Sustainable Management of Groundwater Resources in Central Tunisia: Nitrate Pollution and Health Risk Assessment

Abstract

Degraded groundwater quality, characterized by elevated salinity and nitrate concentrations, poses significant public health concerns, particularly for vulnerable populations such as children. High content of nitrate in drinking water may lead to non-carcinogenic health risks, highlighting the urgent need for sustainable groundwater management strategies to protect both human health and environmental integrity. This study assesses the suitability of groundwater resources in the Regueb Basin for irrigation and drinking purposes, with particular attention paid to nitrate contamination. The Irrigation Water Quality Index (IWQI) indicates considerable spatial variability in groundwater quality, with values varying between 15.86 and 89.55 and a median of 41.69, reflecting differing levels of suitability for irrigation across the basin. Similarly, the Drinking Water Quality Index (DWQI) ranges from 149.16 to 982.42, with a median value of 445.71, suggesting significant concerns regarding groundwater suitability for drinking purposes. The health risk assessment (HHRA) based on the Nitrate Pollution Index (NPI) and the nitrate hazard quotient (HQ__nitrate) reveal substantial risks to human health. NPI values vary between 0.45 and 5.5, with a median of 1.65 indicating varying levels of nitrate pollution. The HQ__nitrate results show that all groundwater samples (100%) pose health risks for children (HQ > 1). For women, 75.61% of HQ values exceed the safe threshold, affecting approximately 80% of the study area, whereas for men, 48.48% of HQ values exceed 1, impacting about 36.67% of the area. Overall, these findings highlight the urgent need for effective groundwater management strategies to mitigate nitrate contamination and ensure the safe and sustainable use of the groundwater resources in the Regueb Basin.

1. Introduction

Groundwater is vital for drinking, agricultural production, and socio-economic development, especially in arid and semi-arid regions where surface water availability is limited [1]. However, increasing anthropogenic pressures have led to progressive groundwater quality deterioration worldwide. Among the various contaminants, nitrate (NO₃⁻) has emerged as one of the most widespread pollutants due to its high mobility in the subsurface and its strong association with agricultural activities, particularly the excessive application of nitrogen fertilizers, as well as wastewater mismanagement [2,3,4].

Globally, elevated nitrate concentrations in groundwater have been identified as a major environmental and public health concern. Excessive nitrate intake poses severe health risks, including methemoglobinemia in infants, adverse reproductive outcomes, and potential long-term carcinogenic effects, as recognized by the World Health Organization [5] and several international reviews [6,7]. Consequently, rigorous assessment and continuous monitoring are vital to guarantee the maintainable use of groundwater resources.

Numerous studies worldwide have documented the growing severity of nitrate contamination in groundwater systems. In Asia, significant nitrate pollution has been reported in semi-arid aquifers of China and India, primarily linked to agricultural intensification and fertilizer misuse [8,9,10]. Similar trends have been observed in Africa, where groundwater nitrate contamination has been widely documented in Nigeria, Ghana, Morocco, and Tunisia, reflecting the combined impacts of agricultural activities and insufficient wastewater management [11,12,13]. In Europe, several studies have documented groundwater nitrate concentrations exceeding the guideline value of 50 mg/L recommended by the WHO for drinking water quality. High nitrate levels have been observed in intensively cultivated areas of Spain [14], Italy [15], and Poland [16]. Similar situations have also been reported in many semi-arid regions worldwide, highlighting the global scale of nitrate pollution and the urgent need for effective groundwater management strategies.

Integrated methods combining the Nitrate Pollution Index (NPI) with Human Health Risk Assessment (HHRA) are increasingly applied to assess nitrate contamination and its potential health effects [17,18,19]. These approaches allow for the measurement of pollution levels, the recognition of at-risk populations, and the establishment of priorities for intervention strategies.

In Tunisia, nitrate pollution is well-documented and constitutes a major environmental challenge, especially in agricultural regions experiencing growing pressure on groundwater resources. Several studies have highlighted alarming nitrate levels across various aquifers [20,21,22], with exceedances of national and international drinking-water standards. Investigations in Cap Bon and southeastern Tunisia attribute nitrate enrichment primarily to fertilizer leaching, animal waste, and uncontrolled abstraction [23]. Recent studies in the Regueb Basin have provided additional insights into groundwater dynamics and hydrogeochemical processes, emphasizing the role of irrigation practices and recharge potential in nitrate distribution [24].

Groundwater represents the primary water resource in many semi-arid regions, including Central Tunisia, where increasing agricultural activities and intensive fertilizer use have led to growing concerns about nitrate contamination. In this context, the present study provides a comprehensive assessment of groundwater quality in the Regueb Basin aquifer (Central Tunisia), with particular emphasis on nitrate contamination and its associated human health risks. While previous studies in Tunisia have mainly focused on hydrogeochemical characterization or general groundwater quality evaluation, integrated assessments that simultaneously examine groundwater suitability for drinking and irrigation, nitrate pollution levels, and related human health risks remain limited, particularly in semi-arid aquifer systems such as the Regueb Basin.

