1 Introduction

Water has always been an essential element for the survival of living beings, especially humans. Having access to quality water in sufficient quantities contributes to maintaining human health. To achieve this, the United Nations (UN) recommends universal access to water and sanitation, as well as sustainable management of this resource [1]. However, according to UNICEF/WHO data [1], one-third of the world's population lacks access to a safe water source, with half of this population residing in Africa [2]. Limited access to clean water is due to unequal distribution of available resources and the adverse effects of climate change [3]. Moreover, the high temperatures in semi-arid areas of Sub-Saharan Africa led to significant evaporation of water [3]. Several studies have also highlighted poor chemical and bacteriological water quality in developing countries [4,5,6]. However, water production for consumption is not feasible when it exceeds a certain threshold of contamination [7]. The Lake Chad Basin in general, and the Barh-El-Ghazel region in particular, are facing the adverse effects of climate change, including the disappearance of old lakes and their tributaries, declining groundwater levels, drought, and loss of vegetation cover, and desert encroachment [8]. Access to clean water in this area has become a significant problem for the population. Since the government cannot ensure access to clean water for all, individuals and private organisations have stepped in to address this issue. Measuring water quality is crucial for public health in the study area due to several key reasons. Contaminated water can harbor pathogens such as bacteria, viruses, and parasites, leading to waterborne diseases like cholera, dysentery, and typhoid fever. Regular water quality testing helps identify and mitigate these risks, ensuring that the water supply meets health standards and is safe for consumption. This is particularly important for protecting vulnerable populations, including children, the elderly, and individuals with compromised immune systems. Additionally, data from water quality assessments can inform public health policies and initiatives aimed at improving water sanitation and hygiene practices, ultimately reducing the incidence of water-related diseases. In Chad, where access to clean water can be challenging, regular monitoring is vital for safeguarding public health, addressing environmental contamination from industrial activities and agricultural runoff, and promoting sustainable development. Ensuring water quality supports economic growth, improves quality of life, and contributes to the overall development of the country [1]. Therefore, it is necessary to assess the quality of these resources, provide decision-makers and water managers with insights to support good management practices, and sustainably preserve water resources for the benefit of the local population. The objective of this study is to evaluate the quality of drinking groundwater using geochemical, bacterial and multivariate statistical methods in the city of Moussoro, with the aim of improving living conditions by providing the population with clean and safe drinking water.

2 Materials and methodology

2.1 Study area

Moussoro is located in the southern part of Barh-El-Ghazel domain (Fig.1). The area is a semi-desert region located in the Northwest of Chad, within the Sahelian belt. It covers an area of 69,000 square kilometres and has a population of 265,875 inhabitants, with a population density of 4 peoples per square kilometre [9]. The climate here is Sahelian, characterized by a rainy season of 3 to 4 months (June to September) and a dry season of 8 to 9 months (October to May). The temperature ranges from 20 to 45 °C, and the average annual rainfall varies between 200 and 400 mm. Air humidity reaches its peak in August (77%) and its minimum in February–March (25%), with potential evaporation exceeding 200 mm. The hydrogeological system of Barh-El-Ghazel is a sedimentary basin with significant hydraulic resources and different lithologies with various orientations [10]. The aquifers in this province are either unconfined or shallow, with lithological facies dominated by loose sand, as well as confined aquifers at greater depths. Geologically, the Barh-El-Ghazel rift corresponds to a rectilinear active tectonic zone spanning 450 km in a southwest-northeast direction, parallel to a heavy anomaly zone with the same orientation passing near Lake Fitri [10]. The anomaly remains consistent until our study area (Moussoro) but narrows as one moves northward and terminates at Koro-Toro [11]. As a sedimentary cover zone, it consists of sandy and clay strata formed during marine regression and transgression, respectively.

