Hydrogeological Characterization and Water Quality Evaluation of Amman-Wadi as Sir Aquifer, Northeastern Jordan

Abstract

Groundwater resources in Jordan are under severe stress due to rapidly increasing water demand and over-abstraction that far exceeds natural replenishment. In addition, water quality is threatened by pollution from the misuse of fertilizers and pesticides, leakage from septic tanks, and illegal waste disposal. This study focuses on the Aqeb, Corridor, and Special Economic Zone wellfields, where hydrological and hydrochemical investigations were carried out. A total of 36 groundwater samples were collected and analyzed for hydrochemical composition, stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H), and trace elements. In addition, two exploration 2D seismic profiles crossing the study area were interpreted, providing critical insights into the activity of the subsurface Fuluk Fault zone and its relationship with the wellfields. The hydrochemical results reveal elevated total dissolved solids and nitrate concentrations, accompanied by more depleted δ¹⁸O and δ²H values in wells located in the central part of the study area. Three distinct hydrochemical groups were identified within the same aquifer, indicating heterogeneity in groundwater chemistry that reflects variations in recharge conditions, flow paths, and geochemical processes. The first group (high Na/Cl with low salinity) likely represents recently recharged waters with limited rock–water interaction. The second group (intermediate Na/Cl and moderate salinity) may be influenced by evaporation, irrigation return flow, or cation exchange. The third group (low Na/Cl with high salinity) suggests the dissolution of sulfate minerals or mixing with deeper mineralized groundwater, possibly facilitated by structural features such as the Fuluk Fault. Seismic interpretation indicates several active near-surface fault systems that are likely to serve as preferential pathways for salinity and nitrate enrichment, linked to intensive agricultural activities and wastewater leakage from nearby septic tanks. The findings emphasize the combined influence of geochemical processes, excessive groundwater abstraction, and structural features in controlling water quality in the region.

1. Introduction

Jordan faces a severe and persistent water scarcity crisis driven by its arid to semi-arid climate and rapid population growth, including recurrent influx of refugees, placing enormous pressure on its already overexploited water systems [1,2,3,4,5,6]. Similar to other countries in the region, the imbalance between water supply and demand represents one of the most critical sustainability challenges [7]. Groundwater serves as the main freshwater source in Jordan, yet over-abstraction has exceeded natural recharge nearly threefold in recent decades [8,9,10]. The resulting decline in groundwater levels across most wellfields has caused wells to dry up, requiring costly deepening, and increasing energy demands for pumping. In many areas, such as northern Jordan, groundwater levels have dropped by 25–75 m between 1995 and 2017, with annual decline rates of 2–5 m [11]. These hydrogeological stresses threaten long-term water security and highlight the urgent need for improved understanding of aquifer behavior under intensive use. Within this context, water hydrochemical and isotopic analyses offer powerful tools to trace groundwater flow, evaluate recharge processes, and identify contamination pathways [12,13,14,15,16]. However, despite numerous local monitoring initiatives, comprehensive regional-scale studies integrating hydrochemistry, isotope geochemistry, and structural controls on groundwater quality remain limited for northern Jordan. Moreover, the mechanisms linking fault activity, salinization, and nutrient enrichment in heavily abstracted aquifers are poorly constrained. Addressing these gaps is essential for developing sustainable groundwater management strategies that could be applicable to other arid and semi-arid regions facing similar challenges.

The present study investigates the hydrochemical and isotopic characteristics of the Amman–Wadi As Sir limestone aquifer (A7/B2), which serves as a critical water source for northern Jordan. The research focuses on the Aqeb, Corridor, and Mafraq Special Economic Zone wellfields that represent major abstraction zones collectively producing about 25 MCM/a (Figure 1). More than forty years ago, when groundwater exploitation in Northeast Jordan started with drilling domestic water wells, groundwater was still plentiful, and wellfields were built rather to economic conditions than on hydrogeological ones. Aqeb Wellfield is stretched in a west–east direction as it follows the old road to Baghdad, and some infrastructure was available. Groundwater flow used to be in a north–south direction but has been reversed recently due to the heavy overexploitation [4,17].

This study integrates hydrochemical, isotopic, and structural approaches to provide new insights into groundwater evolution and fault-controlled vulnerability within an overexploited aquifer system. The findings contribute to broader hydrogeological understanding of water sustainability in dryland environments and align with global efforts to support the United Nations Sustainable Development Goals (SDGs), particularly SDG (6) on clean water and sanitation.

2. Geological–Hydrogeological Framework of the Study Area

The study area is characterized by hot, dry summers and cold winters, with precipitation occurring mainly in short, intense events. Annual rainfall averages 150 to 100 mm/a, with a west to east decreasing trend [24]. Elevations vary from ~700 m above sea level (asl) in the west to ~840 m asl in the eastern parts of the wellfield, while the northeastern extension toward the former volcano Jabel Druze in Syria reaches nearly 1800 m asl.

