Hydrogeochemical characterization and appraisal of groundwater suitability for drinking purposes using water quality index (WQI) in Aksum town and surrounding areas, Tigray, Ethiopia Open Access

Obviously, as the world advances into the 21st century, water has emerged as one of the key and undeniably pervasive issues encountered by society. The natural occurrence and distribution of water on the Earth's surface is 97.3 and 2.7% under the ocean and freshwater, respectively (Tenalem & Tamuru 2001). These figures highlight the limited availability of water for domestic, agricultural, and industrial uses compared to the total amount of water available on the planet. The core problem lies in the failure to recognise and manage water as a finite resource. In general, although groundwater is an essential source of water for various purposes, this valuable resource is increasingly threatened by anthropogenic and geological processes (Tajwar et al. 2023). The quality of water is as paramount as its quantity, as it plays a vital impact in its determination of the appropriateness for different purposes. Therefore, it is essential to guarantee that the available water is free from contaminants to ensure safety and safeguard social well-being (Elkhalki et al. 2023). The WHO estimates that close to 80% of human diseases are attributable to inadequate quality of water (Ram et al. 2021; Saqib et al. 2023; Shuaibu et al. 2024).

According to Masood et al. (2022) and Niyazi et al. (2023), the key factors affecting groundwater chemistry include the interaction between rocks or minerals with water, the composition of the recharge water, the duration of groundwater retention in the aquifer, geochemical possesses or reactions, the dissolution and precipitation of minerals within the saturated zone, and climatic conditions. Additionally, human activities such as uncontrolled domestic and municipal wastes, the application of fertilizers and manures, landfill sites, unplanned urbanization, industrialization, population growth, and overexploitation are key factors affecting groundwater quality (Khan et al. 2020; Kharake & Raut 2020; Masood et al. 2022; Shuaibu et al. 2024).

The water quality index (WQI) was initially introduced by Horton (1965) and later refined by Brown et al. (1970). The WQI serves as a metric that aggregates various indicators of water quality to offer a complete assessment of water conditions. It is widely recognized that evaluating water quality focusing on a single parameter fails to offer a comprehensive understanding. Therefore, the WQI has become a globally accepted tool for water quality assessment (Berhe 2020; Abadi et al. 2024), highlighting the significance of implementing a comprehensive approach to water quality evaluation. Once the groundwater becomes contaminated, remediation or cleaning is expensive, time-consuming, and challenging (Moges & Dinka 2022; Yıldırım 2023). Therefore, continuous monitoring, effective quality of groundwater management, and the enforcement of preventive actions are essential.

Aksum began using groundwater for its water supply in the early 1970s by drilling three wells. However, from the outset, two main issues were identified: poor water quality in some wells and low yield in others. Laboratory test results reported by Abay Engineering (2006), showed that groundwater in the study area has elevated amounts of carbon dioxide (CO₂), iron (Fe), manganese (Mn), total hardness (TH), and total dissolved solids (TDS), exceeding the standards set by World Health Organization (WHO) guidelines. Additionally, the groundwater exhibits lower pH and dissolved oxygen levels. For this reason, several wells in the area have been abandoned. However, the aquifer systems contributing to poor groundwater quality remain poorly understood, necessitating detailed and further studies through the calculation of the drinking water quality index, comparison of ionic molar ratios, and interpretation using analytical and graphical techniques.

Therefore, the primary objective of this research is to conduct a comprehensive assessment of groundwater quality by employing WQIs, specifically the drinking water quality index, to evaluate its suitability for human consumption. Additionally, the study aims to characterize the underlying hydrogeochemical processes that influence groundwater chemistry and quality. This is achieved through a combination of analytical methods, such as the evaluation of ionic concentrations and calculation of ionic ratios, and graphical techniques, including Piper, Schoeller, and Gibbs diagrams.

Topographically, the study area includes both plains and rugged landscapes, with elevations varying between 1,900 and 2,500 m above mean sea level. The drainage pattern is predominantly dendritic (Figure 1).

The Precambrian basement rock is composed of low-grade, altered Neoproterozoic metamorphic formations associated with the Arabian–Nubian Shield (Tadesse et al. 1999; Alemayehu 2011). These rocks are characterized by extensive weathering, fracturing, and shearing with colours ranging from light grey to dark grey and light green (Alemayehu 2011).

