Hydrogeochemistry and Accelerating Salinization of Groundwater in the Saoura Valley Oases (Southwest, Algeria)

Abstract

The Saoura Valley (southwestern Algeria) hosts14 oases that primarily depend on groundwater in an endorheic basin. The hydrogeological system is bisected by the Saoura Wadi into two distinct compartments: an active, interconnected eastern compartment (Mio–Plio–Quaternary alluvial aquifer and terraces of the Great Western Erg) and a passive, fossil western compartment (Guir Hamada and Cambro–Ordovician aquifers). In September 2024, 51 groundwater samples were collected from nine oases. Temperature ranged from 16.2 to 31.4 °C and pH ranged from 7.1 to 7.85. Total dissolved solids (TDS) varied widely (179–4480 mg/L; median of 454 mg/L), with electrical conductivity between 280 and 7000 µS/cm. Three main hydrochemical facies were identified: Ca–Mg–SO₄–Cl (30%), Na–Cl–SO₄ (55%), and hypersaline types in the terminal inferoflux zone. Nitrate concentrations exceeded the WHO guideline (50 mg/L) in 22% of samples, attributed to localized agricultural and domestic inputs. Geochemical evolution is controlled by evaporite dissolution (gypsum, halite), cation exchange, and evaporative concentration, with a downstream salinity gradient from freshwaters near the Great Western Erg toward hypersaline inferoflux. Comparison with historical data (1941, 1963, and earlier studies) indicates a trend of increasing salinization since the 1990s, associated with intensive borehole pumping and irrigation return flow. These findings suggest risks to the long-term sustainability of the Saoura oases.

1. Introduction

Oases constitute anthropogenic ecosystems in hyper-arid environments, where groundwater availability sustains cultivated areas amidst arid regions [1,2]. In the Algerian Sahara, date palm oases have historically supported human settlement and agriculture due to substantial groundwater reserves, estimated at 31–60 × 10¹² m³ in the northern Sahara aquifers alone [3,4]. These systems play a significant socio-economic role by supporting food security, generating income, and limiting rural exodus in arid desert environments [5,6,7,8]. Traditional water harvesting techniques, such as foggaras and shallow wells, combined with established social rules for water sharing, have sustained these oases for centuries [9,10,11].

Since the second half of the 20th century, however, rapid demographic growth and agricultural intensification have initiated a major hydraulic transition. Traditional low-impact structures have been progressively replaced by thousands of deep boreholes and motorized pumps, significantly increasing abstraction rates [12,13]. This shift has led to a regional lowering of water tables, the drying-up of foggaras and traditional wells, and a quantifiable deterioration of groundwater quality, particularly through increasing salinization and nitrate contamination [14,15,16]. In many Saharan oases, overexploitation induces upward migration of saline deep water (up-coning) and the mobilization of ancient evaporitic salts dissolved along flow paths, transforming formerly fresh groundwater lenses into brackish or saline resources within a few decades [17,18,19].

The Saoura Valley (southwest Algeria) illustrates this hydrogeological crisis. Despite early hydrogeological investigations [20,21,22] and more recent studies [15,23,24], the valley has experienced continuous expansion of irrigated perimeters and drilling since the 1990s, with limited updated monitoring of groundwater chemistry over the last decade. The combination of an endorheic setting, evaporite-rich geology, and intense pumping makes the Saoura oases particularly vulnerable to potentially irreversible salinization.

The present study, based on a field campaign conducted in September 2024 (51 sampling points distributed across nine oases), aims to:

➢

Characterize the current hydrochemical status of groundwater in the Saoura Valley and identify dominant geochemical processes;

➢

Establish spatial hydrochemical zonation and facies evolution along flow paths;

➢

Quantify long-term trends (1941–2024) by comparing present results with historical data;

➢

Assess the respective roles of natural (evaporite dissolution, cation exchange, evaporation) and anthropogenic factors (over-pumping, irrigation return flow, fertilizer use) in the observed degradation;

➢

Evaluate the vulnerability of the Saoura oases to salinization and discuss implications for their future sustainability.

By providing an updated and comprehensive hydrochemical dataset for the valley, this work seeks to contribute to the design of necessary groundwater management strategies in a significant oasis system of the Algerian Sahara.

2. Study Area

The Saoura Wadi forms the Saoura Valley, located in southwestern Algeria and constitutinga narrow strip extending from the Abadla depression in the north to the Touat region in the south. This area is bordered by the Great Western Erg dune system and the Ougarta Mountains (Figure 1 and Figure 2). The study area is located approximately between 00°50′ and 02°30′ west longitude and 29°00′ and 30°50′ north latitude.

The Saoura Valley contains several oases along its 250 km course. There are 14 main oases stretching linearly from northwest to southeast: Igli, Mazzer, Béni Abbes, Tamtert, el Ouata, Agdal, Guerzim, Beni Yakhlef, Kerzaz, Timoudi, Ouled Khoudeir, Timgharine, Ksabi, and Hassi Abdellah. This ephemeral river originates at the Igli oasis, representing the confluence of the Guir and Zousfana valleys, and terminates in the Sebkhael Mellah to the south, near Kerzaz, forming an endorheic basin. It should be noted that at Foum El Kheneg, near the Hassi Abdellah oasis, the channel bifurcates into two branches: Wadi Souireg, which transports flows from the Saoura to the immense Sebkhael Melah depression, and Wadi Messaoud, which only flows toward the Touat following the overflow of the primary channel.

2.1. Geology of the Saoura Valley

The Saoura Valley is geologically linked to the Ougarta Mountains, which form the outer zones of the Variscan (Hercynian) orogeny [27,28]. The Saoura Valley forms a depression occupied by palm groves, situated between the aeolian deposits of the Great Western Erg to the east and the Hamada of Guir to the west. This Hamada is a broad plateau with an average altitude of 600 m comprising Tertiary sediments. The map below (Figure 3) illustrates the geology of the study area.

At the edges of the Saoura Valley, three main units are distinguished:

• A Precambrian basement formed by volcanic formations that emerged in the cores of certain anticlinal structures [30,31]. Its largest exposure is within the Sebkhael Melah structure, containing a thick series of turbidites (>3500 m), intercalated with basaltic and andesine flow sand tuffs and intruded by dolerites and monzonites [27].
• The Paleozoic cover accumulates a thickness of +5000 m. There is evidence of folding into tight anticlines and wide synclines [31,32], with Cambro–Ordovician sandstones and quartzites acting as the primary structural framework [27].
• The Cretaceous and Tertiary terranes comprise the Kemkem Plateau and the Hamada of Guir. These consist of sandy clays at the base and silicified lacustrine limestones.
• Quaternary alluvial deposits.