As shown in Figure 2, the exposed geological units in the study area range in age from the Late Cenomanian to the Miocene [25]. The Fuluk Fault system, a major regional fault system, crosses the study area from northwest to southeast. To the west of the fault, the Umm Rijam and Wadi Shallalah formations (B4/B5) together with the Muwaqqar Chalk Marl aquitard (B3) have been eroded, while borehole data confirm their presence to the east of the fault. Groundwater occurs predominantly within the Amman–Wadi As Sir limestone aquifer (A7/B2). In the western part, the basalt aquifer is locally in direct hydraulic connection with the underlying formations (Amman–Wadi As Sir limestone A7/B2).

The geological and hydrogeological sequence within the study area is characterized by the following formations:

• Amman-Wadi As Sir limestone aquifer (A7/B2): This aquifer consists of the uppermost formation in Ajlun group (Amman formation—A7) and the lower part of Balqa group (Wadi As Sir—B2), separated locally by the Wadi Umm Ghudran marly limestone (B1 aquitard). This aquifer unit consists of limestone, dolomitic limestone and dolomite with intercalated beds of sandy limestone, chalk, marl and Phosphatic chert in the upper part of the B2 formation [26,27,28]. The thickness of the A7/B2 within the sub-crop area ranges between 150 m and 450 m [29]. In general, the saturated thickness of the A7/B2 aquifer ranges from 110 m in the western part of the area to 450 m in the eastern parts [26]. It is a fissured aquifer with low-density fissures but with some karstic enlargement of those and is known to be good water bearing if enough fissures are encountered during drilling.
• Muwaqqar Chalk Marl aquitard (B3): This formation overlies the A7/B2 aquifer and consists mainly of yellowish marly limestone and grey-to-black bituminous marly limestone (oil shale). The formation water contains high concentrations of heavy metals (e.g., Mo, Ni, and As) and can pose a risk to groundwater in the A7/B2 aquifer if mobilized. This formation separates the A7/B2 aquifer from the overlying B4/B5 aquifer [25,26,27,28].
• Umm Rijam and Wadi Shallalah aquifer (B4/B5): The B4/B5 aquifer consists of limestone, chalky limestone, chalk, and chalky marl with beds and nodules of brown to black chert from the Tertiary period [28,29,30]. The often-high salinity of the water in this aquifer, combined with its considerable distance from major population centers, limits its utilization in Jordan.
• Basalt aquifer: West of the Fuluk Fault, the basalt aquifer is in direct hydraulic contact with the A7/B2 aquifer and together they form a combined aquifer. East of the Fuluk Fault, the A7/B2 aquifer is separated from the basalt aquifer by the Muwaqqar Chalk Marl formation (B3—aquitard) and the Umm Rijam (B4/B5—aquifer). The thickness of the basalt aquifer in the study area is between 200 and 400 m [29].

Figure 2. Hydrogeological map of the study area (modified after [11]), showing the positions of the seismic sections and the distribution of the studied wells (groundwater contour lines based on [31]).

Figure 2. Hydrogeological map of the study area (modified after [11]), showing the positions of the seismic sections and the distribution of the studied wells (groundwater contour lines based on [31]).

Structurally, the study area is influenced by two main fault systems: the NW–SE-trending Fuluk Fault zone and a series of N–S-striking faults (Figure 2). While depicting only the surface expressions of these buried structures, this study provides, for the first time, a description of their subsurface configuration. The elevated Jabal Druze volcano north of the study area in Syria acts as a major recharge area, generating radial groundwater flow that provides inflow to the region [4,17]. The rainfall is caught in the Basalt cover and its main flow direction used to be south towards the Azraq oasis (southeast corner in Figure 2) before groundwater exploration started.

In the 1980s and 1990s, the first Aqeb wells for domestic water supply were constructed and impact on the basalt and A7/B2 groundwater system grew as further construction of water wells by the elongation of the water pipeline alongside the Baghdad Road continued. Subsequently, private landowners followed and used the fairly shallow water levels of the Basalt Aquifer for agriculture. Nowadays, more than 2800 deep agricultural wells surround the Aqeb wellfield in the extent of Figure 2. The basalt aquifer was still mainly saturated with water and that was extracted further southeast from the target area (wells at Azraq oasis) together with water from the B4/B5 aquifer. West of the Fuluk Fault, water extraction first started in the basalt strata and subsequently moved deeper into A7/B2. Especially, extensive water extraction west of the target area switched the groundwater flow from a north-to-south to a north-to-west direction [4,11,17,28,31].

Groundwater systems were seen as a bottomless bucket of water available for anyone who could afford to drill a well. Groundwater level observation efforts are constantly available, however, focused on single-point observation and only seldom on spatial observations [4,11,17,28,31]. It is clear that observation wells are expensive if depths of more than 500 m are needed. So, static water levels during well inspection were necessary to support the few water level stations in the north to construct contour maps [11,17,31]. Five official monitoring stations and about 20 static water levels are the base of piezometric water level estimation in the target area and observe the activity of >2800 water wells in Figure 2. The contours indicate the current groundwater level situation, which is highly superimposed by water extraction.