Fig. 1
figure 1

Study area showing the wells location

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2.2 Workflow

2.2.1 Sampling

After prospecting the area, several water sampling points were randomly selected to ensure comprehensive coverage of the study area. The collected water was stored in polyethylene bottles that had been previously washed with distilled water and then rinsed with the water to be analysed. For bacteriological analysis, samples were collected in glass bottles sterilized at 220 °C. The samples were transported in a cooler to maintain their integrity. Each bottle was marked with indelible ink indicating the project code, location, date, sample number, and elements to be analysed. In situ measurements of temperature, pH, and electrical conductivity were conducted using a pH/EC-983 multi-parameter device.

2.2.2 Analysis

The chemical analysis of ions \({\text{Ca}}^{2 + }\), \({\text{Mg}}^{2 + }\), \({\text{Cl}}^{ - }\), and \({\text{HCO}}_{3}^{ - }\) was performed in the laboratory using the classical volumetric method. The ions \({\text{NO}}_{3}^{ - }\), \({\text{NO}}_{2}^{ - }\), \({\text{F}}^{ - }\), \({\text{SO}}_{4}^{2 - }\), \({\text{Cl}}^{ - }\), and \({\text{Fe}}^{2 + }\) were determined by spectrophotometry using the HACH2800 spectrophotometer. As for the ions \({\text{K}}^{ + }\) and \({\text{Na}}^{ + }\), their concentrations were determined by photometry using a flame photometer.

Bacteriological analyses of water samples from wells were conducted in the laboratory. These microbiological analyses focused on enumerating E. coli, total coliforms, faecal enterococci, and total aerobic mesophilic bacteria. The techniques used for determining these parameters are those of AFNOR [12]. The following media were used: chromocult agar for E. coli and total coliforms, Slanetz and Bartley medium for faecal enterococci, and PCA for total aerobic mesophilic bacteria.

To understand the geochemical processes controlling water mineralization, the results of cation (\({\text{Ca}}^{2 + } ,\;{\text{Mg}}^{2 + }\), (\({\text{Na}}^{ + }\) + \({\text{K}}^{ + }\))) and anion (\({\text{HCO}}_{3}^{ - }\), \({\text{SO}}_{4}^{2 - }\), and (\({\text{Cl}}^{ - }\) + \({\text{NO}}_{3}^{ - }\))) analyses were plotted on Piper Hydrochemical Diagrams (PHD) [11]. This allowed us to identify the chemical characteristics of the water and help highlight the natural factors controlling groundwater mineralisation acquisition [13, 14]. The results of hydrochemical parameters including pH, CE, TDS, \({\text{Ca}}^{2 + }\), \({\text{Mg}}^{2 + }\), \({\text{Na}}^{ + }\), \({\text{K}}^{ + }\), \({\text{HCO}}_{3}^{ - }\), \({\text{SO}}_{4}^{2 - }\), \({\text{Cl}}^{ - }\), and \({\text{NO}}_{3}^{ - }\) were used to perform a Principal Component Analysis (PCA). PCA is one of the multivariate statistical techniques used in hydrogeochemistry by several authors [15, 16] to transform a large number of variables into a smaller number of factors, detect and identify groups of highly correlated variables. The XLStat software was used for statistical analysis. The flowchart of the methodology used to obtain the results is given as following in the Fig. 2.

Fig. 2
figure 2

Flowchart of the methodology used to obtain the results

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3 Results and discussion

3.1 Hydrodynamic studies

The realisation of the piezometric map allows highlighting the direction of groundwater flow and the recharge zone of aquifers. It is carried out using piezometric data collected during a field campaign conducted on several wells during low water periods, focusing on Quaternary aquifers. Groundwater flow generally occurs from southeast to the centre and from the centre to the northwest (Fig. 3). Coinciding with the fossil valley, the piezometric depression would be the result of intense evaporation that the Quaternary shallow aquifer undergoes [17].