To address this gap, the present study adopts an integrated methodological framework that combines hydrochemical analysis with multiple evaluation tools, including the Drinking Water Quality Index (DWQI), Irrigation Water Quality Index (IWQI), Nitrate Pollution Index (NPI), and Human Health Risk Assessment (HHRA). Specifically, the objectives of this study are to: (i) evaluate the suitability of groundwater for drinking and irrigation purposes, (ii) analyze the spatial distribution and intensity of nitrate contamination, and (iii) assess the potential human health risks associated with nitrate exposure through groundwater consumption. By linking hydrochemical processes, water quality indices, and health risk assessment, this study provides new insights into the spatial distribution of nitrate contamination and contributes to the development of sustainable groundwater management strategies in semi-arid environments.

2. Material and Methods

2.1. Study Area

The study area is located in central Tunisia and is characterized by a semi-arid climate and a complex hydrogeological structure composed mainly of alluvial deposits and carbonate formations (Figure 1). Groundwater resources occur primarily in shallow aquifer systems widely used for domestic and agricultural purposes. Groundwater level generally ranges between approximately 10 and 40 m below ground surface depending on local geological and hydrological conditions. The hydraulic conductivity of the aquifer formations varies according to lithological characteristics, with higher permeability associated with sandy deposits and fractured carbonate formations. Regional groundwater flow generally follows the topographic gradient toward the central depression of the basin [25].

Groundwater samples were collected from 30 wells distributed across the study area. Prior to sampling, wells were purged for several minutes to obtain representative groundwater samples. The samples were collected in pre-cleaned polyethylene bottles, and they were preserved in iceboxes at approximately 4 °C and transported to the laboratory for chemical analysis within 24 h to ensure the reliability of the measurements [5].

The Regueb Basin is located within the Jeffara Plain and is underlain predominantly by Tertiary and Quaternary sedimentary formations. These geological units comprise marls, sandstones, conglomerates, and extensive alluvial deposits, which collectively exert a significant control on groundwater occurrence, storage, and movement. The heterogeneity of these formations, in terms of lithology and permeability, influences aquifer characteristics, recharge dynamics, and vulnerability to contamination. Understanding the geological framework is therefore essential for evaluating groundwater quality and appraising the spreading and transport of nitrate pollutants within the basin [26].

In the Regueb Basin, the hydrogeological framework is predominantly governed by Mio-Plio-Quaternary (MPQ) sedimentary formations composed of unconsolidated sands, gravels, and clays that form the principal aquifer systems [27]. These shallow and highly permeable deposits serve as the main groundwater sources for both drinking and irrigation purposes. The basin lies within a semi-arid climate, marked by scarce and irregular rainfall, high temperatures, and significant potential evapotranspiration, all of which greatly restrict natural groundwater recharge [28]. Groundwater replenishment occurs mainly through diffuse rainfall infiltration, resulting in marked seasonal and inter-annual fluctuations in water levels (Figure 2).

The combination of limited recharge, high climatic stress, and intensive groundwater abstraction has increased pressure on the aquifer system, contributing to declining water tables and heightened vulnerability to contamination, particularly nitrates [29,30]. These conditions highlight the need for combined water management approaches, including the adoption of sustainable pumping strategies, regulation of agricultural water use, and the implementation of artificial recharge techniques to enhance aquifer sustainability and protect groundwater quality in the Regueb Basin [31].

2.2. Data Collection

Thirty water samples were collected from wells systematically distributed across the Regueb Basin to capture diverse land-use types and hydrogeological settings [27] (

Table 1. Statistics of groundwater hydrochemical parameters in the Regueb basin.

Parameter	Unit	Minimum	Maximum	Mean	Standard Deviation	Skewness	Kurtosis
pH	-	7.22	8.08	7.49	0.20	0.44	1.47
TDS	g/L	1.14	6.05	3.15	1.35	0.41	−0.67
Na+	mg/L	165.0	12,489.9	412.48	245.05	2.08	4.57
K+	mg/L	3.7	40.7	14.82	10.35	1.15	0.01
Ca2+	mg/L	63.7	470.0	259.20	142.04	0.14	−1.19
Mg2+	mg/L	41.8	368.3	187.6	95.61	0.28	−0.86
HCO3−	mg/L	54.96	292.6	149.85	72.31	0.56	−1.08
SO42−	mg/L	167.0	1279.3	689.45	268.50	0.16	−0.75
NO3−	mg/L	28.0	129.0	53.39	23.92	1.64	2.34
Cl−	mg/L	137.0	2864.2	1002.61	663.56	0.86	0.39

). Groundwater sampling was carried out in September 2021, corresponding to the late irrigation period, when groundwater quality reflects the cumulative impact of agricultural activities while minimizing short-term variability related to rainfall events. However, it should be noted that sampling during a single period may not fully capture seasonal hydrochemical variations [10]. The samples were analyzed for major cations and anions, including magnesium (Mg²⁺), calcium (Ca²⁺), sodium (Na⁺), potassium (K⁺), bicarbonate (HCO₃⁻), chloride (Cl⁻), sulfate (SO₄²⁻), and nitrate (NO₃⁻). All measurements were conducted at the CRDA Laboratory in Sidi Bouzid following standardized procedures to ensure data reliability [32,33].