2.2. Saoura Hydrogeological System

The aquifers of the Saoura comprise a complex hydrogeological system, divided by the Saoura Wadi into two hydrogeological units (Figure 3). The eastern unit compartment encompasses the Mio–Plio–Quaternary groundwater of the Great Western Erg, which constitutes the primary water resource in the upper Saoura Valley. This system consists ofa sandy–alluvial filling with a variable thickness ranging from 30 m to 100 m. This aquifer extends from the Saharan Atlas to the Tadmaït Plateau. Piezometric data indicate flow toward the Saoura Wadi and toward the south; recharge occurs mainly via the northern wadis (Namous Wadi, El Gharbi Wadi, etc.) and through the infiltration of meteoric water through the Great Western Erg sands.

The alluvial aquifer of the Great Western Erg drains naturally toward the Saoura inferoflux. The large spreads of sand and gravel deposits form the stepped alluvial terraces of the Saoura, which store water infiltrated during surface runoff. The Saoura inferoflux serves as the base level of groundwater.

The western unit, located on the right bank of the Saoura, comprises Tertiary lacustrine limestones, extending from Boudenibe (1150 m) to the Ougarta Range (650 m), with a width of 110 km and a length of 200 km, oriented NW-SE. Aquifer recharge is attributed to the humid periods of the Quaternary and is partially maintained by a system of wadis that incise the plateau (Aïcha Wadi, El Abiod Wadi, Alarfedj Wadi). Piezometric gradients suggest a flow consistent with the general inclination of the NW-SE plateau. The Tertiary aquifer communicates locally with the Paleozoic strata.

The Paleozoic formations constitute a multi-layered aquifer system. The Cambro–Ordovician aquifer represents the lower portion of this system, identified near the villages of Zeghamra and Ougarta.

3. Materials and Methods

3.1. Groundwater Sampling and Analytical Procedures

Asampling campaign was carried out in September 2024 across the Saoura region in the Becher area. Initially, 54 water samples were collected (consisting of 8 boreholes and 46 wells and springs) following standard procedures described by [33]. However, upon laboratory analysis and data validation, 3 samples were excluded from the final dataset, as their Charge Balance Error (CBE) exceeded the ±5% threshold. Consequently, the results and interpretations presented in this study are based on the remaining 51 high-quality samples. During fieldwork, a Garmin eTrex GPS (Global Positioning System) was used to record the geographical coordinates of each sampling point, a digital camera was employed to document site conditions, and a cool box was used to preserve the samples during transport to the laboratory of the National Agency for Hydraulic Resources (ANRH) in Bechar. In situ physico-chemical parameters, including temperature, pH, and electrical conductivity were measured at each site using a HACH HQ40d multi-parameter probe(Hach Company, Loveland, CO, USA), which was calibrated daily using standard buffer solutions (pH 4.0, 7.0, and 10.0) and a 1413 µS/cm conductivity standard. Topographical maps (1:200,000 scale; Béni Abbès and Kerzaz sheets) facilitated field orientation and site mapping. Laboratory chemical analyses of the major ions were performed following standard analytical procedures [34]. Calcium and magnesium concentrations were determined using EDTA complexometric titration. Chloride was measured via the Mohr method, sulfate by conductimetric titration, and sodium and potassium were measured using flame photometry. The potentiometric method (Consort 861) for HCO₃ nitrates were quantified by potentiometry (HI 121) using a dedicated ion-selective electrode. To ensure analytical rigor and reproducibility, a comprehensive QA/QC protocol was implemented. All instruments were calibrated using multi-point analytical-grade standards, with correlation coefficients (R²) exceeding 0.995. For every batch of 10 samples, one procedural blank (using deionized water) and one duplicate sample were analyzed to monitor for contamination and assess analytical precision. The relative percent difference (RPD) for duplicate analyses was maintained within ±5%. While certified reference materials (CRMs) were not available, the accuracy was cross-verified using internal laboratory standards of known concentration. These internal standards were prepared at the ANRH laboratory using high-purity analytical-grade salts (Merck™, Darmstadt, Germany and Sigma-Aldrich™, St. Louis, MO, USA). Multi-ion solutions were prepared gravimetrically using a high-precision balance (±0.0001 g) and calibrated glassware. The accuracy of the analytical methods was confirmed by the recovery rates of these standards, which consistently ranged between 97.5% and 102.5% for all major ions. Furthermore, the reliability of the dataset was validated by the Ion Charge Balance Error (IBE), which remained within the acceptable limit of ±5% for all samples. The analytical results, expressed in milligrams per liter (mg/L), are summarized in

Table 1. Chemical and physical properties of water samples.

