Appraising groundwater quality and probabilistic human health risks from fluoride-enriched groundwater using the pollution index of groundwater (PIG) and GIS: a case study of adama town and its vicinities in the central main Ethiopian rift valley

Abstract

This research's main objective is to identify the level of contamination in drinking water in Adama town and its environs by employing PIG, GIS and HHRA. The physical–chemical parameters of groundwater were determined, and the results were compared to regional and global drinking water quality guidelines. The pH of groundwater is alkaline, and the contents of Ca²⁺, Na⁺, HCO₃⁻, and F⁻ in the majority of samples surpassed the permissible drinking limit. The hydrochemical facies were identified in the following order: Ca–Mg–HCO₃, Na–Ca–HCO₃, and Na–HCO₃. Cation exchange and Rock–water interaction are the major dominant natural mechanisms controlling groundwater chemistry. Using IDW interpolation methods with Arc GIS 10.8, spatial analysis of the physico-geochemical content of water divulged that TDS, pH, TH, EC, Mg²⁺, Ca²⁺, K⁺, Na⁺, Cl⁻, HCO₃⁻, F⁻, and SO₄²⁻ all exhibit a positive trend in the direction of groundwater flow from the upland to the lowland (rift floor). As per PIG, the results show that 57%, 33%, 7% and 3% of the samples were found in the insignificant, low, moderate and high, correspondingly. The total hazard index (THI) is calculated from hazard quotients (HQIntake and HQDermal) results showing 83%, 73%, and 57% of the samples exceed the non-carcinogenic health threat of fluoride THI >1 in drinking water for children, women and men. Children are more susceptible to danger than either males or women, according to the THI data, based on body weights and consumption rates. Similarly, females are also more vulnerable to health risks than men.

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1. Introduction

Over the last two decades, groundwater has become an increasingly important source of water supply in different locations all over the globe. The main source of water for many nations is groundwater. Groundwater is an important aspect of our system, and it is largely used for drinking and agriculture. Groundwater is a primary supply of domestic water in many African cities.^1,2 Rapid population growth, irrigation development, and an expanding tendency of industrialization have all contributed to increased demand for groundwater. Natural processes degrade groundwater quality, but anthropogenic pollution, which comes from sources such as agricultural fertilizers, industrial effluents, municipal wastewater, landfills, and animal wastes, is also a major driver of water quality degradation.^3–5 However, one of the concerns of the twenty-first century is the degradation of water quality.⁶ Groundwater quality is declining worldwide as a result of anthropogenic activities like increasing urbanisation, industrial expansion, and agrarian intensification.^7–11 Polluted groundwater is less obvious than pollution in rivers and lakes, but it is more difficult to clean up. Groundwater quality has a significant impact on plant development and human health. A slight change in water quality indicates that the aquatic ecosystem is not working properly. Because of weathering from source rocks, anthropogenic activities, and local environmental and ecological circumstances, the quality of groundwater is greatly influenced by physical and chemical soluble factors.¹² Precipitation, the geological structure as well as the mineralogy of aquifers and watersheds, as well as the geological procedures occurring inside the aquifer, are only a few of the variables that affect the chemical composition of groundwater.¹³ In order to improve water resources over the long term, it is essential to comprehend the geochemical evolution of groundwater. The physical and chemical characteristics that impact groundwater quality in a given location are substantially influenced by geogenic and anthropogenic activity.

Inadequate surface water supplies, as well as uncertain and frequent monsoon failures, have prompted planners to look to a more reliable water source, groundwater. Overexploitation of this critical water source, on the other hand, has led to a rapid drop in groundwater levels, which is a problem in many regions of the world, particularly in developing and underdeveloped countries. The implications of such overexploitation have led to the establishment of plans for the long-term development of groundwater resources at all levels: local, regional, and global. Inputs from the evaluation of groundwater resources are critical for such planning, and the current work incorporates a rigorous scientific investigation of several groundwater parameters. Although the notion of groundwater quality appears to be straightforward, determining how to study and evaluate it remains a challenge. It is vital to consider the quality and quantity of groundwater resources to meet the growing demand for water caused by population growth, urbanization, and industrialization, particularly in developing countries. Because of groundwater pollution and aquifer vulnerability, urbanization is a difficult issue in developing countries like Ethiopia.^14–17 The quality of water sources is significantly influenced by human activity in the natural system.¹⁸

Contamination of groundwater by geogenic and anthropogenic causes can have serious health and social repercussions.^19–21 The primary cause of waterborne infections, specifically in rural regions of developing nations like India, South Africa, and Ethiopia, is the use of very contaminated drinking water.^22–24 High priority is given in evaluations of the safety of drinking water to the protection of aquatic ecosystem services, the reduction of the possibility of adverse health effects, and the assurance that uncontaminated water sources will be accessible over the course of many years.^25–27 Inverse Distance Weighting (IDW) and Geographic Information system (GIS) interpolation methods are crucial tools for evaluating and monitoring groundwater properties.^28–32 IDW (deterministic process) is a spatial interpolation algorithm that estimates values between measurements. Spline and kriging techniques are less efficient than IDW because they require more computing and modelling time, whereas kriging requires more user input.^19,33–37 Furthermore, this interpolation method fits well with real-world parameters. Researchers have widely used the IDW interpolation method to study the spatial distribution of physicochemical parameters.^15,38

GIS is an effective method for determining the spatial distribution of contaminants plumes and water resources.³⁹ In order to track the distribution, trends, and physicochemical characteristics of water resources, it is a cost-effective approach to transform data into geographical forecasts.

