Uranium Isotopic Fractionation and Hydrogeochemical Controls in Groundwater of the Jabal Sayid–Mahd Adhab Region, Western Saudi Arabia

Abstract

Uranium isotopic composition of shallow groundwater in the Jabal Sayid-Mahd Adhab area of western Saudi Arabia was investigated to evaluate geochemical changes resulting from water-rock interactions. The wide range of uranium concentrations (0.75–29.3 ppb) and ²³⁴U/²³⁸U activity ratios (1.11–3.11) reflect variable redox and uranium dissolution conditions across the aquifer. Samples with high uranium concentrations but low activity ratios suggest a recent release of uranium from mineral phases, which is further enhanced by the presence of fluoride ions. Fluoride’s strong reactivity aids in uranium dissolution by forming stable uranyl-fluoride complexes under open-system leaching conditions. Conversely, higher isotopic ratios in low-uranium samples suggest longer water-rock interaction and preferential leaching of ²³⁴U by alpha-recoil processes. The positive correlation between uranium and salinity parameters further indicates that uranium enrichment is linked to increased ionic strength and the abundance of complex ligands. The relationship between activity ratio ²³⁴U/²³⁸U (AR) and 1/U in the studied samples indicates that uranium behavior in the shallow aquifer is dominated by open-system leaching, with local binary mixing superimposed in a few sites. The findings emphasize that uranium isotopic composition is a valuable tool for identifying localized groundwater mixing and assessing the hydrogeochemical impacts of nearby mineralized areas on the aquifer system. These results represent an essential baseline for future environmental monitoring and for evaluating potential temporal changes in uranium behavior.

1. Introduction

The Jabal Sayid-Mahd Adhab region is significant for mining rare metals and radioactive elements [1,2,3,4,5,6,7,8,9,10,11,12], reflecting the geological diversity of the Arabian Shield, which comprises igneous and metamorphic rocks dating back to the Precambrian era. These rocks have experienced extensive geological processes, including uplift, folding, and volcanic activity, which have modified their chemical and mineralogical characteristics. Felsic igneous rocks, such as granite and rhyolite, represent major sources of uranium. The oxidation of U(IV) to U(VI) enhances uranium solubility and mobility, increasing its potential for leaching [13,14,15,16,17,18]. This mobility leads to the fractionation of uranium isotopes, particularly resulting in ²³⁴U enrichment in aquatic environments [19,20,21,22,23,24]. Although closed systems older than one million years may reach uranium-series equilibrium, disequilibrium commonly occurs because daughter isotopes are more mobile, particularly under varying water–rock interaction conditions. Variations in the ²³⁴U/²³⁸U activity ratios (AR) in groundwater therefore provide significant insights into uranium accumulation, water mixing, and geochemical processes; with global average ratios typically around 1.15 [25,26,27].

This study is the first to investigate the uranium isotopic composition of groundwater in the selected regions, aiming to evaluate uranium concentrations and the ²³⁴U/²³⁸U activity ratio across multiple wells, to provide insights into redox conditions and uranium sources. The results are expected to address several key questions and hypotheses guiding the research: (1) Are there significant variations in the ²³⁴U/²³⁸U activity ratio among different wells in the Jabal Sayid-Mahd Adhab region, and what factors influence these variations in terms of uranium sources and redox conditions? (2) What are the main chemical controls affecting uranium activity in the shallow groundwater of the Jabal Sayid-Mahd Adhab region, and how do water-rock interactions, evaporative processes, and anthropogenic inputs shape the groundwater’s physicochemical characteristics? (3) Does fluoride have a greater influence on uranium mobility than carbonate in arid conditions, and how do soluble uranyl-fluoride species impact uranium mobility in shallow groundwater? (4) Are there indications of natural radioactive contamination in the groundwater samples, and what are the implications for local water management, especially regarding the safety and suitability of groundwater for drinking and irrigation?

2. Geology of the Study Area

The Jabal Sayid-Mahd Adhab area is situated in the western region of Saudi Arabia (Figure 1a) and is part of the larger geological framework of the AS (Figure 1b). It belongs to the Jeddah and Hijaz terranes and lies near the Bir Umq suture zone. This suture zone serves as a boundary between two distinct terranes: the Hijaz and Jeddah terranes. The Jeddah terrane is positioned in the western region of the AS, bordered by several tectonic units: the Hijaz terrane to the north, the Afif terrane to the east, the Asir terrane to the south, and the Red Sea coastal plain to the west. The oldest rock unit in the study area is the Sumayir Formation (831 ± 47 Ma, ref. [28]). This formation consists of basalt, chert, tuffite, and siltstone, as well as minor mafic-ultramafic intrusive rocks, and it is locally interbedded with serpentinite.

The Dhukhr tonalitic complex, with a crystallization age of 811 ± 4 Ma [29], intruded into the Sumayir Formation. The Mahd Group rocks are the most extensive in the study area and comprise sedimentary and volcanic metamorphic formations, featuring alternating basaltic and andesitic lavas along with sedimentary rocks such as chert, limestone, sandstone, and conglomerates. Additionally, the injected tonalites of Hufayriyah and the Ram Ram complex occur within the Mahd rocks. The ages of the Mahd rocks range from 775 to 785 million years [29]. The Ram Ram complex consists of small rock masses, including red granite, granodiorite, diorite, and gabbro. These rocks intrude the Mahd Group and are dated to between 750 and 770 million years [29].

Rocks of the Ghamr Group are prevalent in the central and southern parts of the study area (Figure 1b). Their ages are estimated at 748 ± 22 million years [30]. The rocks of the Furayh Group are widely distributed throughout the study area, comprising mafic and felsic volcanic rocks alongside sedimentary rocks, all exhibiting a low degree of metamorphism.