2.3. Data Analysis

2.3.1. Irrigation Water Quality Index (IWQI)

The irrigation groundwater suitability in the Regueb Basin was assessed using the IWQI. This index integrates individual chemical indicators [34,35] with multiple combined indices [36] to offer a complete assessment of groundwater quality for decision-making purposes. Given the basin’s reliance on groundwater for extensive agricultural activities, it is essential to evaluate chemical constituents that can influence crop productivity and soil health, particularly by affecting the plant cells’ osmotic pressure [37].

The IWQI, defined by [36], incorporates multiple critical water quality parameters, such as sodium adsorption ratio (SAR), electrical conductivity (EC), Kelly ratio (KR), soluble sodium percentage (SSP), sodium content (%Na), permeability index (PI), magnesium hazard (MH), potential salinity (PS), and residual sodium carbonate (RSC). These parameters were calculated using the equations summarized in

Table 2. Classification and weighting of IWQI parameters for irrigation suitability assessment.

Parameters	Range	Class	Score	Weight	Domain (%)	Number of Samples (n)
EC (µS/cm)	<250	Excellent	5	4	–	0
	250–750	Good	4		30.09	11
	750–2250	Permissible	3		1.93	4
	2250–5000	Doubtful	2		67.98	15
SAR	<3	Acceptable	1	3	0.79	2
	3–9	Moderate	2		96.16	26
	>9	Unsuitable	3		3.03	2
Na%	<20	Excellent	1	2	–	0
	20–40	Good	2		54.75	11
	40–60	Permissible	3		45.18	17
	60–80	Doubtful	4		0.07	2
	>80	Unsuitable	5		–	0
MH	<50	Acceptable	1	1	20.02	9
	>50	Unsuitable	2		79.97	21
KR	<1	Acceptable	1	1	85.17	26
	>1	Unsuitable	2		14.83	4
PI	>75	Excellent	3	1	–	0
	25–75	Acceptable	2		19.19	7
	<25	Unsuitable	1		80.81	23
PS	<3	Acceptable	1	1	–	0
	3–15	Moderate	2		1.55	4
	>15	Doubtful	3		98.45	26
SSP	<50	Acceptable	1	1	87.68	26
	>50	Unsuitable	2		12.32	4

, allowing for a quantitative evaluation of irrigation suitability across the study area (Figure 3).

The IWQI was determined using a three-step approach: calculation of the quality rating (qi) for each water quality parameter; assignment of relative weights (wi) according to the significance of each parameter in evaluating irrigation water quality, with the highest weight (4) given to EC, followed by SAR (3), %Na (2), and %MH, SSP, KR, and PI (1); and computation of the overall IWQI. These steps involve calculating the quality rating (qi) for each parameter, as expressed in the following Equation (1):

$$qi=qimax-{\displaystyle \frac{\left(xij-xinf\right)\ast qiamp}{xamp}}$$

(1)

where qimax is the maximum value of qi for the class; xij is the observed value for the parameter; xinf is the value corresponding to the lower limit of the class to which the parameter belongs; qiamp is the amplitude of the class; xamp is the amplitude of the class to which the parameter belongs.

The IWQI is calculated using Equation (2):

$$\mathrm{I}\mathrm{W}\mathrm{Q}\mathrm{I}={\sum }_{i=1}^{n}qiwi$$

(2)

where qi: the quality of the ith parameter; wi: the weighted weight of the ith parameter; n: the number of parameters.

2.3.2. Drinking Water Quality Index (DWQI)

The Drinking Water Quality Index (DWQI) was assessed based on ten groundwater quality parameters EC, pH, Na, K, Cl, TDS, Mg, SO₄, Ca, and HCO₃ following [5] guidelines. The computation was carried out in four sequential steps:

Assigning Weight Values (wi): a weight (wi) from 1 to 5, based on its impact on human health, was attributed to each parameter.

Calculating Relative Weight (Wi):

$$Wi={\displaystyle \frac{wi}{{\sum }_{i=1}^{n}wi}}$$

(3)

where Wi is the relative weight, n is the number of properties and wi is the weight of each water property.

Quality Assessment (qi):

$$qi={\displaystyle \frac{Ci}{Si}}\ast 100$$

(4)

where qi is the quality index, Ci is the quantity of each water property and Si is the desirable limit of each property [5].

DWQI Calculation:

$$\mathrm{D}\mathrm{W}\mathrm{Q}\mathrm{I}={\sum }_{i=1}^n{SI}_i$$

(5)

where SI_i is the subindex of the i parameter and qi is the quality score depending on the quantity of i properties, as shown in Equation (6).

$${SI}_i=Wi\ast qi$$

(6)

2.3.3. Nitrate Pollution Index (NPI)

The Nitrate Pollution Index (NPI) is a metric used to evaluate the extent of nitrate contamination in groundwater [38]. It is defined as follows (Equation (7)):

$$\mathrm{N}\mathrm{P}\mathrm{I}={\displaystyle \frac{C-HAV}{HAV}}$$

(7)

where C, is the measured concentration of nitrates in a sample, and HAV is the value of nitrates acceptable to the human species, which is taken as 20 mg/L.