The Mesozoic sedimentary formations include undifferentiated clastic sedimentary rocks, predominantly reddish sandstones. These sandstones are notable for their reddish colour, extensive weathering, and significant fracturing. They are mainly exposed in the southern, southeastern, and southwestern portions of this research area. The undifferentiated sediments exhibit variability in colour, thickness, texture, and degree of maturity (Alemayehu 2011). Common sedimentary structures in the unit include laminations in siltstone and bedding or cross-bedding.

Basaltic flow dominates the region, characterized by pronounced textural and compositional heterogeneity, with intercalated thin red paleo-soil horizons throughout the stratigraphic sequence (Tadesse 1997). Basalt exhibits a fresh black hue in unweathered sections and a light brownish colour in weathered zones. It has a fine grain size and massive structure, with areas showing extensive fracturing and advanced weathering. Columnar jointing is also prominently observed. In the study area, trachyte and phonolite units occur in smaller outcrops, forming elevated areas (Figure 2). It is bright white to whitish-grey fresh colour, with moderate weathering, and a relatively hard, compact, and massive texture that distinguishes the trachyte unit. In contrast, phonolite is distinguished by dark grey to pink colour, coarse grain size, extensive weathering (mainly due to exfoliation), and significant fracturing caused by jointing. The hydrogeological characteristics and groundwater potential of the area are significantly impacted by the geology, physiography, climate, and geological structures (Ataklti et al. 2024). Volcanic rock formations and the related sedimentary deposits form intricate aquifer systems, where the lithology and internal structural features of the host formations predominantly govern groundwater occurrence and distribution (Alemayehu 2011). The underlying sandstone formation exhibits limited aquifer yield due to the intercalation of clay and siltstone, which restricts aquifer capacity.

Basement rocks generally have low permeability, except in their upper weathered horizons (Alemayehu 2011). Both trachyte and phonolite have negligible groundwater potential due to their steep slopes, limited recharge areas, insufficient porosity, and extremely low permeability (Tadesse 2017). Overall, the hydrogeological characteristics of the area's rocks are primarily controlled by secondary porosity, such as the degree of weathering and fracturing. The primary aquifer in the region is the extensively weathered and fractured segment of basaltic formation (Alemayehu 2011).

In May 2020, 19 samples of water were taken from various sources to ensure representation: three from deep wells, 10 from shallow wells, four from hand-dug wells, one from a spring, and one from surface water (Dam) using cleaned polyethylene bottles (Figure 1). The sample water was collected in 1 L plastic bottles, which were carefully cleaned, labeled, and sealed to ensure sample integrity. Prior to sampling from each well, purging was conducted for 4–5 minutes to flush out stagnant water, and each sample bottle was thoroughly rinsed at least three times using the groundwater to be collected.

To maintain the integrity of the collected water samples, standard procedures for sampling, preservation, transportation, and analysis, and strict adherence to the guidelines set by the American Public Health Association (2017) were maintained. A GARMIN eTrex 10 handheld GPS was used to identify the water sampling sites. During the fieldwork, on-site measurements of key parameters, including pH (measured with HANNA METER (HI 98127) with an accuracy of ±0.1), temperature, and electrical conductivity (EC) (measured with HANNA METER HI 98312) were recorded.

Once collected, all water samples were moved to the laboratory and refrigerated at 4 °C to prevent contamination and minimize the impact of heat and light. The tests were performed at the geochemical laboratory of Mekelle University's School of Earth Sciences. All water samples were analyzed using the instruments listed in Table 1.

Table 1

Methods used for the chemical analysis of the water sample

Parameters .	Analysis methods .
Ca2+, Mg2+, Fe, Mn	Atomic Absorvation Spectrometer
Na+, K+	Flame Photometer (model-JENWAY PEP7)
Cl−, NO3−, ⁠, PO43−, NH4+, F−	UV–VIS spectrophotometer
Alkalinity, CO32−,	Titration methods

Parameters .	Analysis methods .
Ca2+, Mg2+, Fe, Mn	Atomic Absorvation Spectrometer
Na+, K+	Flame Photometer (model-JENWAY PEP7)
Cl−, NO3−, ⁠, PO43−, NH4+, F−	UV–VIS spectrophotometer
Alkalinity, CO32−,	Titration methods

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Both ions were converted to milliequivalents per litre (meq/L). Acceptable discrepancies in electroneutrality should not exceed 5%. For this study, the ion balance error for all water samples was within the acceptable range. In addition, descriptive statistical analysis and correlation analysis between different water quality parameters were performed using the Statistical Package for the Social Sciences (version 20).