The Hadb ash Sharar suite comprises semi-circular bodies of undeformed granite, granodiorite, and gabbro, which are interbedded within the Mahd, Ghamr, and Hufayriyah groups. These rocks are dated at 584 ± 26 million years using the rubidium-strontium dating method [30]. The igneous bodies of the Hadb ash Sharar suite include monzogranites, alkali-feldspar granites, alkali-rich granites, and smaller bodies of granodiorite and gabbro. The pegmatites found in the Jabal Sayid area are part of the Hadb ash Sharar suite rocks, noted for their high radioactivity and richness in titanium, tin, uranium, and rare earth elements. The latest volcanic activity in the study area is represented by the basaltic and andesitic rocks of Harrat Rahat, which date to the Tertiary and Quaternary periods.

3. Uranyl Mineralization

The study area features late alkaline to peralkaline granites that intersect with the volcanic and volcano-sedimentary rocks of the Mahd Group, alongside various outcrops of granodiorite and quartz diorite. It is notable for several mineral-rich regions, including the gold and copper deposits at the Mahd adh Dhahab prospect, as well as uranium, copper, and REE deposits in the Jabal Sayid area. The Al-Dahayeen Plateau, which is located to the south of Jabal Sayid, is also rich in uranium and REEs. These regions contribute significantly to groundwater uranium content and its isotopic composition.

The Jabal Sayid outcrop features pegmatites located in the northern section of an alkaline granite block. These granites are classified as unique AS granites and are part of the Hadb ash Sharar pluton. This pluton includes a core of pink granites surrounded by a broad band of hornblende, biotite, and granodiorite-rich monzogranites.

The main mineral composition of these pegmatites was identified as quartz and microcline, along with varying quantities of aegirine, arfvedsonite, plagioclase, and hematite, as noted by [31]. Additionally, ref. [4] reported several ore minerals in the aplite-pegmatite rocks of Jabal Sayid, such as synchysite, bastnaesite, pyrochlore, zircon, thorite, monazite, fluorite, sphalerite, and sphene. Fluorite is the most abundant mineral, primarily found in weathering zones. Yellow uranyl mineralization is also present as cavity fillers and crusts in the quartz veins and along the fractures of the host rock. Additionally, minerals such as galena, malachite, sphalerite, and calcite are observed. The research conducted by [5] identified kasolite, a Pb-U silicate mineral, commonly found alongside iron oxides, malachite, calcite, and galena. To the south of Jabal Sayid, the alkaline Hadb Ash Sharar granite has reserves of 23 million tons containing 0.13% Nb, 0.13% Ce, over 1.7% Zr, and 134 ppm U [32].

In situ gamma-ray spectrometry measurements in the aplite-pegmatite at Jabal Sayid identified radioactive zones with a maximum eU content of 1550 ppm and eTh of 7974 ppm, as reported by [9]. In comparison, alkali granite has average eU and eTh levels of 12 ppm and 34 ppm, while felsite shows similar values at 11 ppm for eU and 32 ppm for eTh. Pegmatite veins intersecting alkali granite exhibit higher averages of 34 ppm for eU and 101 ppm for eTh. The metamorphosed volcanic rocks of the Mahd Group display the lowest radioactivity, with average eU at 0.8 ppm and eTh at 1.6 ppm and show signs of post-magmatic alterations like silicification and oxidation. Excluding the metavolcanics, a strong positive correlation between eU and eTh across various rock types suggests geochemical coherence during magma crystallization, indicating these elements are mainly incorporated into accessory minerals and unaffected by alteration processes.

Granitic rocks, which have high eU and eTh, do not show elevated values of eU/eTh ratio (less than 1). In contrast, the volcanic metamorphic rocks of the Mahd Group exhibit the highest values of eU/eTh ratio, reaching up to 7. Ref. [5] proposed that hydrothermal solutions related to granitic magma played a key role in the formation of uranyl minerals in the Jabal Sayid region. Some limited redistribution of uranium may have occurred, as suggested by the presence of heterogeneous processes such as kaolinitization and oxidation. Conversely, the high thorium content in the granitic rocks indicates syngenetic formation, with the most differentiated pegmatites exhibiting the highest values of eU and eU/eTh. The formation of kasolite was significantly influenced by fluoride complexes. High-temperature hydrothermal solutions reacted with existing uranium-bearing metamict accessory minerals, such as zircon, uranium-rich thorite, and pyrochlore, resulting in the development of uranous fluoride complexes [5].

4. Materials and Methods

Nineteen groundwater samples were collected and analyzed to assess their major anions and uranium isotopic composition. The investigated wells, with depths ranging from 10 to 34 m, are classified as shallow wells tapping unconfined aquifers. Figure 2 displays a map of the valleys in the Sayid and Mahd Adh Dhahab area, highlighting the locations of the selected wells. The samples were selected from wells near uranium mineralization in this region to investigate the uranium transport and its possible precipitation from groundwater.

4.1. Cation and Anion Determination

In order to investigate the possible uranium speciation in groundwater, various laboratory techniques and instruments were employed to measure the major ion concentrations and physicochemical parameters in the water samples. pH was measured using a Mettler DL25 and the TDS (Total Dissolved Solids)/conductivity using a Mettler Toledo. Silicate, nitrate and phosphate were measured by spectrophotometry; bicarbonate and sulfate by potentiometry; chloride by DPD colorimetry (Diethyl-P-phenylene Diamine) and fluoride were measured by fluoride ion-selective electrode (ISE). Quality assurance and quality control (QA/QC) procedures were applied to ensure the reliability of the anion and cation analyses. Replicate measurements were performed routinely to assess analytical precision, and detection limits were determined according to standard laboratory protocols. The ionic balance error was also calculated to verify the accuracy and internal consistency of the hydrochemical data. All groundwater samples showed acceptable charge balance errors ranging between 2% and 5%, confirming the accuracy of the analyses.