Many dry and semi-arid parts of Ethiopia depend on groundwater supplies for residential uses, irrigation needs, or agricultural output. Ethiopia presently gets more than 80% of its drinking water from subsurface water.^40,41 Groundwater dependence will become even more reliant in the future as demand rises due to population growth and climate change. In addition, the industry's water requirements have increased overall. The groundwater table is being driven down by intense rivalry among consumers in the agriculture, industry, and household sectors. The quality of groundwater is quickly declining as a result of extensive surface water contamination. Furthermore, untreated wastewater discharged through bores and leachate from non-scientific solid waste disposal contaminates groundwater, lowering the quality of freshwater supplies. In both urban and rural regions of Adama Woreda, groundwater serves as the main source of drinking water. Furthermore, despite increased access to water, the amount, quality, and long-term viability of urban water services are major concerns.

In the MER (Main Ethiopian Rift), groundwater is intensively utilised for the provision of drinking water.⁴² The aquifers throughout the RV (Rift Valley) and the highlands have diverse hydrochemistry. Several researchers have observed high F- levels in RV groundwater,^43,44 and it is the primary geological problem for the supply of drinking water. Due to the presence of mica aquifers, amphiboles, and pyroxene, fluoride may be discovered in the groundwater of the research area.⁴⁵ Furthermore, anthropogenic activities like urban sewage, industrial, as well as agricultural intensification are affecting the quality of surface and groundwater in Adama Woreda, (MER). In spite of the fact that millions of people living along the MER depend on groundwater supplies for their survival, there was no groundwater quality monitoring system in place, nor were there any aquifer or well-head protection zones. For the purpose of efficiently developing new groundwater schemes and managing groundwater resources in the area under investigation, it is required to have a regional variation map of the principal cations and anions. The study's goals are to analyze groundwater quality for drinking applications.

The groundwater is in close touch with a variety of minerals, all of which are soluble in water in different ways. Water's utility for various purposes is determined by dissolved minerals. A rift with igneous structures runs through the research area. Groundwater is the principal source of water in this area, serving not only household but also agricultural and industrial needs. With these considerations in mind, the current research sought to better understand the nature of groundwater, its seasonal hydrogeochemical fluctuations, and its suitability for various uses such as irrigation and residential activities. To determine the best way to develop solutions for the water resource challenge, a powerful tool GIS application, the PIG is utilized to determine the quality of water. Using pollution index of groundwater (PIG), Human health risk assessment (HHRA), and GIS methodologies, the primary aim of this research is to examine the safety of using groundwater for drinking reasons, in addition to the dangers that fluoride pollution may cause to human health.

2. Materials and methods

2.1. Portrayal of research site

2.1.1. Location and climate. Adama Woreda is positioned inside the north-central part of the Main Ethiopian Rift (CMER), within the East Showa area of the Oromia. The geographical location of the town ranges from 8027.5′N–8035.7′N and 39 013.5′E–39019′E with an altitude that generally ranges from 1360–2338 m above sea level (Fig. 1). It's located in the East African Rift Valley (EARV), about 99.5 km southeast of the Ethiopian capital city of Finfinnee (Addis Ababa). It's nestled between two mountain ridges in the Great Rift Valley on generally flat lowland, which is located in the Awash River basin. Adama is the fourth largest city in Ethiopia and one of the most important cities in the Oromiya region. This country's second-most populous city has seen tremendous growth. It is the most populous and rapidly urbanising city in the Oromia Region, which is centrally located on the express highway connecting Addis Ababa and Djibouti and has a plethora of industries and manufacturing enterprises, making it a major commercial and transportation hub. In the Adama woreda, the annual rainfall ranges from 1200 mm to 800 mm. It has bimodal rainfall patterns, with the main rainy season, the summer monsoon, extending from June to September, and the dry period extending between October and February. There is a short rainy season between February and May, with an average rainfall of 800 mm. The mean annual temperature ranges between 8 and 28 °C. The month of May has the highest recorded temperature. The mean relative humidity is 61.3% (National meteorological service agency).

Fig. 1 Location of the research site.

2.1.2. Geological and hydrological context. Various lithological units, starting from the tertiary to the quaternary age, are outcropped within the study area. The realm is principally lined by volcanic and substance rocks. The sedimentary rock comprises a deposit cowl and a body of water sequences. The geology of the research area is dominated by geological formations associated with volcanism and alluvial deposits. Volcanic rocks and materials include basalts, trachyte, rhyolitic lava flows and domes, pumice fall, tuffs, ignimbrites, scoria, obsidian, and ash flow deposits (Fig. 2).

Fig. 2 Geological map of the study area.

