep recharge for a residence time greater than 50 years with deep groundwater dating from 25,000 to 42,000 years. Isotopic data confirm a vertically stratified system, with deep aquifers holding fossil water and shallow units showing recent recharge. Recharge estimates vary significantly (0–354 mm/year) depending on methodology, reflecting uncertainties in climatic, geological, and anthropogenic parameters. This review highlights major methodological limitations, including inconsistent data quality, limited spatial coverage, and insufficient integration of socio-economic drivers. To ensure long-term sustainability, future work must prioritize high-resolution hydrogeological mapping, multi-method recharge modeling, dynamic vulnerability assessments, and strengthened groundwater governance. This synthesis provides a critical foundation for improving water resource management in one of Burkina Faso’s most strategic aquifer systems.

1. Introduction

Groundwater represents the largest share of accessible freshwater globally and plays a major role in sustaining human activities [1]. In 2010, global groundwater withdrawals were estimated at approximately 1000 km³, allocated among irrigation (67%), industrial use (11%), and domestic consumption (22%). These withdrawals account for 25% of the total volume of freshwater used worldwide and approximately 10% of the renewable groundwater resources [2,3]. As such, groundwater is of critical importance to economic development, particularly in light of ongoing climate change and its associated impacts [1]. Consequently, improving the understanding of groundwater systems is essential—not only to optimize their multiple uses but also to ensure effective and sustainable resource management [4,5]. Moreover, both the quality and quantity of groundwater are often significantly influenced by anthropogenic activities [6]. For instance, localized nitrate contamination is sometimes attributed to agricultural zones where nitrogen-enriched fertilizers are used [6]. In addition, groundwater quality is also affected by iron concentrations. Although iron is naturally present in the environment, human activities—such as the discharge of organic matter—can alter the redox conditions, thereby affecting iron mobility and concentration in groundwater [7].

In the West African Sahelian context, characterized by an arid climate and relatively high population density, the pressure on groundwater resources is particularly acute [8]. The availability and sustainability of groundwater depend on a variety of factors including climate [9,10], urban expansion [11], and geological conditions [12]. Groundwater systems in this region are thus vulnerable to low rainfall, high evapotranspiration, and the widespread presence of low-permeability rock formations [13]. These challenges are particularly pronounced in Burkina Faso, where both physical and climatic conditions are generally unfavorable to groundwater recharge [14].

In addition, this natural vulnerability is often intensified by anthropogenic activities, most notably agriculture, where the use of pesticides poses a risk of groundwater contamination [15]. Several regions in the country rely heavily on groundwater for drinking water, thereby increasing their exposure to both depletion and degradation of the resource. In some of these areas, agricultural practices have intensified, further raising the likelihood of groundwater pollution [16].

Effective vulnerability assessment requires access to key data such as recharge rates, geological structures, and aquifer properties [17]. However, there remains a significant lack of reliable data on these topics in Burkina Faso, a landlocked country within West Africa, particularly regarding recharge processes, junction zones between bedrock and sedimentary aquifers, and the geometry of aquifers within the sedimentary domain [18]. This gap is especially problematic in the western sedimentary region, which holds strategic importance for the country. In fact, the sedimentary aquifers in this region are the source of many important rivers, including the Mouhoun and the Comoé, which are extensively used downstream for potable water supply, agriculture, and industry.

The concept of vulnerability is closely related to that of risk. Risk is defined as the probability of an event occurring, although the event remains unrealized [19]. It lies at the intersection of two elements: hazard and vulnerability [16,20,21]. Hazard is perceived as a potentially destructive event that is evaluated based on a combination of exposure and harmful effects [22]. Vulnerability, on the other hand, refers not only to actual damage but also to the potential to suffer damage [19]. It encompasses three core dimensions: weakness, sensitivity, and fragility [23].

The literature typically distinguishes between two major types of vulnerability [16,17,24,25]. First, intrinsic vulnerability refers to the natural physical characteristics of an environment, considering its susceptibility to contamination. This type of vulnerability is dependent on the nature of the contaminant and the hydrogeological features of the area. According to Oke [17], assessing intrinsic vulnerability involves evaluating the protective capacity of geological formations against the introduction and transport of pollutants into groundwater [17,25,26,27]. Second, specific vulnerability integrates both natural parameters and the specific properties of contaminants [17,24,25]. It is sometimes referred to simply as “vulnerability” or as part of a risk assessment. This type of vulnerability is evaluated by combining the intrinsic vulnerability of a given area with the potential for contaminant release from human activities. Depending on the resilience or resistance of the geological environment, vulnerability may be exacerbated or reduced according to the physicochemical properties of the contaminant. Specific vulnerability is therefore directly related to the risk of contamination. While intrinsic vulnerability tends to remain constant over time, specific vulnerability is dynamic and evolves with the presence and intensity of pollution sources [17,24,25].

In the specific context of Burkina Faso, relatively few studies have addressed groundwater vulnerability. This scarcity can be attributed to several factors, including a lack of data and diverging opinions on groundwater management. The present study aims to fill this gap by synthesizing existing information and studies related to vulnerability. The focus is on the Kou sub-basin, located in the western part of the Taoudéni sedimentary basin—a region of strategic importance in terms of groundwater resources [18,28]. The Kou sub-basin stands out not only for its geological complexity and hydrogeological significance, but also because it serves as a critical water supply zone for the urban center of Bobo-Dioulasso, the second-largest city in the Haut-Bassins administrative region in Burkina Faso and its surrounding agricultural and industrial areas. Its sedimentary aquifers play a pivotal role in supporting both ecosystem functions and socio-economic development. Yet, this region remains under-documented in terms of vulnerability mapping, recharge dynamics, and pollution risk assessment. A focused review on the Kou sub-basin is therefore critical to improve scientific understanding and support evidence-based water resource management in one of Burkina Faso’s most vital hydrological units.