To obtain a better understanding of the hydrogeochemical process in the study area, several graphical plots were employed, including Piper diagrams, Schoeller plots, Gibbs diagrams, and ionic molar ratios.

The Piper diagram is implemented to classify the groundwater types or hydrochemical facies based on the concentration of major cations and anions. This diagram aids in identifying the dominant ions and understanding the genesis and evolution of groundwater chemistry (Piper 1944; Diédhiou et al. 2023). Schoeller plot was used for the comparative visualization of major ion concentrations in groundwater samples. Both diagrams have been plotted using Aquachem software (version 4).

The WQI, initially formulated by Horton (1965), provides a composite score representing the overall suitability of groundwater for drinking. It combines several parameters to categorize water as excellent, good, poor, very poor, or unfit for potable use (Kachroud et al. 2019; Gautam & Rai 2023; Mechal et al. 2024; Sanad et al. 2024).

The WQI for all groundwater samples was computed based on the parameters EC, TDS, Na⁺, Ca²⁺, Mg²⁺, K⁺, ⁠, Cl⁻, ⁠, NO₃⁻, Fe²⁺, and F⁻ (Table 2).

Table 2

Relative weights of water quality parameters for groundwater and WHO standards referenced to calculate the WQI

Parameter .	WHO standard (2011) .	Weight (wi) .	Relative weight (Wi) .
EC (μS/cm)	1,000	4	0.09
TDS (mg/L)	500	5	0.11
	500	1	0.02
Na+	200	4	0.09
Ca2+	75	3	0.07
Mg2+	50	3	0.07
K+	12.00	2	0.04
Cl−	250	5	0.11
	250	5	0.11
NO3−	50	5	0.11
Fe2+	0.3	4	0.09
F−	1.50	5	0.11
		= 46	= 1

Parameter .	WHO standard (2011) .	Weight (wi) .	Relative weight (Wi) .
EC (μS/cm)	1,000	4	0.09
TDS (mg/L)	500	5	0.11
	500	1	0.02
Na+	200	4	0.09
Ca2+	75	3	0.07
Mg2+	50	3	0.07
K+	12.00	2	0.04
Cl−	250	5	0.11
	250	5	0.11
NO3−	50	5	0.11
Fe2+	0.3	4	0.09
F−	1.50	5	0.11
		= 46	= 1

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According to Shuaibu et al. (2024), the WQI values were derived through a structured four-stage process: assigning weights to each groundwater quality parameter, computation of relative weights, computing the rating scale for each water quality parameter, and computing the sub-index and the overall WQI.

In the first stage, weight values (w_i) were allocated to each parameter based on its comparative significance in determining water's appropriateness for consumption. The allocated weights (w_i) ranged from 1 (indicating minimal impact) to 5 (indicating maximum impact) (Table 2).

This step-by-step methodology ensures a comprehensive evaluation of groundwater quality, integrating multiple parameters into a single index that reflects its suitability for drinking purposes based on the classification stated in Table 3.

Table 4 presents the minimum, maximum, and mean values of the physicochemical parameters of groundwater samples taken from the study area. pH, which measures the balance between acidity and alkalinity balance in a solution, ranged from 6.2 to 7.9, with an average value of 7.14. This indicates a slightly acidic to basic characteristic (Table 4).

Table 4

Statistical results for the chemical analysis of groundwater samples [explanation: EC (μS/cm) and all concentrations (mg/L) except pH]

Parameters .	Minimum .	Maximum .	Mean .	Std. deviation .
pH	6.20	7.90	7.14	0.44
TDS	387.38	2,774.00	702.00	529.13
EC	403.00	3,890.00	856.33	773.10
TH	269.15	1,121.67	440.83	189.73
Ca2+	86.00	232.00	122.28	35.86
Mg2+	12.00	130.00	32.50	26.48
K+	0.72	17.30	4.51	4.92
Na+	3.19	83.69	14.05	18.49
NH4+	0.04	1.23	0.16	0.29
Fe	0.17	0.96	0.40	0.27
Cl−	11.56	65.60	29.70	12.18
	103.50	282.60	186.33	44.21
	66.36	1,117.10	233.55	231.90
NO3−	1.35	96.63	37.82	33.10
F−	0.02	1.92	0.21	0.46

