Arsenic is a metalloid of geological origin [1]. In the environment, its sources include the ongoing use of arsenic-containing compounds including pesticides as well as accidental releases during gold and lead mining and coal combustion. Anthropogenic activities such as artisanal gold mining and agriculture result in its presence in wells and reservoir lakes [1][2]. It is widely believed that arsenic in groundwater originates from the dissolution of sulfide ore bodies or young sedimentary aquifers in alluvial plains and deltas, which are the focus of most arsenic research [3]. In the context of Burkina Faso’s crystalline basement (which accounts for 80% of the country’s geological formations), arsenic is increasingly found at high concentrations in groundwater, most often exceeding the WHO standard. The PDSEAII study (2006) documented such cases in the Bankui, Nakambé, and Yaadga regions. The National Office for Water and Sanitation (ONEA, 2020) also reported cases in the Nakambé and Oubri regions. In February 2026, ONEA identified a new case in the Nazinon region, Zoundweogo Province, specifically in the commune of Nobéré.

In the context of Burkina Faso, very few studies [3] have focused on determining the source of arsenic, and this is what we aim to do through geological sections and water chemistry analyses in the bedrock aquifers of Burkina Faso.

The geology of Burkina Faso comprises three main types of geological formations (Figure 1; [4]), which are:

- The Paleoproterozoic (~2 Ga) e bedrock, consisting of metamorphic rocks also known as Birimian formations and igneous rocks. This bedrock belongs to the Baoulé-Mossi domain of the Man/Léo Ridge in the southern part of the West African craton and accounts for 80% of Burkina Faso’s geological formations.

- The Neoproterozoic sedimentary cover extends across the western, northern, and southeastern margins and belongs to the Taoudéni and Volta basins.

- The Cenozoic formations of the terminal continental margin occupy small areas in the northwestern and far eastern parts of the country.

The Nakambé, Oubri, and Yaadga regions, which are the subject of this study, are almost entirely occupied by the Paleoproterozoic basement. Only a small portion of the Yaadga region, in its northwestern part, is covered by the Terminal Continental formations (Figure 1).

Crystalline rocks are impermeable in their undamaged state. However, tectonic stresses have developed networks of fractures within these compact formations that are sufficiently dense and open to convey and store infiltrating water. Overlying the mechanically formed discontinuities are physicochemical weathering processes that increase the porosity and permeability of the layer of rock exposed to atmospheric agents. The nature and thickness of the weathered overburden are determined by the nature of the bedrock, climatic conditions, and the intensity of fracturing. Under these conditions, the bedrock and its overburden form a two-layer aquifer system comprising a discontinuous, transmissive lower reservoir and a continuous upper reservoir, which, due to its high porosity, serves as a storage reservoir [5].

Figure 1. Location of the study area on the geological map of Burkina Faso [6].

In Burkina Faso, it is these aquifers in fractured environments or networks of fractures and fissures with highly variable widths and lengths that are exploited for village water supply programs, drinking water supply systems (AEP) serving major population centers, and the development of multi-village drinking water supply systems. The sustainability of this reservoir is determined by the location of the piezometric level relative to the top of the bedrock, by the continuity of recharge from weathered rock, and by the interconnection of the various fracture networks [5].

Depending on the geometry of the fissures and fractures and the degree of interconnection among the various fracture networks, these zones can induce regional flow that is sometimes significant [5] and is often responsible for the physicochemical variability of the water.

In this study, we focus on the source and geochemistry of arsenic in the aquifers of the lower confined groundwater table of the bedrock in Burkina Faso.

Log data from boreholes FY13 and F8 showed that their aquifers draw water from Paleoproterozoic basement formations, notably schists, mylonites, and migmatites, which are potentially mineralized with arsenopyrite [4].

Furthermore, the order of abundance of the major ions (Ca²⁺ > Na⁺ > Mg²⁺ > K⁺ and HCO 3 − > SO 4 2 − > NO 3 − > Cl − ) indicates that the waters are calcium bicarbonate-rich and therefore consistent with those of the basement, according to [14].