4.2. Alpha Spectrometry Technique

The determination of uranium isotopes (²³⁸U and ²³⁴U) in groundwater samples was performed using alpha spectrometry following chemical separation through ion exchange. A tracer of 0.03 Bq of ²³²U was added to 200 mL of each water sample, which was then left for three days to reach isotopic equilibrium. Uranium was isolated using AG 1-8X anion-exchange resin in 9 M HCl and subsequently precipitated as uranium fluoride onto a 0.1 µm plastic membrane. The prepared sources were analyzed with an ORTEC Octete Plus alpha spectrometer equipped with eight high-resolution silicon detectors (efficiency 20%–21.5%). Detector calibration and quality assurance were verified using IAEA-certified reference materials.

Laboratory blanks were routinely processed; no additional blank corrections were required beyond the standard QA/QC procedures. Replicate measurements were performed to assess analytical precision, achieving an overall measurement accuracy better than 0.01 Bq L⁻¹. The obtained spectra allowed accurate quantification of uranium concentrations and isotopic ratios, with a detection limit below 0.01 Bq L⁻¹, ensuring reliability and consistency in the uranium determinations.

5. Results and Discussions

5.1. Physicochemical Parameters and Major Ion Concentrations of Groundwater

The measured physicochemical parameters of the groundwater samples provide important insights into the hydrogeochemical environment, water quality, and processes influencing groundwater composition. The pH values vary from 7.14 to 8.43, with an average of 7.92, indicating that the groundwater is neutral to slightly alkaline. This alkalinity implies carbonate buffering, likely resulting from the dissolution of carbonate minerals such as calcite or dolomite. The near-neutral pH also favors the stability of bicarbonate ions in groundwater. The temperature of the samples varies between 23.7 °C and 35.1 °C, with an average of 27.8 °C, reflecting ambient environmental conditions typical of shallow aquifers in semi-arid to arid regions. Slight variations in temperature among samples may indicate differences in depth, recharge timing, or local geothermal gradients.

The total dissolved solids (TDS) values show a wide range (447–22,400 ppm; average 7434 ppm), indicating significant spatial variability in salinity. According to standard water quality classifications, most samples fall into the brackish to saline category. The elevated TDS values suggest intense evaporation, dissolution of evaporitic minerals, and/or mixing with saline water sources. The electrical conductivity (EC) varies from 691 to 34,700 µS/cm (average 11,477 µS/cm), showing a strong positive correlation with TDS, as both parameters reflect the ionic concentration of dissolved constituents. High EC values confirm salinity enrichment, possibly caused by evaporative concentration and mineral dissolution. Generally, the physicochemical characteristics suggest that the groundwater is moderately to highly saline, controlled by water-rock interactions, evaporative processes, and anthropogenic inputs, with limited buffering capacity in some locations.

The variations in anion concentrations help distinguish between natural geochemical evolution and human-induced contamination, as well as infer the hydrological connectivity and recharge characteristics of the shallow groundwater system. The chemical composition of the groundwater shows significant variations in anion concentrations, reflecting natural geochemical processes in this arid region (

Table 1. Major cations, anions and physicochemical parameters of groundwater from 19 wells in the study area.

S. N.	Ca (ppm)	Mg (ppm)	Na (ppm)	K (ppm)	NH4 (ppm)	Cl (ppm)	HCO3 (ppm)	NO3 (ppm)	SO4 (ppm)	F (ppm)	NO2 (ppm)	PO4 (ppm)	SiO2 (ppm)	T (°C)	T.D.S (ppm)	Cond. µS/cm	PH
SW-1	362	112	684	1.3	<0.04	974	149	218	1140	1.18	<0.03	<0.09	36.4	29.4	3630	5580	7.14
SW-2	970	290	3264	9.4	<0.04	5235	180	101	2870	3.10	<0.03	<0.09	26.8	23.7	13,200	20,500	7.53
SW-5	221	89	1119	4.1	<0.04	1073	339	126	1635	1.55	<0.03	<0.09	26.7	26.7	4230	6530	7.88
SW-6	1260	170	1550	0.7	3.46	3972	169	70	2010	1.51	<0.03	0.63	31.9	26.2	9530	14,700	7.89
SW-7	318	99	1347	1.1	<0.04	1060	200	93	2700	1.44	<0.03	<0.09	23.4	28	5350	8240	7.93
SW-8	715	325	3200	25.2	<0.04	3050	819	72	4065	3.74	<0.03	9.70	36.6	27.6	11,360	17,500	7.97
SW-9	480	93	1215	16.6	<0.04	1481	157	71	2300	1.13	<0.03	<0.09	23.9	24.3	5460	8400	7.82
SW-10	268	60	699	0.9	<0.04	479	179	82	1700	3.34	<0.03	0.18	38	26.1	3170	4890	8.11
SW-11	234	105	1539	18.8	<0.04	372	844	79	2970	3.62	<0.03	10	44	26.8	5340	8220	8.25
SW-12	536	168	1664	3.5	<0.04	1296	167	284	3295	3.80	<0.03	<0.09	27.9	28.7	6980	10,770	7.88
SW-13	608	164	1587	3	<0.04	1679	198	422	2770	1.92	<0.03	<0.09	23.5	31.2	7220	11,120	7.83
SW-21	978	400	9970	10.6	<0.04	5698	292	294	18,375	4.02	<0.03	0.56	14.6	25.1	22,400	34,700	8
SW-22	972	438	5792	16.6	1.72	9051	749	73	3348	1.70	<0.03	4.81	40.2	26.7	20,400	31,500	7.86
SW-24	59	10	509	0.2	<0.04	164	325	23	860	2.94	<0.03	<0.09	32.3	27.7	1820	2800	8.18
SW-28	680	276	2096	0.8	<0.04	2599	130	134	2940	1.64	<0.03	<0.09	42.9	31	8870	13,670	7.79
SW-31	83	267	129	9.1	<0.04	345	326	393	460	0.24	<0.03	1.15	34.7	27	2040	3160	8.24
SW-32	8	69	20	0.8	<0.04	63	211	27	51	1.12	<0.03	<0.09	11.5	26.9	447	691	8.43
SW-34	322	28	584	3.1	<0.04	962	179	29	835	1.37	<0.03	0.19	21.6	29.4	3180	4890	8.06
SW-42	1008	131	987	4	0.39	2818	85	46	1125	1.24	<0.03	<0.09	20.5	35.1	6610	10,210	7.65
Ave.	531	173	1998	6.8	1.90	2230	300	139	2918	2.14	<0.03	3.40	29.3	27.8	7434	11,477	7.92