2. Study Area

2.1. Location

The Kou sub-basin is situated in the province of Houet (Figure 1), in the southwestern region of Burkina Faso, and covers an area of approximately 1823 km² [29]. According to the 2019 national census [30], the population of Houet was estimated at 1,510,638 inhabitants, distributed across 310,022 households. The administrative center of the province is the municipality of Bobo-Dioulasso, the economic capital city of the country. The sub-basin primarily encompasses three administrative departments: Bama, Bobo-Dioulasso, and Péni [31,32,33]. However, according to the Mouhoun Water Agency, the boundaries of the sub-basin extend beyond these three departments to also include the municipality of Karangasso-Sambla [34].

2.2. Climate

The climate of the Kou sub-basin is of the Sudanian type [29], characterized by a distinct dry season and a rainy season. Annual rainfall in the region generally varies between 600 mm and 1500 mm (Figure 2), depending on interannual climatic variability [35,36,37]. More recent mapping efforts indicate a narrower precipitation range, from 750 to 1150 mm [31]. In contrast, potential evapotranspiration (PET) values are relatively high, ranging from 1729 mm to 2392 mm [38], which contributes to a significant water deficit. Temperature fluctuations in the basin vary seasonally, with average values between 25 °C and 30.5 °C [38].

2.3. Relief, Land Use/Land Cover, and Soils

Topographically, the Kou sub-basin is characterized by a sandstone plateau, bounded to the east by the Banfora Escarpment, which rises less than 200 m above the surrounding terrain [38,39]. In the southern portion of the sub-basin, the landscape becomes more rugged, marked by hills and rocky outcrops ranging in elevation from 400 to 670 m [39]. Slopes in these areas can reach gradients of 15% to 30%, although much of the basin is dominated by low-gradient terrain (0–2%), particularly in the central and northern floodplains [31]. These low-lying areas are prone to seasonal flooding [38,39].

Land cover within the basin includes patches of forest and natural vegetation, mainly composed of wooded and shrubby savannahs [39]. In addition, the urban area of Bobo-Dioulasso and its peri-urban surroundings represent significant artificialized zones. Agricultural land is widely distributed, with irrigated perimeters located on the alluvial plain, alongside seasonal rainfed croplands. Surface water bodies such as the Kou River, floodplains, and areas of bare or eroded soils also occupy a substantial portion of the basin. In 2023, the Mouhoun Water Agency produced an updated land use map (Figure 3), underscoring the central role of land occupation in determining groundwater vulnerability. The current land use distribution within the sub-basin is as follows: rainfed agricultural lands, 1279 km²; artificial territories (urban and industrial zones), 201 km²; forests and semi-natural environments, 168 km²; protected areas, 80 km²; irrigated perimeters, 78 km²; open areas with or without vegetation, 54 km².

Soil classification identifies five dominant soil types: poorly developed soils, vertisols, tropical ferruginous soils, ferralitic soils, and hydromorphic soils [38] in the Kou Sub-basin (Figure 4).

2.4. Hydrology

The Kou sub-basin forms part of the larger Volta River Basin, a transboundary watershed that spans several West African countries, including Mali, Burkina Faso, Côte d’Ivoire, Ghana, and Togo [31,39]. Within Burkina Faso, the Volta Basin is primarily drained by the Mouhoun and Nakanbé rivers. One of the major tributaries of the Mouhoun is the Kou River, which plays a central hydrological role in the sub-basin.

The Kou River (Figure 5) is a perennial watercourse with a total length of approximately 70 km. It originates at an altitude of 500 m in Kodara, located within the commune of Péni. The river is fed by multiple springs, among which the Nasso/Guinguette and Pesso/Desso springs are the most significant. The Nasso/Guinguette spring is particularly noteworthy, as it exhibited the highest discharge among the springs, with a flow rate of 6000 m³/h recorded in 2016 [39].

The hydrographic network in the sub-basin is relatively dense and plays a vital role in both surface and groundwater interactions. These water bodies serve not only as drainage structures but also as recharge and discharge zones for the underlying aquifers. The permanent flow of the Kou River and its tributaries reflects the contribution of baseflow from the aquifers, particularly during the dry season, when surface runoff is minimal.

The dynamics of the Kou River sustain various socio-economic activities downstream, including domestic water supply, agriculture, and industry. Moreover, the hydrological regime of the river is directly influenced by climatic variability and land use changes, which can modify both runoff patterns and aquifer recharge rates. Therefore, the Kou River serves as a hydrological backbone of the sub-basin, linking surface processes with subsurface storage and flow mechanisms.

2.5. Geology

The Kou sub-basin lies within the Taoudéni sedimentary basin, which extends across western Burkina Faso and comprises nine major geological formations. Among these, five are specifically present within the Kou valley: the Kawara-Sindou Sandstones (GKS), the Fine Glauconitic Sandstones (Gfg), the Quartz Granule Sandstones (Ggq), the Siltstones, Argillites, and Carbonates of the Guena-Souroukoundinga Formation (Sac1), and the Fine Pink Sandstones (Gfr) [39,40,41,42].

In addition to these primary sedimentary units, a range of superficial formations are also observed across the basin. While the study by SOGREAH [42] treats these as distinct geological formations, Sauret [38] considers them collectively as surface deposits. These include lateritic crusts, weathered formations, and alluvial deposits. Notably, dolerites and gabbro-doleritic intrusions are considered by some authors [39,43] to represent an independent formation, rather than being subsumed under superficial deposits.

A significant advancement in geological understanding came from the BUMIGEB study conducted in 2018 [44], which produced a more detailed lithostratigraphic map at a scale of 1:200,000—an improvement over the earlier 1:1,000,000-scale representations (Figure 6). This updated mapping effort refined the characterization of the subsurface units and proposed a revised stratigraphy based on field observations, lithological analysis, and structural interpretation.

The revised geological framework includes a complex system of doleritic and gabbroic intrusions, often occurring as sills and subhorizontal dikes, some of which contain magnetite and quartz. Within the group known as the “Falaise”, the Takalédougou Formation is composed of fine glauconitic sandstones, while the Kawara-Sindou Formation features coarser sandstones and conglomerates. A lower sandstone series, referred to as the “Lower Sandstone Formation”, contains silt-rich sandstone layers. Further, the group of formations associated with the Bobo structural unit includes various fine-grained lithologies such as the “Passe de Fo” formation, which consists of siltstones and quartzitic sandstones; the Samandéni-Kiébani Formation, composed of siltstones, argillites, and carbonates; the Bonvalé Formation, formed by fine pink sandstones; and the Guena-Souroukoundinga Formation, also dominated by siltstones, argillites, and carbonates [44].