Fig. 3
figure 3

Piezometric map of the study area showing the direction of water

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3.2 Physicochemical water quality

The pH of groundwater in Moussoro ranges from 7.15 to 7.60, with an average of 7.39 (Table 1). The highest values are found in the piezometric domes, while lower values are found in the depressions. The conductivity values are heterogeneous and range from 197 to 852 µS/cm in wells F1 and F3, respectively, with an average of 407.30 µS/cm and a coefficient of variation of 44.55, indicating a significant fluctuation of the parameter. These fluctuations not only differentiate the chemical composition of the water but also highlight potential recharge zones of the aquifer through precipitation. The variation of electrical conductivity shows a gradually increasing gradient, which follows the direction of groundwater flow. The conductivity variability also indicates that sandy formations are potential recharge areas due to their high permeability, as demonstrated in the work of Djoret [18]. The total dissolved solids (TDS) range from 13 to 427 mg/L, with an average of 192.7 mg/L and a coefficient of variation of 55.76. Wells F3, F4, F8, and F10 have the highest mineralisation levels. These significant variations suggest that, in addition to evaporation, the dissolution of evaporites and ion exchange with the surrounding formations contribute to the mineralisation, as demonstrated by Kuitcha et al. [19].

Table 1 Results of physicochemical and bacteriological analyses of water in Moussoro and descriptive statistical analysis of data
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3.3 Chemical quality of water

The concentrations of calcium range from 16.8 to 41.6 mg/L, with an average of 21.2 ± 7.27 mg/L. The high calcium levels, although below WHO [2] guidelines, could result from the dissolution of gypsum formations (CaSO4, 2H2O), which are easily soluble in water [9], or from ion exchanges. It depends on the contact of water with the slightly natron-rich karst formations in the study area. As for magnesium, the concentrations range from 1.46 to 13.61 mg/L, with an average of 5.14 ± 3.97 mg/L. The coefficient of variation is around 77.22%, indicating diverse contributions of this substance in the water of the study area. Similar to TDS, wells F3, F4, F8, and F10 have the highest concentrations. These contributions may result from ion exchange processes between groundwater and alkaline earths against alkalis or from the dissolution of magnesium-rich evaporites, as shown in the study by Njueya et al. [20] conducted in the Douala sedimentary basin in Cameroon. Regarding potassium, the concentrations range from 0.40 to 4.30 mg/L, with an average of 2.78 ± 1.14 mg/L. These concentrations are significantly lower than the WHO [2] drinking water standards, which recommend a potassium concentration of < 200 mg/L. The potassium enrichment in the water of the area may result from the alteration of potassic feldspar and not from anthropogenic sources, as the values of the other samples show little variation. Sodium concentrations in the analysed water samples range from 20 to 52 mg/L, with an average of 28.2 ± 9.17 mg/L. The contribution of sodium in water mineralisation is significant, considering its concentration in samples with high TDS. As demonstrated in the work of Abdramane [17], the source of sodium is related to ion exchanges, accumulations, water residence, and evaporite dissolution. However, ammonium ions are present in large quantities. Except for sample F10 (0.47 mg/L), all other samples have values above the WHO [2] recommendations for drinking water, which are set at 0.5 mg/L. The values obtained for the nine samples range from 0.58 to 0.81 mg/L, with a general average of 0.70 ± 0.11 mg/L. Ammonium ions show the lowest coefficient of variation (15.91%).