). The chloride concentration ranges from 63 to 9051 ppm (average 2230 ppm), indicating considerable spatial variability. These elevated values, especially those exceeding 1000 ppm, suggest strong effects of evaporation and contamination from domestic sources. The bicarbonate levels (85–844 ppm; avg. 300 ppm) are moderate, implying that carbonate mineral dissolution and CO₂ from soil respiration significantly contribute to groundwater chemistry. This also suggests active recharge from areas where infiltration interacts with carbonate-rich sediments. The nitrate content (23–422 ppm; avg. 139 ppm) is well above the natural background level (typically <10 ppm), pointing to a clear anthropogenic impact and surface contamination due to the shallow depth of the aquifer. The sulfate concentration (51–18,375 ppm; avg. 2918 ppm) dominates the anionic composition, which may be attributed to oxidation of sulfide minerals that occur in the nearby Precambrian rocks and mining areas, dissolution of gypsum or anhydrite, and industrial inputs. Extremely high sulfate values could also indicate evaporative concentration in arid and semi-arid conditions. The fluoride concentration (0.24–4.02 ppm; avg. 2.14 ppm) exceeds the recommended limit for drinking water (1.5 ppm) in several samples. This suggests fluoride-bearing mineral dissolution (e.g., fluorite, biotite) or evaporation effects enhancing ion accumulation. The silicate concentration (11.5–44 ppm; avg. 29.3 ppm) reflects silicate mineral weathering (such as feldspars and clays), contributing to the overall ionic balance but remaining within normal ranges for groundwater interacting with silicate rocks. As expected in such an arid area, the low or undetectable concentrations of phosphate, ammonium, and nitrite concentrations in most samples indicate limited organic or nutrient pollution, except at a few localized sites (Table 1).

The distribution of major cations in the studied shallow groundwater reflects the dominant geochemical processes in arid environments (Table 1). Calcium concentrations range between (7.8–1260 ppm; avg. 530.6 ppm), indicating dissolution of gypsum and, at some sites, interaction with carbonate minerals. Magnesium shows a narrower and lower range (10.3–438 ppm; avg. 173.4 ppm), suggesting contributions from Mg-bearing minerals such as mafic minerals and dolomite, and partial removal by cation-exchange processes [33]. Sodium is the most abundant cation in many samples, with a wide range of (20.1–9970 ppm; avg. 1997.5 ppm), highlighting strong effects of halite dissolution, intense evaporation, and cation exchange with Na⁺ on clay surfaces [34]. Potassium remains consistently low (<0.2–25.2 ppm; avg. 6.8 ppm), reflecting limited mobility and fixation in clay and feldspar minerals.

The distribution of major cations (Ca²⁺, Mg²⁺, Na⁺ + K⁺) and anions (HCO₃⁻ + CO₃²⁻, SO₄²⁻, Cl⁻ + F⁻) on the Piper diagram (Figure 3) helps identify the dominant hydrochemical facies and understand the geochemical processes controlling groundwater composition [35]. Most samples plot within the sodium–chloride and calcium–sulfate water fields, indicating strong influence of evaporite dissolution, particularly halite and gypsum, which is typical of arid areas. The very wide sodium range drives many samples toward the Na⁺ + K⁺ corner of the cation triangle, producing a dominant sodium–chloride to calcium sulfate water type in the central diamond. The limited presence of bicarbonate-rich facies reflects minimal recent recharge and weak interaction with carbonate minerals. The clustering of points toward the chloride and sulfate vertices also signifies intense evaporation. This diagram demonstrates that shallow groundwater in the region is largely controlled by evaporation, dissolution of evaporitic minerals, and ion exchange processes, leading to chemically evolved water with elevated salinity. This classification is crucial for evaluating groundwater suitability for drinking and irrigation and for understanding the hydrogeochemical evolution of shallow aquifers in arid environments.

5.2. Uranium Behavior and Speciation in Groundwater

The examined wells, with depths between 10 and 34 m, tap into the upper unconfined aquifer. This aquifer is directly affected by surface processes such as rainfall infiltration, evapotranspiration, and potential human activities. Groundwater at this depth reflects recent recharge and is more sensitive to variations in physicochemical conditions than deeper confined aquifers. In such shallow groundwater systems, uranium behavior is strongly affected by near-surface geochemical interactions. Spearman’s rank correlations indicate that uranium behavior in this groundwater system is mainly controlled by the degree of groundwater salinity and the prevailing geochemical conditions, rather than by temperature or pH alone (

Table 2. Spearman’s rank correlation matrix showing the relationships between the variables determined in groundwater.