Ignimbrite is exposed in the northern and central part of the investigation area and forms the major part of the volcanic rock. It is made of welded tuff (ignimbrite) and a non-welded pyroclastic case (ash and tuff). It is heavily welded, and characterized by developed vertical joints and breaks that offer high to moderate permeability. Basalts are vesicular basalts that have been filled with carbonate minerals. The Adama unit (Nazareth group) is made up of a dense sequence of welded ignimbrites with fiamme pumice ash and acidic lava flows, as well as domes with basaltic units. Between lava domes and ignimbrites, one is interlayered. Rhyolites, light-coloured trachyte, and dark obsidian layers comprise these rock units. They have distinct flow structures and are characterised by columnar joints in some places. In general, these rocks are porphyritic, with a felsic to glassy groundmass containing plagioclase, quartz, pyroxene, and alkali feldspar phenocrysts. The soil profile in the area has largely been influenced by pedogenic processes associated with volcanism. The lacustrine sediments were deposited in this great ancestral lake^43,46 and are found in the lakes and on the shoulders of the Pliocene-Pleistocene ignimbrite that pervades the rift in general and Adama and the environs in particular. These fine-grained deposits are typically yellowish-brown in colour, have fine stratification, and frequently contain a high proportion of volcanic matrix. The hydrogeology of Adama woreda is blanketed with volcanic rocks of various and variable hydraulic characteristics. The primary aquifers in this area are relatively permeable quaternary scoriaceous basalts with high number one hydraulic conductivity; deep wells may contain ignimbrites and sloppily fractured basalts.⁴⁷ Apart from weathered and fractured zones, the Nazaret unit (welded ignimbrites) has poor hydraulic conductivity. Along with weathered and fractured zones, the locally acidic volcanic gadgets of the quaternary bede gebaba volcanic unit exhibit low permeability. These are much less fractured and faulted formations.⁴⁸

2.2. Sample collection and analysis

Water analyses have been executed via way of means of the use of general procedures “American Public Health Association”.⁴⁹ For the purpose of characterising and mapping the appropriateness of the groundwater throughout the study site, a total of thirty wells were selected from a wide variety of sites. A global positioning system (GPS device) was used to record the sample sites and altitude. During water samples collection, handling, protection, and analysis, general strategies endorsed with the aid of using the⁴⁹ had been observed to make sure facts were quality and consistent. The parameters together with pH and electric conductivity (EC) have been measured within side the discipline even as the attention of chemical constituents together with Mg²⁺, Ca²⁺, K⁺, Cl⁻, Na⁺, NO₃⁻, SO₄²⁻ and F⁻ in groundwater samples turned into decided with inside the laboratory. The sampling maintenance and evaluation have been executed according to the standard methods (Table 1) prescribed via way of means of the.⁴⁹ In the current study, duplicates have been used for QC (Quality Control) and QA (Quality Assurance). To check the accuracy of the analytical findings, IBE (“Ionic Balance Error”) turned into calculated (eqn (1)). All major anions and cations have been taken in milliequivalents according to a litre (meq L⁻¹) and the IBE values have to now no longer surpass the secure restriction of ±5%.

IBE (%) = (∑Cations − ∑Anions)/(∑Cations + ∑Anions) × 100 (1)

Table 1 Analytical techniques adopted to analyze physicochemical parameters⁵⁰

Parameters	Units	Analytical techniques
Hydrogen ion (pH)	Range	Field kit (IONIX portable water quality meter) in situ
Electrical conductivity (EC)	(μS cm^−1)
Total dissolved solids (TDS)	(mg L^−1)
Sodium (Na^+) & potassium (K^+)		Flame photometric
Calcium (Ca^2+) & magnesium (Mg^2+)		EDTA titrimetric
Chloride (Cl^−)		AgNO3 titrimetric
Bi-carbonate (HCO3^2−)		H2SO4 titrimetric
Sulphate (SO4^2−)		Turbidity (UV-Visible spectrophotometer)
Fluoride (F^−)		Ion chromatography

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2.3. Pollution index of groundwater (PIG)

It is widely accepted and used by researchers around the world to appraise the water quality index,^51–53 which was initially formulated and proposed by ref. 54. It is a numerical representation of the extent of contaminants in groundwater based on the relative influence of physicochemical (pH, TDS, major cations, and anions) parameters.²⁶ On the basis of the WHO drinking standards, PIG was calculated using the method suggested by ref. 54.

PIG enables quantifying the relative effect of each chemical variable, such as pH, TDS, Cl⁻, Mg²⁺, SO₄²⁻ etc., individually on the overall groundwater quality. For the present study, the estimation of PIG was done in 5 steps. In the 1st step, relative weight (from 1–5) was apportioned to every chemical variable (Table 2). The highest weight (R_w = 5) was assigned to the parameter having the highest significance and its relative effect on public health. Similarly, the lowest weight (R_w = 1) was assigned to the parameter having the lowest effect on human health.⁵⁴ In “the next step, the Wp (weight parameter) was estimated for each chemical variable to evaluate its relative impact on groundwater quality (eqn (2)):

Wp = R_w/∑R_w (2)