Sample	Oasis	T °C	pH	Electrical Conductivity (EC) (milliS)	Dry Residue (DR) (mg/L)	Ca2+ (mg/L)	Mg2+ (mg/L)	Na+ (mg/L)	K+ (mg/L)	Cl− (mg/L)	SO4− (mg/L)	NO3− (mg/L)	HCO3− (mg/L)	Total Hardness (TH) (mg/L CaCO3)	Total Alkalinity (TAC) (mg/L CaCO3)	Ion Balance (%)
Obs1	Igli	25.3	7.6	0.8	530	42	42	45	9.1	120	95	48	119	277.8	98	−2.2
Obs2		26.3	7.4	0.7	480	76	16	35.5	7	90	70	28	122	255.7	100	2.9
Obs3		26.1	7.4	0.7	440	76	16	35.8	7	95	75	9	122	255.7	100	3.5
Obs4		24.2	7.4	0.8	500	84	13	45	6	65	160	0	113	263.3	93	2.5
Obs5		25.9	7.7	0.7	450	59	21	45	6.8	110	78	14	125	233.8	102	−1.4
Obs6		23.1	7.6	0.9	560	67	16	80	7.6	140	80	48	107	233.2	88	1.20
Obs7	Beni Abbès	24.3	7.6	1.1	740	63	16	150	5.2	220	80	19	183	223.2	150	−0.3
Obs8		20.5	7.7	1.2	780	76	16	145.5	8.3	185	180	12	195	255.7	160	−2.9
Obs9		25.4	7.4	1.2	800	72	34	124.6	12.2	160	275	9	110	319.8	90	−0.2
Obs10		26.7	7.6	0.8	510	76	26	37.9	6.6	105	83	53	119	296.8	98	1.7
Obs11		24.8	7.4	1.1	700	118	24	60	6.2	120	205	45	146	393.5	120	−0.6
Obs12	Tamtart	23.9	7.5	1	620	72	21	90	5.4	120	200	34	104	266.3	85	−2.2
Obs13		25.3	7.4	0.9	600	84	24	60	5.7	90	210	1.8	146	308.6	120	−2.2
Obs14		26.6	7.6	1.2	780	80	21	120.9	6.5	160	240	35	107	286.2	88	−2.9
Obs15		31.4	7.5	1.7	1050	122	26	171.8	3.3	295	230	15	128	411.7	105	1.1
Obs16		30	7.5	1.6	970	97	58	133.6	2.8	185	380	21	119	481.1	98	0.3
Obs17		26.1	7.7	0.6	391	59	16	33.5	3.5	50	100	26	104	213.2	85	1.7
Obs18	el Ouata	16.2	7.7	1.6	1100	114	24	194.4	7.6	150	385	75	125	383.5	102	2.6
Obs19		19.6	7.5	1.2	800	84	50	90	11.8	148	280	53	113	415.7	93	−0.7
Obs20		25.5	7.7	0.9	560	84	24	43.6	5.9	116	110	35	107	308.6	88	2.1
Obs21		21.3	7.8	1.1	710	80	24	105.8	7.9	100	260	25	156	298.6	128	−1.9
Obs22		24.2	7.6	1.1	670	34	47	107.6	7.9	100	260	21	140	278.4	115	−1.9
Obs23		24.3	7.8	1	620	84	18	85	6.7	108	180	26	146	283.9	120	−0.3
Obs24	Beni Yakhlef	24.4	7.7	0.5	350	46	13	40	2.7	60	70	17	125	168	102	−2.8
Obs25		24	7.6	0.5	340	46	13	32.5	3	50	28	30	156	168	128	−1.8
Obs26		24.8	7.6	0.5	340	59	5	28.9	2.2	60	30	59	76	167.9	62	1.7
Obs27		25.1	7.6	0.6	380	46	13	37	2.2	54	50	35	122	168	100	−0.9
Obs28	Kerzaz	25.9	7.8	0.5	300	42	5	40	2.4	70	50	14	82	125.5	67	−3.1
Obs29		22.9	7.7	0.5	290	42	5	31.4	6.8	50	60	3	79	125.5	65	0.6
Obs30		25.8	7.5	0.4	280	38	5	29.4	2.1	33	50	21	92	115.5	75	−2.4
Obs31		20.9	7.4	0.5	340	51	13	31.4	2.4	58	65	40	70	180.9	57	2.7
Obs32	Timoudi	26.7	7.7	0.5	320	38	16	32	2.3	35	45	23	137	160.8	112	1.3
Obs33		17.4	7.5	0.5	340	46	11	34	2.2	40	40	31	125	160.2	102	2.5
Obs34		23.2	7.8	3.1	2000	257	74	260	10.7	395	925	40	37	946.5	30	−1.8
Obs35		26.2	7.8	0.7	430	67	8	50	3	80	120	15	82	200.2	67	−0.7
Obs36	Ouled Khoudéir	19.3	7.7	1	600	63	21	106	11.2	72	240	1.3	162	243.8	133	0.4
Obs37		23.8	7.7	1.1	730	63	39	115	10.2	86	310	25	156	317.9	128	−0.9
Obs38		24	7.6	0.7	430	63	13	45	6.2	70	130	25	104	210.9	85	−3.5
Obs39		24.5	7.7	0.6	410	59	18	40	8.5	63	120	9	113	221.5	93	0.9
Obs40		20.8	7.5	3.1	1945	168	95	335	47	530	620	85	110	810.7	90	1.5
Obs41		25.8	7.4	3.1	1950	181	111	300	42	530	650	60	92	909	75	2.1
Obs42		22.6	7.5	3.3	2150	173	97	391.2	44.9	620	620	80	101	831.4	83	2.1
Obs43		27.3	7.7	3	1880	122	84	326.9	45.3	500	640	85	95	650.6	78	−3.3
Obs44		26.4	7.7	2.4	1500	84	89	247.1	51.3	380	470	77.5	110	576.3	90	0.1
Obs45		26.6	7.4	2.6	1640	126	105	247.8	51.3	470	470	75	76	747	62	2.9
Obs46		26.4	7.6	1.7	1160	105	45	170	46.1	280	260	70	82	447.5	67	5.2
Obs47		20.4	7.6	1.2	1150	135	34	209.2	12.8	320	300	65	110	477.1	90	2.3
Obs48	Ksabi	26	7.8	1.2	740	84	42	95	8.5	142	280	2	125	382.7	102	0.4
Obs49		21.8	7.6	1	600	80	29	49.1	5.5	85	198	5	125	319.2	102	0.1
Obs50		21.7	7.6	1.2	800	54	39	150.4	10.5	96	330	3	168	295.4	138	1.4
Obs51		26.1	7.7	1.2	790	63	37	130	10.4	80	310	11	214	309.7	175	−1.1

and were analyzed to evaluate the hydrochemical characteristics of the Saoura Valley groundwater and its suitability for irrigation and domestic uses.

Quality, reliability, and ionic balance represent three key criteria in hydrochemical studies for the interpretation and evaluation of surface and groundwater chemistry [35]. Ionic balance testing is commonly used to assess the internal consistency of chemical analyses by comparing the sum of measured cations with the sum of measured anions expressed in milliequivalents per liter meq/L [36]. When ionic equilibrium is achieved, the analytical results are considered internally consistent; however, it is recognized that ionic balance does not, by itself, guarantee absolute analytical accuracy, as compensating errors in cation and anion measurements may occur. Therefore, ionic balance assessment constitutes a fundamental preliminary step in hydrochemical data validation. In this study, Charge Balance Error (CBE) was calculated for all water samples (Table 1). The majority of samples fall within acceptable limits (±5%), supporting the overall suitability of the data for further interpretation. Although the cation–anion correlation (Figure 4) exhibits a non-zero intercept, suggesting a minor systematic analytical bias, the acceptable CBE values indicate that this offset does not significantly preclude the identification of the hydrochemical processes [37]:

$$\mathrm{C}\mathrm{B}\mathrm{E} (\%)={\displaystyle \frac{\mathsf{\Sigma }\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}-\mathsf{\Sigma }\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}}{\mathsf{\Sigma }\mathrm{c}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}+\mathsf{\Sigma }\mathrm{a}\mathrm{n}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{s}}}\times 100$$

The analytical quality of the 51 chemical analyses was verified by the ion balance. All samples fell within the internationally accepted limit of ±5% [34], with errors ranging from −4.82% to +4.91% and a mean absolute error of ±2.35% (Figure 4). This distribution confirms excellent analytical accuracy and high reliability of the hydrochemical dataset (Figure 4).