	Ca	Mg	Na	K	Cl	HCO3	NO3	SO4	F	PO4	SiO2	T	TDS	EC	Ph	U
Ca	1
Mg	0.712 **	1
Na	0.740 **	0.786 **	1
K	0.241	0.465 *	0.452	1
Cl	0.932 **	0.779 **	0.861 **	0.379	1
HCO3	−0.341	0.113	0.105	0.433	−0.129	1
NO3	0.104	0.468 *	0.363	0.193	0.195	−0.004	1
SO4	0.572 *	0.688 **	0.942 **	0.518 *	0.688 **	0.211	0.375	1
F	0.260	0.308	0.648 **	0.250	0.331	0.328	0.232	0.741 **	1
PO4	0.149	0.377	0.238	0.487 *	0.140	0.585 **	−0.038	0.327	0.298	1
SiO2	−0.035	0.239	0.191	0.122	−0.014	0.211	0.137	0.311	0.283	0.439	1
T	−0.036	−0.129	−0.243	−0.330	−0.205	−0.325	0.066	−0.222	−0.206	−0.309	−0.061	1
TDS	0.888 **	0.835 **	0.956 **	0.433	0.961 **	−0.024	0.284	0.835 **	0.479 *	0.218	0.070	−0.198	1
EC	0.886 **	0.836 **	0.958 **	0.430	0.961 **	−0.024	0.290	0.840 **	0.487 *	0.218	0.081	−0.209	1.000 **	1
Ph	−0.598	−0.333	−0.382	−0.043	−0.592	0.626 **	−0.311	−0.218	0.127	0.513 *	0.055	−0.177	−0.496	−0.496	1
U	0.337	0.311	0.528 *	0.228	0.381	0.056	0.247	0.577 **	0.714 **	−0.071	0.030	−0.102	0.447	0.460 *	−0.253	1

** Indicates significance at the 0.01 level; * Indicates significance at the 0.05 level.

). The weak negative relationship between uranium and pH (r = −0.253) suggests limited influence of alkalinity within the studied range (7.14–8.43), possibly due to limited adsorption or precipitation of uranyl species, while the very low correlation with temperature (r = −0.102) confirms that temperature variations have little effect on uranium solubility. In contrast, the moderate positive correlations with EC and TDS (r = 0.460 and 0.447, respectively) imply that uranium enrichment is associated with waters of higher ionic strength (Figure 4a).

The geochemical data reveal significant correlations between uranium and certain anions, indicating the main geochemical controls on uranium mobility and speciation in the studied groundwater. The area is characterized by an arid climate and the presence of nearby uranium and sulfide-bearing rocks, which together influence groundwater chemistry. In neutral to alkaline and oxidizing conditions, common in arid and semi-arid terrains, uranium mobility is strongly influenced by complexation with carbonate, fluoride, and other oxyanions. The strong positive correlation between uranium content and fluoride (r = 0.714) suggests that fluoride complexation plays a dominant role in uranium mobilization and stability in solution (Figure 4b). In alkaline to slightly alkaline groundwater, typical of arid regions, uranium predominantly occurs as uranyl–fluoride complexes (e.g., [UO₂F₃]⁻, [UO₂F₂]⁰), which are relatively soluble and stable under oxidizing conditions [36]. According to a study using systematic Raman Spectroscopy of the complexation of uranyl with fluoride [37], the uranyl ion (UO₂²⁺) forms several stepwise complexes with fluoride in aqueous solution, namely UO₂F⁺, UO₂F₂ (aq), UO₂F₃⁻, UO₂F₄²⁻, and UO₂F₅³⁻. The formation constants of these complexes were determined through detailed spectroscopic analysis. These findings suggest that fluoride can form stable, stepwise complexes with uranyl in aqueous solutions, which has important implications for uranyl speciation, mobility, and stability in fluoride-bearing natural waters or radioactive waste streams. In the studied groundwater, the relatively high fluoride activity enhances uranium solubility and transport in the groundwater system. Recent field and experimental studies support the idea that fluoride plays a critical role in uranium mobilization in groundwater, especially in arid to semi-arid areas. For example, ref. [38] demonstrated that in the southern Punjab alluvial aquifers, competitive ion exchange between sediments and groundwater can co-mobilize fluoride and uranium, with the process being influenced by ionic strength and bicarbonate dynamics. This finding reinforces the observed strong correlation between uranium concentration and fluoride, suggesting that uranyl-fluoride complexes significantly enhance uranium solubility and transport in oxidizing, fluoride-rich settings. In such environments, fluoride is one of the key ligands closely associated with uranium, particularly in settings with high total dissolved solids and evaporation [39]. According to ref. [5], fluoride speciation is proposed as one of the primary forms influencing the migration of uranium and REEs in the Jabal Sayid area, based on the pH of the fluids.

Weak correlations of uranium with chloride (r = 0.381) and bicarbonate (r = 0.056) suggest a minor role for chloride or carbonate complexation (Table 2). Although bicarbonate commonly forms soluble uranyl–carbonate complexes (e.g., [UO₂(CO₃)₂]²⁻, [UO₂(CO₃)₃]⁴⁻), the low correlation implies that carbonate activity is limited. Uranyl–carbonate complexes are reported to be important for uranium mobility under oxidizing, alkaline conditions [39]. They also noted that uranium often covaries with fluoride and other oxyanions under these conditions, implying that the speciation and transport of uranium depend on the relative abundance of competing ligands such as carbonate, fluoride, and sulfate. They emphasized that pH, redox state, and solute chemistry are essential controls on uranium distribution and speciation.

The hypothesis of competitive ion exchange driving co-mobilization of fluoride and uranium is supported by [38]. They found that fluoride and uranium become exchangeable in sediments, and that increases in ionic strength, as well as variations in bicarbonate, influence their mobilization into groundwater. This study provides field evidence that fluoride plays a significant role in controlling uranium mobility in certain sedimentary systems. In shallow aquifers under arid and semi-arid conditions, carbonate and fluoride processes are suggested to interact, with fluoride release (and thus speciation) enhanced when carbonate minerals precipitate [40].