Table 2 Relative weight of chemical parameters (after ref. 54)^a

Chemical parameters	Drinking standard, Ds	Relative weight (Rw)	Weight parameter (Wp)
a Note: All parameters in mg L^−1 except pH.
pH	7.5	5	0.1316
TDS	500	5	0.1316
F^−	1.5	5	0.1316
Cl^−	250	3	0.0789
SO4^2−	150	2	0.0526
NO3^−	45	4	0.1053
HCO3^2−	300	5	0.1316
Na^+	200	4	0.1053
K^+	10	1	0.0263
Mg^2+	30	2	0.0526
Ca^2+	75	2	0.0526
		∑Rw = 38	∑Wi = 1.000

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In the 3rd step, the status of content, denoted by Sc, was estimated for individual parameters (eqn (3)):

Sc = C/D_s (3)

where C is: the concentration of a specific quality variable for a particular spot, and D_s is the threshold limit of that specific variable with respect to drinking quality.⁵⁴

In the next step, overall water quality (Ow) was evaluated by multiplying the Wp and Sc (eqn (4)):

Ow = Wp × Sc (4)

In the fifth and final step, the PIG was computed through summation of all the Ow values (eqn (5)):

PIG = ∑Ow (5)

During the PIG appraisal, the relative influence of an individual chemical parameter from each sampling location is taken into consideration. The individual impact of a particular pollutant on the overall groundwater quality becomes clear through the values of Ow for a particular contaminant. When a specific contaminant's Ow value exceeds 0.1, it contributes 10% to the PIG value of 1.0.^51,54 The categorization of PIG based on its values is given in Table 3.

Table 3 Classification of groundwater quality as per PIG values

PIG values	Classification of groundwater quality
<1.0	Insignificant pollution
1.0–1.50	Low pollution
1.50–2.0	Moderate pollution
2.0–2.50	High pollution
>2.50	Very high pollution

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2.4. Human health risk assessment (HHRA)

Health risk is the severe toxic effects on the human that results from pollution.⁵⁵ Fluoride in drinking water interacts population in three different pathways such as intake, dermal and inhalation.⁵⁶ However, inhalation way of health risk is not considered in this study because of the absence of toxicological information like fluoride reference dose and transfer percentage from water to air. US EPA (“U.S. Environmental Protection Agency”) suggested that health risk assessment mainly made from four methods like exposure assessment, risk characterization, dose–response assessment, and hazard identification^1,57 was followed in the current study. Based on the two pathways discussed above, DAD and CDI (mg kg⁻¹ day⁻¹) were computed to estimate the receiving doses via individual pathways. CDI stands for chronic daily intake and DAD stands for dermally absorbed dose. Calculating the non-carcinogenic risk linked with the drinking of water via the CDI pathway requires the use of eqn (6):⁵⁸

CDI = C × IR × ED × EF/BW × AT (6)

here the term ‘CDI’ stands for ‘chronic daily intake’ (mg kg⁻¹ day⁻¹); C stands for the fluoride content in groundwater (mg L⁻¹); IR stands for ingestion rate (L day⁻¹: 0.90 L day⁻¹ for infants and 1.5 L day⁻¹ for men and women); The exposure duration is ED (years: 30 for men and women, and 12 for children, correspondingly); EF stands for exposure frequency (365 days per years for children, males, and females); BW is the average body weight (in kilogrammes; for men, women, and children, correspondingly: 70, 55, and 15); AT stands for the average time (days: 10 950 for men and women, 4380 for children, correspondingly). Dermal contact pathway was computed with eqn (7):

DAD = TC × K_i × EV × ED × EF × SSA × CF/BW × AT (7)

DAD stands for dermally absorbed dose (mg kg⁻¹ day⁻¹); TC; contact duration (h day⁻¹) 0.4 for children, men and women. K_i; dermal adsorption parameters (cm h⁻¹: 0.001 cm h⁻¹); EV; bathing frequency (times per day) 1 considered as time in a day). SSA; skin surface area (cm²) 12 000 and 16 600 cm² for children and female, male correspondingly); CF; conversion factors (0.001). ED stands for exposure duration (years: 30 and 12 for male, female and children, correspondingly). EF stands for exposure frequency (days per year) 365 days for children, male and female); BW indicate body weight (kg: 55, 70, and 15 for female, male, and children correspondingly), and AT stands for average time (days: 10 950 for female and male, 4380 children, correspondingly). HQ_oral and HQ_dermal hazard quotient for the fluoride health risk evaluation is calculated using eqn (8) and (9).

RfD indicates the reference dosage of a particular contaminant. The RfD of fluoride (0.04 mg kg⁻¹ day⁻¹). The total hazard index (THI) which indicates the non-carcinogenic risk is evaluated by s hazard quotients (HQ_oral + HQ_Dermal) and are computed by eqn (10):

THI_i = HQ_oral + HQ_dermal (10)

Based on the THI values, <1 value indicates no major risk of non-carcinogenic” effects. Although, in the case of THI value that surpasses THI > 1 are risks to inhabitants.⁵⁸

2.5. Software used

The places of every well have been taken into the GIS surroundings and the consequences of every parameter analysis have been introduced to the involved wells. Spatial analyst, a prolonged module of ArcGIS 10.8 became used to discover the spatial conduct of the groundwater pleasant parameters. GIS application software An IDW (Inverse Distance Weighting) spatial distribution map for water quality indicators was made using ArcGIS 10.8.^18,19 The concentration of geochemical parameters and the approximate groundwater quality index for the preparation of spatial maps are very convenient and efficient decision-makers in the field of water resources. The geochemical characterization became performed via way of means of the usage of AqQA software.