Hydrochemical facies were classified using DIAGRAMMES software, while Microsoft Excel was used for data organization and calculations. Origin for was employed for graphical plotting, and CorelDRAW for figure preparation. The obtained results are summarized in Table 1. These data facilitated evaluation of the hydrochemical characteristics and suitability of Saoura Valley groundwater for various uses.

A non-exhaustive inventory of water points within the Saoura oases was compiled (Table 1), incorporating previous hydrogeological and hydrochemical studies [38,39,40]. The survey included all existing water points (wells, springs, boreholes, and foggaras) (Table 1). It should be noted that this inventory is not exhaustive.

3.2. Multivariate Statistical Analysis

Multivariate statistical analysis provides a quantitative and independent method for classifying groundwater samples [41]. This approach identifies relationships between hydrochemical variables and groundwater samples, as previously demonstrated [42].

To interpret the factors governing to the chemical composition of the groundwater samples, principal component analysis (PCA) and hierarchical cluster analysis (HCA) were applied using the R programming language V4.5.2 [43]. IBM SPSS Statistics V26 facilitated the PCA.

4. Results and Discussion

4.1. Hydrochemical Characterization and Facies Type of Groundwater in the Saoura Valley

The hydrochemical analysis results are summarized in Table 1. A total of 51 groundwater samples were collected from nine oases (Igli, BéniAbbès, el Ouata, Tamtert, BéniYakhlef, Timoudi, Kerzaz, Ouled Khoudeir, and Ksabi), including 8 deep boreholes tapping the regional aquifer system and 46 wells and springs exploiting shallow alluvial and carbonate aquifers. This sampling strategy was designed to capture both vertical and lateral hydrochemical variability across the hydrogeological units of the study area.

The groundwater samples display a wide range of salinity and ionic composition, reflecting variable degrees of water–rock interaction, groundwater residence time, and evaporative concentration under hyper-arid climatic conditions. Groundwater temperatures range from 16.2 to 31.4 °C (mean 24.1 °C), consistent with deep circulation and shallow groundwater exposure in arid environments. The groundwater samples exhibit slightly alkaline pH values (7.4–7.8, mean 7.6). While this range is common in many sedimentary aquifers, it is consistent with the chemical signature of the study area, where the dominance of Ca²⁺, Mg²⁺, and HCO₃⁻ suggests mineralization influenced by the weathering of limestone and dolomitic formations [44].

Electrical conductivity (EC) varied from 400 to 3300 µS/cm (mean 1198 µS/cm), while total dissolved solids (TDS) ranged between 280 and 2150 mg/L (mean 775.41 mg/L). Most samples (78%) were classified as freshwater (TDS < 1000 mg/L), while 11 samples were in the brackish range, and 1 sample from Timoudi (obs30) was saline, reflecting longer flow paths and evaporite interaction.

Major cation chemistry was dominated by Na⁺ (28.9–391.2 mg/L, mean 124.93 mg/L) in the majority of samples, followed by Ca²⁺ (34–257 mg/L, mean 83.92 mg/L) and Mg²⁺ (5–111 mg/L, mean 29.53 mg/L). Among anions, SO₄²⁻ predominates (28–925 mg/L, mean 241.14 mg/L), followed by Cl⁻ (33–620 mg/L, mean 181.41 mg/L) and HCO₃⁻ (37–214 mg/L, mean 119.80 mg/L). These results highlight the influence of gypsum, anhydrite, and halite dissolution within sulfate- and evaporite-bearing formations of the region.

The Piper trilinear diagram (Figure 5) identifies three dominant hydrochemical facies:

• Na-Cl-SO₄ (55% of samples): This facies is in el Ouata, Tamtart, Ouled Khoudéir, and Timoudi (obs34), characterized by high concentrations of Na⁺, Cl⁻, and SO₄²⁻. These samples plot near the Na⁺ and Cl⁻ + SO₄²⁻ apices, indicating halite and gypsum dissolution accompanied by evaporative enrichment [45].
• Ca-Mg-SO₄ (30%): This type is in Beni Abbès, Ksabi, and Igli, with elevated Ca²⁺, Mg²⁺, and SO₄²⁻. These data point toward the Ca²⁺ + Mg²⁺ and SO₄²⁻ vertices. The elevated sulfate levels (reaching 380 mg/L in Tamtart and 310 mg/L in Ksabi) indicate the dissolution of gypsum/anhydrite dissolution within sulfate-bearing formations [46].
• Ca-Mg-HCO₃ (15%): This facies is in Beni Yakhlef and Kerzaz, characterized by low TDS and a relative dominance of Ca²⁺ and HCO₃⁻. These samples are situated in the Ca²⁺-HCO₃⁻. Situated in the left-central field of the diamond, these waters reflect more recent recharge within carbonate aquifers [47].

Overall, the distribution of hydrochemical facies reflects a general geochemical evolution along the regional groundwater flow path, from upstream oases (Igli and Beni Abbès) toward downstream areas (Timoudi, Ouled Khoudéir, and Ksabi). Groundwater chemistry evolves from Ca–Mg–SO₄ to Na–Cl–SO₄ types due to increasing residence time, evaporite dissolution, evaporative concentration, and cation exchange processes. The occurrence of Ca–HCO₃ waters in Beni Yakhlef and Kerzaz, despite their intermediate position, indicates localized recharge through carbonate formations or wadi infiltration, locally modifying the regional flow system [48].

The Gibbs diagrams (Figure 6) identify the controlling mechanisms:

TDS vs. Na⁺/(Na⁺ + Ca²⁺): Most samples plot within the rock dominance domain, indicating water–rock interaction as the primary salinity source. However, high-salinity samples from Timoudi (obs34) and Ouled Khoudéir (obs40–45) shift toward the evaporation dominance zone, showing that evaporative concentration intensifies salinity in downstream areas [49].

TDS vs. Cl⁻/(Cl⁻ + HCO₃⁻): Samples are distributed along the rock dominance trend, though high-salinity points exhibit higher Cl⁻ ratios (>0.8), consistent with halite dissolution and evaporation–crystallization processes in saline end-members [50].

