Groundwater Recharge Evaluation Using Stable Isotopes and the NETPATH Model in Al-Madinah Al-Munawarah Province, Saudi Arabia

Abstract

In this study, hydrogeochemistry and environmentally stable isotopes were employed to examine the processes involved in recharging aquifer systems and the changes in the groundwater chemistry caused by the interaction between the water and the aquifer matrix. Based on data derived from 113 groundwater wells, various tools and techniques, including stable environmental isotopes Oxygen-18 and Deuterium (δ¹⁸O and δD) for 33 samples and geochemical modeling with NETPATH, were used to evaluate the recharge mechanism and the evolution of the groundwater, combining GIS with hydrological and hydrochemical methods. The results revealed that groundwater from the Quaternary was the main source for irrigation; the water quality was categorized as relatively fresh to saline, with the total dissolved solids (TDSs) ranging from 261.3 to 8628.56 mg/L, exhibiting an average value of 2311.68 mg/L. The results of the environmental isotope analysis showed that the range of oxygen δ¹⁸O isotopes in the groundwater was from −5.65‰ to +0.39‰, while the range of hydrogen δD isotopes was from −32.60‰ to 4.73‰. Moreover, the δ¹⁸O–δD relationship indicated that the groundwater samples fell around the global meteoric precipitation line, showing a strong relationship, with a coefficient (R²) of approximately 0.82. The NETPATH model revealed that the dissolved chemical species within the groundwater system primarily originated from processes such as mineral weathering and dissolution, ion exchange, and evaporation.

1. Introduction

At present, water resource scarcity is a critical issue worldwide; this is due to growing water demands for different purposes and the impact of ongoing climate change [1,2]. The effective management of water resources, particularly groundwater, requires the performance of various studies and measurements in order to assess its sustainability and achieve its maximum utilization. The assessment of both the amount and quality of groundwater is important for the advancement of society and in creating a foundation for future water resource planning. In the Kingdom of Saudi Arabia (KSA), there is a restricted supply of non-renewable water resources, which differs across different areas. The province under consideration in this work (Al-Madinah Al-Munawarah) is situated in the north–northwest of the KSA and is known to have average potential for water resources. The lack of significant water-bearing aquifers in this region is a result of its natural geography and limited rainfall, being primarily encircled by crystalline rocks. There are numerous basins in this region (Figure 1). Hence, the calculation and determination of the water quantities, such as rainwater, that recharge the groundwater aquifers are crucial in effectively managing the groundwater resources. Numerous research studies have been carried out concerning the geophysical and hydrogeological characteristics and quality of the groundwater in Al-Madinah Al-Munawarah Province [3,4,5,6,7,8,9,10]. However, there have been no detailed attempts to estimate the groundwater recharge or the effects of rock–water interactions on the groundwater composition (groundwater evolution during flow path). Knowledge regarding these issues could aid in determining the salinity of the basins’ beds and groundwater. There are many methods available to study groundwater recharge. The selection of such methods [11,12,13] could be challenging due to the varying assumptions and limitations of each technique. Hence, it is advisable to utilize various approaches so as to reduce the ambiguity and enhance the conceptual understanding of groundwater recharge in the examined area [14,15].

The study of groundwater recharge, especially in arid regions, is a challenging task, as the recharge amounts tend to be lower than those determined with the methods used in previous works [16,17]. The recharge flux from precipitation directly into groundwater aquifers may be smaller, and it fluctuates due to the arid climate and the low permeability of the soil [18]. The primary goals of this study were to analyze the groundwater chemistry, measure the hydrogeochemical changes, identify the recharge sources using environmental isotopes, and evaluate the changes in the groundwater chemistry caused by the interaction between the groundwater and aquifer matrix using the NETPATH geochemical model. The results and methods were examined in relation to their accuracy and ease of application. Moreover, the outcomes of this research—specifically the assessment of the current recharge rate—can be applied in comparable semi-arid regions globally to promote sustainable development. The main research objective was to establish a plan for the recharging of groundwater in Al-Madinah Al-Munawarah so as to promote improved groundwater management and ensure the sustainability of the region’s water resources. The specific goals were as follows:

• To determine the chemical and isotopic compositions of the groundwater, as well as understanding the geochemical evolution and mixing processes in the groundwater;
• To understand how various conditions and factors affect groundwater resources;
• To study the relationship between the groundwater quality, water-bearing formations, and recharge sources, which is essential for the appropriate management and utilization of groundwater.