3. Results and discussion

3.1. Variation of physico–chemical ions

Physico–chemical ionic concentrations and statistical metrics such as minimum, maximum, and average as well as consuming water quality standards^59,60 are given in Table 4.

Table 4 Statistical overview of the various physicochemical ions in groundwater

Ions	Minimum	Maximum	Mean	Std. dev.	CES, 2013	WHO, 2017		% of the samples exceeding the limit
						Most desirable	Not permissible
pH	6.5	8.2	7.5	0.5	6.5–8.5	6.5 to 8.5	<6.5 and >8.5	0
EC (μS cm^−1)	110.7	3040.0	794.4	646.7	—	<1500	>1500	13
TDS (mg L^−1)	55.4	1814.0	483.6	406.8	1000	<500	>1500	3
TH (mg L^−1)	5.9	717.3	122.4	127.7	—	<100	>500	—
Ca^2+ (mg L^−1)	2.2	224.0	34.7	41.4	75	<75	>200	3
Mg^2+ (mg L^−1)	0.1	38.4	8.7	8.5	50	<50	>150	0
Na^+ (mg L^−1)	9.5	562.3	178.6	157.0	200	<200	>200	50
K^+ (mg L^−1)	1.5	52.0	19.1	12.2	1.5	<10	>10	80
Cl^− (mg L^−1)	5.8	192.0	42.8	51.7	250	<200	>600	0
SO4^2− (mg L^−1)	4.3	243.2	40.8	50.6	250	<400	>400	0
HCO3^2− (mg L^−1)	143.0	1171.0	458.0	245.9	—	<300	>600	20
F^− (mg L^−1)	1.0	10.5	4.2	3.3	1.0	<1.5	>1.5	83

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3.2. Physical characteristics of ground water

The pH of the groundwater in the area under investigation ranged from 6.5 to 8.2, with an average value of 7.5; this suggests that all samples were of a nature that was moderately acidic to alkaline, which is in conformity with the guidelines made by the WHO for normal groundwater quality (6.5 to 8.5). Fig. 3a depicts an interpolated spatial variation map of groundwater pH. The pH level is highest in the north-eastern and southern parts of the research area. High pH values are found in RV aquifers, where Na-rich igneous rock serves as a major aquifer.^1,47 The high pH values found in the investigation area's northern parts were caused by underground water moving through faults in the deep ground.

Fig. 3 Spatial variation maps of pH (a), EC (b), TDS (c), and TH (d).

The EC varied between 110.7-3040 μS cm⁻¹, with mean values of 794.4 μS cm⁻¹ (Table 4). According to WHO guidelines, the prescribed limit for EC is 1500 s cm⁻¹; however, with the exception of sampling site number 25, the limit was surpassed (Fig. 3b). In the research region, high EC values may be induced by agricultural practices that add certain salts to the ground as well as wastewater discharge from industrial and urban areas (Amanial, 2015).⁶¹ The TDS values varied between 55.4 and 1814 mg L⁻¹ with an average of 483.6 mg L⁻¹ and most of the samples illustrated TDS values within the desirable limit of 1000 mg L⁻¹ as per^59,60 standards (Table 4). Samples 5, 9, 14, and 17 surpassed the limit (Fig. 3c) of 1000 mg L⁻¹ and were unsuitable for drinking. Higher TDS content is owing to salts from thick soil and weathered rock, as well as longer groundwater residence times in contact with aquifers.

3.3. Chemical characteristics of groundwater

3.3.1. Major cations. Calcium levels in the Adama woreda range between 2.2 and 224 mg L⁻¹, with a mean of 34.7 mg L⁻¹. Analogously, the Mg²⁺ content varies considerably and is ranging between 0.1 and 38.4 mg L⁻¹, with an average of 8.4 mg L⁻¹ in the commended area. As per WHO drinking water norms, almost all samples of groundwater are safe to drink for Mg²⁺ and Ca²⁺, except for one sample.¹⁷ Ca²⁺ and Mg²⁺ concentrations were found to be elevated in the central as well as south-eastern regions of the investigation area (Fig. 4a and b) as a consequence of the weathering of basic volcanic rocks comprising mafic minerals (pyroxene and olivine), which comprises a wider portion of the study site. Sodium is the most abundant cation in the region, particularly in groundwater samples from the rift floor. The Na⁺ content varies between 9.5 and 562.3 mg L⁻¹, with a mean of 178.6 mg L⁻¹. The values reflect the rising trend of aquifers from the highlands to the rift floor (Fig. 4c). As per the WHO's drinking water norms, more than half of the samples of the groundwater in the research region are under the permitted range. The high rock–water interactions with the abundant acidic volcanic rocks found along the flow path from highland to lowland (rift floor) in the Woreda stimulate the discharge of Na⁺ into groundwater. K⁺ content in the area groundwater samples displays a similar pattern with Na⁺ and varies from 1.5 to 52 mg L⁻¹. K⁺ content is typically greater in the research region, with nearly 80% of samples exceeding the permissible level for potable water. The major sources of elevated sodium concentrations in groundwater are weathering, natural underground salt deposits, and the disintegration of sodium-bearing minerals and rocks via cation exchange. The positive trend rift ward (Fig. 4d) might be attributed to weathering of K⁺-bearing minerals (K-feldspars/orthoclase) in the host rock in the direction of groundwater flow. K⁺ levels in groundwater samples from the Woreda act similarly to Na⁺ and range from 1.5 to 52 mg L⁻¹, with 80% of samples surpassing the permitted limit for drinking water. The propensity of a positive trend rift ward (Fig. 4d) might be attributed to weathering of K⁺ bearing minerals (K-feldspars/orthoclase) from host rocks in the direction of groundwater flow.