Ion ratios support these interpretations: a Na⁺/Cl⁻ ratio > 1 in 55% of samples (notably in Kerzaz and Beni Yakhlef) suggests silicate weathering or cation exchange [51]; a Ca²⁺/SO₄²⁻ ratio ≈ 1 in sulfate-rich waters is consistent with gypsum equilibrium [52]; and (Ca²⁺ + Mg²⁺)/(SO₄²⁻ + HCO₃⁻) ≈ 1 indicates a charge balance dominated by carbonate and sulfate mineral dissolution [53].

In summary, the groundwater chemistry reflects a continuum from dilute recharge waters (Ca-HCO₃) in upland oases (Beni Yakhlef, Kerzaz) to saline Na-Cl-SO₄ waters in lowland discharge zones (Ouled Khoudéir, Timoudi), This evolution is controlled primarily by evaporite dissolution, cation exchange, and evaporative concentration, supplemented by minor anthropogenic nitrate inputs in agricultural areas [54].

The observed hydrochemical facies distribution is closely controlled by the hydrogeological compartmentalization of the Saoura Valley. In the eastern active compartment, groundwater circulating within the Mio–Plio–Quaternary sandy–alluvial aquifer of the Great Western Erg exhibits a progressive chemical evolution along the regional flow path toward the Saoura wadi and southern discharge zones. Recharge occurring through wadi infiltration and diffuse percolation of meteoric waters produces low-mineralized Ca–Mg-HCO₃ facies in upstream oases, such as Beni Yakhlef and Kerzaz. As confirmed by their lower TDS and Na/Cl molar ratios near 1.0, these waters reflect relatively reflecting limited water–rock interaction within carbonate-bearing formations.

Down-gradient, increasing residence time, interactions with evaporitic sediments, and ion-exchange processes within sandy and alluvial deposits lead to the development of Ca–Mg–SO₄ facies, particularly in intermediate oases such as Igli, Beni Abbès, and Ksabi. In the terminal lowland areas (Ouled Khoudeir, el Ouata, Tamtert, and Timoudi), where groundwater converges toward the lower flux of the Saoura and evaporation is enhanced under hyper-arid climatic conditions, Na–Cl–SO₄ facies dominate. These saline waters reflect halite and gypsum dissolution combined with cation exchange (Ca²⁺ replaced by Na⁺) and evaporative concentration, as confirmed by Piper and Gibbs diagrams.

In contrast, the western passive compartment, developed within Tertiary lacustrine limestones, shows signatures influenced by ancient recharge and prolonged residence times. The predominance of sulfate and carbonate-rich facies here is consistent with structural control of flow and mineralized host rocks. Overall, the hydrochemical patterns across the Saoura system clearly reflect the combined effects of aquifer lithology, flow dynamics, and climatic constraints.

4.2. Correlation Matrix and Hierarchical Cluster Analysis

The Pearson correlation matrix (Figure 7) identifies the geochemical relationships among the major ions in the Saoura groundwater [55,56]. The results indicate trends associated with the dominant hydrogeochemical processes in the aquifer system.

Strong correlations (r > 0.90) were observed between Na⁺–Cl⁻ (0.95), Na⁺–SO₄²⁻ (0.90), and Cl⁻–SO₄²⁻ (0.83), which, coupled with the TAC–HCO₃⁻ (1.0) and TH–conductivity (0.97) correlations, highlight the dominant role of evaporite dissolution (halite and gypsum) and carbonate equilibria in controlling mineralization [52,57]. The strong link between Ca²⁺ and SO₄²⁻ (0.86) and Mg²⁺ (0.87) further supports the concurrent dissolution of sulfate-bearing minerals [58].

Significant to strong correlations (r = 0.70–0.90) were observed between Ca²⁺–Mg²⁺ (0.87) and K⁺–Cl⁻ (0.85), suggesting salinity enrichment processes and secondary cation exchange reactions occurring within clay-rich layers [59]. In contrast, the significant correlation between K⁺ and NO₃⁻ (r = 0.7) likely reflects common anthropogenic sources, particularly agricultural activities involving the application of potassium and nitrate-based fertilizers, as well as domestic wastewater inputs [60].

Moderate to weak correlations (r = 0.35–0.50), including NO₃⁻–SO₄²⁻ (0.50) and Ca²⁺–residue (0.81), indicate more localized or less systematic geochemical behavior, potentially linked to variations in fertilizer application or soil–water interactions [61]. In contrast, parameters with insignificant correlations (r < 0.35), such as those between HCO₃⁻ and Cl⁻ (−0.22) or SO₄²⁻ (−0.22), exhibit limited interdependence, implying distinct sources or independent geochemical controls [62].

Principal component analysis (PCA) was applied to a standardized dataset of 14 physicochemical parameters from 51 groundwater samples to reduce dimensionality while preserving maximal variance (Figure 8;

Table 2. Principal component loadings and the proportion of variance explained by the five rotated components using Kaiser-normalized varimax rotation.

	PC1	PC2	PC3	PC4	PC5
T	0.010	−0.094	−0.044	0.978	−0.054
pH	−0.053	0.044	0.986	−0.045	−0.107
EC	0.967	−0.084	−0.019	0.034	0.209
DR	0.970	−0.085	−0.025	−0.011	0.216
Ca	0.890	−0.279	−0.108	−0.116	−0.097
Mg	0.885	−0.052	−0.077	0.128	0.307
Na	0.933	0.031	0.025	−0.047	0.271
K	0.711	−0.069	−0.019	0.125	0.610
Cl	0.903	−0.130	−0.078	0.081	0.312
SO4	0.961	−0.101	0.057	−0.075	0.026
NO3	0.378	−0.301	−0.200	−0.179	0.736
HCO3	−0.127	0.977	0.027	−0.052	−0.106
TH	0.961	−0.175	−0.100	0.012	0.122
TAC	−0.126	0.977	0.031	−0.064	−0.102

) [63]. Following Kaiser-normalized Varimax rotation, the first five components explained a significant portion of the total variance, with the first two principal components accounting for 65.4% (PC1: 47.9% and PC2: 17.5%).

PC1 (54.8%) represents a mineralization and salinity axis, dominated by exceptionally high positive loadings for dry residue (DR: 0.970), Electrical Conductivity (EC: 0.967), Sulfate (SO₄: 0.961), Total Hardness (TH: 0.961) and Sodium (Na: 0.933). Strong loadings for Chloride (Cl: 0.903), Calcium (Ca: 0.890), and Magnesium (Mg: 0.885) further confirm that PC1 reflects primary geogenic ion enrichment driven by water–rock interactions [64]. In this rotated space, pH (−0.053) shows a weak negative loading (−0.165), confirming its relative independence from the main mineralization process [65].