Significant correlations between uranium and total dissolved solids (r = 0.447), as well as between uranium and sulfate (r = 0.557), indicate that evaporitic conditions and sulfate mineral dissolution (possibly from gypsum or the oxidation of sulfide minerals) further contribute to uranium mobility (Figure 4a,c). The areas with high sulfate concentration tend to maintain oxidizing groundwater conditions, which favor uranium in its hexavalent form (U(VI)) [39]. Oxidized uranium is more likely to remain soluble, and in the presence of high sulfate, the redox environment is less likely to reduce uranium to the less soluble U(IV). This is consistent with the significant correlation observed between uranium and sulfate, indicating that sulfate is not only a marker of the geochemical environment but an indirect factor enhancing uranium mobilization. The very low correlations with nitrate (r = 0.247) and silicate (r = 0.030) indicate that these species have little influence on uranium behavior, reflecting their minor abundances and weak complexation tendencies with uranium (Table 2). The overall geochemical pattern, where SO₄²⁻ > Cl⁻ > HCO₃⁻ > NO₃⁻ > SiO₃²⁻ > F⁻, characterizes a sulfate–chloride water type typical of arid-zone groundwater influenced by evaporitic and surface contamination processes. In such an environment, uranium speciation is primarily governed by oxidizing conditions and complexation with fluoride and sulfate ions, with limited buffering by carbonate equilibria.

5.3. Uranium Isotopic Composition of Groundwater

The ²³⁴U/²³⁸U activity ratio (AR) is a valuable tool for interpreting groundwater flow and mixing of groundwater from different aquifers [20,41]. While ²³⁸U can be dissolved from rocks through chemical processes like oxidation and dissolution, ²³⁴U can be dissolved via both chemical and physical processes. These physical processes include the effects of alpha particle emissions and radiation damage in the mineral crystal structure.

The relationship between the activity ratio and uranium concentration in groundwater provides valuable insights into the geochemical processes controlling uranium mobility and sources. Typically, uranium in groundwater exists primarily as the isotopes ²³⁸U and ²³⁴U, and their activity ratio (²³⁴U/²³⁸U) often deviates from unity due to alpha recoil and preferential leaching. Low uranium concentrations accompanied by high activity ratios usually indicate prolonged water-rock interaction or leaching from old aquifer materials, where recoil processes enhance the enrichment of the daughter isotope. Conversely, high uranium concentrations associated with low activity ratios indicate recent uranium input. Understanding these ratios is crucial for assessing uranium mobility in groundwater systems, which has implications for water quality, environmental monitoring, and the management of uranium resources.

The activities of ²³⁸U and ²³⁴U in the studied groundwater samples vary significantly among the wells (

Table 3. The isotopic composition of uranium in groundwater from 19 wells in the study area.

Sample Number	238U Bq/L	234U Bq/L	Total U (ppb)	234U/238U Activity Ratio
SW-1	0.092 ± 0.013	0.188 ± 0.022	7.532 ± 1.065	2.038 ± 0.372
SW-2	0.358 ± 0.063	0.500 ± 0.083	29.309 ± 5.113	1.396 ± 0.337
SW-5	0.063 ± 0.016	0.090 ± 0.020	5.129 ± 1.297	1.435 ± 0.482
SW-6	0.044 ± 0.014	0.075 ± 0.019	3.584 ± 1.138	1.714 ± 0.697
SW-7	0.159 ± 0.028	0.209 ± 0.035	13.012 ± 2.301	1.313 ± 0.318
SW-8	0.275 ± 0.039	0.497 ± 0.064	22.486 ± 3.171	1.807 ± 0.345
SW-9	0.047 ± 0.011	0.103 ± 0.018	3.883 ± 0.908	2.160 ± 0.631
SW-10	0.169 ± 0.126	0.203 ± 0.144	13.825 ± 10.346	1.200 ± 1.237
SW-11	0.080 ± 0.010	0.135 ± 0.014	6.552 ± 0.803	1.680 ± 0.270
SW-12	0.196 ± 0.064	0.400 ± 0.114	16.008 ± 5.255	2.045 ± 0.890
SW-13	0.118 ± 0.040	0.367 ± 0.096	9.617 ± 3.298	3.115 ± 1.347
SW-21	0.253 ± 0.152	0.282 ± 0.165	20.738 ± 12.409	1.111 ± 0.930
SW-22	0.071 ± 0.014	0.143 ± 0.023	5.787 ± 1.175	2.028 ± 0.526
SW-24	0.135 ± 0.048	0.211 ± 0.066	11.060 ± 3.916	1.563 ± 0.739
SW-28	0.055 ± 0.010	0.074 ± 0.012	4.467 ± 0.809	1.357 ± 0.329
SW-31	0.010 ± 0.006	0.015 ± 0.007	0.838 ± 0.524	1.500 ± 1.189
SW-32	0.011 ± 0.008	0.014 ± 0.009	0.882 ± 0.671	1.333 ± 1.317
SW-34	0.009 ± 0.009	0.018 ± 0.012	0.747 ± 0.758	2.000 ± 2.393
SW-42	0.151 ± 0.018	0.261 ± 0.027	12.379 ± 1.467	1.723 ± 0.272
Ave.	0.121 ± 0.036	0.199 ± 0.050	9.886 ± 2.970	1.711 ± 0.770

Bq/L = Becquerel per liter; ppb = parts per billion.

). ²³⁸U ranges from 0.009 to 0.358 Bq/L with a mean value around 0.121 Bq/L. ²³⁴U ranges from 0.014 to 0.500 Bq/L, averaging about 0.199 Bq/L. The total uranium concentration varies from 0.75 to 29.3 ppb, showing large spatial variability. The activity concentrations of ²³⁴U and ²³⁸U in the studied region are illustrated in Figure 5a. A positive correlation is observed between the activity concentrations of ²³⁴U and ²³⁸U in the analyzed samples (Figure 5b). This correlation is associated with the leaching of both uranium isotopes into the groundwater as it flows through the faults and fissures within the reservoir rocks [42,43]. The activity ratio ²³⁴U/²³⁸U ranges from 1.11 to 3.11, with an overall average of about 1.71 (Figure 5c).