2. Study Area

The study area was Al-Madinah Al-Munawarah Province, which has an area of approximately 152,000 km² and is situated in the northwest of the KSA (Figure 1). Its lack of significant water-bearing aquifers is a result of its natural geography and limited rainfall, being primarily encircled by crystalline rocks. There are numerous basins in this region. The rainfall distribution in the province ranges between 40 mm (west) and 80 mm (east), as shown in Figure 2a, with infrequent and minimal precipitation and frequent extended dry periods. The wide range of rainfall quantities is caused by the high-altitude mountainous terrain of the region. These conditions could positively affect the renewal of the groundwater aquifers in the region and enhance the groundwater quality. Geologically, the western portion of the Arabian Shield is composed of Proterozoic rocks, which were exposed to folding structures, metamorphic processes, granitization, and intrusion in multiple tectonic phases before the Cambrian period [3]. In Al-Madinah Al-Munawarah Province, different rocks from the upper Proterozoic era are present, which are classified as the Alysa and Furayh groups, separated by a minor angular unconformity (Figure 2b). The Alys group comprises mafic volcanic rocks, volcanic rocks composed of fragments from past eruptions, and tuff containing multiple layers of silicic lava [3,4,5,6,7]. The present work was part of our research on groundwater sustainability in semi-arid regions. The Al-Madinah Al-Munawarah region was chosen based on its importance as a groundwater resource in the KSA. In this province, groundwater is extracted from different geological formations, including Quaternary alluvial deposits, Tertiary and Quaternary (Harrat) rocks, and the elevated weathered and fractured sections of Proterozoic basement rocks [8].

3. Materials and Methods

The research methodology was as follows:

1. Field measurements and collection of groundwater samples: In this study, the depth to groundwater was measured in 113 groundwater wells, and 103 water samples were collected for the chemical analysis. Among these, 33 samples were selected as representatives for the isotope analysis. The portable Magellan GPS 315 (San Dimas, CA, USA), was used to determine the water samples’ positions and identify the UTM coordinates of the study area (Figure 1). The groundwater samples were filtered using 0.45 μm Millipore membrane filters, followed by acidification. The water samples were stored at 4 °C until they were taken to the laboratory for physicochemical analysis and examination. Physicochemical parameters including the temperature, pH, EC, and TDSs were measured at the site.

2. Laboratory work: Standard Methods for the Examination of Water and Wastewater [19] were used to determine the main cations and anions in all 103 groundwater samples (Figure 1). Various ions, including (Ca⁺²) and (Mg⁺²), were quantified using titration, while (Na⁺) and (K⁺) were analyzed using flame photometry (Jenway, Models PFP7, Cole-Parmer Ltd., Staffordshire, United Kingdom). The Cl⁻ levels were determined through titration with code chloride 9253, and solution of silver nitrate (AgNO₃ (4500-Cl argentometry method, [20]), while the SO₄²⁻ levels were measured using the turbidity with a spectrophotometer. The alkalinity (carbonate and bicarbonate) concentrations were estimated by titration against 0.01N H₂SO₄ using Phenolphthalein and methyl orange indicators with code (2320). Minor and trace elements, for example SiO₂, was measured using Inductively Coupled Plasma Optical Emission Spectrometry (ICP-AES, Shimadzu ICPE-9000), thermo-Jarrell elemental company, (Columbia, MD, USA). Before analysis, the ICP-AES was calibrated using Accu Trace Reference Standard and Accu Trace Blank, while I⁻, Br⁻, and PO₄³⁻ were measured using ion chromatography (ICS-1100 systems, Ramsey, MN, USA). The charge balance error (CBE) was calculated using Equation (1). The result indicated that the analytical error in the determined ion concentrations (meq/L⁻¹) fell within 5%.

$$\mathrm{CBE}=\frac{\sum \mathrm{Cations}-\sum \mathrm{Anions}}{\sum \mathrm{Cations}+\sum \mathrm{Anions}}\times 100$$

(1)

Moreover, stable isotopes for deuterium and oxygen-18 were examined for 33 representative groundwater samples using a PICARRO L2130-i high precision isotopic analyzer by Hydrogeology Group, Institute of Geological Sciences—Frei University Berlin—Germany. The isotopic compositions were expressed as the differences between the measured ratios of each sample and the Vienna Standard Mean Ocean Water (VSMOW) reference value, divided by the measured ratio of the used reference. The results are presented as per mil deviations from the SMOW.