Parameters .	Minimum .	Maximum .	Mean .	Std. deviation .
pH	6.20	7.90	7.14	0.44
TDS	387.38	2,774.00	702.00	529.13
EC	403.00	3,890.00	856.33	773.10
TH	269.15	1,121.67	440.83	189.73
Ca2+	86.00	232.00	122.28	35.86
Mg2+	12.00	130.00	32.50	26.48
K+	0.72	17.30	4.51	4.92
Na+	3.19	83.69	14.05	18.49
NH4+	0.04	1.23	0.16	0.29
Fe	0.17	0.96	0.40	0.27
Cl−	11.56	65.60	29.70	12.18
	103.50	282.60	186.33	44.21
	66.36	1,117.10	233.55	231.90
NO3−	1.35	96.63	37.82	33.10
F−	0.02	1.92	0.21	0.46

TH, total hardness.

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EC is a parameter that reflects water's ability to conduct an electrical current (Ram et al. 2021; Demelash et al. 2023), varied between 403 and 3,890 μS/cm, with an average of 856.33 μS/cm (Table 4). The lowest value (403 μScm) was recorded in a hand-dug well (HDW6), while the highest value (3,890 μS/cm) was observed in a deep well (DPW3) located in Adikerni.

The TDS level in the study area varied from 387.38 mg/L in a hand-dug well (HDW6) to 2,774 mg/L in a deep well (DPW3) in Adikerni (Table 5). Approximately 94.7% of the collected water samples were categorized into freshwater, whereas the rest 5.3% were categorized as brackish water (Table 5).

Water hardness, mainly attributed to the presence of Ca²⁺ and Mg²⁺, is represented as the equivalent concentration of CaCO₃ in mg/L (Demelash et al. 2023). Total hardness in the samples varied between 269.15 mg/L of CaCO₃ in a hand-dug well (HDW6) and 1,121.67 mg/L of CaCO₃ in a deep well (DPW3), with a mean of 440.83 mg/L of CaCO₃ (Table 6).

The classification system proposed by Hem (1989) was used to categorize water hardness in the study area. About 94.7% of the samples were identified as very hard water, while only one sample (SRW) fell under the moderately hard category (Table 6).

Groundwater primarily contains dissolved ions, which are commonly referred to as major ions including Ca²⁺, Mg²⁺, Na⁺, K⁺, ⁠, Cl⁻, and (Freeze & Cherry 1979). The most abundant in the area is calcium (Ca²⁺), having concentrations that range from 86 mg/L in a hand-dug well (HDW6) to 232 mg/L in a deep well (DPW3), with a mean value of 122.28 mg/L (Table 5). Magnesium (Mg²⁺) is the second most dominant cation, varying between 12 mg/L in a hand-dug well (HDW3) and 130 mg/L in a deep well (DPW3), with an average value of 32.5 mg/L. The Na⁺ concentrations range between 3.19 mg/L in SW10 and 83.69 mg/L in DPW3, with a mean of 14.05 mg/L. Potassium (K⁺), the least abundant cation, ranges between 0.72 mg/L in SW8 and 17.3 mg/L in DPW3, with a mean of 4.51 mg/L (Table 4).

Among the anions, bicarbonate is the most predominant, ranging between 66.36 mg/L in HDW6 and 1,117.1 mg/L in DPW3, with a mean of 233.55 mg/L. Sulphate (⁠⁠) is the second most prevalent anion, with concentrations between 103.5 mg/L in SW15 and 282.6 mg/L in SW12, with a mean of 186.33 mg/L (Table 4).

However, nitrate (NO₃⁻) levels in several locations exceeded the permissible limits, having a peak concentration of 96.63 mg/L observed in SW12. The high nitrate levels in shallow groundwater are mainly associated with anthropogenic activities, including overuse of fertilizers, pesticides, and manures, as well as unmanaged domestic and municipal waste, uncontrolled sewage discharge, and leaky landfills (Alemayehu 2011).

Five distinct hydrochemical facies or water types were identified through the analysis of a Piper diagram. The results indicate that 57.8% of the samples are categorized into the Ca–Mg–SO₄–HCO₃ type, 15.8% of the samples are classified within the Ca–Mg–HCO₃–SO₄ type, 15.8% samples are categorized under the Ca–SO₄–HCO₃ type, 5.3% of the samples are identified as part of the Ca–SO₄ type, and the remaining 5.3% of the samples are classified within the Ca–Mg–HCO₃ type (Table 7).