The temperatures measured in situ ranged from 31.81˚C to 32.57˚C during the September field campaign and fall within the temperature range for deep boreholes (31 to 34 ^˚C) documented by Yameogo (2008). Their slight decline after September is attributable to recharge from rainfall. pH values ranging from 6.5 to 8.5 are also indicative of closed systems, particularly those of deep aquifers [15].

The calcium bicarbonate pole on the Piper diagram is a characteristic of bedrock aquifers, according to [14]. Neither the cation nor the anion triangles indicate a particular dominance of any single ion, suggesting that the water’s mineralization is natural and, consequently, that the arsenic also has a natural origin.

These results corroborate those of the PEEN final synthesis report (2010), which indicated that the Birimian volcano-sedimentary formations are among the primary sites with high arsenic concentrations, as is the case with the shales in the Yatenga region (As = 98.7 µg/L for FY13). According to [1], arsenic and its derivatives (arsenides, sulfides, oxides, arsenates, and arsenites) are rare in nature but occur in some mineralized zones, and arsenopyrite (FeAsS) is the most abundant arsenic-bearing mineral in these zones. Authors such as [16] have also reported high arsenic concentrations (185 - 1025 mg/kg) in soils overlying the mineralized bedrock at the Ashanti Mine in central Ghana, which may have resulted from the excavation and exposure of arsenopyrite. Indeed, [17]-[19] have shown that arsenic is of geological origin, generally present in volcanic sediments, sulfide minerals, metal oxides, and upwelling from hydrothermal sources, which, through various biogeochemical processes, can dissolve in water and reach aquifers and watercourses.

Where arsenopyrite-rich veins are present in the geological environment, arsenic concentrations are locally high due to water-rock interactions. These concentrations may decrease or disappear over time following the complete dissolution of the vein.

However, in cases where arsenopyrite mineralization is disseminated (e.g., the Koupéla granite), there is a strong correlation between the rock’s mineral components and arsenic. Thus, an increase in As concentration may be observed during low-water periods when groundwater has more time to interact with the rocks.

The positive correlation between Ca²⁺, Mg²⁺, HCO 3 − , SO 4 2 − , Na⁺, and As, as well as the very strong correlation (0.98) between conductivity and bicarbonates, indicates that the water’s mineralization is natural and that the arsenic likely originates from the bedrock.

The very strong correlation (0.98) between conductivity and bicarbonates indicates that bicarbonates constitute the bulk of the mineralization and that this mineralization is natural.

Furthermore, since the analysis of major elements showed that the water is generally potable in terms of its physicochemical parameters, the very strong correlation between conductivity and bicarbonates indicates that the water’s alkalinity consists primarily of hydrogen carbonates HCO 3 − [15].

Indeed, according to these authors, in bedrock aquifers, alkalinity stems primarily from the weathering of silicate minerals. Soil water rich in CO₂ attacks aluminosilicates, releasing constituents such as Na⁺, Ca²⁺, K⁺, and Mg²⁺ into solution. The main product of this weathering process—and generally the first to form—is kaolinite, according to the following reactions.

These equations confirm the strong correlations between electrical conductivity, Ca²⁺, Mg²⁺, HCO 3 − , SO 4 2 − , Na⁺, and As, and also confirm that the arsenic originates from the rocks of the mineralized basement.

The principal component analysis (PCA) shows that the F1 (60.011%) and F2 (20.4%) axes account for more than 80% of the variance (Table 4) and confirm the natural mineralization of the water.

Table 4. Correlation between variables and factors.

Accounting for 60% of the variance, the F1 axis represents the first dimension of the PCA. On this axis, electrical conductivity, Ca²⁺, Mg²⁺, HCO 3 − , SO 4 2 − , Na⁺, and As are correlated (Table 5 and Figure 8 of the active variables). This confirms that arsenic originates from the same source as these mineral elements (the bedrock, which in this case is the basement).