2.3.4. Human Health Risk Assessment (HHRA)

Following the framework established by the United States Environmental Protection Agency (USEPA), the non-carcinogenic health risk from drinking water consumption can be estimated using Equation (8) [39]. Accordingly, the non-carcinogenic risk resulting from drinking water consumption is expressed as follows:

$$CDI={\displaystyle \frac{C\ast IR\ast ED\ast EF}{ABW\ast AET}}$$

(8)

where CDI: chronic daily dose (mg/kg per day); IR: human ingestion rate (L/day); EF: frequency of exposure (days/year: 365 days); C: the concentration of a particular contaminant in groundwater (mg/L); ABW: medium body weight (Kg); ED: duration of exposure (years); AET: medium duration of exposure (days) = (ED × 365) [18].

The exposure parameters used for the Hazard Quotient (HQ) calculations for children, men, and women are summarized in

Table 3. Exposure parameters used for Hazard Quotient (HQ) calculations.

Parameter	Description	Child	Man	Woman	Reference
IR (L/day)	Water ingestion rate	0.78	2.5	2.5	[38]
ED (years)	Exposure duration	12	64	67
EF (days/year)	Exposure frequency	365	365	365
ABW (kg)	Average body weight	15	65	55
AET (days)	Average exposure time (ED × 365)	4380	23,360	24,455

.

These parameters were adopted from commonly used exposure assumptions in groundwater health risk assessment studies.

The present study assesses the non-carcinogenic health risk associated with nitrate (NO₃⁻) in drinking water, primarily quantified through risk quotient values (HQ__nitrate) [13] which is calculated using Equation (9):

$${HQ\_}_{nitrate}={\displaystyle \frac{CDI}{RfD}}$$

(9)

where HQ is the nitrate risk quotient, CDI: the chronic daily dose (mg/kg day), and RfD is the reference dose of the contaminant (USEPA) [mg/(kg per day)⁻¹].

In the case where HQ < 1, harmful effects of exposure cannot be expected. However, if HQ > 1, it represents a potential risk of presence of additional non-carcinogenic substances and exposure to harmful effects.

3. Results and Discussion

3.1. Statistical Analysis of Groundwater Hydrochemical Parameters

The physico-chemical properties of the sampled groundwater, encompassing pH, Salinity (TDS), major dissolved ions (SO₄²⁻, Cl⁻, HCO₃⁻, NO₃⁻, K⁺, Na⁺, Ca²⁺, and Mg²⁺) are summarized in Table 1. This table presents the minimum, maximum, mean, standard deviation, Skewness and Kurtosis values calculated from a dataset of 30 groundwater samples. The results indicate that groundwater in the study area is neutral to slightly alkaline, with low pH variability, reflecting a chemically buffered system dominated by carbonate equilibria. Salinity shows moderate to high values and relatively symmetrical distributions, suggesting spatially variable but widespread mineralization within the aquifer. Major ions display contrasting statistical behaviors. Calcium, magnesium, bicarbonate, and sulfate have moderate dispersion and near-symmetrical distributions, indicating consistent control by water–rock interaction, particularly dissolution of carbonate and sulfate minerals. In contrast, sodium and chloride exhibit high variability and strong positive skewness, pointing to localized salinity sources such as evaporate dissolution and evaporation processes, which affect only part of the aquifer. Nitrate concentrations show pronounced positive skewness and high kurtosis, revealing the presence of extreme values superimposed on a lower background level. This pattern is characteristic of non-uniform anthropogenic inputs, most likely related to agricultural activities. Overall, the statistical characteristics suggest that groundwater chemistry is primarily governed by natural geochemical processes, while salinity hotspots and nitrate enrichment reflect superimposed human influence.

3.2. Groundwater Quality Assessment

3.2.1. Assessment of Irrigation Water Quality Using IWQI

Electrical conductivity (EC) is a key parameter for evaluating water salinity and its appropriateness for irrigation. In the Regueb aquifer, EC values ranged between 1200 and 24,180 µS/cm, with an average of 7641 µS/cm. Following the classification of [40], EC is divided into five categories: excellent (<250 µS/cm), good (250–750 µS/cm), permissible (750–2250 µS/cm), doubtful (2250–5000 µS/cm), and unsuitable (>5000 µS/cm). In the study area, 50% of the samples fell into the doubtful category, 36.67% were classified as good, and 13.33% as permissible, reflecting a generally moderate but variable salinity risk for irrigation (Figure 4a) [31].

The SAR reflects the balance between sodium and calcium plus magnesium ions, serving as an indicator of potential soil sodicity. In the Regueb aquifer, SAR values ranged from <3 to >9, with the majority of samples (96.16%) classified as moderately suitable (3–9), 6.66% as acceptable (<3), and 6.66% as unsuitable (>9) (Figure 4b). High SAR values can negatively affect soil structure by displacing calcium and magnesium ions, thereby reducing soil permeability [41,42].