Table 7

Hydrochemical facies of water samples determined by the predominant cations and anions

Sample ID .	Cation .	Anion .	Hydrochemical facies .
HDW6	Ca2+ > Mg2+ > Na+ > K+	> > Cl−	Ca–SO4
SW1, SW3, SW5, SW9, SW10, SW12, DPW1, DPW2, SRW, HDW1, HDW7			Ca–Mg–SO4–HCO3
HDW3, SW8, SW13			Ca–SO4–HCO3
DPW3		> >	Ca–Mg–HCO3
SW14, SW15, SP			Ca–Mg–HCO3–SO4

Sample ID .	Cation .	Anion .	Hydrochemical facies .
HDW6	Ca2+ > Mg2+ > Na+ > K+	> > Cl−	Ca–SO4
SW1, SW3, SW5, SW9, SW10, SW12, DPW1, DPW2, SRW, HDW1, HDW7			Ca–Mg–SO4–HCO3
HDW3, SW8, SW13			Ca–SO4–HCO3
DPW3		> >	Ca–Mg–HCO3
SW14, SW15, SP			Ca–Mg–HCO3–SO4

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Ca²⁺/Mg²⁺ molar ratio: The molar ratio of Ca²⁺/Mg²⁺ is a commonly applied marker to detect the dominant geochemical processes influencing groundwater, for example, the dissolution of calcite and dolomite minerals or the weathering of silicates (Berhe et al. 2021; Niyazi et al. 2023). A Ca²⁺/Mg²⁺ ratio of approximately 1 suggests the dissolution of dolomite, whereas a ratio between 1 and 2 shows calcite dissolution. Ratios exceeding 2 signify the contribution of silicate mineral weathering to the presence of calcium and magnesium in the groundwater (Berhe et al. 2021; Aweda et al. 2023; Diédhiou et al. 2023; Niyazi et al. 2023).

Na⁺/Cl⁻ molar ratio: The sodium-to-chloride molar ratio (Na⁺/Cl⁻) is a critical parameter for evaluating the origin of Na⁺ and Cl⁻ in groundwater. Under typical rock-water interaction processes, halite dissolution governs sodium concentrations, resulting in a Na⁺/Cl⁻ molar ratio close to 1.

In the study area, one sample (SRW) indicated Na⁺ released from weathering of silicate minerals and two samples (SW15 and HDW1) reflected the presence of the dissolution of halite. Notably, 84.2% of the samples fell below the 1:1 line, indicating a predominance of Cl⁻ over Na⁺ (Figure 5(b)).

/Cl⁻ molar ratio: The /Cl⁻ molar ratio was analyzed to assess the outcomes of human-induced actions on groundwater quality in the study area. All collected samples exhibited /Cl⁻ molar ratios exceeding 0.05.

Ca²⁺+Mg²⁺ vs. scatter plot: The connection between (Ca²⁺ + Mg²⁺) and was examined to recognize processes including ion exchange and reverse ion exchange (Figure 5(d)). Points deviating to the left, where HCO₃⁻ + SO₄²⁻ exceed Ca²⁺ + Mg²⁺, suggest ion exchange dominance. Conversely, points shifting to the right, where Ca²⁺ + Mg²⁺ exceed HCO₃⁻ + SO₄²⁻, indicate reverse ion exchange processes (Demelash et al. 2023; Abadi et al. 2024). In this research, 84.2% of groundwater samples were plotted to the right, and 15.8% of the samples aligned along the 1:1 line.

Na⁺+K⁺ vs. TZ⁺ and Ca²⁺+Mg²⁺ vs. TZ⁺ scatter plots: The scatter plots of Na⁺ + K⁺ vs. total cation (TZ⁺), and Ca²⁺ + Mg²⁺ vs. TZ⁺ (Figures 5(e) and 5(f)) revealed that the entire set of water samples fell below the equiline (1:1). This reveals the predominant contribution of cations from ion exchange processes or silicate mineral weathering (Aweda et al. 2023; Demelash et al. 2023). Moreover, Saraswat et al. (2019) noted that samples falling along the equiline in the Ca²⁺ + Mg²⁺ vr. TZ⁺ plot suggests that Ca2 + and Mg2+ (Figure 5(f)) primarily dominate the total cation concentrations.

Pearson correlation analysis was conducted to understand the connection between various hydrogeochemical factors of groundwater and the results are presented in Table 8. Statistical analysis was carried out on the concentrations of major ions and physicochemical parameters to pinpoint the relationships and variations among the water samples. The moderate to strong correlations among different parameters are highlighted in bold (Table 8).