3. Office work:

• We examined the existing geological, meteorological, and hydrogeological studies, as well as the recent methodologies utilized in identifying and estimating groundwater recharge sources.
• An assessment of different hydrogeochemical processes was performed through a range of tools and techniques, including hydrochemical facies evaluation diagrams (HFED), and geochemical models such as NETPATH.
• The global meteoric water line (GMWL) and mixing water line (MWL) were used for sample categorization based on the data for oxygen-18 and deuterium. The groundwater samples’ isotopic compositions were expressed as per mil ‰ deviations from the Standard Mean Ocean Water (SMOW). In line with [21], these variances were expressed as δD for deuterium (²H) and δ¹⁸O for oxygen-18, applying the following equations:

$${\delta }^{18}\mathrm{O}‰={\displaystyle \frac{{}^{18}\mathrm{O}/{}^{16}\mathrm{O}_{\mathrm{Sample}}-{}^{16}\mathrm{O}_{\mathrm{SMOW}}}{{}^{18}\mathrm{O}/{}^{16}\mathrm{O}_{\mathrm{SMOW}}}}$$

(2)

$$\delta \mathrm{D}‰={\displaystyle \frac{{}_1{}^2\mathrm{H}/{}_1{}^2{\mathrm{H}}_{\mathrm{s}\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{l}\mathrm{e}}\mathrm{}-\mathrm{}{}_1{}^2\mathrm{H}/{}_1{}^2{\mathrm{H}}_{\mathrm{S}\mathrm{M}\mathrm{O}\mathrm{W}}}{{}_1{}^2\mathrm{H}/{}_1{}^2{\mathrm{H}}_{\mathrm{S}\mathrm{M}\mathrm{O}\mathrm{W}}}}$$

(3)

Positive results for δ¹⁸O and δD indicate that a water sample is more enriched in these parameters associated with the SMOW, while negative values indicate that these parameters are reduced in comparison to the standard. The worldwide meteoric water δ¹⁸O and δD levels have been shown to exhibit a linear correlation, which can be represented by the equation given below [21]:

δD = 8 δ¹⁸O + 10

(4)

Data depicted on the GMWL are assumed to be of meteoric origin, while those that deviate are assumed to indicate either enrichment or depletion. This is based on large quantities of meteoric water collected at different latitudes.

• NETPATH version 2 is a geochemical model, developed in [22], that quantifies the mass balance and mass transfer reactions responsible for the evolution of the groundwater chemistry along specific flow paths. This model facilitates the calculation of the activity of dissolved species within a system; these may be either introduced (through dissolution) or removed (upstream) from the original solution (via rainfall). Specific constraints, encompassing both chemical and isotopic data, are established to determine the final composition of the water (downstream). Additionally, this model aids in distinguishing the impacts of seawater intrusion from subsequent geochemical alterations along the flow path direction. The selection of phases or minerals is based on these constraints, the characteristics of the aquifer matrix, and the types and states of the phases, determined through a comparison with a thermodynamic database (WATEQ program). Chloride is treated as a conservative ion; therefore, when calculating the mixing ratio, chloride phases are neglected. Furthermore, mixing proportions can also be derived from the δ¹⁸O and δD values, which enable the verification of the results obtained through the chloride analysis.
• GIS technology was used to obtain data on the characteristics and quality of the groundwater.

4. Results and Discussion

4.1. Groundwater Aquifer and Movement

Quaternary water-bearing formations serve as a primary resource in groundwater extraction for irrigation, representing valuable sources in the studied province [23]. Lithologically, they consist of gravel and sand, ranging from coarse to fine, and conglomerates. The thickness of the Quaternary aquifer increases in the downstream areas of wadis, reaching a depth of approximately 100 m. The groundwater aquifer starts at the top of the basement rocks and extends to the free-water surface. Low-permeability materials, lava, and inter-lava hydraulic fluctuations generate certain semi-confined areas. The variation in the depth to groundwater from the ground surface is 10.57 to 177 m (Figure 3a). Therefore, as illustrated in Figure 3b, a groundwater head distribution map was constructed using the depth-to-water data for 2022 (for 113 wells). Based on this map, the groundwater head in the far south of the province experiences southwesterly flows due to the decreasing water head, with high potential for water yielding in the southern well fields. Rainfall is the source of aquifer recharge along the basin, as indicated by the heads in the north, which align with the flow direction to the southwest of the province down-gradient. The water flow in the central region of the province shows a westward direction, which is in line with the presence of the Red Sea drainage point within the aquifer system. Accordingly, the evolution of the groundwater chemistry through these different flows is significant; this is discussed in the next section.