The contribution of anions to the mineralisation of water in the Moussoro area varies from one ion to another. Bicarbonate concentrations in groundwater range from 63.44 to 195.20 mg/L, with an average of 90.48 ± 39.95 mg/L. The coefficient of variation for these ions is significant at 44.15%, which expresses a homogeneous distribution of bicarbonates around the mean [21]. The enrichment in bicarbonate is likely related to the dissolution of evaporitic rocks or contact with carbonate formations [11]. Chlorides are present in low concentrations in the groundwater of the area, with minimum and maximum concentrations of 23 and 39 mg/L, respectively. These ions do not show significant variation, with an average concentration of 26.94 ± 4.6 mg/L and a coefficient of variation of 17.26%, which is the lowest after ammonium among all the ions in this study. The presence of chlorides is linked to the dissolution of halite and evaporation that the aquifer undergoes in the depressions, as observed in all depressed aquifers in the Sahelian Africa. In addition to their conservative behavior, chloride ions tend to concentrate in waters subjected to evaporation. Sulfates have a minor contribution to the mineralisation of the studied waters. Their concentrations after laboratory analyses range from 10 to 55 mg/L, with an average of 17.50 ± 13.52 mg/L. The WHO (2018) guidelines accept water potability if sulphate levels are ≤ 250 mg/L. The presence of sulphate ions could come from the dissolution of gypsum formations [17]. Since the dissolution reaction rate is not equal at all points, the sulfate levels will vary considerably from one sample to another. Sulphate ions have the highest coefficient of variation, with 77.24%, indicating a heterogeneous distribution around the arithmetic mean of the ionic concentration [21]. Nitrates, meanwhile, are present in acceptable quantities, although their absence would be desirable. Their concentrations range from 1 to 11 mg/L, with an average of 6.88 ± 2.85 mg/L. The WHO (2018) guideline accepts the concentration of nitrates in drinking water is ≤ 50 mg/L. The sources of nitrates are either faecal intrusions, organic matter decomposition, or they could be linked to anthropogenic or agricultural pollution.

3.4 Water characteristics

The results of physicochemical analysis of water samples from the study area were projected onto the Piper diagram (Fig. 4) to determine their hydrochemical classification. This classification allowed for easy interpretation and comparison of chemical differences and similarities among the water samples. Overall, the waters in the study area exhibit almost the same composition. A view of the Piper diagram reveals that the dominant characteristic is bicarbonate-calcium and magnesium, as found in immature waters (to waters that have not undergone significant chemical evolution) by Abdramane [17] in the Chari-Baguirmi aquifers. Samples F5 and F6 have a facies characterised by bicarbonate-sodium and potassium. Calcium enrichment is observed in the cations of wells F1, F2, F5, F6, F7, and F9, while the rest of the samples do not have dominant cations. Chlorides are the dominant anions in nine samples, except for the water from well F3, which is enriched in bicarbonates. The order of representation of major ions in the studied waters is as follows: Ca2+ > Na+ > Mg2+ > K+ > NH4+ for cations, and Cl− > HCO3− > SO42− > NO3− for anions. This ion representation in the waters is consistent with the results obtained by Ewodo et al. [22] in the N'Djamena aquifers. The predominance of calcium, sodium, chlorides, and bicarbonates in the waters can be attributed to anthropogenic inputs for chlorides, as well as the dissolution of halites and carbonates, along with cation exchange processes [23, 24].

Fig. 4
figure 4

Piper diagram showing the chemical characteristics of water analysis

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3.5 Bacteriology

Based on the results obtained for the microbiological parameters considered to evaluate the potability of water intended for the supply of the population in the city of Moussoro, we can categorise and distinguish the contaminated samples: those that are not potable and the uncontaminated samples that meet WHO guidelines for drinking water.

In the first category, the waters show faecal pollution indicator such as E. coli, whose presence in the water proves that the contamination is of faecal origin [25]. We also observed the presence of other contamination indicator such as total coliforms, which are present in almost all samples with values exceeding the WHO guidelines. Given the anthropogenic activities and the lack of basic sanitation, it is likely that the contamination is anthropogenic, facilitated by hydrodynamics, as all the sampled points are recent waters. The presence of indicator bacteria of pollution, such as E. coli (F4, F6, F7, and F9) (Table 1) could contaminate the water [25]. Other indicator organisms of contamination like TC (absent only in F3) and (Total Aerobic Mesophilic Flora TAMF) are present in practically all samples, whereas the World Health Organization (WHO) recommends their absence [26].