The results show an exceptionally strong correlation (r = 1) between TDS and EC, suggesting that EC is a function of TDS or that water conductivity is influenced by TDS (Berhe et al. 2021; Singh et al. 2023; Sanad et al. 2024). Besides, both EC and TDS exhibit moderate to strong correlations with Ca²⁺, Mg²⁺, Na⁺, NH₄⁺, ⁠, Cl⁻, and F⁻ (Table 8). This indicates that TDS and EC are often controlled by or derived from these cations and anions (Berhe et al. 2021; Demelash et al. 2023).

As shown in Table 8, moderate to strong correlation coefficients were detected between with Ca²⁺ (r = 0.77), Mg²⁺ (r = 0.98), Na⁺ (r = 0.94), and K⁺ (r = 0.65). This suggests various processes, including water-rock interaction, mainly silicate weathering, ion exchange, and making a dominant ion (Berhe et al. 2021; Demelash et al. 2023).

According to Demelash et al. (2023), the poor relationship between EC with NO₃⁻ and shows that the source of these ions is related to anthropogenic. Furthermore, the weak correlation coefficient between K⁺ with and NO₃⁻ suggests a non-geogenic origin, likely due to agricultural activities (e.g., fertilizers, pesticides) and unmanaged domestic and municipal waste (Deshpande & Murkute 2023). Table 8 also indicates a very high correlation coefficient between Ca²⁺ and Cl⁻, NO₃⁻ and Mg²⁺ with Cl⁻, NH₄⁺, F⁻ and Na⁺ with NH₄⁺, F⁻.

Based on the findings of Berhe et al. (2017), Niyazi et al. (2023), and Diédhiou et al. (2023), one of the most critical factors influencing the hydrochemical characteristics of groundwater during its flow and residence time within the aquifer involves the ion exchange process occurring between groundwater and the host rock.

If both CAI₁ and CAI₂ are positive, this shows an exchange of Na⁺ and K⁺ from the groundwater with Ca²⁺ and Mg²⁺ from the host rocks. Conversely, if both CAI₁ and CAI₂ are negative, it implies the exchange of Ca²⁺ and Mg²⁺ from the groundwater with Na⁺ and K⁺ from the host rocks (Demelash et al. 2023). In this study, 73.68% of the samples showed positive values, suggesting the exchange of Na⁺ and K⁺ from the water with Ca²⁺ and Mg²⁺ from the rocks. However, the rest 26.32% of the water samples had negative values, signifying the reverse exchange of Ca²⁺ and Mg²⁺ from the water with Na⁺ and K⁺ from the host rocks (Figure 6).

The main goal of calculating WQIs is to assess the potability of groundwater by considering the collective impact of various physicochemical parameters (Gebrerufael et al. 2019; Singh et al. 2023). The WQI method has been widely applied globally to assess the water quality in different regions (Alogayell et al. 2023; Liu & Li 2023; Tesema et al. 2023; Shaibur et al. 2024).

In the current investigation, the calculated WQI values for the water samples ranged between 25.6 and 215.94, with a mean of 69.09 (Table 9). From the WQI results, the groundwater samples taken from the research site are classified into three groups: excellent, good, and very poor water quality for drinking. Specifically, the groundwater samples are distributed as follows: excellent (15.8%), good (78.9%), and very poor (5.3%) (Table 10). The 5.3% categorized as very poor water quality represents only one sample (Table 9). It is a deep well, and the elevated concentrations of ions could be a result of the geogenic process (rock–water interaction) that is due to the long residence time of circulation. However, the water samples categorized as good are from shallow and hand-dug wells, whose quality is influenced by both geogenic processes and likely by anthropogenic activities such as agricultural runoff (fertilizers and pesticides).

The physicochemical analysis of groundwater samples from the study area reveals significant spatial variations in water quality, reflecting the influence of geological formations, hydrogeochemical processes, and anthropogenic activities.

The observed pH variation is influenced by lithology, groundwater recharge processes, and interactions with atmospheric CO₂. Groundwater in carbonate-rich formations typically exhibits near-neutral to slightly alkaline pH due to the dissolution of minerals like calcite and dolomite. In contrast, slightly acidic pH may result from organic matter decomposition, redox reactions, or anthropogenic factors such as agricultural runoff. Low pH values are often associated with the absence of carbonate minerals in the aquifer and pollution sources like landfills or mine drainage (Masarik et al. 2006).

