Gravity Flow System at Sulaimani, Kurdistan Region, Iraq: Groundwater and Isotopic Geochemistry and Their Implications for Groundwater Protection

Abstract

Groundwater recharge sources and residence times in the Sulaimani–Warmawa Sub-basin, located in the Kurdistan Region of Iraq, were assessed through an integrated hydrogeological, hydrochemical, and isotopic investigation. The study area, located around Sulaimani City, is characterized by a semi-arid climate with precipitation predominantly occurring during winter and early spring. Hydrochemical results indicate groundwater types ranging from Ca–HCO₃ to Mg–Ca–HCO₃, accompanied by a progressive increase in electrical conductivity along the regional flow path. Stable isotope signatures (δ²H and δ¹⁸O) show that groundwater is primarily recharged by winter precipitation derived from both Eastern Mediterranean and Persian Gulf air masses. Two groundwater groups were identified based on isotopic composition and tritium content: recently recharged groundwater and older groundwater, represented by two samples. Tritium values ranging from 0.8 to 4.9 TU correspond to minimum residence times from less than 10 years to approximately 40 years. Higher tritium concentrations near recharge zones reflect recent infiltration, whereas lower values indicate older groundwater with limited modern recharge. The piston flow model provided the best fit for tritium data, suggesting limited mixing and relatively rapid subsurface flow. Samples with higher salinity likely reflect reduced flushing in low-permeability zones, resulting in elevated dissolved solids. Hydraulic-data-based estimated groundwater flow velocities yielded lower values compared to tritium-based estimates, implying preferential flow in karstified formations. The relatively short groundwater residence times mean there is high vulnerability to contamination, emphasizing the need for careful land-use planning and groundwater protection strategies.

1. Introduction

Groundwater is a crucial source of fresh water, particularly in semi-arid regions and developing countries where clean water resources are scarce. In many regions, the demand for water is increasing due to population growth leading to an imbalance in the availability of clean drinking water. The problem of the imbalance in the supply and demand of clean drinking water is being intensified by several factors, including climate change, a significant rise in industrial activities, and the increasing consumption of water for agricultural and household purposes [1,2]. To effectively address the water supply and demand issues in these regions, it is important to use application of environmental isotopes, or hydrogeological analysis, or age dating groundwater, rather than scientific knowledge. This can be achieved by establishing a comprehensive understanding of the recharge processes, the residence time of the groundwater, and renewability of the groundwater resources [3]. However, due to the complexity of the hydrogeological systems, this process can face significant challenges [4].

Many studies have been conducted using environmental tracers, such as stable and radioactive isotopes, for determining the sources of groundwater recharge and its residence time [5,6]. For instance, Kpegli et al. [7] developed a groundwater-flow model for southern Benin in West Africa. In another study, the hydrogeochemistry and isotopes (²H, ³H, and ¹⁸O) were used to identify water–rock interactions in karst water systems in the Pingyi-Feixian Region in China [8]. Several additional studies have been carried out in different regions of the world. These studies have employed environmental tracers to identify the source and origin of groundwater, for example, in Piraquara de Fora Bay, Brazil [9], the Yucatan Peninsula, Mexico [10], and south-eastern Brazil [11].

Recharge zones can potentially be determined by analyzing stable isotopes in conjunction with hydrogeological and hydrogeochemical data [12,13]. By comparing the δ²H and δ¹⁸O isotope ratios, along with other hydrogeochemical data, the relationship between the surface and groundwaters of rivers and lakes can be determined [14,15]. For example, isotopic fractionation in water caused by evaporation and condensation allows tracing of the water’s origin and mixing history [6,16]. Furthermore, the residence period and the presence of percolation-favourable zones may be determined by correlating isotopic data with the mineralization of groundwater [17,18].

So far, only a limited number of studies have been conducted on groundwater ages in the Kurdistan Region of Iraq. However, stable isotope analysis has recently been carried out in the area of interest [19]. Nevertheless, research using tritium (³H) to examine the relationship between rain, surface, and groundwater was still lacking in this area.

Tritium (³H), the heaviest hydrogen isotope, is naturally produced and also generated through nuclear weapons testing [9,20,21]. Annual releases of ³H at approximately 10 TBq were also reported by [22,23] for pressured water reactors in Korea and China, respectively. Tritium (³H) has been utilized for various purposes, such as the determination of water mixing, groundwater recharge, and assessing the flow velocity and residence time of groundwater in aquifers [6,21].