This refined lithological and structural interpretation of the Kou sub-basin highlights its stratigraphic heterogeneity and the complexity of its sedimentary architecture. Such geological variability plays a key role in shaping the hydrogeological behavior of the basin, influencing not only aquifer distribution and hydraulic connectivity but also groundwater flow paths, recharge mechanisms, and contaminant transport processes.

2.6. Hydrogeology

The hydrogeological framework of the Kou sub-basin has been defined primarily through studies conducted by SOGREAH [42]. These investigations suggest that the basin hosts a single, continuous aquifer system, with groundwater flow predominantly oriented from the southwest to the northeast [43]. This conceptual model was adopted due to the limited availability of lithological data and the lack of deep geophysical surveys or reliable piezometric measurements. As such, the actual number and extent of aquifers remain uncertain.

The apparent absence of impermeable layers between the various geological units—coupled with their lithological similarity, particularly among the sandstones and sandy deposits—supports the hypothesis of a continuous, stratified sedimentary aquifer system, likely lacking significant clay barriers [43]. Furthermore, hydrochemical data show an evolution of the groundwater facies from calcium-magnesium bicarbonate upstream to more bicarbonate-enriched waters downstream. This trend is consistent with the interpretation of a single, interconnected aquifer system in which water mixing occurs along the flow path.

Residence time estimates also support this interpretation. Groundwater in the fine sandstones exhibits relatively short residence times of about 50 years, whereas water stored in siltstones and argillaceous formations may have residence times exceeding 4000 years. This range reflects slow, continuous flow throughout the basin, reinforcing the concept of a regional, multilayered, yet hydraulically connected aquifer. The lack of evidence for significant compartmentalization or substantial piezometric differences between formations further supports the hypothesis of a single, extensive groundwater body.

Based on porosity characteristics, Dakouré [43] distinguished three types of aquifers in the Kou basin. The first type is the discontinuous aquifer, in which groundwater is stored and transmitted exclusively through fractures in hard basement rocks, dolerites, limestones, sandstones, and argillites. The second type, the semi-continuous aquifer, allows groundwater to circulate not only through fractures but also through pore spaces in materials such as schist crusts, cemented sandstones, and carbonates. The third type is the continuous aquifer, where groundwater is contained within a porous or highly fractured medium—often resulting from intense weathering—and is typically found in alluvium, sandy layers, and friable sandstones.

Further hydrogeological investigations by Ouédraogo [41], Sogreah [42], Talbaoui [45], Sauret [38] and Yofe/Tirogo [39] identified six main aquifer units in the Kou sub-basin. The Kawara-Sindou Sandstone aquifer (GKS) is confined and rests on the crystalline basement, with an estimated thickness of around 100 m. This aquifer yields flow rates of up to 65 m³/h, with transmissivity values around 1.14 × 10⁻³ m²/s and a storage coefficient of approximately 1.5 × 10⁻⁴. Wells can access this aquifer at depths of up to 90 m, with static water levels ranging from 16 to 40 m.

The Fine Glauconitic Sandstone aquifer (Gfg) is also confined, with an estimated thickness between 100 and 150 m. Transmissivity values within this aquifer are variable, with a minimum of 2.2 × 10⁻⁶ m²/s and a similar storage coefficient (1.5 × 10⁻⁴). Flow rates typically range between 50 and 80 m³/h, with an influence radius of approximately 500 m and drawdowns between 50 and 100 m. The Quartz Granule Sandstone aquifer (Ggq) lies beneath the Gfg aquifer, as suggested by interpretations of borehole logs and electrical conductivity measurements. Although its boundaries remain poorly defined, water levels can reach depths of up to 60 m in some areas, but are generally shallower near the Nasso/Guinguette springs, where levels are below 20 m. To date, the top of this aquifer has not been precisely identified.

The Upper Fine Pink Sandstone aquifer (Gfr) may be either unconfined or confined, depending on the local context. This aquifer generally hosts shallow groundwater, with water levels less than 20 m below the surface. The Siltstone–Argillite–Carbonate aquifer (Sac1) is confined and has a saturated thickness of approximately 80 m. Based on three pumping tests, transmissivity values range from 9 × 10⁻³ to 6 × 10⁻² m²/s, with a storage coefficient of about 5.5 × 10⁻⁴. This aquifer is separated from the Ggq unit by a series of red or yellow clay layers, which vary in thickness from about ten meters to more than 100 m.

The alluvial aquifer is a shallow, unconfined aquifer formed by alluvial deposits (primarily clay and laterite) associated with the Kou River. In the upstream part of the basin, the aquifer comprises clayey-sandy deposits approximately 15 m thick and exhibits low permeability, with hydraulic conductivity values around 10⁻⁵ m/s. Downstream, the alluvial deposits become more permeable, consisting of alternating clay, laterite, and weathered sandstone layers up to 45 m thick, with hydraulic conductivity increasing to about 2.2 × 10⁻⁴ m/s. This aquifer is hydraulically connected to the Kou River, with bi-directional exchange depending on seasonal variations. During the dry season, the upstream alluvial zone contributes approximately 9.3 × 10⁵ m³ to the river, while the downstream zone contributes about 2.2 × 10⁵ m³.

Structural features also play a significant role in groundwater circulation. A major SW–NE-trending collapse fault links the Ggq and Sac1 aquifers near the Nasso spring zone. These fractures are believed to be the origin of both the Nasso and Pesso/Desso springs, with surrounding zones exhibiting high permeability. Additional fault systems oriented NW–SE have also been identified and are associated with major groundwater discharge points [35]. Geoelectrical surveys conducted by Talbaoui [45] as part of the Water resource development Program (in French, “Valorisation des ressources en eau”—VREO Program) revealed a network of such faults, which may act as preferential pathways for infiltration and drainage. These structural discontinuities are particularly relevant in the context of accidental or diffuse pollution, as they significantly increase the vulnerability of the aquifers.