Elevated TDS concentrations in some deep wells in Aksum's groundwater system are associated with prolonged water-rock interactions (Tefera 2004; Alemayehu 2011). The findings suggest that deep groundwater has significantly higher cation concentrations compared to shallow groundwater, consistent with the previous studies by Alemayehu (2011), which links this to the dissolution of silicate minerals like plagioclase, pyroxene, and olivine.

Elevated sulphate levels in some locations are likely linked to anthropogenic influences. Chloride (Cl⁻) is the least dominant anion, ranging from 11.56 mg/L in SW8 to 65.6 mg/L in DPW3, with a mean of 29.7 mg/L (Table 4).

In general, the hydrochemical analysis indicates that deeper groundwater exhibits higher concentrations of major cations and anions compared to shallow sources, mainly due to extended water-mineral interaction. Elevated nitrate and sulphate levels in some areas underscore the impact of human activities, specifically agricultural practices and improper waste management. These results emphasize the significance of long-term groundwater resource management practices to mitigate anthropogenic contamination and preserve the quality of water within the study area. This dominance is attributed to the weathering of silicate minerals such as amphibolite, pyroxene, olivine, plagioclase feldspar, and clay minerals, which contribute significantly to the concentrations of Ca²⁺ and Mg²⁺ in the groundwater (Alemayehu 2011). On the anions side, weak acids (⁠⁠) are more abundant than strong acids (⁠ and Cl⁻). The abundance of bicarbonate is primarily associated with the interaction of groundwater with CO₂ derived from soil organic matter decay and magmatic sources. This CO₂ reacts with water to produce elevated levels of bicarbonate in the system (Alemayehu 2011). The findings of hydrogeochemical facies imply that calcium, magnesium, bicarbonate, and sulphate are the principal ionic species influencing the chemical characteristics of groundwater within the study area. Gibbs diagram indicates the governing mechanisms of hydrogeochemical processes and depicts the water-rock interaction as the dominant process in the study area.

The Ca²⁺/Mg²⁺ ratios show that 68.4% of water samples exhibited ratios greater than 2, reflecting the significant influence of silicate mineral weathering and the remaining 31.6% of the samples displayed ratios between 1 and 2, suggesting the dissolution of calcite mineral (Figure 5(a)). The relatively low Na⁺ concentrations (Figure 5(b)) might be due to base-exchange processes or contamination due to human-induced actions (Diédhiou et al. 2023; Singh et al. 2023). The higher SO₄^2–/Cl⁻ ratios (>0.05) are mainly related to agricultural return flow water, often linked to the use of gypsum-based fertilizers (Figure 5(c)). The Ca²⁺ + Mg²⁺ vs. HCO₃⁻ + SO₄²⁻ scatter plot shows that most groundwater samples have resulted from reverse ion exchange processes and some 15.8% of the samples reflect the influence of silicate, carbonate, and sulphate mineral dissolution occurring in the aquifer system (Figure 5(d)).

The existence of nutrients such as NO₃⁻ and NH₄⁺ from agricultural/irrigation activities, and domestic and municipal contaminations can also influence TDS and EC concentrations. The high correlation between Na⁺ and Cl⁻ shows the influence of human-induced activities such as agricultural practices (fertilizers, pesticides, and manures), and municipal and domestic waste. Human activities have a significant impact on the groundwater of the study area; this can be supported by the presence of a very high correlation coefficient between Ca²⁺ and Cl⁻, NO₃⁻ and Mg²⁺ with Cl⁻, NH₄⁺, F⁻ , and Na⁺ with NH₄⁺, F⁻ (Table 8).

In this study, positive values (73.68%) of CAI suggested the exchange of Na⁺ and K⁺ from the water with Ca²⁺ and Mg²⁺ from the rocks and the remaining samples (26.32%) had negative values, implying the reverse exchange of Ca²⁺ and Mg²⁺ from the water with Na⁺ and K⁺ from the host rocks (Figure 5).

The principal purpose of this research was to evaluate the suitability of water for drinking purposes and to characterize the hydrogeochemical processes influencing its quality and composition. To achieve this, various ionic ratios, graphical plots (Piper, Schoeller, and Gibbs diagrams), statistical analyses, and WQI assessments were employed. The key findings of the study are summarized below:

The Schoeller diagram demonstrated that the predominant sequence of major ions in groundwater is Ca²⁺ > Mg²⁺ > Na⁺ + K⁺ and > > Cl⁻.