The highest total uranium concentration is observed in sample SW-2 (29.3 ppb), which also shows one of the highest ²³⁸U activities (0.358 Bq/L) but a comparatively lower activity ratio (1.396). This indicates less isotopic fractionation, possibly reflecting recent uranium mobilization from host minerals without strong preferential leaching of ²³⁴U. In contrast, samples with lower uranium contents but higher ²³⁴U/²³⁸U ratios (e.g., SW-13 = 3.115) suggest older groundwater-rock interaction and enhanced alpha-recoil effects, which enrich ²³⁴U in solution relative to ²³⁸U (Table 3).

The range of the ²³⁴U/²³⁸U ratio confirms that the groundwater is typical and aligns with global values between 1 and 2 [26,27,41]. Groundwater from these wells exhibits normal ²³⁴U/²³⁸U isotopic ratios, indicating that the waters are relatively young and contain low uranium concentrations. These findings suggest that uranium is not largely dissolving in the study area, even under oxidizing conditions. This is likely due to its concentration in refractory minerals such as zircon, monazite, and xenotime [4], which are resistant to weathering, especially in low-temperature environments. In addition, the low weathering rate limits the migration of uranium in the study area [9]. Consequently, only minimal redistribution of labile uranium has occurred, occasionally forming uranyl precipitation associated with alteration processes. This interpretation is reinforced by the presence of kasolite along fractured zones in the aplite-pegmatite of the Jabal Sayid area [5]. Uranyl mineralization in the Sayid-Mahd Adhab region likely originates from the precipitation of minerals near the surface due to circulating oxic groundwater. The ongoing preferential leaching of ²³⁴U from uranyl mineralization, driven by recoil processes, suggests the existence of weakly circulating groundwater. This type of groundwater encourages the preferential mobilization of ²³⁴U, resulting in mineralization that is low in ²³⁴U [7].

The variability in uranium concentration and isotopic ratios reflects the influence of both the geochemical environment and hydrochemical composition. In groundwater where fluoride concentrations are elevated, uranium tends to form stable uranyl-fluoride complexes that enhance uranium solubility even under slightly alkaline and oxidizing conditions [38,40]. This explains why samples such as SW-2, SW-8 and SW-21, which likely occur in areas of higher TDS and fluoride, show elevated uranium contents (Table 1 and Table 3). These samples show lower ²³⁴U/²³⁸U activity ratios (~1.1–1.4), which indicate recent mobilization of uranium from mineral surfaces or adsorption–desorption processes driven by high fluoride and salinity. On the other hand, higher ratios (>2) suggest prolonged contact with aquifer rock and alpha-recoil effects.

According to ref. [20], the ²³⁴U/²³⁸U activity ratios typically increase as uranium concentration decreases, producing a positive relationship when the activity ratio is plotted against the reciprocal of uranium concentration (1/U). This pattern reflects preferential mobilization of ²³⁴U through recoil and leaching processes in groundwater. The relationship between the ²³⁴U/²³⁸U AR and the reciprocal of total uranium concentration was evaluated to assess whether uranium isotopic variations are governed by open-system α-recoil addition or by water mixing processes (Figure 6a). In this figure, samples exhibiting high uncertainties above 50% in ²³⁴U and ²³⁸U measurements were excluded to avoid misleading interpretation. In the analyzed groundwater samples, the regression analysis yielded the equation:

AR = 1.73 − 0.054 (1/U)

The scatter of data points supports the open-system recoil-addition model. The samples with low total U (<5 ppb) such as SW-9 and SW-34 exhibit high activity ratios (>2), whereas samples with high uranium (>20 ppb) such as SW-2 and SW-21 display lower ratios (1.111–1.396). Correlation with the hydrochemical data shows that samples with higher fluoride and TDS contents tend to have lower activity ratios (Table 1 and Table 3), while low-salinity waters exhibit higher ratios due to longer residence times and cumulative ²³⁴U enrichment by recoil.

Minor deviations from the trend, particularly for samples such as SW-8 and SW-13, may reflect partial mixing between two water components: (1) a fluoride-rich, high-U/low-AR water (SW-8 type), and (2) a low-U/high-AR water (SW-13 type). These samples are located north of Jabal Sayid area close to sample SW-12 which has intermediate composition (Figure 2 and Figure 6a). A distinct linear relationship is observed between the activity ratio and the reciprocal of uranium concentration in these three groundwater samples (Figure 6b), expressed by the regression equation:

AR = 0.71 + 23 (1/U)

This strong positive correlation indicates a binary mixing trend between two contrasting endmembers [20]. The intercept value (~0.71) represents the uranium-rich endmember, which likely corresponds to groundwater in contact with uranium mineralization zones present in Jabal Sayid. This source shows high uranium concentrations and an activity ratio close to equilibrium. With increasing distance from these mineralized zones, uranium concentrations decrease while the activity ratio increases, reflecting progressive mixing with uranium-poor water enriched in ²³⁴U due to preferential leaching and recoil effects. It is noteworthy that this linear trend was only observed in these three closely spaced samples, whereas other samples from the wider region did not show similar behavior. This suggests that the mixing process is localized, possibly controlled by small-scale hydrological or geochemical interactions around the uranium-bearing rocks. These findings highlight the significance of uranium isotopic composition as a sensitive tracer for detecting localized groundwater mixing and for assessing the hydrogeochemical influence of nearby mineralized zones on the aquifer system.