At the hydrogeological scale, one of the most critical parameters is the recharge of groundwater resources. According to Scanlon et al. [12], infiltration represents the downward movement of water from the soil surface toward the subsurface. In unsaturated zones, terms such as net infiltration, drainage, or percolation are often used to describe this process. While sometimes used interchangeably with “recharge”, true groundwater recharge specifically refers to water that percolates beyond the root zone and reaches the saturated zone of an aquifer [46].

Numerous studies have attempted to estimate groundwater recharge in the Kou basin (

Table 1. Groundwater recharge estimates in the Kou Sub-Basin.

Reference	Year	Method	Estimate (mm·year−1)
SOGREAH [42]	1994	Water balance method (Thornthwaite)	248
Dakouré [43]	2003	Water balance (Thornthwaite)	75–120
		Volume variation from piezometric fluctuations	<16
		Hydrological reservoir modeling	127
		Hydrogeological modeling (Processing-MODFLOW)	0–47
Derouane [40]	2008	Hydrogeological modeling (MODFLOW)	20–160
		Water balance method (Thornthwaite)	26–42
Sauret [38]	2013	Water balance method (Thornthwaite)—1961–2010	354–200
	2013	Water balance method (FAO-56 Penman–Monteith) (1961–2010)	288–69
Dao [48]	2015	Water balance method (Thornthwaite)	26.13
Yofe/Tirogo [39]	2016	Water balance method (Thornthwaite)	60–150
		Hydrogeological modeling (Visual MODFLOW)	95–115
Kouanda [47]	2019	Hydrogeological modeling (SWAT)	124

), starting with the work of SOGREAH [42]. Most of these studies employed the water balance method, using Thornthwaite’s approach to estimate potential evapotranspiration. However, results vary widely—from 0 to 354 mm/year—depending on the assumptions, timeframes, and methodologies used [38,39,40,42,43,47,48]. Thornthwaite’s method calculates potential evapotranspiration (PET) using empirical coefficients, and its annual resolution does not adequately capture seasonal variability or aquifer storage dynamics. Furthermore, it fails to consider critical factors such as land use changes, intensive pumping, and spatial variability in aquifer properties.

2.7. Population Growth, Dynamics, and Socio-Economic Activities

The Kou sub-basin, located within the Houet province, plays a central role in water supply for the city of Bobo-Dioulasso, which relies heavily on groundwater for potable water provision [16]. Bobo-Dioulasso, the second-largest city in Burkina Faso and often referred to as the country’s economic capital, is a major hub of population growth and economic activity. The sub-basin is extensively exploited by several stakeholders, including the National Office for Water and Sanitation (ONEA), irrigated agricultural schemes, and industrial actors such as LAFI, BRAKINA, SOBBRA, SN-CITEC, and CIMASSO [16,49].

The region is also a major destination for internal migration, with the local economy increasingly dependent on industry, agriculture, and livestock production [38]. According to the Mouhoun Water Agency [34], agriculture contributes 25.1% to the gross domestic product (GDP) of the Hauts-Bassins region, while non-market services and manufacturing industries account for 16.2% and 15.5%, respectively.

Groundwater resources within the Kou sub-basin are extracted to meet a range of demands, including domestic, agricultural, industrial, and mining uses. For domestic supply, potable water is mainly delivered through drilled boreholes operated by ONEA. The volume of water withdrawn by ONEA has increased significantly over the years. In 2001, the extraction rate was 250 m³/h, rising to 915 m³/h in 2005, and reaching 1250 m³/h by 2010 [38]. By 2014, withdrawals had climbed to 40,000 m³/day—or approximately 1666.66 m³/h—demonstrating a substantial increase of over 30% in potable water demand between 2010 and 2014 [39]. In rural areas, additional withdrawals for drinking water were estimated in 2016 at approximately 10,000 m³/day, or 27.4 m³/h [39].

A study commissioned by the Government of Burkina Faso in 2023 reported that the volume of water abstracted from boreholes operated by the National Office for Water and Sanitation (in French, “Office National de l’Eau et de l’Assainissement”—ONEA) in Bobo-Dioulasso increased from 3 million m³/year in 2010 to over 10 million m³/year in 2021—an increase of more than 60% [50]. By 2022, withdrawals had reached 13,622,054 m³ [51]. For the broader sedimentary region of western Burkina Faso (excluding areas served by ONEA), groundwater withdrawals in 2023 were estimated at around 12,000 m³/day [50], compared to 15,353,360 m³/year for the same region reported by CACI/GERTEC [51].

Agricultural activities are concentrated in the alluvial plain of the Kou River. Irrigated farmland is supplied by both the alluvial and deeper aquifers. Annual withdrawals for irrigated perimeters are estimated at 2.7 million m³ (equivalent to 308.22 m³/h) from the alluvial aquifer and 4.15 million m³ (476.7 m³/h) from deeper aquifers [38,39]. These volumes are subject to seasonal variation, depending on the type of crops and the area cultivated. For irrigated farming zones in Bobo-Dioulasso, withdrawals from groundwater sources were estimated at 7943.79 m³/year [51], while agricultural activities within the alluvial plain of the Kou consumed approximately 5,245,000 m³/year.

Withdrawals by industrial and public-service actors, such as the National Oil Company (in French, “Société Nationale Burkinabé d’Hydrocarbures”—SONABHY) and the National Electricity Company (in French, “Société Nationale d’Electricité du Burkina”—SONABEL), remain poorly quantified, largely due to the confidential nature of water use data [38]. However, estimates from [39] suggest total industrial withdrawals were around 1650 m³/day, with 1500 m³/day attributed to manufacturing companies and 150 m³/day to bottled water production facilities—equivalent to a combined rate of 68.5 m³/h. More recently, water abstraction by sachet water production units was estimated at 864 m³/day in 2023, totaling approximately 429,240 m³/year for the entire Hauts-Bassins region [51].