3. Methods and Analysis

3.1. Water Sampling and Analysis

Groundwater samples were collected from public production wells to evaluate water quality, identify geochemical processes, and assess the influence of fault systems on groundwater contamination. The samples were collected between January and February 2019 from three wellfields: Aqeb (27 samples), Corridor (4 samples), and Mafraq Economic Zone (5 samples). All investigated wells tap the Amman–Wadi As Sir limestone aquifer (A7/B2). The wells were strategically drilled along the main road to facilitate connection with the primary water pipeline that supplies the main pumping station in Mafraq Governorate. The basic characteristics of the sampled wells are summarized in

Table 1. Identification and general hydrogeological characteristics of the investigated wells. EC: electrical conductivity, DWL: dynamic water level, and bgl: below ground level.

Well Name *	Well ID	Well Depth (m)	pH	EC (µS/cm)	Temperature (°C)	Production (m3/h)	DWL (m bgl)
Aqeb km 91.5B	AL4000	333	8.05	610	15.6	66.2	272.00
Aqeb km 93	AL1485	305	7.97	690	34.5	90	-
Aqeb km 93.5B	AL5043	450	7.98	610	35.2	73	-
Aqeb km 94.5	AL3004	450	7.78	556	34.8	-	327.84
Aqeb km 95B	AL4085	438	7.98	537	35.3	-	-
Aqeb km 101C	AL5119	430	8.18	2010	32	-	315.08
Aqeb km 102A	AL5154	425	8.18	2160	32.6	-	329.40
Aqeb km 102.5A	AL5058	500	7.88	1820	28.7	68.5	352.68
Al Bustaneh (103D)	AL5161	420	8.18	1076	27.3	30.6	352.40
Aqeb km 104B	AL5156	450	8.15	1140	30.7	75	336.00
Aqeb km 107	AL2689	395	8.13	1135	30	-	324.92
Aqeb km 109B	F 4171	451	7.93	2320	28.7	-	-
Aqeb km 109.5	F 4396	500	8.34	601	31.5	78.2	340.00
Aqeb km 110	F 1333	390	8.2	2090	17.3	30.4	300.00
Aqeb km 111C	F 4334	415	7.84	2000	31.5	-	-
Aqeb km 112D	F 4337	452	7.94	1726	28	128.3	317.29
Aqeb km 113A	F 4312	781	8.24	434	29.1	86	-
Aqeb km 114A	F 1443	634	8.46	395	17.5	80.2	342.00
Aqeb km 116	F 1440	615	8.05	348	23.2	125.9	331.90
Aqeb km 117	F 1361	451	8.36	326	29	8	-
Aqeb km 118	F 4386	451	8.34	301	29.2	6.3	-
Aqeb km 119	F 4393	438	8.55	286	28.2	-	-
Aqeb km 122	F 1442	625	8.38	284	26.5	125.4	344.70
Aqeb km 123	F 1441	705	8.34	352	29.9	130.5	351.50
Aqeb km 124B	F 3946	441	8.41	281	31.1	64.5	-
Aqeb km 133A	F 4357	N/A	8.28	538	33.5	134.5	-
Aqeb km 140	F 3935	421	8.52	447	35.2	-	291.40
Economic 1	AL3908	424	7.97	504	29.3	94	363.00
Economic 3	AL3910	470	8.12	450	20.9	65	389.00
Economic 4	AL3914	500	8.04	1048	23.3	58	314.00
Economic 5	F 4264	500	8.34	1024	30.2	68.8	306.35
Economic 6	AL5046	502	7.87	1106	29.1	-	-
Corridor 1	AL3475	395	7.99	1114	26.8	-	-
Corridor 17	AL3768	386	8.24	600	14.8	56	308.00
Corridor 3A	AL5093	N/A	8.33	397	28	-	232.84
Corridor 7	AL3449	321	8.28	342	31.2	-	214.60

Notes: * Water wells are located parallel to the overland road to Bagdad. Hence, they were originally named after the kilometer post alongside. Letters behind the kilometer mark count the number of wells replaced at the same spot. Well ID is according to the Water Information System of the Ministry of Water and Irrigation.

.

At each site, a full set of bottles was collected to cover different analytical purposes: (i) 100 mL unfiltered samples for stable isotope analysis of oxygen and hydrogen, (ii) 100 mL filtered and acidified samples (1 mL of HNO₃) for cation analysis, and (iii) 500 mL unfiltered samples for the determination of anions and heavy metals. Field parameters including water temperature (°C), pH, and electrical conductivity (EC) (µS/cm) were measured on-site using a WTW Multi 3430 set G device. All samples were stored in cool conditions until laboratory analysis. The distribution of sampling sites is shown in Figure 2.