Table 5. Relationship between observations and axes.

The strong correlation between sulfates and As (0.74) indicates that As originates from sulfide minerals. The angular deviation between the two in Figure 8 can be explained by dissolution, which led to the decoupling of the two parameters [3]. Indeed, the retention of arsenic through adsorption is a possible process. Arsenic’s high affinity for metal oxides [10] means that arsenic would normally undergo much greater retention through adsorption than SO₄. Furthermore, if arsenides or secondary oxides are the sources of arsenic in groundwater, it is possible that the SO 4 2 − released during the initial oxidation of sulfide may have been leached from the aquifer long ago [1].

Figure 8. Active variables on the axes.

Arsenic concentrations in aqueous media can also be explained by oxidation and/or precipitation reactions depending on atmospheric conditions [20], as shown by the following equations:

4 FeAsS + 13O 2 + 6 H 2 O = 4 Fe 2 + + 4 HAsO 4 2 − + 4 SO 4 2 − + 8 H +

Here, arsenic is released, along with iron and sulfate.

The oxidation of pyrite, FeS₂, by oxygen can be described by the following reaction:

FeS 2 + 7 / 2 O 2 + H 2 O = Fe 2 + + 2 SO 4 2 − + 2 H +

Arsenic treated with nitric acid yields arsenic acid, which is present only in an aqueous medium. This slow oxidation reaction is sometimes described as dissolution.

The poor correlation between iron and As indicates that the As originates from other sulfides such as orpiment (As₂S₃) or realgar (As₂S₂) in mineralized zones [1] or in the form of native arsenic, arsenides, and arsenates.

Observations 1 and 2 (4.231 and 2.568) in Table 5 and Figure 9 confirm the correlations between axis F1, Ca²⁺, Mg²⁺, HCO 3 − , SO 4 2 − , Na⁺, and As. These results confirm that axis F1 is the primary axis of mineralization and that the arsenic in the studied boreholes originates from the mineralized basement.

Figure 9. Relationship between observations, variables, and axe.

Arsenic speciation analysis also showed that As(III) is the predominant form, indicating that we are in a reducing environment (bedrock aquifers).

Furthermore, according to [1], the symptoms caused by prolonged exposure to arsenic appear to vary among individuals, population groups, and geographic areas. There is therefore no universal definition of arsenic poisoning, which complicates the assessment of the burden of disease. In the mineralized bedrock, the predominance of the more toxic As(III) as is the case here could explain the manifestations of arsenic-related toxicity (cancers, hyperkeratosis, melanosis, and pigmentation) in the northern region, and more specifically in Yatenga Province [21]. In this region, arsenic concentrations often exceed the WHO standard of 10 µg/L. High concentrations of up to 1630 µg/L have sometimes been recorded and are associated with Birimian volcanic-sedimentary formations mineralized with sulfides, including arsenopyrite [3]. This is the case for the FY13 borehole in Ouahigouya (98.7 µg/L).

Based on well logs, physicochemical analysis, and arsenic speciation, this study shows that the arsenic in the studied waters originates from the mineralized basement. The As(III) form, one of the most toxic Ar, predominates because it is characteristic of reducing environments such as the aquifers under study. The study also shows that arsenic speciation which is not taken into account by legislation and knowledge of the source of arsenic are essential for understanding the symptoms resulting from exposure.

Author Contributions

Conceptualization, Kabore Zourata; methodology, Kabore Zourata; software, Kabore Zourata; validation, Kabore Zourata, Naba Seta, and Nakolendoussé Samuel; formal analysis, Naba Seta; investigation, Kabore Zourata; resources, Kabore Zourata; data curation, Nakolendoussé Samuel; writing—original draft preparation, Kabore Zourata; writing—review and editing, Naba Seta; visualization, Naba Seta; supervision, Naba Seta; project administration, Kabore Zourata; funding acquisition, Kabore Zourata. All authors have read and agreed to the published version of the manuscript.