Tritium can be used to date groundwater, assuming that the initial tritium content is known. However, this is not always the case, and simultaneous application of tritium and ³He is used for dating because ³H is the product of tritium decay [6,20].

The area of study is the Sulaimani–Warmawa Sub-basin (SWSB), located in the Kurdistan Region of Iraq near Sulaimani City (Figure 1A). From a hydrological viewpoint, it is a sub-basin that is a part of the Sulaimani-Sharazoor basin in the Sulaimani Governorate [24]. It is a semiarid area with a Mediterranean climate, and most of the precipitation falls in the winter months. Groundwater is a vital water resource in the area, given that most streams are perennial and typically experience declining water levels and reduced discharge during late summer. From an economic viewpoint, the region is important because of its huge oil reserves, and several towns are booming due to the oil industry. The main city located inside the SWSB is Sulaimani, with approximately 790,000 inhabitants mostly dependent on groundwater for water supply. The Sub-basin has been studied by [19], and the principal processes responsible for groundwater chemistry evolution have been identified.

This study had two main objectives: (1) to determine the possible sources and timing of recharge, and (2) to estimate the groundwater flow velocities, residence times, and groundwater vulnerability to contamination by using an integrative approach comprising hydrogeological, hydrogeochemical, and isotopic data. These objectives should enhance the understanding of local aquifer dynamics and contribute to sustainable water resource management and environmental conservation in the Iraqi Kurdistan.

2. Geology and Hydrogeology

The study area, known as the Warmawa-Sulaimani Sub-basin (SWSB), is located in the High Folded Zone. The SWSB is a part of the Sulaimani-Sharazoor basin in the Sulaimani Governorate, Kurdistan Region of Iraq (Figure 1A,B). The study area covers approximately 1500 km². Stevanovic and Markovic [24] stated that the region is characterized by a hot, dry summer, and a cold winter. The winter in the area is cold, and most of the precipitation occurs during this season, with a maximum of over a hundred mm in January [25]. The mean annual precipitation, air temperature, and evaporation are 668.5 mm, 21 °C, and 2298.8 mm, respectively. The Tanjero River is the principal river in the basin and usually becomes dry in August until the beginning of the new precipitation period.

The outcropping geological units comprise early Cretaceous, Tertiary, and Quaternary sedimentary deposits [26,27]. The oldest unit of the Early Cretaceous age is the Balambo Formation, which is composed of radiolarian limestone with thin and well-bedded sections. In addition to the Qamchuqa and Kometan formations, which consist of massive grey dolomite and dolomitic limestone, as well as white-weathered and light-grey, thin-bedded, and locally silicified limestone. The Late Cretaceous formations are characterized by Bluish-grey marl and marly limestone of the Shiranish Formation and Silty marl, siltstone, shale, sandstone, conglomerate, and sandy or silty organic detrital limestone of the Tanjero Formation. The sequence is followed by alternating flysch and carbonate sediments of the Kolosh, Sinjar Gercus, and Pila Spi Formations. The Quaternary sedimentary deposits are composed of alluvial fans and floodplain sediments around the Tanjore River and its tributaries: gravel, sand, silt, mud, sandy, and silty clay soil (

Table 1. Geological formations in the study area, after [19,25,26,27,28].

Age	Geological Units	Lithological Description	Aquifer Type
Quaternary	Quaternary deposits	Alluvial fans and flood plain sediments: gravel, sand, silt, mud, sandy, and silty clay soil	Quaternary intergranular aquifer (QIA)
Tertiary	Pila Spi	Cyclic deposits of claystone, marl, gypsum, and sandstone with occasional limestone	Karstic-fissured aquifer (KFA)
	Gercus	Alteration of red claystone, marl, gypsum and sandstone few limestone	Aquiclude
	Sinjar	Well-bedded limestone, dolomite, marly limestone, and sandstone	Karstic-fissured aquifer (KFA)
	Kolosh	Dark grey claystone, shale, and siltstone with rare conglomerate	Aquiclude
Late Cretaceous	Tanjero	Silty marl, siltstone, shale, sandstone, conglomerate, and sandy or silty organic detrital limestone	Tanjero aquifer (TA)
	Shiranish	Bluish-grey marl and marly limestone	Aquiclude
Lower Cretaceous	Qamchuqa	Grey massive dolomites and dolomitic limestone	Karstic-fissured aquifer (KFA)
	Kometan	White-weathered, light grey, thin-bedded, and locally silicified limestone	Kometan aquifer (KA)
	Balambo	Thin well bedded radiolarian limestone	Karstic-fissured aquifer (KFA)

and Figure 2).