Laboratory analyses were performed at BGR, Hannover, employing a combination of Ion Chromatography (IC) and Inductively Coupled Plasma–Mass Spectrometry (ICP-MS). Major and minor ions were quantified using a DIONEX ICS-3000 IC (Dionex Corporation, Sunnyvale, CA, USA (Cl, Br, F, NO₃, NO₂, and SO₄), a SPECTRO ARCOS ICP-OES (SPECTRO Analytical Instruments GmbH, Kleve, Germany) (Na, K, Ca, Mg, P, Fe, Mn, Al, and Si), and a UNICAM UV 300 photometer (Thermo Spectronic, Birmingham, UK) (NH₄). Standard calibration procedures were applied daily, following DIN 32645, to ensure the accuracy of quantification. Bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) concentrations were determined using a SCHOTT Titroline Alpha Plus automatic titration system, following a modified DIN 38409-H7-2, with titration curves used for species identification and quantitation. Phosphate contributions to alkalinity were derived from ICP-OES results. Uranium and trace elements were analyzed with an AGILENT 7500 ICP-MS. Analytical precision was assessed through daily quality control standards, while accuracy was verified using proficiency tests and reference solutions in accordance with DIN ISO 11352:2013-03. Detection limits and analytical precision depend on necessary dilution factors and are indicated by significant digits for the parameters in

Table 2. Major ion concentrations (mg/L) and summary statistics for groundwater samples from the investigated wells.

Ion	Number of Samples: 36
	Minimum (mg/L)	Maximum (mg/L)	Average ± Std (mg/L)
${Ca}^{2+}$	6.5	130	40.5 ± 36.2
${Mg}^{2+}$	2.0	132.0	34.3 ± 35.2
${Na}^{+}$	33.4	185.0	70.4 ± 35.4
$K^{+}$	3.0	14.2	7.0 ± 3.5
${Cl}^{-}$	24.5	567.0	174.1 ± 171.5
${SO}_4^{2-}$	15.4	207	70.3 ± 59.6
${NO}_3^{-}$	4.1	48.9	16.4 ± 11.2
${HCO}_3^{-}$	62.5	149	94.7 ± 23.6

and

Table 3. Minimum and maximum concentrations of selected heavy metals and trace elements (µg/L) in groundwater samples from the investigated wells, compared with the Jordanian drinking water standard (JS 286/2015 [32]); “-” indicates that no guideline value is specified.

Element	Min (µg/L)	Max (µg/L)	Max Limit in the Jordanian Standard (µg/L)
Ag	<0.003	0.357	100
As	0.120	1.18	10
B	21.0	70.0	2400
Ba	0.150	172.0	1000
Cd	<0.002	0.117	3
Co	0.005	0.044	-
Cr	3.97	28.6	50
Li	0.30	4.5	-
Mn	0.11	16.9	400
Mo	0.93	10.0	90
Ni	<0.2	4.2	70
Pb	<0.03	0.57	10
Sb	<0.005	2.76	20
Rb	1.96	12.8	-
Sr	53.8	1333.0	-
Ti	<0.04	0.67	-
U	0.16	1.48	-
V	6.75	111.0	-
Zn	0.9	143.0	4000
Zr	0.005	0.02	-

and Appendix A and Appendix B. Charge balance error calculations were performed on all water samples and were usually better than ±3%.

The hydrochemical results were subsequently processed and visualized in AquaChem software (version 2014.2, © Waterloo Hydrogeologic, Canada), including the preparation of Piper diagrams to characterize groundwater composition and trends and saturation indices.

Stable isotope analyses of δ²H and δ¹⁸O were performed simultaneously using a Picarro L2120-i cavity ring-down (CRD) laser spectrometer following samples vaporization. Each sample was measured a minimum of four times, and the reported values represent the arithmetic mean value. The results are expressed in delta notation in per mill (‰) relative to the Vienna Standards Mean Ocean Water (VSMOW). Quality control of the raw data included automated screening for organic contamination using ChemCorrect software (Version 1.2.0 default settings), as well as corrections for memory effects and instrumental drift. Normalizing was carried out against the VSMOW/SLAP scale. The external reproducibility, expressed as standard deviation of a routine quality-check standard across all runs, was better than ±0.20‰ for δ¹⁸O and ±0.8‰ for δ²H. Corrections for memory effects and instrumental drift were conducted by replicate injections and by including ten laboratory standards randomly into each measuring sequence.

3.2. 2D Seismic Sections

Structural features, including the NW–SE-trending Fuluk Fault zone and N–S-striking faults, are examined for their potential role in controlling salinity and nutrient levels through preferential flow and contaminant migration pathways (Figure 1).

Between 1981 and 1996, the Ministry of Energy and Mineral Resources (MEMR) of Jordan, together with international petroleum companies, conducted several surveys. This study utilizes seismic profiles acquired in 1986 by MEMR and Geophysical Service Inc. Two profiles, oriented N-S and E-W, intersect the Fuluk Fault (Figure 2). The data were recently reprocessed and interpreted using IHS Markit KINGDOM (2018), with further details provided in Al Hseinat and AlZidaneen [21].