The Kou sub-basin is subject to increasing pressure from domestic, agricultural, and industrial water demands. The rising trend in withdrawals reflects growing socio-economic development but also underscores the urgent need for sustainable water resource management strategies to prevent overexploitation and degradation of groundwater reserves.

3. Synthesis of Vulnerability Studies

3.1. Literature Search Methodology

This review was conducted through a systematic search of both national and international digital databases, including ScienceDirect, Web of Science, and Google Scholar. The search strategy was designed to capture both foundational and recent research related to groundwater vulnerability in the study area.

Relevant studies were identified using a combination of thematic keywords such as “Kou”, “aquifer”, “vulnerability”, and “risk”, along with Boolean operators (OR, AND) to refine the queries. Two sets of search strings were developed—one in French and the other in English—to ensure a comprehensive coverage of both local and international scientific literature. The French search query was defined as follows: “aquifère du Kou” OR “Kou”) AND (“vulnérabilité” OR “risque” OR “recharge” OR “surexploitation”) AND (“changement climatique” OR “pression anthropique” OR “pollution”).

The English equivalent was defined as: “Kou aquifer” OR “Kou”) AND (“groundwater vulnerability” OR “risk assessment” OR “recharge rate”) AND (“climate variability” OR “anthropogenic pressure”).

The temporal scope of the search spanned from 1990 to 2024, thereby including both seminal contributions and recent advances in the field. Initial selection was based on titles, followed by a more detailed screening of abstracts to identify the most relevant publications. Both peer-reviewed scientific literature and grey literature were included, particularly those focusing on groundwater resources in the Kou sub-basin or the broader Taoudéni sedimentary basin. Additionally, studies addressing groundwater quality, recharge estimation, and hydrodynamic parameters were retained, while irrelevant or duplicate entries were excluded. The resulting body of literature provided a multi-thematic basis for this synthesis, incorporating insights from hydrogeology, hydrology, climatology, anthropogenic pressures, and groundwater vulnerability.

3.2. Summary of Earlier Studies Conducted in the Kou Sub-Basin

3.2.1. Groundwater Vulnerability-Related Studies

Several studies have been conducted over the past three decades to assess the vulnerability of groundwater in the Kou sub-basin, employing various methodological approaches and targeting different vulnerability dimensions. These studies have used parametric methods such as DRASTIC, GOD, and APSU, as well as numerical modeling tools (e.g., MODFLOW/MT3D), to produce vulnerability maps and simulate pollutant transport.

Table 2. Groundwater vulnerability studies in the Kou Sub-Basin.

Reference	Year	Methodology	Type of Vulnerability Assessed	Key Results/Vulnerability Index	Spatial Highlights
SOGREAH [42]	1994	Stratigraphic analysis of superficial layers	Intrinsic	Vulnerability map produced based on superficial thickness	High vulnerability in the upstream area of the Nasso springs (alluvial plain)
Kam [32]	2007	DRASTIC method	Intrinsic + Specific (pesticides)	Intrinsic: Index range 50–170 Pesticides: 0–230+	High vulnerability near rivers and urban fringe of Bobo-Dioulasso
Yanogo [52]	2008	DRASTIC, Improved DRASTIC, and GOD methods	Intrinsic + Specific + Risk	DRASTIC: 0–116 Improved DRASTIC: 110–208 GOD: 0–0.49	Elevated risk in urban-industrial zones and downstream agricultural areas
Bieupoudé & Gardin [53]	2008	APSU (Belgian method)	Risk and Transport Dynamics	4 maps: transfer time, dilution duration, global vulnerability, and vulnerability–land use overlap	Vulnerability highest in lowlands (alluvial plain) and northern Bobo-Dioulasso; dilution time > 10 years in Nasso
Talbaoui [45]	2009	MODFLOW/MT3D modeling + Fluorescein tracer injection	Risk-based contamination modeling	Pollutant travel time: 18–20 years to Nasso sources	Pollution from industrial zones and roads can eventually reach water supply springs

summarizes key information from these studies, including the methodology used, the type of vulnerability assessed (intrinsic, specific, or risk-based), the range of vulnerability indices produced, and the major spatial trends identified. This synthesis highlights areas of convergence among studies and underscores zones of particular concern within the basin—especially around urban and agricultural hotspots.

3.2.2. Groundwater Quality and Geochemical Assessments-Related Studies

Several studies have investigated the hydrochemical characteristics of groundwater in the Kou sub-basin and its surrounding sedimentary formations (

Table 3. Groundwater quality and geochemical studies in the Kou and Taoudéni basins.

Reference	Year	Geographical Focus	Major Findings	Indicators of Contamination
Dakouré [43]	2003	Taoudéni Basin (Burkina Faso)	Ca-HCO3 facies dominant; generally potable water. Some exceedances in K+, Fe, and NO3− in isolated samples across formations.	Localized pollution from agriculture (nitrates, iron)
Huneau et al. [54]	2011	Taoudéni Basin (multi-country)	Hydrochemistry varies by lithology. Higher bicarbonate, Ca, Mg in carbonate units. Sulfates high in Sac1/Sac2. Nitrates mostly low.	Some samples exceed 160–300 mg/L NO3− in Gfg and CT
Taupin [55]	2017	Western Burkina Faso	EC 1 highly variable (58–1856 µS/cm), reflecting both geology and nitrate contamination. HCO3–Mg facies predominant, some shifts to more alkaline types.	Nitrates and K+ indicate anthropogenic sources
Service [56]	2018	Bobo-Dioulasso area	37% Ca-Mg-HCO3, 39% Ca-Mg-Cl-SO4, others minor. 65.71% potable, but 41.23% show anomalies (high turbidity, NO3− up to 68.9 mg/L).	Industrial and agricultural impacts
Kouanda [47]	2019	Upper Mouhoun Basin	Dominant Ca-Mg-HCO3 facies; evolution toward Cl−-rich waters (Ca-Mg-Cl-SO4) in some areas due to ion exchange.	Anthropogenic influence inferred
Kutangila et al. [28]	2024	Western Sedimentary Basin (Burkina Faso)	Two dominant facies: Ca–Mg–HCO3 and Na–K–HCO3. EC averages ~283 µS/cm. Isolated high EC and nitrate levels in some zones.	Mix of natural hydrolysis and localized pollution

Note: ¹ EC: electrical conductivity.