According to the lithology and hydrogeologic characterization, five types of aquifers exist in the Sulaimani–Warmawa Sub-basin [19,24,30,31] (Figure 3). Detailed hydrogeological characteristics are available in

Table 2. Hydrogeological characteristics of the aquifers in the study area [24,32,33,34,35].

Aquifer/Rock Unit	Quaternary	Tanjero	Kometan	Qamchuqa	Pila Spi
Aquifer Type	QIA	TA	KA	KFA	KFA
Depth of the aquifer (m)	0–60	0–210	20–155	155–400	10–150
Thickness of the aquifer, (m)	5–50	60–240	100–125	40–80	30–100
Specific Capacity (m2/s)	4 × 10−4	4 × 10−5	3 × 10−3	1.5 × 10−4	2.5 × 10−4
Transmissivity (m2/s)	2.5 × 10−4	8.4 × 10−5	1.9 × 10−3	1.4 × 10−4	1.4 × 10−4
Hydraulic Conductivity (m/s)	8.2 × 10−6	1.1 × 10−6	2.3 × 10−5	1.7 × 10−6	6.2 × 10−6
Surface extension (km2)	577	417	100	50	40
Effective Recharge (%)	14	12	31	31	27
Min. and Max. Discharge (L/s)	1.5–5	0.5–3	6–16	4–12	5–14
Type of flow	Diffuse	Diffuse	Conduit	Diffuse–Conduit	Diffuse–Conduit

. The Karstic-fissured aquifer (KFA) is represented by Balambo, Qamchuqa, Pila Spi, and Sinjar formations, which are mostly composed of limestone, dolomite, dolomitic limestone, and chalky marl. Qamchuqa aquifer is a well karstified and highly fissured, thick, semi-confined aquifer underlain by Sarmord formation (aquiclude). The Kometan aquifer (KA) is represented by the Kometan Formation, which is mostly composed of limestone [19,32]. Kometan formation is well karstified with confined to semi-confined conditions and overlain by Shiranish formation [24]. The aquifer is composed of carbonate rocks and contains a large amount of groundwater, varying in space and time.

Aquicludes are represented by Shiranish, Kolosh, and Gercus Formations [19,28].

The Tanjero Formation is the principal, widespread, and main productive aquifer in the study area in terms of quality and quantity of groundwater [19,30,34]. In addition, the Quaternary Intergranular Aquifer (QIA) is represented by Quaternary deposits, which are mostly composed of sand, silt, rock fragments, pebbly, sandy, silty, and clayey, and cover large parts of the study area [19,30,36].

General direction of groundwater flow is from the mountains in the north and northwest towards Sulaimani and the Tanjero River (Figure 3), which later drains into Darbandikhan Lake in the southeast [19].

3. Material and Methods

In this study, a total of 25 groundwater samples were analyzed to improve understanding of groundwater dynamics in the region. Ten groundwater samples were collected from existing boreholes during fieldwork conducted in April 2022, with the objective of filling spatial data gaps identified in a previous study by [19]. The field parameters, including pH, electrical conductivity (EC), and temperature, were measured in the field after pumping several well volumes. The water wells were pumped and on-site measurements of pH, EC, and temperature were taken using portable meters. Wells were purged to remove stagnant water, and samples were collected using a flow-through cell to prevent contamination. Values of pH, EC, and temperature were determined with calibrated instruments, and data accuracy was ensured through calibration and validation with laboratory standards. The groundwater samples have been taken for the determination of main cations, anions, and trace elements, as well as for the stable isotope (²H and ¹⁸O) analyses. Collected samples for cation and trace element analyses were filtered in the field by the 0.45 mm Millipore filters, acidified using Ultrapure HNO₃, and stored in 50 mL HDPE bottles. Cation and trace element measurements were conducted in the analytical facilities of Charles University in Prague by using the ICP-MS technique. The analytical error of each solution analysis was less than 2%. The anion concentrations were measured using HPLC and Dionex ICS, San Jose, CA, USA. The ferrous iron was determined by titration with potassium dichromate. Alkalinity values were obtained through HCl titration using the Gran plot to determine the endpoint. The QA/QC was carried out according to established protocols. About 10% of samples were examined twice, and the obtained results were consistent. The newly collected data were merged with some samples from the previous study of [19] and used for interpretation.