4. Results

4.1. Water Type

The hydrochemical analysis reveals that the cation order in the sampled groundwater is ${Ca}^{2+}$ > ${Mg}^{2+}$ > (${Na}^{+}$+$K^{+}$), while the dominant anion sequence is ${Cl}^{-}$ > ${HCO}_3^{-}$ > ${SO}_4^{2-}$ (Figure 3). Summary statistics of the analyzed ions are provided in Table 2, and the complete dataset of major ion concentrations for all samples is presented in Appendix A. The hydrochemical facies of groundwater, classified according to Langguth [33], are illustrated in the Piper diagram (Figure 3). Although nitrate levels in the sampled wells remain below the permissible limits defined by the Jordanian drinking water standard (286/2015) (50 mg/L), elevated concentrations were detected in some locations, ranging from 4.1 to 48.9 mg/L. The spatial distribution of nitrate concentrations across the wells is depicted in Figure 4.

Saturation indices (SIs) for calcium carbonate were also calculated. Values below zero indicate undersaturation, suggesting relatively young water with short residence times, enriched in CO₂ and showing limited interaction with the aquifer matrix. In contrast, SI values above zero reflect oversaturation, implying longer residence times, extensive water–rock interaction, and the potential for carbonate scaling. The spatial variability of the calculated SI values is shown in Figure 5.

4.2. Water Salinity

Total dissolved solids (TDSs) in the groundwater range between 180 and 1485 mg/L. According to the Jordanian drinking water standard 286/2015, most samples fall within freshwater category; however, several wells exceed the 1000 mg/L threshold. The spatial distribution of salinity (Figure 6) shows marked heterogeneity, with relatively low concentrations (<400 mg/L) in both the eastern and western zones, and the highest values concentrated in the central part of the study area. In some wells, elevated values of TDS coincide with increased nitrate concentrations. Linear regression analysis revealed a statistically significant (F < 0.05) positive relationship between TDS and NO₃ with R² = 0.91 (adjusted R² = 0.9) suggesting a strong covariance is present. Furthermore, the p-value of (<0.05) for TDS suggests that it is a significant predictor of nitrate in the groundwater samples. These associations likely reflect the influence of wastewater infiltration from septic tanks or the application of fertilizers in nearby agricultural lands in which both TDS and nitrate show a similar behavior.

4.3. Heavy-Metal Concentrations

Heavy metals were analyzed in all collected groundwater samples to evaluate their potential impact on water quality. Table 3 summarizes with the permissible limits set by the Jordanian drinking water standard No. 286/2015. The results indicate that all measured elements remain well below the allowable thresholds, suggesting that the heavy-metal concentration does not pose a concern in the study area. The complete dataset for heavy metals and rare-earth elements is provided in Appendix B.

4.4. Multivariate Statistical Analysis

The groundwater chemical composition was investigated through Principal Component Analysis (PCA) using the open-source PAleontological Statistics Software (PAST, v. 4.17 [34]). PCA is utilized to identify the main parameters governing the covariance in groundwater chemical composition. Since the variables are measured in different units, all variables were normalized. PCA was conducted twice: The first run identified that, based on the Kaiser criterion (eigenvalue > 1), the first two principal components (PC 1 and PC 2) are considered primary and explain 64% of the total variance in the data. PC 1 (42.4%) is characterized by strong positive correlations for major ions and TDS, including Na, Ca, Mg, Cl, SO₄, NO₃, K, and SiO₂ as well as elements like Sr, Cs, and Y. PC 2 (21.6%) is defined by a specific suite of trace elements, including As, U, B, W, Mo, V, HCO₃, and Li. The second PCA analysis was conducted using the variables identified in the first PCA analysis. The Kaiser–Meyer–Olkin (KMO) was 0.74, and the Bartlett’s Sphericity Test (p-value) was <0.05. The PCA biplot (Figure 7) shows that the hydrochemical variables support a structured distinction between different primary processes controlling the hydrogeochemistry of the samples.

Based on the PCA results (

Table 4. PCA summary and explained variance.

	PC 1	PC 2		PC 1	PC 2
K	0.28	0.04	As	−0.20	0.29
Na	0.21	0.17	B	−0.07	0.40
Cl	0.28	0.10	Cs	0.23	0.15
Mg	0.27	0.08	Li	0.002	0.39
Ca	0.27	0.11	Mo	−0.18	0.30
SO4	0.28	0.09	U	−0.12	0.37
HCO3	−0.22	0.25	V	−0.16	0.23
NO3	0.28	0.04	Y	0.21	0.19
SiO2	0.22	−0.22	Eigenvalue	11.4	5.3
Sr	0.26	0.18	Cumulative Eigenvalue	11.4	16.7
TDS	0.28	0.11	Variance (%)	59.8	27.9
			Cumulative Variance (%)	59.8	87.7

), the first two components reflect the primary processes controlling the groundwater hydrogeochemistry variance, together explaining (87.7%) of the variance. PC 1 (59.8%) has strong positive correlations with TDS, Cl, SO₄, Ca, Mg, Na, and K, suggesting a process of general groundwater mineralization driven by the dissolution of evaporites, carbonates, and silicates from the A7/B2 aquifer. Furthermore, the positive loading of Nitrate (NO₃) is an indicator of anthropogenic contamination, possibly from agricultural return flow (fertilizers) or sewage, which is a common issue in heavily utilized aquifers like the A7/B2 in Jordan.