). These analyses provide essential insight into both natural geochemical processes and anthropogenic impacts affecting water quality. Early work by Dakouré [43] identified dominant calcium-bicarbonate facies across the basin, generally indicating acceptable drinking water quality under World Health Organization (WHO) standards. However, certain samples exhibited elevated levels of potassium, iron, and nitrates, suggesting localized contamination across multiple aquifer formations.

Subsequent studies (e.g., Huneau et al. [54]) confirmed that groundwater chemistry in the Taoudéni basin varies according to lithology. In carbonate-rich units (e.g., Sac1 and Sac2), weathering processes such as hydrolysis result in higher concentrations of bicarbonate, calcium, and magnesium. In contrast, formations such as Gfr, Ggq, Gfg, and GKS exhibited more intermediate mineralization levels, reflecting a mix of siliceous and calcareous cementation. Sulfate concentrations are generally low but may rise (30–60 mg/L) in Sac1 and Sac2, due to the presence of gypsum crystals. Nitrate levels are generally low, except in the Gfg and superficial formations, where local pollution from latrines and livestock facilities may lead to values of 160–300 mg/L.

Taupin [55] further explored regional variation in electrical conductivity (EC), finding significant heterogeneity, particularly in crystalline formations. High EC values were often correlated with elevated nitrate concentrations, suggesting anthropogenic contamination. Similar patterns were observed in a localized study of Bobo-Dioulasso [56], where 41.23% of the sampled wells exhibited signs of pollution—particularly in central and peri-urban zones with intense agricultural or industrial activity. While 65.71% of the wells met potable water standards, elevated turbidity and nitrate/nitrite peaks were also noted.

Later work by Kouanda [47] in the upper Mouhoun basin confirmed the predominance of Ca-Mg-HCO₃ facies but also detected shifts toward chloride-rich types (Ca-Mg-Cl-SO₄) in certain zones, likely reflecting ion exchange and anthropogenic inputs. Most recently, Kutangila et al. [28] identified two dominant hydrochemical facies: calcium–magnesium–bicarbonate types resulting from carbonate dissolution and silicate hydrolysis, and sodium–potassium–bicarbonate types potentially linked to anthropogenic activities.

3.2.3. Isotopic Analyses and Groundwater Residence Time-Related Studies

Isotopic investigations have provided key insights into groundwater origin, recharge processes, and residence times in the Kou sub-basin and the surrounding sedimentary formations (

Table 4. Isotopic and residence time studies.

Reference	Year	Methods Used	Major Findings	Implications
Dakoure [43]	2003	δ18O, δ2H, 3H, 14C	Sedimentary aquifers show depleted isotopic signatures near GMWL; meteoric origin with minimal evaporation	Indicates deep recharge; residence time > 50 years
			Basement/superficial aquifers enriched in heavy isotopes; some evidence of evaporation/mixing	Suggests recent recharge or pollution near urban areas
Huneau et al. [54]	2011	δ18O, δ2H, 3H, 14C, 13C	Low/absent 3H in deep layers, detectable 3H in shallow systems	Confirms stratified residence times across aquifer layers; residence time 25,000–42,000 years
Kouanda [47]	2019	δ18O, δ2H, 3H	Strong interaction between surface water and groundwater. Tritium found in shallow zones only	Stratified recharge: recent in shallow wells, older in deep aquifers
Kutangila et al. [28]	2024	δ18O, δ2H, 3H	Defined three compartments: (1) recent recharge zone, (2) intermediate mixing zone, (3) deep, old water	Highlights compartmentalization and varying vulnerability levels

). These studies rely on stable isotopes (δ¹⁸O and δ²H), radioactive tracers (³H, ¹⁴C), and noble gases to assess the temporal dynamics of groundwater systems. It is worth noting that many of the isotopic datasets reviewed extend beyond the Kou sub-basin and encompass the broader Taoudéni sedimentary basin in Burkina Faso, which covers approximately 45,000 km². Consequently, the spatial scope of sampling provides regional-scale insights, although finer-scale sampling within the Kou sub-basin remains a priority for future studies.

Research by Huneau et al. [54] using stable isotope signatures indicated that most groundwater samples from the Taoudéni sedimentary basin plot along or slightly below the Global Meteoric Water Line (GMWL), suggesting meteoric origin with minimal evaporation prior to recharge. In contrast, samples from superficial and crystalline aquifers, especially those near Bobo-Dioulasso, exhibit enrichment in heavy isotopes—pointing to either partial evaporation before infiltration or mixing with shallow, possibly polluted waters. Isotopic results also revealed notable differences between sedimentary and basement aquifers. The former typically showed lighter isotopic signatures and more depleted δ-values, consistent with deeper and older groundwater. Deuterium excess values, averaging around +6‰, further confirmed the predominance of rainfall-derived recharge. Tritium measurements (³H) provided insight into groundwater age. In general, low or undetectable tritium levels were recorded in sedimentary aquifers, suggesting residence times greater than 50 years [43]. In contrast, the presence of detectable tritium in some superficial aquifers indicated more recent recharge events, potentially within the last few decades. The analysis also revealed vertical stratification in residence times, with younger water in the upper sandstone layers and older water at greater depths in siltstone and clay-rich formations.

In addition to these studies, Kouanda [47] carried out a comprehensive assessment of the upper Mouhoun basin, incorporating stable isotope analyses (δ¹⁸O and δ²H) and tritium measurements to investigate groundwater recharge mechanisms and age distribution. Tritium concentrations varied by depth, with shallow wells exhibiting detectable levels (indicating recent recharge), while deeper boreholes showed no measurable tritium, pointing to older groundwater. The study confirmed a vertically stratified aquifer system, with rapid infiltration in alluvial zones and slower recharge in confined sedimentary formations.