Fig. 4 Spatial variation map of Ca (a), Mg (b), Na (c) and K (d).

3.3.2. Major anions. The concentration of HCO₃⁻ the dominant anion in the Woreda ranged between 143 to 1171 mg L⁻¹ “with a mean value of 458 mg L⁻¹. The spatial variation map of HCO₃⁻ in the research region reveals that the western, northwest, and southwest portions (Fig. 5c) are noticeably higher due to carbonate rock formation and silicate hydrolysis. Additionally, the chemical process between underground water and silicate minerals resulted in an elevated level of HCO₃⁻ in the groundwater.⁶² In the mer and research area, the interaction of dissolved CO₂ with acidic volcanic rocks resulted in elevated levels of HCO₃⁻ in groundwater.⁶³ The levels of Cl⁻ and SO₄²⁻ in the woreda ranged from 5.8 to 192 mg L⁻¹ with a mean value of 257.8 mg L⁻¹ and 4.3 to 243.2 mg L⁻¹ with a mean value of 40.8 mg L⁻¹, correspondingly (Table 4). In the region, the concentrations of both parameters in groundwater are typically low. All water samples in the research area are safe to consume as per who drinking water quality criteria. The high content of Cl⁻ and SO₄²⁻ was noticed in the south and southwest of the research area (Fig. 5a and b) owing to interaction with the lithosphere and the impacts of evaporation, which can be attributed to anthropogenic influences such as agricultural activity, exaggerated usage of inorganic fertilizers, city municipal waste disposal, agricultural runoff, landfill leachate, household sewage, industrial effluent release, and power generation.

Fig. 5 Spatial variation map of Cl⁻ (a), SO₄²⁻ (b), HCO₃⁻ (c), and F⁻ (d).

The F⁻ content in the study site varies from 1 to 10.5 mg L⁻¹, with a mean of 4.2 mg L⁻¹ (Table 4). Most water samples were found to be unsuitable for humans to consume, as per WHO norms. Unfit samples are most widely found in the southern and southeastern parts of the research area (Fig. 5d). When the level of F⁻ in natural water is between 0.5 and 1.5 mg L⁻¹, the enamel of the teeth is strengthened. F⁻ Concentrations in natural drinking water above 4 mg L⁻¹, above 1.5 mg L⁻¹, and above 10 mg L⁻¹ are associated with skeletal fluorosis, dental fluorosis, and crippling skeletal fluorosis, correspondingly.^64,65 Fluoride levels in groundwater may be elevated as a result of fluoridated water salts percolating through acidic volcanic rocks (unwelded tuffs, lacustrine sediments, pyroclastic deposits, rhyolitic lava flows and ignimbrite) and geothermal water in this investigated region.^44,66–68 The content of groundwater is overhauled as it moves down from the highlands to the lowlands of the rift valley. In highland aquifers, F⁻ content is typically low, but it increases along the groundwater flow line in rift floor aquifers.

3.4. Hydrochemical facies (piper plot)

The Piper tri-linear plot, which was developed by ref. 69, has been utilised in order to determine the various hydrochemical facies and the geochemical development of groundwater chemistry in accordance with their dominating ions. In the current research area, the Piper tri-linear plot (Fig. 6) shows that Na⁺ and Ca²⁺ are the dominant cations relative to Mg²⁺. Although, Cl⁻, and HCO₃⁻ are the more prominent anions when compared with SO₄²⁻. The major hydrochemical facies are of the order of Ca–Mg–HCO₃, Na–Ca–HCO₃, and Na–HCO₃. The mixed Ca–Na–HCO₃ facies illustrated that the groundwater primarily reacts with basic volcanic aquifers and Na–HCO₃ in the acidic volcanic aquifers that are widespread in the study area.

Fig. 6 Hill piper plot for the groundwater.

3.5. Pollution index of groundwater (PIG)

The assessment and quantification of groundwater contamination shall be carried out considering all the relative contributions of the physicochemical parameters. If the total groundwater quality parameter, i.e., Ow, exceeds 0.1, is 10% of the total PIG value of 1.0, values less than 1.0 are classified as insignificant pollution.^54,70 Exceeding Ow values provides ample information on the extent of pollution in the aquifer system.⁵³ In this basin, PIG values range from 0.56 to 2.56 with a mean value of 1.04. It can be said that these parameters had the most adverse impact on the quality of groundwater. The site-wise spatial variation map of PIG is displayed in Fig. 7. The majority of the study area is occupied by insignificant pollution, moderate pollution occupied 3 wells, and a small portion of the southeast, south of the basin is occupied by low pollution. However, the PIG values show that only 33%, 57% of the samples (1–1.5, <1) were found in the ‘low pollution zone’ and ‘insignificant pollution zone,’ while 3%, 7% of the samples were found in the high and moderate pollution, correspondingly (Table 5).