PC2 (15.5%) is clearly defined by the carbonate system, exhibiting strong positive loadings for both bicarbonate (HCO₃: 0.977) and total alkalinity (TAC: 0.977). The low loadings for salinity markers on this axis suggest that PC2 effectively isolates the natural buffering processes and carbonate weathering from the regional salinization trend.

The higher-order components capture specific secondary processes: PC3 (0.986) is almost exclusively defined by pH, indicating independent acidity/alkalinity controls. PC4 is dominated by Temperature ((T): 0.978), indicating independent thermal influences on the groundwater system. PC5 shows strong associations for Nitrate(0.736) and Potassium (0.610), highlighting agricultural runoff and fertilizer application as a distinct chemical signature separate from the natural mineral baseline [66]. The PCA biplot identifies clear hydrochemical groupings: (1) highly saline samples like obs34 (high positive PC1), (2) bicarbonate-rich waters (high positive PC2), and (3) a cluster of relatively fresh waters near the origin. These results confirm that the groundwater chemistry is primarily controlled by evaporite dissolution, with secondary impacts from alkalinity buffering and agricultural inputs [67].

Hierarchical cluster analysis (HCA) was performed using Ward’s linkage method with Pearson correlation to identify patterns in the hydrochemical dataset comprising 5151 groundwater samples from the study area. Two dendrograms were generated: one for chemical variables and one for samples.

The variable dendrogram (Figure 9) identified two primary clusters at a high dissimilarity level of approximately 1.0. The first major cluster is subdivided into two distinct groups at a dissimilarity level of roughly 0.15:

➢

Group A comprises Cl⁻, Na⁺, EC, and K⁺, with very low dissimilarity (approx. 0.05 to 0.2). The extremely tight association between Cl⁻ and Na⁺ confirms halite dissolution as a dominant process, while their link to EC reflects their primary role in the total mineralization of the groundwater [68]. The inclusion of K⁺ in this group suggests a common association with saline enrichment, potentially from agricultural runoff or secondary mineral weathering [66].

➢

Group B comprisesMg²⁺, SO₄²⁻, and Ca²⁺. The close linkage between Mg²⁺ and SO₄²⁻ (dissimilarity < 0.15) suggests a shared origin from the dissolution of gypsum and magnesium-bearing minerals within the aquifer matrix [68].

The close association of these parameters across both groups (dissimilarity < 0.4) confirms the progressive influence of evaporite dissolution as the dominant control on the ionic strength of the evolved waters. In contrast, the second major cluster consists solely of HCO₃⁻, which remains isolated at the highest dissimilarity level (1.0). This isolation indicates that bicarbonate-driven processes, such as carbonate mineral dissolution (calcite/dolomite), represent a distinct hydrochemical signature primarily dominant in low-salinity recharge samples, completely independent of the regional salinization path [69]. Overall, the HCA confirms that while natural mineralization (evaporites and carbonates) governs the base chemistry, the grouping of K⁺ and Cl⁻ with the main salinity cluster highlights how agricultural inputs are integrated into the final hydrochemical signature of the terminal oases.

The sample dendrogram (Figure 10) categorized the 51 observations into two primary clusters at a significant linkage distance of approximately 27. These groupings correspond to the spatial and geochemical evolution of the groundwater: Cluster 1 (indicated in orange) comprises groundwater samples from the terminal discharge zones, specifically Timoudi (obs34) and the Ouled Khoudéir group (obs40–46). Within this branch, obs34 is identified as a distinct outlier, branching off at a linkage distance of roughly 8, which highlights its unique status as the most saline end-member in the study area. Cluster 2 (indicated in green) includes the low-to-moderate salinity samples from Igli, Beni Yakhlef, Kerzaz, Beni Abbès, and Tamtart. These samples represent the Ca–HCO₃ and Ca–Mg–SO₄ facies. The sub-branching within this group (at distances < 10) reflects localized variations in carbonate buffering and minor mineral dissolution along the upstream and intermediate sections of the flow path. The HCA results are consistent with the Piper diagram facies and Gibbs plots, indicating a hydrochemical gradient from carbonate-buffered recharge waters (rock dominance) to evaporite and evaporation-influenced discharge waters. The clustering quantifies the transition from freshwater, carbonate-dominated systems to saline, waters characterized by chloride- and sulfate enrichment at the terminal ends of groundwater flow paths.

4.3. Geochemical Ratios and Water Origin

The analysis of characteristic ionic ratios identifies the geochemical processes controlling groundwater chemistry in the Saoura Valley [70,71]. The ionic composition reflects a complex interplay of lithological interactions and intensive evaporation:

• Mineral Weathering and Evaporitic Influence: The Na⁺/Cl⁻ relationship (Figure 11) indicates that the majority of samples plot along the 1:1 line, reflecting the influence of the region’s evaporitic lithology [72] rather than a single mineral source. This is complemented by gypsum/anhydrite dissolution, as evidenced by the distribution of Ca²⁺ and SO₄²⁻ near the 1:1 line. However, several samples from Ouled Khoudéir and Timoudi deviate toward an excess of calcium, suggesting additional mineral sources or cation exchange processes [73].
• Carbonate Weathering and Deficits: Standard carbonate weathering typically follows a 1:2 molar trajectory between calcium and bicarbonate. However, the samples exhibit a severe HCO₃⁻ deficit, deviating significantly from both the 1:1 meq/L equiline and the 1:2 trajectory in the Ca²⁺ vs. HCO₃⁻ and Mg²⁺ vs. HCO₃⁻ plots [74]. This confirms that limestone and dolomite dissolution are secondary to evaporitic inputs. While the Ca²⁺ vs. Mg²⁺ relationship shows some samples near the 1:1 line, supporting some dolomitic limestone dissolution [75], the broad scatter indicates mixed calcium sources.
• Mixed Geochemical Controls: The SO₄²⁻ vs. Cl⁻ relationship shows a clear linear trend, indicating a common evaporitic origin for these ions, with higher concentrations observed in the terminal discharge zones [76]. The significant excess of Cl⁻ relative to SO₄²⁻ in high-salinity end-members like obs34 and obs40–45 highlights the impact of evaporative concentration and halite dissolution as the primary drivers of terminal salinity [77]. Finally, the SO₄²⁻/Cl⁻ ratio indicates a predominance of sulfate, consistent with the contribution of gypsum dissolution alongside halite in the terminal flow paths [78].