4.2. Groundwater Chemistry

Table 1. Descriptive statistics derived from geochemical analysis of 103 groundwater samples.

Parameter	Unit	Minimum	Maximum	Mean	WHO for Drinking Water (2022) [27]
T	°C	24	31	27	27–34
pH	-	6.48	8.5	7.56	6–8.5
EC	µS/cm	582	14050	3697.99	1000
TDS	mg/L	261.3	8628.56	2311.68	600
Na+	mg/L	41.9	1754.58	474.61	200
Ca2+	mg/L	16.08	854.01	228.61	100–300
Mg2+	mg/L	1.99	550.64	77.19	100–200
K+	mg/L	6.39	75.6	11.55	Less than 10
CO32−	mg/L	3	51	15.2	50–200
HCO3−	mg/L	30.5	1265.6	190.23	150–350
Cl−	mg/L	13.42	3186.35	617.43	200–300
SO42−	mg/l	35.05	3143.55	807.91	250
NO3−	mg/L	0.02	359.47	61.11	50
PO43−	mg/L	0.01	5.16	0.38	Less than 0.1
I−	mg/L	0.01	1.27	0.08	Less than 0.1
Br−	mg/L	0.06	5.82	0.68	Less than 0.1
F−	mg/L	0.02	3.9	0.61	1.5
SiO2	mg/L	5.45	130.22	26.08	Less than 30
Total Hardness	mg CaCO3/L	81.64	4395	833.76	500
Alkalinity	mg/L	44.99	1029.78	170.3	200

displays the chemical properties of the groundwater in Al-Madinah Al-Munawrah Province. The majority of the groundwater was neutral to slightly alkaline, as indicated by the pH range of 6.48 to 8.5, with a mean of 7.56 (Table 1). The primary factors influencing the temperature of groundwater are the geothermal gradient and the air temperature at the land’s surface [24]. The average groundwater temperature in the province was roughly 27.3 °C, although it varied from 24 to 31 °C. The total dissolved solids (TDSs) are typically employed for the measurement of the quality of groundwater [25]. According to Piper [26], groundwater is categorized as fresh, brackish, or saline. The findings presented in Table 1 indicate that the total dissolved solid (TDS) values in the groundwater in this study varied between 261.3 and 8628.56 mg/L, with an average of 2311.68 mg/L. This suggests that the water ranged from fresh to brackish, according to [25]. Based on the total dissolved solid (TDS) levels in the area, a map was constructed to illustrate the locations of higher-salinity areas (Figure 4). This map shows that the southeastern and southern areas of the region have higher TDS levels. The major ions (Na⁺, Mg²⁺, Ca²⁺, SO₄²⁻, and Cl⁻) typically exhibited positive correlations with the total dissolved solids (TDSs), as illustrated in Figure 5. The correlation coefficients (R²) were notably high, ranging from 0.69 to 0.90, showing that the rises in their concentrations were aligned along the flow direction from the upstream to the downstream areas of the basins, as depicted in Figure 1. Meanwhile, for CO₃²⁻ and HCO₃⁻, the relationship with the TDSs was weak because the freshwater in the study area originated from precipitation, and the higher salinity was due to the water–rock interactions and seawater intrusions in some areas.

4.3. Hydrogeochemical Facies of Groundwater

The modified Piper diagram, as introduced by Chadha [28] and illustrated in Figure 6, shows the major ions’ chemistry, offering insights into hydrogeochemical facies and the evolution of groundwater based on the relative concentrations of major ions. In this study, approximately 52% of the groundwater samples exhibited a Na-Cl water type, indicative of seawater intrusion (refer to Figure 6). Conversely, 47% of the samples fell within Field 2, characterized by water with Ca-Mg-SO₄/Cl, which reflects the influence of ion exchange processes. The remaining 1% of the groundwater samples were situated in Field 1, where Ca and Mg were the predominant cations and HCO₃⁻ was the predominant anion, attributed to recharge from rainfall. These findings indicate that both geochemical and physical processes, including dissolution, leaching, and evaporation, along with precipitation recharge and water–rock interactions, are significant factors that influence the water quality variations in the study area.