The %Na indicates the relative proportion of sodium in irrigation groundwater. In this study, %Na values ranged from below 20% to 91.38%, with 56.67% of samples falling into the permissible category (40–60%), 36.66% classified as good (20–40%), 6.66% as doubtful (60–80%), and none as excellent (<20%) (Figure 4c). High sodium content in irrigation groundwater can impair soil permeability and adversely impact crop development [31,37]. Magnesium Hazard (MH) evaluates the effect of magnesium on soil quality. MH values ranged from <50% to >50%, with 70% of samples exceeding the threshold and thus considered unsuitable for irrigation, while 30% were within the acceptable range (<50%) (Figure 4d). High MH can increase soil alkalinity and negatively impact crop productivity [10].

Potential Salinity (PS) evaluates the risk of soil salinization from chloride and sulfate ions. PS values ranged from <3 to >15, with 86.67% of samples classified as doubtful (>15), 13.33% as moderate (3–15), and none as acceptable (<3) (Figure 5a). Prolonged use of water with high PS can degrade soil fertility and crop yield [43,44].

Permeability Index (PI) assesses the impact of water chemistry on soil permeability. PI values ranged from <25 to >75, with 76.67% of groundwater samples classified as unsuitable (<25), 23.33% as acceptable (25–75%), and none in the excellent category (>75) (Figure 5b). High Na⁺ and HCO₃⁻ concentrations reduce soil permeability, affecting crop growth [29,35].

Kelly’s Ratio (KR) measures the dominance of sodium relative to calcium and magnesium in water. In the Regueb aquifer, KR values ranged from below 1 to above 1, with 86.67% of samples considered suitable for irrigation (<1) and 13.33% deemed unsuitable (>1) (Figure 5c). KR is commonly employed to assess the sodium hazard in irrigation water [37,45].

Soluble Sodium Percentage (SSP) represents the proportion of sodium relative to other cations. SSP values varied from <50% to >50%, with 86.67% of samples suitable for irrigation (<50%) and 13.33% unsuitable (>50%) (Figure 5d). Proper management of SSP is essential to maintain soil structure and irrigation efficiency [40,42].

Based on the IWQI, the groundwater’s classification (

Table 4. Classification of groundwater samples for irrigation suitability based on IWQI.

IWQI Class	IWQI Range	Quality Level	Domain (%)	Samples (n)	Reference
Low pollution	<22	Low	2.95	5	[34]
Medium pollution	22–37	Medium	42.72	11
High pollution	>37	High	54.31	14

) reveals that, overall, groundwater quality in the study area is largely unsuitable for irrigation. Only 16.67% of the samples are categorized as low-restriction (IWQI < 22), highlighting the limited availability of high-quality water for irrigation purposes.

The moderate restriction class (IWQI = 22–37) accounts for 36.67% of the samples, suggesting that groundwater can be used for irrigation under controlled management practices. In contrast, the high restriction class (IWQI > 37) is dominant, comprising 46.67% of the samples and covering 54.31% of the study area, indicating a high risk of soil salinization and reduced crop productivity (Figure 6).

The observed deterioration in groundwater quality is likely driven by natural mineralization processes within the aquifer, the semi-arid climate, and intensive agricultural practices. These results underscore the importance of implementing sustainable groundwater management strategies, such as optimized irrigation techniques and the cultivation of salt-tolerant crops, to support long-term agricultural sustainability [31,34,41].

3.2.2. Drinking Water Quality Evaluation Using DWQI

The hydrochemical properties of groundwater in the Regueb aquifer are presented in

Table 5. Classification of groundwater quality based on nitrate concentration.

Parameter	Range	Class	Domain (%)	Samples (n)
TDS (mg/L)	840–2000	Moderate	26.66	8
	2000–4000	Inadequate	46.66	14
	4000–8000	Poor	26.66	8
	>8000	Unsuitable for drinking	–	0
pH	<7.62	Neutral	56.67	17
	>7.62	Slightly alkaline	43.33	13
Ca2+ (mg/L)	<200	Acceptable	56.67	17
	>200	Unacceptable	43.33	13
Mg2+ (mg/L)	<50	Poor	3.33	1
	50–150	Acceptable	30.00	9
	>150	Unacceptable	66.67	20
NO3− (mg/L)	<45	Low risk	38.04	11
	45–100	High risk	60.76	18
	>100	Very high risk	1.18	1
Na+ (mg/L)	115–230	Acceptable	1.54	1
	230–460	Moderate	68.68	21
	460–920	Inadequate	27.70	8
	920–1840	Poor	2.06	1
K+ (mg/L)	<12	Acceptable	48.63	15
	>12	Unacceptable	51.36	15
SO42− (mg/L)	24.05–580	Moderate	17.51	5
	580–1150	Inadequate	82.25	24
	1150–2240	Poor	0.23	1
	>2240	Unsuitable	–	0
HCO3− (mg/L)	<120	Acceptable	29.17	9
	>120	Unacceptable	70.83	21
Cl− (mg/L)	175–350	Acceptable	1.55	3
	350–700	Moderate	21.75	8
	700–1400	Inadequate	62.29	11
	1400–2800	Poor	14.40	6
	>2800	Unsuitable	0.08	2

. Total dissolved solids (TDS) vary between 840 and 16,926 mg/L, with a mean value of 5348 mg/L, surpassing the WHO guideline of 1000 mg/L in the majority of samples. Elevated TDS levels correspond to high contents of Ca²⁺, Mg²⁺, Na⁺, K⁺, HCO₃⁻, Cl⁻, and SO₄²⁻. Spatial analysis indicates that 40% of the samples are moderately mineralized, while 60% fall into categories ranging from unsuitable to undrinkable, particularly in the vicinity of Sebkhat Mecheguia (Figure 7a) [41,46].