Fig. 7 Spatial distribution of PIG.

Table 5 Results of pollution index of groundwater

S. no.	PIG values	Classification of GWQ	No. wells	Percentage (%) of wells
1	<1.0	Insignificant pollution	1, 2, 6, 7, 11, 12, 13, 15, 16, 19, 20, 23, 24, 25, 26, 27, 30	57
2	1.0–1.5	Low pollution	3, 4, 8, 10, 17, 18, 21, 22, 28	33
3	1.5–2.0	Moderate pollution	5, 29	7
4	2.0–2.5	High pollution	14	3
5	>2.5	Very high pollution	—	0

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3.6. Human health risk assessment (HHRA)

The presence of a substantial level of fluoride in human drinking water may provide a number of health risks. The THI was used in this study to show the overall non-carcinogenic health risk of fluoride in drinking water. Table 5 depicts the HQ results for children, women, as well as men on the basis of the intake and dermal pathway. The observed HQ_oral values ranged from 0.61 to 6.43 for women, 0.50 to 5.22 for men, and 0.70 to 7.32 for children The HQ_dermal values for men ranged from 5.46 × 10⁻⁵ to 5.73 × 10⁻⁴, for women 5.85 × 10⁻⁵ to 6.14 × 10⁻⁴, and for children ranged 3.75 × 10⁻⁵ to 3.94 × 10⁻⁴. The THI ranges from 0.70 to 7.32 for children, 0.61 to 6.44 for women, and 0.50 to 5.22 for men. The result revealed that 83% for children, 73% for women, and 57% for men of the samples surpassed the THI > 1 (Table 6 & Fig. 8). Based on body weights and ingestion rates, children are at a greater risk than men and women, according to THI results. Comparative studies conducted in various parts of the world have been identified. For example, in South India,⁷¹ China,⁷² and Iran,⁷³ children and women face a greater risk than males due to their lower body weights. Individual body weight is a primary indicator of health effects, as men's heavier weight protects them from health risks when compared to women and infants. The highest fluoride content in groundwater contributes to health issues like skeletal and dental fluorosis, which predominantly affect infants.³³ Regular consumption of fluoride-contaminated water raises the risk of developing fluorosis, skeletal and tooth decay, and spinal disorders.

Table 6 For children, women, and men, the health hazards associated with drinking water ingestion and skin contact were estimated

S. no	HQ oral			HQ dermal			THQ total
	Children	Women	Men	Children	Women	Men	Children	Women	Men
1	1.74	1.53	1.24	9.38 × 10^−5	1.46 × 10^−4	1.37 × 10^−4	1.74	1.53	1.24
2	1.12	0.98	0.79	6.00 × 10^−5	9.36 × 10^−5	8.74 × 10^−5	1.12	0.98	0.79
3	5.92	5.21	4.22	3.19 × 10^−4	4.97 × 10^−4	4.64 × 10^−4	5.92	5.21	4.22
4	3.07	2.70	2.19	1.65 × 10^−4	2.57 × 10^−4	2.40 × 10^−4	3.07	2.70	2.19
5	7.32	6.43	5.22	3.94 × 10^−4	6.14 × 10^−4	5.73 × 10^−4	7.32	6.44	5.22
6	1.05	0.92	0.75	5.63 × 10^−5	8.78 × 10^−5	8.19 × 10^−5	1.05	0.92	0.75
7	2.30	2.02	1.64	1.24 × 10^−4	1.93 × 10^−4	1.80 × 10^−4	2.30	2.02	1.64
8	5.85	5.15	4.17	3.15 × 10^−4	4.91 × 10^−4	4.59 × 10^−4	5.86	5.15	4.17
9	4.88	4.29	3.48	2.63 × 10^−4	4.10 × 10^−4	3.82 × 10^−4	4.88	4.29	3.48
10	1.39	1.23	0.99	7.50 × 10^−5	1.17 × 10^−4	1.09 × 10^−4	1.39	1.23	0.99
11	0.70	0.61	0.50	3.75 × 10^−5	5.85 × 10^−5	5.46 × 10^−5	0.70	0.61	0.50
12	2.23	1.96	1.59	1.20 × 10^−4	1.87 × 10^−4	1.75 × 10^−4	2.23	1.96	1.59
13	0.70	0.61	0.50	3.75 × 10^−5	5.85 × 10^−5	5.46 × 10^−5	0.70	0.61	0.50
14	6.34	5.58	4.52	3.41 × 10^−4	5.32 × 10^−4	4.97 × 10^−4	6.34	5.58	4.52
15	1.25	1.10	0.89	6.75 × 10^−5	1.05 × 10^−4	9.83 × 10^−5	1.25	1.10	0.89
16	1.39	1.23	0.99	7.50 × 10^−5	1.17 × 10^−4	1.09 × 10^−4	1.39	1.23	0.99
17	1.12	0.98	0.79	6.00 × 10^−5	9.36 × 10^−5	8.74 × 10^−5	1.12	0.98	0.79
18	6.69	5.88	4.77	3.60 × 10^−4	5.62 × 10^−4	5.24 × 10^−4	6.69	5.88	4.77
19	2.26	1.99	1.61	1.22 × 10^−4	1.90 × 10^−4	1.77 × 10^−4	2.26	1.99	1.61
20	1.74	1.53	1.24	9.38 × 10^−5	1.46 × 10^−4	1.37 × 10^−4	1.74	1.53	1.24
21	4.74	4.17	3.38	2.55 × 10^−4	3.98 × 10^−4	3.71 × 10^−4	4.74	4.17	3.38
22	6.97	6.13	4.97	3.75 × 10^−4	5.85 × 10^−4	5.46 × 10^−4	6.97	6.13	4.97
23	2.09	1.84	1.49	1.13 × 10^−4	1.76 × 10^−4	1.64 × 10^−4	2.09	1.84	1.49
24	0.91	0.80	0.65	4.88 × 10^−5	7.61 × 10^−5	7.10 × 10^−5	0.91	0.80	0.65
25	0.98	0.86	0.70	5.25 × 10^−5	8.19 × 10^−5	7.65 × 10^−5	0.98	0.86	0.70
26	0.84	0.74	0.60	4.50 × 10^−5	7.02 × 10^−5	6.55 × 10^−5	0.84	0.74	0.60
27	1.39	1.23	0.99	7.50 × 10^−5	1.17 × 10^−4	1.09 × 10^−4	1.39	1.23	0.99
28	2.23	1.96	1.59	1.20 × 10^−4	1.87 × 10^−4	1.75 × 10^−4	2.23	1.96	1.59
29	6.97	6.13	4.97	3.75 × 10^−4	5.85 × 10^−4	5.46 × 10^−4	6.97	6.13	4.97
30	1.25	1.10	0.89	6.75 × 10^−5	1.05 × 10^−4	9.83 × 10^−5	1.25	1.10	0.89
Min	0.70	0.61	0.50	3.75 × 10^−5	5.85 × 10^−5	5.46 × 10^−5	0.70	0.61	0.50
Max	7.32	6.43	5.22	3.94 × 10^−4	6.14 × 10^−4	5.73 × 10^−4	7.32	6.44	5.22
Mean	2.91	2.56	2.08	1.57 × 10^−4	2.45 × 10^−4	2.28 × 10^−4	2.91	2.56	2.08