A conceptual illustration (Figure 7) shows groundwater flow northward from the uranium mineralization zone through the shallow aquifer system. The three wells (SW-8, SW-12, and SW-13) are located approximately 4, 8, and 10 km north of the source, respectively. The schematic plot shows that uranium concentration gradually decreases, while the activity ratio (²³⁴U/²³⁸U) increases with distance from Jabal Sayid, indicating progressive uranium leaching and isotopic fractionation along the groundwater flow path.

5.4. Environmental Implications

The observed enrichment of uranium in fluoride-rich groundwater has important environmental and health implications for local communities. In the studied area, several groundwater samples contain uranium concentrations approaching WHO [44] guideline values of 30 µg/L (≈0.37 Bq/L for ²³⁸U), particularly in wells characterized by elevated fluoride, salinity, and high TDS levels, which reflect intensive mineral dissolution.

From a health-risk perspective, chronic ingestion of uranium-bearing water may lead to chemical nephrotoxicity, as prolonged exposure above the WHO limit has been associated with kidney dysfunction. Similarly, fluoride levels above the recommended guideline of 1.5 mg/L can cause dental and skeletal fluorosis. The coexistence of elevated uranium and fluoride in several samples highlights a co-exposure scenario, where populations may simultaneously be subjected to both nephrotoxic (U) and fluorotic (F⁻) risks. Such co-occurrence is geochemically reasonable, as both elements can be released from U–F–bearing minerals present in the Jabal Sayid area and their mobility is enhanced through uranyl–fluoride complexation, which increases uranium stability in solution.

The positive correlation between fluoride and uranium concentrations further reinforces the geochemical link between these species, indicating that fluoride complexation plays a key role in enhancing uranium mobility relative to other anions. Consequently, communities relying on shallow groundwater in these zones may be exposed to combined chemical hazards, which require careful risk assessment.

With respect to the radiological risk associated with uranium in groundwater, the annual effective dose (AED) and the internal hazard index (H_in) were evaluated. Based on the average uranium concentration (9.886 µg/L, Table 3), the corresponding activity was estimated as 0.247 Bq/L using a conversion factor of 0.025 Bq per µg of U. The AED was calculated following [45]

AED = A_U × IR × DCF

where A_U is the activity of uranium in Bq/L, IR (Ingestion Rate) represents the annual volume of drinking water consumed by an adult (commonly assumed to be 730 L/year, equivalent to 2 L/day), and DCF (Dose Conversion Factor) represents the effective radiation dose received per unit of ingested activity, expressed in Sv/Bq. For uranium ingestion, the commonly used value is 4.5 × 10⁻⁸ Sv/Bq according to [45]. The resulting AED is 0.0081 mSv/yr, which is well below the recommended limit of 0.1 mSv/yr. The internal hazard index was also calculated as:

H_in = A_U/370

The resulting internal hazard index value is 6.7 × 10⁻⁴, indicating no significant radiological risk from uranium ingestion

From a management perspective, the results underscore the need for regular monitoring of uranium and fluoride, particularly in areas with elevated TDS. Mitigation strategies such as adsorption on activated alumina, bone char, ion exchange, or reverse osmosis may be required to ensure that drinking water remains within safe limits. Special attention should be given to sample SW-2, which is located at a considerable distance from the known uranium occurrences in the Sayid area yet shows the highest uranium concentration in the dataset. This anomaly suggests additional mobilization pathways or localized geochemical controls that warrant further investigation.

6. Conclusions

The present study investigates the chemical controls on uranium activity in the shallow groundwater of the Jabal Sayid-Mahd Adhab region, Saudi Arabia. The physicochemical characteristics suggest that the studied shallow groundwater is moderately to highly saline, controlled by water-rock interactions, evaporative processes, and anthropogenic inputs. The combined isotopic and hydrochemical evidence indicates that uranium mobility in the studied shallow groundwater is governed primarily by highly soluble uranyl–fluoride complexes, as demonstrated by strong positive correlations between fluoride and uranium activities. The relatively short residence time and contact with the oxidizing zone promote the formation of the uranyl–fluoride speciation.

In the Sayid-Mahd Adhab region, uranium concentrations ranged from 0.75 to 29.3 ppb, typical for oxidizing groundwater. The average activity concentrations of ²³⁴U and ²³⁸U in water samples were found to be 0.2 Bq/L and 0.12 Bq/L, respectively. The ²³⁴U/²³⁸U activity ratios, ranging from 1.11 to 3.11, suggest the presence of relatively young waters with minor water-rock interaction contributions, while high ratios in some wells reflect alpha-recoil and isotopic fractionation which enhance ²³⁴U solubility.

The positive correlation between uranium and TDS, together with the weak AR–1/U correlation, confirms that uranium enrichment occurs mainly in fluoride-rich, high-salinity zones, whereas elevated activity ratios mark older, low-salinity recharge waters. The obtained isotopic data support the open-system recoil-addition model. Minor deviations from this model may reflect partial mixing between two water components: (1) a fluoride-rich, high-U/low-AR water, and (2) a low-U/high-AR water. The localized mixing occurs in some wells in the northern Jabal Sayid area. Overall, the ²³⁴U/²³⁸U (AR) vs. 1/U relationship in the studied samples indicates that uranium behavior in the shallow aquifer is dominated by open-system leaching and fluoride-induced solubility, with local binary mixing superimposed in a few sites. Based on the average uranium concentration, the annual effective dose (0.0081 mSv/yr) and hazard index (6.7 × 10⁻⁴) are far below the recommended safety limits, indicating negligible radiological risk.

These findings underscore the importance of fluoride in controlling uranium solubility and mobility, particularly in high-salinity zones. For future research, we recommend modeling uranium speciation under variable Eh–pH conditions, as well as investigating dynamic groundwater mixing and the potential impacts of climate change or anthropogenic influences on uranium behavior in the region. Regular monitoring and targeted water-treatment strategies are essential to ensure safe groundwater use.