Groundwater pH in the Regueb aquifer varied from 7.32 to 8.18, with an average of 7.59, indicating neutral to slightly alkaline conditions, consistent with the underlying sandstone and basalt lithology. All samples fall within the WHO recommended range of 6.5–8.5. Neutral water dominates 56.67% of the samples, covering 66.67% of the study area, while slightly alkaline water accounts for 43.33% (Figure 7b) [20,47].

Calcium (Ca²⁺) concentrations in the Regueb aquifer ranged from 40 to 720 mg/L, with a mean of 268.8 mg/L, and 43.33% of the samples surpassed the WHO standard of 200 mg/L (Figure 7c). Magnesium (Mg²⁺) levels varied between 43.2 and 662.4 mg/L, averaging 255.52 mg/L, with 66.67% of samples surpassing the permissible limit of 150 mg/L, indicating widespread water hardness (Figure 7d) [8,11].

Nitrate (NO₃⁻) concentrations in the Regueb aquifer varied from 29 to 130 mg/L, with a mean value of 52.93 mg/L. A significant portion of the samples falls into high to very high health-risk categories, potentially leading to conditions such as methemoglobinemia and gastrointestinal disorders (Figure 7e) [4,6,48].

Sodium (Na⁺) concentrations ranged from 115.1 to 1156 mg/L, largely influenced by irrigation return flows, soil leaching, wastewater intrusion, and the mixing of brackish water (Figure 8a). Potassium (K⁺) levels varied between 5 and 47.7 mg/L, with a mean of 19.51 mg/L, and 46.67% of samples exceeded the recommended limit of 12 mg/L (Figure 8b) [4,49].

Elevated sulfate concentrations (4368 mg/L) may be related not only to natural mineral dissolution but also to anthropogenic activities such as the use of sulfate-based fertilizers, irrigation return flows, and wastewater infiltration (Figure 8c).

Bicarbonate (HCO₃⁻) remains within safe limits across the study area (Figure 8d). Chloride (Cl⁻) ranges from 213 to 6035 mg/L, indicating strong salinity influence and potential anthropogenic contamination (Figure 8e) [17,50].

Based on the DWQI, the groundwater quality is predominantly poor to unfit for human consumption for human consumption, with 66.67% of samples classified as unfit (DWQI > 300), covering 75.88% of the study area (

Table 6. Groundwater suitability for drinking based on DWQI.

DWQI Range	Water Quality Class	Domain (%)	Samples (n)	References
<50	Excellent water	–	0	[51]
50–100	Good water	–	0
100–200	Poor water	1.31	4
200–300	Very poor water	22.8	6
>300	Unfit for human consumption	75.88	20

). Only a minor fraction corresponds to poor water quality, confirming the limited suitability of groundwater for drinking in the Regueb aquifer (Figure 9) [12,13].

Overall, the findings demonstrate that groundwater quality in the Regueb aquifer is influenced by the interplay of natural mineralization, semi-arid climatic conditions, and agricultural activities. These results underscore the critical need for regular water quality monitoring, effective treatment measures, and sustainable groundwater management practices to minimize health risks and ensure the provision of safe drinking water, corroborating observations from previous studies [1,41,46].

3.2.3. Nitrate Contamination Assessment Using the NPI

The NPI classification indicates that the majority of groundwater samples fall within moderate to sensitive pollution categories, confirming the widespread impact of nitrate contamination in the Regueb aquifer (

Table 7. NPI values and categories of groundwater from the Regueb aquifer.

NPI Range	Pollution Level	NPI Class	Domain (%)	Samples (n)
<0	Clean	1	–	0
0–1	Light pollution	2	17.65	10
1–2	Moderate pollution	3	53.2	12
2–3	Sensitive pollution	4	20.42	5
>3	Very sensitive pollution	5	8.71	3

). NPI values ranged from 0.45 to 5.5, with a mean of 1.65, reflecting spatial variability in nitrate pollution throughout the study area.

Based on NPI classification, 33.33% of samples are lightly polluted (0–1), 40% are moderately polluted (1–2), 16.67% are sensitive (2–3), and 10% are very sensitive (>3). Spatially, moderate pollution dominates 53.2% of the study area, highlighting areas of concern for water quality (Figure 10) [12,22,40].

The NPI results indicate that nitrate contamination is widespread, with more than half of the area affected by moderate to very sensitive pollution levels. This reflects the combined impact of agricultural activities, inadequate sanitation, and domestic wastewater discharge.

Agricultural runoff is likely the main contributor, as nitrate-rich fertilizers are commonly applied in the study area. Uncovered septic tanks and sewage intrusion also elevate Nitrate concentrations in shallow aquifers, while leachate from open dumps may further exacerbate pollution [48,49].