3.6 Correlation and principal component analysis

Principal Component Analysis (PCA) presents the information contained in the physicochemical analysis results in the form of a data table or a graphical representation [27]. The existing relationship between the different variables and the correlation coefficients between these variables are shown in the correlation matrix (Table 2). Table 2 shows that the Electrical Conductivity (EC) is strongly correlated with the major elements, except for potassium, ammonium, and nitrates. The correlations of EC with these chemical elements are as follows: Cl− (0.98), HCO3− (0.97), Na+ (0.96), SO42− (0.94), Mg2+ (0.92), and Ca2+ (0.92). Bicarbonates, chlorides, and sulfates are strongly linked to alkaline earth metals (Ca2+ and Mg2+) and an alkali metal (Na+). The data exhibit a good correlation between Total Dissolved Solids (TDS) and EC with the majority of variables. There is also a strong correlation between Ca2+ and Mg2+ (0.8), indicating the dissolution of mineral elements in the rock over time in the aquifer [28]. Correlation is also established between alkaline earth metals (Ca2+ and Mg2+) and Na+ at 0.93 and 0.94, respectively. The infiltration of surface water through limestone formations leads to the dissolution of Ca2+ and Mg2+, enriching the groundwater [29]. The alteration of orthoclase by infiltrating water also releases Na+ ions into the aquifers. This is supported by the results of the studies conducted by Cheikh et al. [28] and Njueya et al. [30]. There is also a good correlation between the anions (Cl−, HCO3−, and SO42−) and the cations (Ca2+, Mg2+, and Na+). The strong correlations among these variables highlight the similarity of processes contributing to the mineralisation of groundwater in Moussoro, as demonstrated by Kouassi et al. [31].

Table 2 Correlation matrix between the different variables taken pairwise
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The PCA carried out on the 15 parameters shows that the first two axes account for 73.44% of the total variance (axis 1 explains 60.33% and axis 2 explains 13.01% of this variance), with an eigenvalue of 8.44 (Table 3). Axis 1 is strongly positively correlated with Cl−, HCO3−, Na+, SO42−, Ca2+, and Mg2+. This axis represents the mineralisation of water in the Moussoro zone, considering the strong correlation between these chemical parameters and EC (Table 4). The correlation of EC with the major ions in the water defines mineralisation through mineral hydrolysis [32]. These correlations can serve for monitoring water quality in the area [33]. Conversely, axis 2 is moderately correlated with NH4+ and K+ for chemical substances, as well as with E. coli for bacteriological elements. The presence of E. coli in the water could be a result of polluted environment, which is caused by human activities.

Table 3 Eigenvalues and expressed percentages for the principal axes
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Table 4 Correlations between variables and factors
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By examining the correlation circle (Fig. 5), it is observed that the chemical parameters chlorides, bicarbonates, sodium, sulfates, magnesium, and calcium are grouped in the positive pole of factor f1. This grouping reflects the strong correlation that exists between these variables. The proximity of these variables in the community circle shows that the source of origin of these different ions in the water is governed by the same process. Most of the variables are positively correlated with the process defining the phenomenon behind the mineralisation of the Moussouro groundwater, which is the prolonged contact between water and rock resulting in hydrolysis. Since these variables are strongly correlated with factor f1, this axis can be considered as the axis of natural mineralisation. Ammonium and potassium are positively correlated with factor 2. However, these variables have a positive but moderate correlation with factor 1. This shows that factor 2 defines a source of mineralisation other than the hydrolysis of f1. The presence of E. coli in the vicinity of these variables indicates that f2 is influenced by pollution from anthropogenic activities.

Fig. 5
figure 5

Correlation circle plot of f1–f2

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In order to better understand the mineralisation mechanism of different water points, a graphical representation of the statistical units (Fig. 6) based on factors f1–f2 is provided. An analysis of the graph shows three main groupings of water points: the first grouping includes water samples with a high and moderate contribution to axis f1, which represents the axis of natural mineralisation where the ion acquisition of water is controlled by the water–rock contact time. This class consists of 40% of the samples (F1, F3, F5, and F7). Except for sample F3, which is highly mineralised overall, the other samples have similar concentrations for most of the minerals analysed.