PC 2 (27.9%) is defined by a specific suite of trace elements, suggesting a specific, geologically controlled, source or process. Strong positive correlations are observed for a group of trace elements that include Lithium (Li), Boron (B), Uranium (U), Molybdenum (Mo), Arsenic (As), and HCO₃. This association suggests a signature of deep-circulating fluids that have leached these elements from deeper rocks, often mobilized by elevated pH conditions (indicated by HCO₃) [35] or fault-controlled flow. The positive loadings for As and Mo are related to the geological context where the Muwaqqar Chalk Marl aquitard (B3), which overlies the A7/B2 aquifer, is rich in heavy metals like As and Mo, suggesting its influence on the groundwater in the area. In addition, the negative loading of SiO₂ on PC 2, and its positive loading on PC 1, suggests that the primary source of silica is related to general silicate weathering (PC 1) and inversely related to the deep/tectonic-influenced water (PC 2).

The PCA results coupled with the spatial distribution of the samples signify the effect of the Fuluk Fault in the study area. PC-1-dominant samples are characterized by high mineralization and anthropogenic contamination (NO₃), consistent with a more confined aquifer system with slower flow and higher residence time, while the PC-2-dominant samples are characterized by a tectonic/deep fluid signature (Li and B) and the mobilization of trace elements (As, Mo) from the Muwaqqar Chalk Marl (B3) aquitard. This suggests that the Fuluk Fault acts as a major structural control, facilitating the upward migration of deep-circulating, tectonically influenced fluids and/or the localized release of Muwaqqar Chalk Marl (B3)-derived elements into the hydraulically connected combined aquifer west of the fault (Figure 8).

4.5. Stable Isotopes

Stable isotopes of oxygen (δ¹⁸O) and hydrogen (δ²H) are widely applied as tracers of groundwater recharge conditions, flow dynamics, and evaporative processes [36,37]. The isotopic composition of the groundwater in the study area shows δ¹⁸O values ranging from −6.19‰ to −6.93‰, with an average of −6.49 ± 0.20‰, and δ²H values between −31.0‰ to −33.8‰, averaging −32.3‰ ± 0.8‰. Most of the data points graphically cluster around the EM-MWL line and above the GMWL, suggesting that the groundwater originates from local meteoric water that has not undergone significant isotopic fractionation commonly caused through processes such as prolonged evaporation. This suggests rapid infiltration and recharge process consistent with the short and intense precipitation events in the study area. The tight clustering suggests a common recharge source and a relatively uniform process of infiltration. More depleted δ¹⁸O values were generally associated with less saline samples, suggesting recharge influence and variable flow paths within the aquifer system. The blue circled cluster (lighter isotopes) likely represents the younger, less-evolved groundwater recharged at higher elevations or during colder periods. This is also observed by their lower scores on PC 1 (mineralization). Red circled cluster (heavier isotopes) represent the older, more-evolved groundwater that has undergone deep circulation, water–rock interaction, and is likely associated with the fault-controlled upwelling. These samples generally have positive scores on PC 2 (geogenic control) and some of them show positive PC 1 (mineralization) scores.

The distribution of samples in a δ¹⁸O vs. δ²H diagram is presented in Figure 9, while the relationship between δ¹⁸O and TDS is shown in Figure 10. Linear regression analysis showed that a statistically significant (F < 0.05) positive relationship between TDS, δ¹⁸O, and δ²H values with R² = 0.44, 0.32 (adjusted R² = 0.42, 0.3), respectively, is present suggesting covariance. All groundwater samples exhibit high deuterium excess (DE = δ²H − 8 * δ¹⁸O), indicating secondary processes affecting the isotopic composition. The complete isotope dataset is provided in Appendix A.

4.6. Hydrogeochemical Processes

The diverse processes that control the groundwater characteristics can be further investigated through bivariate plots of major ions.

4.6.1. Silicate Weathering

As identified by the PCA results, silicate weathering is inversely related to the deep circulation-influenced water. Figure 11a shows the correlation between Na⁺ and Cl⁻, indicating that most of the “eastern” samples cluster around the equiline with only three deviated samples. The cluster around the 1:1 line probably suggests that halites are dissolved in groundwater of equal amounts of Na and Cl. The “West” samples on the other hand show a wide spread of point with clusters around the 1:1 line and deviating below and above the line. This suggests variable behavior for these samples in terms of the deposition of halites in groundwater. Samples that fall below the 1:1 line with Na/Cl ratio (>1) imply that silicate weathering is probably the source of sodium in the groundwater [38]. This can be further investigated using the biplot of HCO₃⁻ vs. (Ca⁺² + Mg⁺²) (Figure 11b) in which the samples would typically plot close to the (0.5) line to indicate carbonate or silicate weathering as primary sources for Ca⁺² and Mg⁺² in the groundwater or in the plot region of (<0.5) to indicate ion exchange or enrichment of HCO₃⁻ as primary processes [39]. For the current study, the figure shows that most of the “East” samples are located in the region below the (0.5) line, suggesting dominance of the latter processes, while the “West” samples show varied distribution below and above the (0.5) line, suggesting more complex processes controlling the source of these elements in the groundwater.