---

Fig. 8 Calculated THI for all woreda groundwater samples to evaluate health risks for children (a), women (b), and men (c).

3.7. Recommendations for future management of groundwater quality

A detailed hydrogeochemical study in the research area noticed that most people are in the need of groundwater for their day to day primary activities and it is quite important to monitor the assessment of the health threat due to the presence of high level of concentration of fluoride in certain areas. Finally, the research study proposed the following remedial measures to be taken as follows.

• The study results are either to treat fluoride-rich groundwater as reverse osmosis (RO) treatment to provide community people with safe drinking water supplies and avoid major health risks of fluorosis.

• Suggest effective remedial measures of large and small scale rainwater harvesting and the construction of artificial recharge structures in the basin which will be an effective way to recharge the groundwater so that it will dilute the concentration of fluoride.

4. Conclusion

This research investigation delineates the aptness of groundwater quality using various criteria such as PIG, HHRA, GIS, and the human health risks linked with fluoride in ingestion water. This research aimed to provide valuable hydrogeochemical information that is vital for the efficient and well-planned use of available groundwater resources which is non-harmful to human beings. The following concise description of this study's main findings:

• The groundwater is found slightly acidic to nearly alkaline in the research area. Na⁺ followed by Ca²⁺ resulted as dominant cations while HCO₃⁻, followed by Cl⁻ and SO₄²⁻ also resulted as dominant anions in the basin. Ca–Mg–HCO₃, Na–Ca–HCO₃, as well as Na–HCO₃ are dominant water types.

• As per PIG, the results show that 57%, 33%, 7% and 3% of the samples were found in the insignificant, low, moderate and high, correspondingly.

• The THI is calculated from hazard quotients (HQ_Intake and HQ_Dermal) results showing 83%, 73%, and 57% of the samples exceed the non-carcinogenic health threat of fluoride THI > 1 in drinking water for children, females and males. Children are more susceptible to danger than either males or women, according to the THI data, based on body weights and consumption rates. Similarly, females are also more vulnerable to health risks than men.

The final outcome of this research tried to provide valuable information to decision-makers on potential groundwater management practices and to lay the foundations for groundwater monitoring stations for the assessment of groundwater quality in central Ethiopia.

Data availability

Data will be made available on request.

Author contributions

Hassen Shube and Shankar Karupannan: conceptualization, methodology, investigation, supervision, writing- original draft preparation, validation, writing- reviewing and editing; Nafyad Serre Kawo: assist in manuscript writing and interpretation of data; Mohammed Haji and Abraham Mechal: data curation, writing- reviewing and editing; Balamurugan Panneerselvam: data curation, visualization, writing- reviewing and editing; Ashu Fekadu: helps in the interpretation of data – writing-reviewing and editing.

Conflicts of interest

The authors confirm no conflict of interest.

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