The measurements of ²H and ¹⁸O isotopes were conducted at the Czech Geological Survey in Prague, Czech Republic using a LWIA 3000 laser analyser, Los Gatos Research (San Jose, CA, USA). The accuracy of the analysis was 0.4‰ for δ²H and 0.12‰ for δ¹⁸O. The data were standardized by the international standard (V-SMOW) and presented using standard δ-notation [20]. Also, in this case, stable isotope data of 3 samples were merged with data from the previous study by [18] and used for interpretation.

In addition, 13 groundwater samples were taken for tritium analysis in March 2022, plus one rainfall sample (composite sample of several rains in a single day) and one sample of surface water from the Tanjero River inside Sulaimani City. The ³H analyses were performed in the Mediterranean Environment and Modelling of Agro-Hydrosystems (EMMAH) of the University of Avignon in France. A Quantulus 1220 ultra-low liquid scintillation spectrometer, Wallac Oy, Turku, Finland. was used to measure the amount of tritium present in the groundwater samples after electrolytic enrichment by low-level liquid scintillation counting with the detection level in the order 1TU.

The following equation was used for calculation of groundwater apparent age [21],

t = −17.93 ln a_t³H/a₀³H

(1)

where a_t is the measured tritium activity in a sample and a₀ is its initial activity.

The piston flow tritium model TracerLPM [35] was adapted for the study area by applying Equation (2):

C_s(t) = C₀ e ^−ƛt

(2)

where

C_s(t): concentration of tritium at time t at which the analysis was performed [TU];

C₀: initial concentration [TU];

λ: tritium decay constant [t − 1];

T: period of time between recharge and analysis [t].

The piston flow model assumes that the recharged water shows only limited mixing and preserves its isotopic signature [37]. The model relies on restrictive assumptions that tracer transport occurs mostly by advection and that no/or limited mixing takes place [38]. A thorough evaluation of their impacts on piston flow age estimates is limited by the deficiency of data and the lack of supporting evidence, such as detailed solute transport modelling. However, the tracer-derived piston flow ages are reasonable for limited available data.

The programme Statistica 13.3 was used to analyze the data and create graphs. The USGS GW_Chart Calibration Plots were used to produce the Piper Diagram, and ArcGIS. 10.7.1 was used to create the maps.

4. Results

4.1. Principal Ions and Trace Elements

In general, the Ca-HCO₃ groundwater type is transformed into the Mg-Ca-HCO₃ groundwater type with increasing EC along the flow path as observed in the Piper diagram (Figure 4), but there are some outliers with very high Na, SO₄, and Cl concentrations. The parameters of water chemistry are given in

Table 3. Water chemistry data of samples collected in April 2022.