The health risks associated with nitrate are significant. Groundwater with High nitrate levels poses potential hazards such as methemoglobinemia in infants, hypertension, and gastric disorders [41,48]. From a management perspective, areas classified as sensitive and very sensitive require priority monitoring, improved sanitation, controlled fertilizer use, and possible water treatment before consumption.

The spatial pattern of NPI corresponds closely with land use and anthropogenic pressure, emphasizing that human activities are the principal drivers of nitrate contamination in the Regueb aquifer. These findings are consistent with studies in similar semi-arid regions, where agriculture and inadequate wastewater management are major contributors to groundwater nitrate pollution [1,17,50].

3.2.4. Health Risk Assessment (HQ)

The risk quotient (HQ) quantifies potential non-carcinogenic health risks. Oral HQ values differ by age and gender: children (1.53–6.06, mean 2.48), women (0.82–3.69, mean 1.5), and men (0.54–2.3, mean 1.05) (

Table 8. Distribution of groundwater samples by Hazard Quotient (HQ) class for different population groups.

HQ	Range	Class	%Class
Child	<1	Acceptable	0
	>1	Unacceptable	100
Women	<1	Acceptable	24.39
	>1	Unacceptable	75.61
Man	<1	Acceptable	51.52
	>1	Unacceptable	48.48

, Figure 11). All water samples pose unacceptable risks for children (HQ > 1). For women, 24.39% of HQ values are safe (HQ < 1, covering 20% of the area), while 75.61% are unsafe. For men, 51.52% are safe (63.33% of the area), and 48.48% are unsafe.

The HQ results indicate that children are the most vulnerable group, with all samples exceeding the recommended safety limit (HQ > 1), consistent with their higher water intake per body weight. Women also show significant exposure (HQ > 1 in 80% of cases), likely due to similar consumption patterns and local water use. Men are comparatively less exposed, but nearly one-third of samples exceed safe HQ levels, indicating localized health risks [22,39].

These findings suggest that nitrate contamination in the Regueb aquifer poses a serious public health concern, especially for infants and women [52]. Mitigation strategies should prioritize children and women through measures such as alternative water sources, water treatment, and regular monitoring of nitrate levels [41]. The spatial distribution of HQ aligns with areas of High nitrate concentration identified in NPI and DWQI analyses, confirming that agricultural runoff, septic tanks, and wastewater are key contributors to health risk [53].

3.3. Potential Sources of Groundwater Contamination

Analysis of land use, irrigation practices, and population activities in the study area indicates that nitrate contamination of groundwater arises from multiple interrelated sources. Agricultural activities, particularly intensive vegetable and cereal cultivation, represent the main contributor, as excessive use of nitrogen-based fertilizers enhances nitrate leaching through the soil into shallow aquifers [26]. Similar trends have been reported in southern Spain and Italy, where fertilizer application strongly correlates with elevated nitrate levels in groundwater [14,15]. In addition, domestic wastewater from peri-urban settlements, often untreated or inadequately managed, infiltrates into the aquifer via septic tanks or effluent leaks, adding localized nitrate loads and microbial contamination [16,54]. Livestock farming further contributes organic nitrogen through manure application and direct waste deposition near recharge zones, which is subsequently mineralized into nitrate, a pattern observed also in Poland and other European semi-arid regions [55]. While anthropogenic activities dominate, natural hydrogeological factors such as nitrogen-rich soils or shallow permeable layers can enhance nitrate accumulation in certain areas [56]. Comparisons with other semi-arid regions worldwide confirm that intensive fertilization, insufficient wastewater management, and livestock operations consistently lead to elevated nitrate concentrations [57]. This multi-factorial understanding underscores the importance of integrated land and water management strategies, including controlled fertilizer use, improved wastewater treatment, and protection of recharge zones, to effectively reduce groundwater contamination and safeguard human health.

3.4. Human Health Risks

To evaluate the implications of this contamination for local populations, a human health risk assessment was conducted based on nitrate concentrations measured in groundwater. The non-carcinogenic health risk was estimated using the Hazard Quotient (HQ) approach for different population groups, particularly adults and children. The results indicate that some wells may pose potential health risks, especially for children, who are more vulnerable to nitrate exposure due to their lower body weight and relatively higher water intake. These findings are consistent with the drinking water guidelines established by the WHO, which recommend a maximum nitrate concentration of 50 mg/L to prevent adverse health effects such as methemoglobinemia. Comparable health risk assessments conducted in agricultural aquifers in Tunisia, Spain, Italy, and Poland have similarly reported elevated nitrate concentrations and associated health concerns linked to intensive farming activities [14,15,16].

3.5. Global Comparison

To further contextualize the severity of groundwater contamination in the study area, the results were compared with those reported from other semi-arid and European agricultural regions. The nitrate concentrations observed in this study are comparable to, and in some cases exceed, levels reported in intensively cultivated aquifers worldwide. Studies from Spain and Italy, for example, have documented significant nitrate accumulation associated with fertilizer application and groundwater–surface water interactions [14,15], while research in Poland has highlighted the role of agricultural nitrogen inputs in increasing nitrate levels in groundwater systems [16]. These international comparisons confirm that nitrate pollution is a widespread environmental issue affecting both semi-arid and temperate agricultural regions. Integrating the findings of the present study within this global context highlights the critical influence of intensive agricultural practices and insufficient nutrient management on groundwater quality and emphasizes the need for sustainable land and water management strategies. Such measures include controlled fertilizer application, improved wastewater treatment systems, and the protection of groundwater recharge zones to mitigate contamination and safeguard water resources and public health.