Figure 5. Major ions’ concentrations (mg/L) versus total dissolved solids (TDSs) in mg/L for groundwater samples collected in Al-Madinah Al-Munawarah Province.

Figure 5. Major ions’ concentrations (mg/L) versus total dissolved solids (TDSs) in mg/L for groundwater samples collected in Al-Madinah Al-Munawarah Province.

4.4. Groundwater Recharge Mechanism

The ideal tracer isotopes for the identification of groundwater recharge and mixing sources are oxygen δ¹⁸O and hydrogen δ²H. As they are components of water molecules and do not participate in geochemical reactions, they offer valuable insights into the physical processes that impact groundwater, such as evaporation and mixing processes [25,26]. The calculated stable isotopes of deuterium and oxygen-18 in 33 selected groundwater samples, as shown in Figure 7, are listed in

Table 2. Results of deuterium and oxygen-18 analyses of 33 selected groundwater samples in Al-Madinah Al-Munawarah Province.

Sample No.	δ18O‰	dδD‰	Stdev δ18O‰	Stdev dδD‰
MAD56	−5.66	−32.61	0.09	0.67
MAD58	−4.27	−30.23	0.09	0.83
MAD78	−3.71	−12.34	0.03	0.61
MAD101	−3.71	−18.88	0.04	0.74
MAD91	−3.17	−13.07	0.05	0.43
MAD100	−3.17	−15.44	0.07	0.47
MAD146	−2.9	−13.72	0.05	0.20
MAD70	−2.6	−7.83	0.02	0.49
MAD124	−2.56	−8.6	0.1	0.59
MAD90	−2.54	−5.13	0.03	0.06
MAD102	−2.52	−7.39	0.1	0.49
MAD92	−2.49	−9.17	0.05	0.31
MAD105	−2.48	−6.03	0.07	0.37
MAD22	−2.24	−4.67	0.12	0.46
MAD159	−2.05	−9.63	0.08	0.35
MAD8	−2.01	−2.02	0.11	0.45
MAD85	−1.96	−9.64	0.08	0.76
MAD143	−1.92	−7	0.03	0.47
MAD99	−1.85	−13.69	0.11	0.54
MAD119	−1.84	−10.45	0.05	0.36
MAD164	−1.69	−5.71	0.08	0.45
MAD32	−1.65	−3.11	0.05	0.26
MAD36	−1.65	−1.93	0.1	0.48
MAD31	−1.55	−2.16	0.04	0.39
MAD129	−1.45	−5.68	0.09	0.4
MAD43	−1.33	−3.84	0.02	0.08
MAD4	−1.18	−2.8	0.08	0.35
MAD171	−0.8	−1.66	0.06	0.14
MAD53	−0.78	2.94	0.07	0.24
MAD55	−0.72	0.17	0.11	0.23
MAD153	−0.63	3.32	0.09	0.69
MAD175	0.35	4.74	0.07	0.33
MAD112	0.39	1.69	0.08	0.39

.

According to the findings of the environmental isotope analysis, the range of oxygen δ¹⁸O isotopes in the measured groundwater was −5.65‰ to +0.39‰, with an average of 2.07‰, while the range of hydrogen δ²H isotopes was −32.60‰ to 4.73‰, with an average of −7.62‰. The average values of this study are close to the results of Al Harbi et al. [3], which were −2.3‰ and −7.8‰ for δ¹⁸O and δ²H, respectively. According to the δ¹⁸O–δ²H relationship (Figure 8), the majority of the groundwater samples fell around the global precipitation line, suggesting that they were mostly of meteoric origin [21] and clearly indicating young groundwater formed during the Holocene. Particularly upstream of the province’s basins, relatively heavy or highly enriched isotope signatures with positive deuterium and oxygen-18 values were identified. These are typical isotope signatures for groundwater that was formed in a warm climate and close to recent precipitation, confirming that the recharge resulted from recent meteoric precipitation. By plotting the environmental isotopic data of Wadi Malal, which was studied by Al Harbi et al. [3], fortunately there is a big match between them and that of this study, where both data are located more or less in the same local meteoric line. Also, by plotting the average isotopic value of rain in Riyadh, which was reported by Michelsen et al. [29], it is very close to the global precipitation line, as shown in Figure 8. The slope of the local meteoric water line for the study area is 6.07, which is less than the global precipitation line and is close to Riyadh’s, which equals 5.22, as reported by [29]. Finally, the groundwater dynamics and interactions are demonstrated in Figure 9, which plots the δ¹⁸O values against the chloride concentrations in the groundwater samples. The predominant trend observed is a rise in the chloride concentration, accompanied by minimal variation in the isotopic values. The increase in the chloride levels may indicate an increase in the groundwater residence time, which is mainly attributed to enhanced interactions between the groundwater and the rocks forming the aquifer system [30].