Recent work by Kutangila et al. [28] integrated isotopic, geochemical, and noble gas data to distinguish between three hydrogeological compartments: (i) a superficial, fast-circulating system with young water; (ii) an intermediate aquifer showing moderate mineralization and mixed isotopic signals; and (iii) a deep, slow-flow system with highly mineralized, older water. Their findings highlight the complex vertical and lateral heterogeneity of the Kou sub-basin’s groundwater system, with implications for both resource sustainability and pollution vulnerability.

3.3. Current Knowledge on Groundwater Vulnerability in the Kou Sub-Basin

The Kou sub-basin has been the focus of multiple hydrogeological and environmental studies over the past three decades, which together provide a nuanced understanding of groundwater vulnerability, quality, recharge processes, and residence times. This body of research confirms that the basin hosts a complex and stratified aquifer system, comprising both unconfined and confined units with highly variable hydraulic and geochemical characteristics.

3.3.1. Groundwater Vulnerability

Various studies employing parametric and modeling methods—most notably DRASTIC, GOD, APSU, and MODFLOW/MT3D—have converged on the finding that the Kou sub-basin exhibits moderate to high vulnerability to pollution, particularly in low-lying alluvial areas, urban peripheries, and zones with intensive agricultural activity. These vulnerable zones coincide with shallow water tables, permeable soils, and proximity to pollution sources, such as wastewater discharge points, pesticide use, and industrial operations. The methodological diversity among studies allows for a multidimensional view of vulnerability—ranging from intrinsic physical susceptibility to specific contaminant-related risks and transport dynamics. However, differences in index scaling and cartographic resolution complicate direct comparisons across studies.

3.3.2. Groundwater Quality

Hydrochemical analyses consistently reveal a dominant calcium–magnesium–bicarbonate facies, characteristic of water–rock interaction processes in sedimentary aquifers composed of sandstones and carbonates. However, localized geochemical anomalies—such as elevated concentrations of nitrates, potassium, iron, and turbidity—have been reported, especially in superficial formations near urban and agricultural zones. These anomalies reflect both natural lithological controls and anthropogenic pressures, including inadequate sanitation infrastructure and intensive fertilizer or pesticide application. Though many samples remain within WHO drinking water standards, the presence of high-nitrate zones (up to 300 mg/L) indicates point-source and diffuse contamination risks that may intensify with increasing population and land-use change.

3.3.3. Residence Time and Aquifer Stratification

Isotopic analyses (δ¹⁸O, δ²H, ³H, ¹⁴C, ¹³C) have provided critical insights into groundwater age and flow paths. Groundwater in the deeper sedimentary aquifers generally shows depleted isotopic signatures and an absence of tritium, suggesting recharge under cooler, possibly pre-modern climatic conditions, with residence times exceeding 50 years—potentially even centuries in some confined zones. In contrast, shallow aquifers and alluvial systems contain younger water, as evidenced by enriched isotopic signatures and the presence of tritium. These findings support a vertical stratification of recharge dynamics and highlight the hydrogeological compartmentalization of the system. The juxtaposition of recent and fossil water implies that vulnerability to surface-derived pollution varies significantly with depth and location.

3.3.4. Recharge Mechanisms and Uncertainty

Estimates of groundwater recharge vary widely, ranging from 0 mm·year⁻¹ to over 350 mm·year⁻¹, depending on the method, temporal scale, and climatic assumptions employed. Earlier studies based on Thornthwaite’s empirical evapotranspiration model tend to overestimate recharge, whereas physically based models such as SWAT or MODFLOW offer more spatially and temporally nuanced estimates. Recharge appears to be concentrated in areas with favorable surface–subsurface connectivity, such as fractured sandstones and alluvial deposits along the Kou River. However, a lack of direct infiltration measurements, coupled with uncertainties regarding soil hydraulic properties, lithological transitions, and anthropogenic abstraction rates, continues to hinder precise recharge quantification. As a result, current recharge estimates should be interpreted as approximations with high uncertainty margins, especially in the context of climate variability and land-use transformation. Although recent studies have adopted more physically based modeling approaches, such as MODFLOW [38,39,40,43] and SWAT [47], their potential remains underutilized in the Kou sub-basin. To improve the reliability of recharge estimates, future research should prioritize rigorous model calibration and validation using multi-source field observations, including soil moisture measurements, piezometric data, and tracer studies. This will allow for a more realistic representation of land–atmosphere–aquifer interactions and reduce the epistemic uncertainty that currently hampers water balance assessments.

3.3.5. Integrated Outlook

The Kou sub-basin exhibits a heterogeneous hydrogeological system that is moderately to highly vulnerable to pollution in specific zones, despite its overall high-quality water profile. The compartmentalized nature of the aquifer, with both recent and ancient waters coexisting, reinforces the need for differentiated management strategies according to depth, geology, and land use. Current knowledge provides a strong foundation for groundwater protection and monitoring, yet important gaps and uncertainties remain—particularly regarding recharge mechanisms, contaminant transport modeling, and the cumulative impact of anthropogenic pressures (

Table 5. Key groundwater findings in the Kou Sub-Basin.

Theme	Major Findings	Implications
Vulnerability	Moderate to high in shallow, agricultural, and peri-urban zones. High-risk zones near Kou River, Nasso springs, and Bobo-Dioulasso.	Targeted protection needed; vulnerability mapping must account for land use and geological layering.
Water Quality	Dominant Ca–Mg–HCO3 facies; local nitrate and iron pollution; EC heterogeneity linked to both geology and human activities.	Pollution risks from agriculture and urban runoff; need for localized water quality monitoring.
Residence Time	Deep aquifers: >50 years (fossil water), no tritium. Shallow aquifers: recent recharge detected. Stratification confirmed via isotopes.	Differential management by aquifer depth; deep aquifers less vulnerable but slower to recharge.
Recharge Mechanisms	Estimated at 0–354 mm·year−1 depending on method. Localized recharge in fractured zones and alluvial corridors. Uncertainty remains high.	Recharge estimates must be refined using multi-method approaches; avoid over-abstraction assumptions.