3.6. Safeguarding Health and Livelihoods Through Groundwater Management

Nitrate-induced groundwater degradation in semi-arid regions has considerable socio-economic impacts, especially in areas like Central Tunisia, where groundwater is considered the main source for both irrigation and drinking water. Several studies conducted in Tunisian aquifer systems have shown that intensive agricultural activities, combined with inadequate sanitation and limited natural recharge, have led to a deterioration of groundwater quality, directly affecting human well-being and local economic stability [21]. From a public health standpoint, water with High nitrate content presents substantial risks, especially for infants, pregnant women, and other vulnerable groups. Prolonged exposure to nitrate-contaminated groundwater has been linked to methemoglobinemia and other chronic health effects, which can elevate healthcare costs and strain local medical infrastructure [7,30]. In rural areas, where alternative water supplies are scarce, households are often forced to purchase bottled water or invest in water treatment systems, representing a substantial financial burden for low-income communities [10,48].

Agriculture, which constitutes a central component of the regional economy in Central Tunisia, is also adversely affected. While nitrate-rich groundwater may temporarily enhance crop growth, prolonged irrigation with contaminated water can generate soil salinization, nutrient imbalance, and declining soil fertility, ultimately reducing agricultural productivity. Recent studies have shown that intensive irrigation practices influence nutrient dynamics and nitrate leaching, with excessive water application increasing nitrate movement through soil profiles and potentially degrading soil quality and crop yields if not optimally managed [58]. Furthermore, research Highlights that salinity and sodicity often linked to poor-quality irrigation water affect a significant portion of global irrigated lands, impairing soil fertility and crop productivity, which poses substantial risks to agricultural sustainability and food security [59]. These findings underscore the intricate connections between groundwater quality, irrigation practices, and long-term agricultural viability in semi-arid environments.

Climate change acts as an additional stressor, intensifying these socio-economic challenges. Rising temperatures, reduced precipitation, and increased evapotranspiration rates decrease groundwater recharge and promote the concentration of dissolved pollutants, including nitrates [28]. Prolonged drought periods increase groundwater abstraction for irrigation, accelerating aquifer depletion and water quality deterioration. These combined pressures heighten the risk of water scarcity, rural poverty, and population displacement in semi-arid regions [30].

Addressing the socio-economic impacts of nitrate pollution requires integrated groundwater management strategies that combine water quality protection, sustainable agricultural practices, and social equity considerations. Encouraging efficient irrigation practices, controlling fertilizer application, and establishing long-term groundwater monitoring programs are crucial measures to safeguard public health and support sustainable socio-economic development in semi-arid regions like Central Tunisia [41].

4. Conclusions & Recommendations

Groundwater in the Regueb Basin shows moderate to High nitrate contamination, with 76.67% of sampled wells exceeding the WHO guideline value of 50 mg/L. Hazard Quotient calculations indicate that children are the most at-risk group, followed by women and men. Based on these findings, groundwater monitoring programs should prioritize high-nitrate wells. Sustainable groundwater management practices, including improved fertilizer application, controlled irrigation, and protection of recharge areas, are recommended to minimize nitrate leaching and safeguard human health and agricultural productivity.

The Irrigation Water Quality Index (IWQI) assessment discloses that more than half of the aquifer is unsuitable for irrigation, highlighting risks of soil salinization, reduced permeability, and declining crop productivity. Assessment of drinking water quality using the DWQI shows that approximately two-thirds of the total samples are unsuitable for human consumption, with elevated levels of TDS, nitrates, and other major ions exceeding WHO standards. Based on the results of the Nitrate Pollution Index (NPI) and the Human Health Risk Assessment (HQ), groundwater monitoring programs should prioritize wells showing nitrate concentrations exceeding drinking water standards. Furthermore, improved fertilizer management and controlled irrigation practices are recommended to minimize nitrate leaching and protect groundwater resources in the Regueb Basin.

These findings underscore the combined effects of natural mineralization, semi-arid climate, intensive agriculture, and inadequate sanitation on groundwater quality. The socio-economic implications are substantial, affecting public health, agricultural sustainability, and local livelihoods.

To mitigate these challenges, integrated groundwater management strategies are essential, including the adoption of efficient irrigation practices, regulation of fertilizer application, water treatment, and long-term monitoring programs. These strategies are vital to protect human health, maintain soil fertility, and encourage the sustainable management of groundwater resources in semi-arid areas such as Central Tunisia.

To lessen the impacts of the contaminated groundwater and safeguard sustainable water use in the Regueb aquifer, it is recommended to (i) establish long-term groundwater monitoring programs for key contaminants (nitrates, salinity, TDS), (ii) promote efficient irrigation practices and salt/nitrate-tolerant crops to protect soil and maintain productivity, (iii) optimize fertilizer use and improve sanitation to reduce anthropogenic nitrate pollution, and (iv) implement affordable water treatment and community awareness programs to safeguard public health.