According to the Piper diagram, five hydrochemical facies or water types were identified: Ca–Mg–SO₄–HCO₃ (57.8%), Ca–Mg–HCO₃–SO₄ (15.8%), Ca–SO₄ (5.3%), Ca–SO–HCO₃ (15.8%), and Ca–Mg–HCO₃ (5.3%), where Ca–Mg–SO₄–HCO₃ is the dominant water type. These results indicate that calcium, magnesium, and bicarbonate are the key constituents of the groundwater in the study area.

The water quality analysis of the study area reveals that groundwater is influenced by a combination of geogenic processes and anthropogenic activities. Thus, the Gibbs diagram results emphasize that the water and rock interaction mechanisms predominantly govern the groundwater chemistry. Scatter plots of ionic ratios such as Ca²⁺/Mg²⁺, Na⁺/Cl⁻, (Ca²⁺ + Mg²⁺)/(⁠ + ⁠), Ca²⁺ + Mg²⁺ vs. TZ⁺ (total cation), and Na⁺ + K⁺ versus TZ⁺ (total cation) analyses, highlight the influence of silicate mineral dissolution, ion exchange, and reverse ion exchange processes. The findings of this study reveal that groundwater quality in the study area is influenced by both agricultural activities and domestic wastewater discharges. Human-induced activities play a role in increased concentrations of specific elements (ions such as Cl⁻, NO₃⁻, SO₄^2–, NH₄⁺, and F) in the groundwater. The extensive use of fertilizers and agrochemicals in the surrounding farmlands contributes to elevated levels of nitrates and other ions, indicating agricultural impact. For instance, the higher SO₄^2–/Cl⁻ ratios (>0.05) are mainly related to agricultural return flow water, often linked to the use of gypsum-based fertilizers, and a predominance of Cl⁻ over Na⁺ in most of the samples also shows the effect of anthropogenic activities. In addition, untreated or poorly managed domestic wastewater, particularly from hotels and urban sewerage systems, significantly affects groundwater quality, introducing contaminants such as high levels of nitrate and ammonium. However, the concentrations of major ions such as Ca²⁺, Mg²⁺, K⁺, and Na⁺, in groundwater are mainly controlled by geogenic processes.

Correlation coefficient analysis demonstrated a strong relationship between TDS and EC, indicating that EC is primarily influenced by TDS. Moderate to strong correlations were observed between EC, TDS, and ions such as Ca²⁺, Mg²⁺, Na⁺, ⁠, Cl⁻, NH₄⁺, and F⁻, suggesting that these ions frequently regulate the EC and TDS levels. Furthermore, high correlations Ca²⁺, NO₃⁻, and Mg²⁺ with Cl⁻, NH₄⁺, and F⁻ with Mg²⁺ and Na⁺, suggest significant contributions of NO₃⁻, Cl⁻, NH₄⁺, and F⁻ from anthropogenic sources to hydrochemical characteristics.

The CAIs (CAI-I and CAI-II) revealed that 73.68% of the samples showed positive values, indicating an exchange of Na⁺ and K⁺ from the groundwater with Ca²⁺ and Mg²⁺ from the host rocks. Conversely, 26.32% of the samples exhibited negative values, representing an indirect exchange of Ca²⁺ and Mg²⁺ from the groundwater with Na⁺ and K⁺ from the host rocks.

The WQI values for groundwater samples varied between 25.6 and 215.94, with a mean of 69.09. The classification of water quality indicated that it was excellent (15.8%), good (78.9%), and very poor (5.3%). The very poor water quality, observed in a single deep well, is likely due to prolonged rock-water interaction. In contrast, the good-quality water from shallow and hand-dug wells is influenced by both geogenic processes and anthropogenic activities, such as agricultural runoff. Future groundwater management strategies should consider the integration of nature-based solutions, particularly in urbanized areas, to mitigate anthropogenic pollution. Approaches such as phyto-purification, filter strips, and the planting of native vegetation can enhance natural filtration processes, reducing contaminant loads and improving groundwater quality. These solutions offer sustainable and cost-effective alternatives that align with ecosystem-based management strategies, contributing to long-term water resource protection.

The authors express their gratitude to Mekelle University for the research support.

A research grant was provided to the first author by Mekelle University, and their contribution is appreciated.

I, on behalf of the team, declare that there are no competing financial interests or personal relationships that might have influenced the findings in this paper.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.