4.5. Groundwater Evolution During the Flow Path

In this study, the NETPATH model was utilized to assess geochemical reactions and mixing with additional sources, based on the chemical and isotopic analyses of the groundwater samples [31,32]. This model estimates the net geochemical reactions along the subsurface flow path of groundwater and accounts for the observed variations in the groundwater chemistry between the initial and final selected wells. Four models were constructed to describe the evolution and salinization processes, as well as mixing between the groundwater at the upstream watershed, that present downstream during the flow path (Figure 7). In this study, carbonate, bicarbonate, sulfur, sulfate, calcium, magnesium, sodium, and chloride ions, as well as silica, strontium, and δ¹⁸O, were included as constraints in the NETPATH geochemical model (

Table 3. Constraints, phases, and processes used in NETPATH models.

Constraints	Phases: Dissolution (−) or Precipitation (+)	Processes (Models)
Calcium, Carbon, Magnesium, Potassium, Sodium, Sulfur, Chloride, Silica, Strontium, and δ18O	Albite, Sepiolite, Alunite, Calcite, Chlorite, Sylvite, Dolomite, Ca-Montmorillonite, K-Mica, Illite, Gypsum, SiO2, Halite NaCl, Anorthite, Exchange. This is in line with the geological composition of the study area [4,5,6,7].	Reaction and/or Evaporation and Mixing

).

As the input phases, the key minerals contained in the aquifer sediment served to represent hydrolysis and the interaction between the aquifer matrix and groundwater. The minerals that predominate in terrestrial alluvial deposits include calcite, dolomite, halite, gypsum, montmorillonite, illite, anorthite, alunite, and chlorite. The study area (Al-Madinah Al-Munawarah Province) has hot desert climate conditions and climatically has been classified as BWh [33]. So, the evaporation constraint was chosen to replicate the effects of aridity on groundwater salinization. To approximate the probability of recharging from rainfall, a mixing parameter was also applied in this model. The mixing ratios calculated using the NETPATH model in the final wells were dependent on the availability of necessary data and whether these data could be used to generate viable models. For instance, the mixing ratios in profiles A and B were determined based on the chloride concentrations, while those in profiles C and D were generated solely based on the δ¹⁸O values. The model findings suggest that the interactions between the groundwater and the rocks in the aquifer, along with mixing, are the primary factors influencing the geochemistry of the groundwater in Al-Madinah Al-Munawarah Province, as detailed in

Table 4. Results of NETPATH models of water–rock interactions (mmol/L) and mixing (percent) to represent groundwater evolution in Al-Madinah Al-Munawarah Province.

Model Type	Profile	Initial Water 1	Initial Water 2	Initial Water 3	Initial Water 4	Final Water	Mixing Percentage (%)				Phases Precipitated (+) or Dissolved (−)
							Initial Water 1	Initial Water 2	Initial Water 3	Initial Water 4	Kao	Hal	Sep	Exc	Cal	Dol	Gyp	Si	Mont	Syl	Alb
Reaction and/or Evaporation	A	Rain	MAD78	MAD70	MAD53	MAD55	-	-	-	-	2.24	15.87	3.08	5.65	−0.83	0	13.76	0	0	0	−4.49
	B	Rain	MAD91	MAD124	None	MAD43	-	-	-	-	0	0	−1.07	−0.71	−5.79	3.17	0	2.45	0	0.03	0
	C	Rain	MAD143	MAD99	None	MAD4	-	-	-	-	0	0	2.12	−0.91	1.82	−1.15	−0.11	−5.85	0	0.07	0
	D	Rain	MAD100	MAD102	None	MAD92	-	-	-	-	16.5	0	6.33	0	10.1	−6.55	3.72	0	−14.16	0.3	0
Mixing	A	Rain	MAD78	MAD70	MAD53	MAD55	0	44.7	12.1	43.2	-	-	-	-	-	-	-	-	-	-	-
	B	Rain	MAD91	MAD124	None	MAD43	6.4	86.4	7.2	None	-	-	-	-	-	-	-	-	-	-	-
	C	Rain	MAD143	MAD99	None	MAD4	24.6	4.9	70.5	None	-	-	-	-	-	-	-	-	-	-	-
	D	Rain	MAD100	MAD102	None	MAD92	3.3	12.1	84.6	None	-	-	-	-	-	-	-	-	-	-	-

Note: Kao = kaolinite; Hal = halite; Sep = sepiolite; Exc = exchange; Cal = calcite; Dol = dolomite; Gyp = gypsum; Si = silica; Mont = Ca-montmorillonite; Syl = sylvite; Alb = albite. Well locations for initial water 1–4 and final water.