4.6.2. Ion Exchange Processes

Previous studies (e.g., [40,41]) report that the plot (HCO₃⁻ +SO₄²⁻) vs. (Ca⁺² + Mg⁺²) can be useful in distinguishing ion exchange processes in groundwater. Figure 11c shows that the “East” samples generally cluster in the lower part around the equiline, while the “West” samples, similar to previous observations, show a wider spread along the equiline. Two “East” samples and six “West” samples plot below the line. This behavior suggests that limestones (calcite and dolomite) and evaporites (gypsum and anhydrite) could be the primary sources for these processes [42]. Furthermore, to identify the particular limestone dissolution, Marghade et al. [43] report that plotting Ca⁺² vs. Mg⁺² (Figure 11d) could provide an indicator of the dominant process. The results show that all the samples plot within the calcite dissolution region, suggesting that this is the main dissolution process in the study area.

These results agree with the geological/hydrogeological characteristics of the study area, the presence of limestone beds, and the nature of the aquifer strata on either side of the fault as well as the presence of the Muwaqqar Chalk Marl (B3 aquitard) in the eastern side.

4.7. Seismic Interpretation

The stratigraphic framework adopted in this study is based on a seismo-stratigraphic interpretation previously developed for the adjacent Dead Sea region to the west of the study area (Figure 1; [21]). Additional calibration was derived from the North Highland-01 (NH-01) exploration well (Figure 1; [44]), which provided direct lithological and stratigraphic control.

To reconstruct the tectono-stratigraphic evolution, two seismic profiles were examined, allowing the identification of both depositional sequences and structural deformation, including faulting, from the Late Cretaceous to the present (Figure 12 and Figure 13).

Within these profiles, seven principal seismic units were recognized and dated: Lower Ajlun (Cenomanian–Early Turonian), Wadi As Sir (WAS, Mid–Late Turonian), Lower Rajil (WUG, Late Coniacian), Upper Rajil (WUG, Santonian), Hamza (ASL, Early Campanian), Hazim (ASL, Mid–Late Campanian), Amman–Al Hisa (AHP, Late Campanian), Muwaqqar (MCM, Maastrichtian–Paleocene), Umm Rijam (UR, Early Eocene), Wadi Shallalah (WSC, Late Eocene), and Basalt (Ba, Miocene). An angular unconformity is identified between the MCM and UR units, marked on the seismic profiles by a dashed red reflector at approximately 0.21 s TWT, truncating the uppermost MCM deposits (Muwaqqar Formation).

In the central part of the study area, the S–N and SW–NE oriented seismic profiles shown in Figure 12 and Figure 13 intersect the NW–SE trending Fuluk Fault. These sections reveal multiple normal faults cutting through the entire succession, from Ordovician strata up to Cenozoic deposits, delineating a half-graben system. Several of these faults appear to exhibit recent tectonic activity, extending to and displacing the present-day surface topography (Figure 12 and Figure 13).

5. Discussion

The investigated wellfields are distributed across an extensive arid region. A distinctive feature of Jordan’s hydrogeological setting is that most aquifers are not directly linked to surface catchments but extend beneath a wide portion of the country. Consequently, the wells within the study area tap into a shared system. This system was traditionally referred to as the “basalt aquifer system”; however, the basalt units are largely unproductive and instead act as recharge zone for the underlying limestone aquifer (A7/B2 aquifer), which constitutes the main water-bearing formation currently exploited.

Groundwater levels in the region are notably deep, with average pumping lifts exceeding 300 m. Dynamic water levels in the examined wells vary between 210 and 390 m bgl. Although the A7/B2 aquifer remains the primary source, its saturated thickness has declined markedly, particularly over the past three decades, and many areas of the aquifer have already been depleted.

Hydrochemical conditions across the wellfields are highly variable regardless similar recharge mechanisms. Electrical conductivity and nitrate concentrations, for example, show substantial differences, especially in wells located in the central part of the study area. The relationship between total dissolved solids (TDSs) and major ions (Figure 14) indicates strong correlations with calcium (R² = 0.96) and magnesium (R² = 0.92) among cations, and with chloride (R² = 0.99) and sulfate (R² = 0.96) among anions. The strong association between TDS and the divalent cations (Ca²⁺ and Mg²⁺) points to significant rock–water interaction processes within the aquifer. Conversely, the high correlation with sulfate in wells of elevated salinity is attributed to the reduction of bicarbonate concentrations, which makes chloride and sulfate the dominant anions in such conditions. These processes were also identified through the PCA analyses where PC 1 denoted rock–water interactions and mineralization, while PC 2 marked the geogenic processes responsible for the enrichment of certain elements.

The sodium-to-chloride ratio (Na/Cl ratio) in the sampled wells ranges between 0.25 and 2.65. When plotted against TDS, three distinct groups can be identified: the Na/Cl ratio in relation with TDS (Figure 15) shows three groups: (i) water with a high Na/Cl ratio (generally >1) and low salinity, (ii) water with a medium Na/Cl ratio (≈0.5) and moderate water salinity, and (iii) water with a low Na/Cl ratio (<0.5) and elevated salinity.