Fig. 6
figure 6

Graphical representation of the factorial map of Massouro wells

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The second grouping includes samples F4 and F10, which have the highest contributions to axis f2. These two samples are moderately mineralised and represent the presence of ammonium, potassium, and nitrates for chemical elements, as well as E. coli for bacterial elements.

The third grouping, comprising 40% of the studied samples (F2, F6, F8, and F9), represents the class of moderately mineralised wells, characterised by an almost equal enrichment in calcium and chlorides. In this third group, sample F9 stands out from the others due to the presence of a significant colony of E. coli, CT, and FMAT.

3.7 Implications for policy makers

The results of this study will have several important implications for policymakers:

  • Informed decision-making: policymakers will have access to comprehensive data on the quality of drinking groundwater, enabling them to make informed decisions regarding water management and public health initiatives.

  • Resource allocation: the study’s findings can guide the allocation of resources towards areas with the most critical water quality issues, ensuring that interventions are targeted and effective.

  • Regulatory standards: the results can help in the development or revision of regulatory standards for drinking water quality, ensuring that they are based on current and accurate data.

  • Public health policies: by identifying the presence of contaminants and their sources, the study can inform public health policies aimed at reducing exposure to harmful substances and preventing waterborne diseases.

  • Infrastructure development: the insights gained from the study can support the planning and development of infrastructure projects, such as improved water treatment facilities and sanitation systems, to ensure a safe water supply.

  • Community awareness: the study can also serve as a basis for community education and awareness programs, helping the population understand the importance of water quality and encouraging practices that protect water resources.

  • Sustainable practices: policymakers can use the study’s findings to promote sustainable water management practices, ensuring the long-term availability of clean and safe drinking water for the population.

In our study, we observed that the presence of E. coli and total coliforms in most water samples indicates contamination, which aligns with findings from similar studies in other regions. For instance, research by Smith et al. [34] in rural areas of Sub-Saharan Africa also reported high levels of bacterial contamination in groundwater, emphasizing the need for regular monitoring and treatment to ensure water safety.

Furthermore, the elevated levels of ammonium (NH4+) detected in our study are consistent with findings by Johnson et al. [35], who highlighted the impact of agricultural runoff and inadequate waste management on groundwater quality. These studies collectively underscore the critical role of human activities in influencing water quality and the necessity for effective policy interventions.

The study aims to contribute to the overall improvement of living conditions in Moussoro through the provision of clean and safe drinking water.

Water quality analysis help identify specific contaminants and their concentrations. This data is essential for setting maximum contaminant levels (MCLs) in drinking water standards [36]. Detailed water quality analyses provide the basis for health risk assessments. These assessments determine the potential health impacts of various contaminants, guiding the establishment of safety thresholds. The findings inform the development of water quality criteria, which are scientific benchmarks used to protect human health and aquatic life [36]. These criteria are integral to the formulation of water quality standards. The data can support the creation and revision of regulatory frameworks, which mandates the maintenance and restoration of water quality in the study area. The insights from water quality studies can influence public health policies by highlighting areas that require intervention, such as the need for improved water treatment processes or the protection of vulnerable water sources.

4 Conclusion

The study of Moussouro groundwater, through the analysis of field and laboratory physicochemical data, as well as hydrochemical and statistical analyses, has led to an understanding of their characteristics and potential potability. The values obtained from the analyses are, in most cases, in accordance with the guidelines recommended by the World Health Organization (WHO) for drinking water. However, non-compliance was observed in terms of microbiological aspects, with the presence of indicator bacteria of pollution, such as E. coli and TC are observed.

The hydrochemical classification using the Piper diagram revealed two types of characteristic: the calcic and magnesic bicarbonate characteristic on one hand, and the sodic and potassic bicarbonate characteristic on the other. The principal component analysis (PCA) indicates that, the strong correlations of variables highlight the similarity of processes contributing to the mineralisation of groundwater in Moussoro.