). Addressing these challenges requires an integrated, multi-scale approach that combines hydrogeological monitoring, isotopic and geochemical tracing, land-use regulation, and sustainable abstraction practices.

3.4. Methodological Limitations

Despite the considerable body of work dedicated to the hydrogeological characterization and vulnerability assessment of the Kou sub-basin, a number of methodological limitations persist, which constrain both the comparability and reliability of results across studies.

A major challenge stems from the diversity of methods employed in the assessment of groundwater vulnerability. Parametric approaches such as DRASTIC [32,52], GOD [52] and APSU [53], as well as numerical transport models like MODFLOW/MT3D [45] have been applied, each with distinct assumptions, weighting schemes, and spatial resolutions [57]. These methods are often context-dependent, and the lack of locally calibrated parameters—especially for methods initially developed in temperate climates—limits their representativeness in the Sahelian context. Moreover, discrepancies in index calculation, data quality, and spatial coverage complicate attempts to synthesize findings across studies [17,24,25]. It is also important to highlight that most vulnerability assessments reviewed in this study applied parametric models originally developed in temperate regions without recalibrating parameter weights for local hydrogeological and climatic conditions. Future studies in the Kou sub-basin would benefit from locally calibrated or adapted versions of these models to enhance their predictive accuracy and relevance. This includes refining weight assignments based on field data, geostatistical analyses, and stakeholder inputs.

A recurring limitation across studies is the insufficient availability of high-resolution hydrogeological data. Many assessments rely on regional geological maps, limited borehole data, and sparse geophysical investigations [38,39], which undermines efforts to accurately delineate aquifer boundaries or identify protective geological layers. The vertical and lateral heterogeneity of sedimentary formations—particularly the transitions between permeable sandstones and low-permeability argillites—remain poorly constrained, making it difficult to develop detailed conceptual or numerical models [41,43].

Groundwater recharge estimation is another area marked by methodological inconsistencies. Most studies [38,39,40,43,47] have relied on climatic water balance models using the Thornthwaite method, which does not fully account for temporal variability, land use change, or subsurface heterogeneity. Physically based models, such as MODFLOW or SWAT, have been used more recently [40,43,47], but often suffer from limited calibration due to the lack of reliable field data. Consequently, recharge estimates vary significantly across studies—ranging from less than 16 mm/year to more than 350 mm/year—and should be interpreted with caution.

Isotopic studies [28,43,47,54] have provided valuable insights into groundwater origin and residence times, but these investigations are often based on limited sample sizes and spatial coverage. This restricts the capacity to detect temporal changes or distinguish between short-term fluctuations and long-term hydrogeological trends. Moreover, many vulnerability assessments are static in nature, failing to account for dynamic drivers, such as population growth, urban expansion, and evolving land use patterns [16,38].

Another methodological weakness is the limited consideration of uncertainty in model results. Few studies include sensitivity analyses, confidence intervals, or validation exercises using independent datasets, despite the recognized importance of uncertainty quantification in risk-based resource management [17,24].

Finally, the integration of socio-economic and institutional dimensions remains underdeveloped. Although some authors acknowledge the impact of agricultural intensification and industrial pressures [15,39], most assessments focus solely on physical parameters, neglecting human behavior, governance frameworks, and policy instruments that influence groundwater vulnerability. While this review highlights the insufficient integration of socio-economic and institutional drivers—such as agricultural intensification, industrial development, and groundwater governance—these aspects were often overlooked in the original studies.

4. Conclusions and Perspectives

This review has synthesized three decades of hydrogeological research focused on the Kou sub-basin in southwestern Burkina Faso, with particular attention to groundwater vulnerability, quality, recharge mechanisms, and residence times. The Kou sub-basin, located within the Taoudéni sedimentary basin, represents a hydro-strategic zone that supplies drinking water to the rapidly growing city of Bobo-Dioulasso, supports irrigated agriculture, and sustains ecological flows along the Kou River.

The collective findings from previous studies reveal a complex, stratified aquifer system, composed of both unconfined and confined sedimentary units. The region’s dominant water type is calcium–magnesium–bicarbonate, though localized pollution by nitrates and iron has been observed, particularly in superficial aquifers and urban fringes. Vulnerability assessments, employing methods such as DRASTIC, GOD, APSU, and MODFLOW/MT3D, consistently indicate moderate to high susceptibility to contamination, especially in alluvial plains, recharge zones, and areas subjected to anthropogenic pressure. Isotopic investigations have confirmed vertical and lateral heterogeneity in groundwater residence times. Deeper aquifers tend to contain older, tritium-free water—suggesting residence times exceeding 50 years—while shallower aquifers exhibit signatures of recent recharge. Recharge estimates remain uncertain, with values ranging from 16 mm/year to over 350 mm/year, depending on methodology. These discrepancies reflect the limitations of current models and the need for improved observational data and coupled modeling approaches.

Altogether, the evidence highlights that while the Kou sub-basin still offers high-quality groundwater resources, it is increasingly vulnerable to overexploitation and pollution. Rapid urbanization, unregulated drilling, agricultural intensification, and climate variability pose significant risks to the long-term sustainability of this vital water resource.

Several priorities emerge for improving both scientific understanding and groundwater governance in the Kou sub-basin. First, there is a clear need to enhance subsurface characterization through high-resolution geophysical surveys and deep borehole drilling, which would allow for a more accurate delineation of aquifer geometry and interconnectivity. Complementary to this, recharge estimation methods must be strengthened by integrating multi-method approaches that combine field measurements, isotopic tracing, remote sensing, and physically based hydrological models. Moreover, vulnerability assessments should evolve from static, index-based maps toward dynamic models capable of simulating future scenarios under different land use and climate change conditions. A better understanding of the interactions between surface water and groundwater—particularly in zones of exchange along the Kou River—is also essential, as these areas are critical both for recharge and for potential contaminant transfer. Finally, improved groundwater governance is urgently needed. This requires coordinated action among scientific institutions, local authorities, water agencies, and communities to implement protective regulations, monitor withdrawals, and embed groundwater sustainability into broader land use and development planning.