.

The models of the water–rock interaction effectively illustrate the development and salinization processes of the groundwater situated in the upstream watershed of the basins within the province throughout its flow path (refer to Figure 7). The findings derived from the geochemical modeling of the water–rock interactions reveal that, as the groundwater slopes, calcite undergoes dissolution while other minerals are precipitated, and cation exchange takes place (as observed from profile A, transitioning from initial waters 1–4 to the final water at the MAD55 site; see Table 4). The groundwater in profile B was formed through the dissolution of sepiolite, exchange with calcite, and the precipitation of other minerals, leading from initial waters 1–3 to the MAD43 site (final water). The dissolution of exchange, dolomite, gypsum, and silica was predominantly indicated by groundwater found in the south (profile C), as well as dolomite and Ca-montmorillonite (profile D), with the precipitation of other minerals (Table 4). This result supports the possibility of recharge from precipitation, as discussed in the stable isotope analysis, in addition to the contribution from water with a lower isotopic concentration, specifically from fractured basement water [34,35].

The four model scenarios (profiles A, B, C, and D) were combined to simulate the recharging and mixing from the upstream of the basins toward the downward region via the groundwater flow path. From the starting waters to the final waters, the estimated recharge and mixing percentages were 44.70%, 12.10%, and 43.20% for profile A; 6.40%, 86.40%, and 7.200% for profile B; 24.60%, 4.90%, and 70.50% for profile C; and 3.30%, 12.10%, and 84.60% for profile D.

In conclusion, the results of this study quantify the relationship between surface water (rainfall) and groundwater in Al-Madinah Al-Munawarah Province. They show that the groundwater is continually recharged in this region, where the government and farmers are focusing on conserving and managing the groundwater for agricultural and household purposes. It is expected that this study will contribute to a decision support system (DSS) with the best groundwater quality and yield, as well as providing suggestions for land use and groundwater extraction in this region. Regarding scientific goals, this study could serve as the basis for other scholars’ investigations of groundwater management aimed at the protection of groundwater and its sustainable utilization in arid areas in the KSA.

5. Conclusions

This research was conducted in Al-Madinah Al-Munawarah Province, which is one of the most significant regions in Saudi Arabia. Recently, this province has attracted attention due to flash floods and the plans to develop groundwater resources. Our study employed hydrogeochemistry and environmentally stable isotopes to determine the processes by which the aquifer system is recharged and the changes in the groundwater chemistry caused by the interaction between the water and the aquifer matrix. This knowledge is essential in promoting sustainable development in the region. The primary supply of groundwater for its many uses is the Quaternary aquifer. In this work, a GIS was combined with contemporary hydrological and hydrochemical methods to assess the recharge rates and groundwater evolution through a variety of tools and techniques, such as stable environmental isotopes (δ¹⁸O and δD) and geochemical modeling with NETPATH. The groundwater in the upstream region of the basins typically ranged from fresh to brackish, while the lower reaches predominantly displayed brackish to saline conditions. The variations in the water quality within the study area were influenced by a combination of geochemical and physical processes, including dissolution, leaching, and evaporation, along with contributions from precipitation and interactions between the groundwater and the rocks forming the aquifer system. The isotopic data (δ¹⁸O–δ²H) suggested that the groundwater was mostly of meteoric origin and younger, formed during the Holocene. The results derived from the NETPATH geochemical model revealed that the primary origin of the dissolved chemical species within the groundwater system in the study area consisted of processes such as mineral weathering and dissolution, ion exchange, and evaporation. Several additional factors that influence the groundwater include human activities and renewal from the underlying fractured basement aquifer. By identifying the sources of groundwater recharge and the factors affecting its quality, decision makers can formulate strategies for the monitoring and management of this resource so as to ensure its protection and sustainable use.