Overall, these ionic relationships demonstrate that groundwater salinity originates from the collective contribution of evaporitic salts and carbonate weathering, significantly intensified by hyper-arid climatic conditions in the lowland oases.

4.4. Ion Exchange Processes and Hydrochemical Evolution

Beyond mineral dissolution, cation exchange serves as a critical secondary mechanism modifying the groundwater chemistry along the Saoura Valley flow path. The bivariate plot of (Na⁺ + K⁺ − Cl⁻) versus (Ca²⁺ + Mg²⁺− (SO₄²⁻ + HCO₃⁻) was utilized to evaluate cation exchange reactions (Figure 12) [79].

Statistical analysis of the primary sample population, the data exhibits a strong linear correlation (y = −1.089x + 0.63; R² = 0.736). This near-unit slope (−1.089) indicates that cation exchange is a dominant process controlling the relationship between alkaline earths and alkali metals in the system. The distribution reveals a dual-mode exchange system:

• Direct Cation Exchange (Softening): A significant portion of the samples, particularly those from the more evolved downstream zones like Ouled Khoudéir (e.g., obs40, obs41, obs42, obs45, obs46), plot in the upper-left quadrant. This indicates a direct exchange process where dissolved Ca²⁺ and Mg²⁺ are adsorbed onto clay surfaces, releasing Na⁺ into the groundwater [80]. This process is a key driver for the transition toward Na–Cl–SO₄ facies observed in the terminal oases [81].
• Reverse Cation Exchange (Hardening): Conversely, a subset of samples, including those from Ksabi (obs50, obs51) and el Ouata (obs18), plots in the lower-right quadrant. This indicates reverse cation exchange, where Na⁺ in solution is adsorbed onto clay surfaces, causing the release of Ca²⁺ and Mg²⁺ in the groundwater [82].
• Process Dominance and Equilibrium: In zones where samples cluster near the origin, such as certain points from Kerzaz and Beni Yakhlef (e.g., obs24, obs27, obs32, obs33), the chemical signature suggests a state of relative equilibrium where cation exchange is less pronounced compared to the primary influence of mineral dissolution [83]. However, as samples move away from the origin along the regression line, the strong correlation demonstrates that cation exchange becomes the dominant modifier of the major ion chemistry [84].
• Localized Anomalies: While most samples follow the predictable exchange trend, the wide vertical dispersion of samples like obs45 and obs41 suggests that in specific high-salinity locations, the exchange processes are intensified by high ionic strength or localized variations in the clay content of the alluvial matrix.

Ultimately, the distribution of the majority of samples along the unit slope (−1.089) confirms that cation exchange effectively regulates the cationic balance, facilitating the progressive sodium enrichment observed parallel to the hydraulic gradient in this clay-rich, evaporative system [85].

4.5. Temporal Evolution and Drivers of Groundwater Quality

To evaluate the long-term hydrochemical evolution of Saoura groundwater, a comparative analysis was performed using historical data from 1941 and 1963, extracted from the works of Merzougui (2021) [74], alongside our 2024 results. Due to the availability of continuous historical records, the Beni Abbès oasis located centrally within the study area was selected as a representative sentinel site (Figure 1). The comparison focuses on key parameters (RS, Cl⁻, Na⁺, Ca²⁺, Mg²⁺, K⁺, SO₄²⁻). The results demonstrate that the groundwater is in a non-stationary state, with a clear evolution of hydrochemical facies over time. This site-specific evolution serves as a proxy for the broader spatial zoning and geochemical shifts observed across the other eight oases in the 2024 sampling campaign since 1963 (Figure 1). Stacked bar charts of major ion composition across six hydrogeological units indicate a significant temporal increase in total mineralization (RS) and salinity-related ions, with notable acceleration in the rate of change observed between 1990 and 2024 (Figure 13).

In the Infero-Flux aquifer, RS increased from ~2500 mg/L in 1963 to over 9000 mg/L in 2024, driven primarily by elevated Na⁺ and Cl⁻ concentrations exceeding 3000 and 4000 mg/L, respectively, alongside sustained high SO₄²⁻ (~2000 mg/L). The Alluvial aquifer (Terrace 3) exhibited a near-doubling of RS from ~4000 mg/L in 1941 to ~8000 mg/L in 2024, with Na⁺ and Cl⁻ dominating the ionic load and Ca²⁺/Mg²⁺ contributions remaining subordinate.

The Alluvial aquifer (Terrace 2) exhibited a similar trend, with RS increasing from ~700 mg/L in 1963 to ~1300 mg/L in 2024, primarily characterized by Na⁺–Cl⁻ enrichment. In contrast, the Alluvial aquifer (Terrace 1) showed moderate decreases in mineralization during intermediate years (1990) before a significant salinity increase in 2024 (RS ~800 mg/L), indicating episodic recharge influences [75]. The Alluvial aquifer of the Great Western Erg maintained consistently low mineralization (RS < 300 mg/L) between 1990 and 2024. This unit exhibited balanced Ca²⁺–HCO₃⁻ (inferred from low Cl⁻/SO₄²⁻ ratios), identifying it as a persistent freshwater lens [76]. Finally, The Hamada of Guir aquifer showed persistently high salinity, increasing from about 1200 mg/L in 1941 to nearly 1300 mg/L in 2024. The dominance of Na⁺, Cl⁻, and SO₄²⁻ indicates a strong long-standing influence of water–rock interaction. Although the absence of isotopic tracers and trace halides (e.g., Br⁻) limits the precise identification of salinization processes through ionic ratios, the hydrochemical composition, geological setting, and arid climate suggest that evaporite dissolution is the primary source of salinity. Evaporation further contributes to the concentration of dissolved salts. Nitrate concentrations provide additional context: generally low NO₃⁻ values indicate a mainly natural origin of salinity, while locally higher values reflect minor anthropogenic inputs. Future studies using isotopes and halide tracers are recommended to better distinguish salinization mechanisms [77].

Historically, while the subsoil of the Saoura wadi (Figure 14) was characterized by high freshwater saturation, the system has experienced a transition from natural, slow-paced mineralization to rapid, human-induced salinization. The freshwater influx from the Great Western Erg aquifer has established a vital hydrochemical boundary (Figure 13). Ultimately, qualitative deterioration is governed by a hierarchy of factors: natural processes (evaporative concentration and geological inputs from gypsiferous formations) provide the baseline, while anthropogenic activities (overexploitation-induced drawdown post-1990, agricultural return flows, and irrigation with saline water) act as the primary accelerants. This has resulted in a hydrochemical evolution from Ca–HCO₃ freshwater facies to Na–Cl (SO₄) dominant types, reflecting the overprinting of primary recharge signatures by accelerated evaporite dissolution and evaporative enrichment.