The occurrence of a low Na/Cl ratio at higher salinities suggests that halite dissolution is not the principal source of salinity. This interpretation is supported by the relatively weak correlation between sodium and chloride concentrations (Figure 16; R² = 0.66).

The presence of three hydrochemical groups within the same aquifer indicates heterogeneity in groundwater chemistry, likely reflecting different recharge conditions, flow paths, and geochemical processes (e.g., [45,46,47]). The first group (high Na/Cl with low salinity) may represent recently recharged waters with limited rock–water-interaction. The second group (intermediate Na/Cl and moderate salinity) could be influenced by evaporation, irrigation return flow, or cation exchange [48]. The third group (low Na/Cl with high salinity) points toward the dissolution of sulfate minerals such as gypsum or anhydrite and/or mixing with deeper mineralized groundwater, possibly facilitated by structural features like the Fuluk Fault [49]. These results are also supported by the PCA analysis that showed more than one hydrogeochemical process is controlling variance in the groundwater composition.

Since many agricultural wells in the area are used for irrigation, a Wilcox [50] diagram was plotted (Figure 17) to assess the suitability of groundwater for agricultural purposes. The water samples analyzed were categorized as S1, indicating a low sodium (alkali) hazard. In terms of salinity, however, they fall into the C2 and C3 classes, reflecting medium-to-high salinity hazards.

Beyond salinity and sodium concerns, groundwater quality is further challenged by contamination issues. High nitrate concentrations in several wells are likely associated to local anthropogenic inputs. The absences of centralized sewage system in surrounding villages contributes to wastewater seepage, primarily from defective septic tanks and unauthorized discharges into wadis. Historical records (2013 to 2014) show total coliform counts above critical levels (>2400) in three wells, indicating microbial contamination consistent with sewage leakage, agricultural practices, and livestock activities.

Apart from human-induced sources, the hydrogeological setting is likely to contribute to groundwater contamination. The presence of the Fuluk Fault and another fault systems may enhance both vertical and lateral transport of pollutants by modifying groundwater flow patterns (Figure 12 and Figure 13). While the degree of its present activity remains uncertain, the fault’s structural characteristics can still increase aquifer susceptibility to pollution by creating preferential pathways for the infiltration of wastewater and other surface-derived contaminants. This is also emphasized by the results of PC 2 that shows geogenic control over the groundwater composition in the area and probably reflects the role of the Fuluk Fault zone in the mobilization of solute between the different geological units.

Fault zones often comprise highly fractured and permeable damaged zones, which can act as preferential pathways for rapid contaminant transport, bypassing natural attenuation mechanisms. Research has shown that such zones may channel fluid and pollutant movement more efficiently than the surrounding host rock (e.g., [51,52,53,54]), while studies in fractured carbonates confirm that contaminants migrate more quickly along fractures compared to porous media (e.g., [55,56]). The Cretaceous to recent tectonic evolution of the fault systems related to Sirhan Graben System is attributed to the SAFB, Irbid Rift, and DSTF [19,20,21,22,57].

6. Conclusions

This study presented an integrated hydrogeological, hydrochemical, isotopic, and geophysical assessment of the Aqeb, Corridor, and Special Economic Zone wellfields in northeastern Jordan, which exploit the Amman–Wadi As Sir limestone (A7/B2) aquifer. The results highlight that the isotopic signatures indicate a constant precipitation source. Furthermore, the groundwater composition in the area is governed by the interplay of natural processes and human activities.

Elevated concentrations, and association, of TDS and nitrate, particularly in wells located in the central part of the study area, are possibly attributed to intensive agricultural practices and wastewater leakage from septic tanks. Although generally within the Jordanian drinking water standards, localized exceedances indicate growing stress on groundwater quality. Stable isotope data confirmed the presence of multiple recharge pathways and emphasized the influence of structural features in shaping groundwater flow and chemistry.

Interpretation of seismic profiles revealed active fault zones, most notably the Fuluk Fault, which likely act as preferential pathways for contaminant migration and mixing with deeper mineralized groundwater. The identification of three hydrochemical groups within the aquifer further reflects heterogeneity in recharge conditions, flow paths, and geochemical processes.

Overall, the findings demonstrate that groundwater over-abstraction, structural heterogeneities, and inadequate management of anthropogenic activities have increased the vulnerability of this strategic aquifer system. While heavy-metal concentrations remain within permissible limits, the spatial variability in salinity and nitrate underscores the urgent need for improved groundwater monitoring, stricter control of agricultural inputs, and enhanced protection measures to ensure the long-term sustainability of the resource.

From a management perspective, the results provide a foundation for developing targeted groundwater protection and monitoring strategies. Regular monitoring along fault zones is recommended as these structures may act as conduits for the vertical movement of saline or contaminated water. Implementing buffer zones around high-risk recharge areas and reducing fertilizer use through controlled agricultural practices could help limit diffuse pollution. Additionally, prioritizing the protection of recharge and low-salinity zones would support sustainable resource utilization. The integration of these measures into local and national groundwater management policies would strengthen resilience against contamination and overexploitation.