Overall, the hydrochemical evidence indicates that groundwater mineralization is dominantly controlled by natural geochemical processes. Evaporite dissolution (halite, gypsum, and anhydrite), together with evaporative concentration under hyper-arid conditions, represents the fundamental mechanism governing salinity evolution, as supported by Piper facies distribution, Gibbs diagrams, ionic ratios, PCA, and HCA results. Cation exchange constitutes a secondary but significant process, responsible for progressive sodium enrichment along flow paths. In contrast, direct anthropogenic chemical influences, mainly reflected by localized nitrate enrichment, play a subordinate role in the overall salinity budget and remain spatially limited to shallow aquifers. However, the physical impact of modern overexploitation has shifted the balance, accelerated natural mineral mobilization and causing the dramatic salinity spikes observed in the recent 1990–2024 window. While these relative contributions are inferred qualitatively, their precise quantitative apportionment would require isotopic tracers and reactive transport modeling, which were beyond the scope of the present study.

4.6. Vulnerability and Pollution Index

The analysis of nitrate concentrations (NO₃⁻) across the nine oases demonstrates significant spatial heterogeneity (Figure 15). The boxplot ordered in geographic order (north → south) indicates that nitrate levels vary extensively both between and within oases. Northern oases, including Igli and Beni Abbès, exhibit moderate concentrations, whereas central and southern oases, particularly el Ouata, Beni Yakhlef, and Ouled Khoudir, display higher median values and frequent outliers. Several samples in these areas surpass the WHO drinking water guideline of 50 mg/L, signaling nitrate contamination.

The individual bar plot illustrates this variability, indicating that exceedances occur intermittently across the region rather than in localized clusters. This pattern suggests mixed and localized nitrate sources, including intensive agricultural fertilizer application, infiltration of untreated domestic wastewater, and soil leaching processes. Extremely high values in specific wells are consistent with point source contamination scenarios.

However, in the absence of isotopic tracers (δ¹⁵N–NO₃⁻ and δ¹⁸O–NO₃⁻) and detailed land-use mapping at the well scale, it is not possible to quantitatively discriminate between agricultural and domestic nitrate sources. Future investigations integrating nitrate isotopic signatures, high-resolution land-use data, and well-specific vulnerability parameters would allow for more robust source apportionment and would significantly strengthen the assessment of anthropogenic impacts on groundwater quality.

In summary, the combined visualization indicates the need for groundwater protection measures in the region, especially in oases with recurrent threshold exceedances. The observed nitrate distribution demonstrates the vulnerability of shallow aquifers to anthropogenic impacts, suggesting a requirement for improved management of agricultural application and sanitation systems.

5. Conclusions

The Saoura Valley (southwestern Algeria) comprises a 250-km long sequence of 14 oases representing a significant agro ecosystems of the Sahara. These oases are supplied by a complex hydrogeological system categorized into two distinct compartments: an active eastern compartment recharged by the Mio–Plio–Quaternary aquifer of the Great Western Erg and its alluvial terraces, and a western compartment formed by the Guir Hamada and Cambro–Ordovician aquifers.

The 2024 survey of 51 groundwater samples exhibited a wide range of mineralization (TDS 260–2145 mg/L; EC 400–3300 µS/cm) and identified hydrochemical zonation along the flow path: fresh to moderately mineralized Ca–Mg–HCO₃ and Ca–Mg–SO₄ waters in upstream oases (Beni Yakhlef, Kerzaz) into Na–Cl–SO₄ and saline Na-dominated waters in the terminal inferoflux and southern terraces (Ouled Khoudéir, Timoudi). Groundwater chemistry is primarily governed by natural geogenic processes, specifically the dissolution of halite and sulfate-bearing minerals (gypsum/anhydrite), as evidenced by strong PC1 loadings (0.90–0.97) and ionic ratios plotting near the 1:1 equiline. Cation exchange serves as a critical secondary modifier, with a near-unit regression slope of −1.089 (R² = 0.736) confirming a dual-mode exchange system that facilitates progressive sodium enrichment. These natural drivers are increasingly overprinted by anthropogenic impacts; PCA (PC5) and HCA (Group A) both isolate a distinct K–NO₃–Cl association. Nitrate concentrations exceed the WHO limit of 50 mg/L in several samples (e.g., obs40, obs44), identifying agricultural return flows as a significant subordinate driver. Comparison with historical data (1941–2024) reveals that while salinization is a long-standing natural feature of the valley’s arid cycle, the rate of salinization has increased significantly since the 1990s. This modern acceleration spatially aligns with the extensive construction of deep boreholes and the decline of traditional foggaras. The freshwater lens supplied by the Great Western Erg, which formerly formed a continuous hydrochemical boundary along the valley, is currently contracting. It is being replaced by brackish and saline water through a combination of natural diffusive salt transport and human-induced up-coning of deeper saline groundwater triggered by localized over-pumping. These trends indicate a potential decline in the long-term sustainability of the Saoura oases. In the absence of integrated management strategies, including regulated groundwater abstraction, artificial recharge assessments, enhanced irrigation efficiency, and rehabilitation of traditional water-harvesting systems, large parts of the cultivated area may become unsuitable for datepalm cultivation and associated crops within the next two decades. The Saoura aquifer system illustrates the sensitivity of Saharan oasis systems; the observed contraction of the freshwater lens and the downstream migration of saline fronts provide clear evidence of the impacts of climate variability and unregulated modernization of water use, demonstrating the requirement for adaptive, data-driven governance of groundwater resources in arid regions.

From a management perspective, priority actions in the Saoura Valley include regulating groundwater abstraction, improving irrigation efficiency, and protecting recharge zones connected to the Great Western Erg. Artificial recharge using seasonal floodwaters may help partially restore the freshwater lens, while the rehabilitation of traditional foggara systems could reduce pressure on deep aquifers. Establishing a long-term groundwater monitoring network would further support adaptive and sustainable management of the oasis system.

Further investigations are required to better constrain the origin of groundwater salinization in the Saoura Valley. The use of advanced chemical and isotopic tracers, such as stable water isotopes (δ¹⁸O, δ²H), halide ratios (Cl/Br), and sulfate and nitrate isotopes (δ³⁴S–SO₄, δ¹⁵N–NO₃), would allow clearer discrimination between evaporation, evaporite dissolution, and anthropogenic inputs. Integrating these tools with long-term monitoring would provide more conclusive insight into salinity sources and their evolution.