Parameter	Unit	dL	WS06	WS08	WS09	WS10	WS12	WS15	WS18	WS21	WS25	WS26
Well depth	m		53	170	24	85	171	130	71	125	66	108
Hydraulic head	m		750.6	579.3	580.5	588	608	533.3	720	823.7	512.2	712.7
pH			7.52	8.02	7.29	7.85	7.89	7.67	7.88	7.37	7.46	7.53
Temperature	C		22.8	22	23	19.7	24.7	17.6	23	24	19	23.5
EC			781	706	827	660	422	555	983	974	1123	1217
Alkalinity as HCO3	mg/L		342	377	334	290	204	325	418	314	345	307
sp.0.05M HCl			2.24	2.47	2.19	1.9	1.34	2.13	2.74	2.06	2.26	2.01
Ca		<0.001	109	83.3	94.5	67.2	52.2	56.5	104	130	159	174
K		<0.05	0.74	2.62	3.66	0.81	0.39	1.02	0.87	0.46	0.86	1.79
Mg		<0.001	19.6	28.7	19.1	19.5	12.6	11.6	39.9	23.3	20.3	30.9
Na		<0.05	10.8	6.76	45.8	28.0	4.34	38.3	42.1	16.8	9.19	10.5
Cl		<0.1	40.3	11.5	64.7	22.9	5.36	8.93	51.4	35.4	54.1	66.8
SO4		<0.2	44.6	27.7	73.8	34.9	15.7	14.0	81.7	184	53.0	161
NO3		<0.2	28.5	23.9	1.63	37.0	27.9	1.79	23.1	35.4	160	207
F		<0.2	0.57	0.37	<1	0.63	0.33	0.63	<1	<1	0.39	<1
Al		<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05	<0.05
PO4		<0.4	<0.8	<0.8	<2	<0.8	<0.4	<0.8	<2	<2	<0.8	<2
Br		<0.2	<0.4	<0.4	<1	<0.4	<0.2	<0.4	<1	<1	<0.4	<1
Mn		<0.001	<0.001	<0.001	0.002	<0.001	<0.001	<0.001	0.005	<0.001	0.005	<0.001
Fe		<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005	<0.005
Li	Y (μg/L)		5.34	7.39	4.80	6.80	3.89	13.20	8.26	4.69	9.06	6.85
Cr			4.26	7.29	2.29	2.32	2.41	1.86	3.31	2.89	3.64	4.55
Mn			1.08	2.39	3.47	0.91	0.39	13.39	4.43	2.02	6.22	2.04
As			0.65	0.47	1.03	0.46	0.41	0.48	0.56	0.43	0.69	0.35
Co			0.33	0.15	0.50	0.11	0.10	0.27	0.22	0.25	0.39	0.27
Ni			2.86	1.27	3.01	0.83	1.71	1.12	2.91	1.56	3.37	2.88
Cu			1.92	0.69	4.11	4.95	17.45	2.09	0.83	0.89	19.57	0.89
Zn			17.69	62.48	70.02	372.73	120.41	16.24	215.42	15.80	57.35	15.13
Rb			0.27	0.46	0.87	0.54	0.46	1.43	1.77	0.57	0.40	1.08
Sr			763.47	931.89	788.72	1118.18	617.27	3447.96	1727.37	1643.19	1034.42	839.40
Cd			0.06	0.06	0.06	0.06	0.05	0.06	0.06	0.05	0.07	0.04
Ba			72.37	100.67	112.99	52.03	32.47	654.52	78.15	74.71	142.41	78.58
Ce			0.01	0.12	0.01	0.04	0.02	0.12	0.01	0.01	0.10	0.02
Pb			0.88	0.55	0.81	0.63	0.90	2.10	0.11	0.26	1.75	0.28
U			0.84	1.07	0.70	1.32	0.73	0.22	1.98	1.27	1.93	1.18
Sc			3.79	3.72	3.44	3.46	3.24	3.02	3.99	3.52	3.66	3.60
Y			0.02	0.08	0.02	0.02	0.01	0.09	0.03	0.03	0.12	0.02
Sn			0.03	0.07	0.05	0.04	0.03	0.04	0.03	0.04	0.24	0.03
Sb			4.40	1.10	6.97	5.95	2.98	1.04	1.15	12.11	1.80	1.51
Cs			0.00	0.01	0.01	0.02	0.01	0.06	0.02	0.03	0.01	0.01
La			0.01	0.08	0.01	0.02	0.01	0.09	0.01	0.01	0.06	0.02
Th			0.00	0.01	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.00

. The pH values range from 7.29 (WS09) to 8.02 (WS08) and the average pH value is 7.64. Values of electrical conductivity range from 422 to 1217 μS/cm, with an average of 824.8 μS/cm. The average value of Ca is 103 mg/L with concentrations from 52.2 to 174 mg/L. Concentrations of Mg are from 11.6 to 39.9 mg/L, averaging 22.6 mg/L. Concentrations of Na are from 6.76 to 45.8 mg/L, with an average of 21.3 mg/L, and concentrations of K are from 0.39 to 3.66 mg/L, with an average of 1.32 mg/L. Concentrations of HCO₃ are from 204 to 418 mg/L with an average of 325 mg/L. Concentrations of SO₄ are from 14 to 184 mg/L, averaging 69.1 mg/L. Concentrations of Cl are from 5.36 to 66.8 mg/L, averaging 36.1 mg/L. Concentrations of Sr are up to 1.72 mg/L, as expected for the groundwater chemistry dominated by the dissolution of carbonates. The principal geogenic contaminant is Ba, with a concentration of up to 0.65 mg/L, previously identified by [18], but concentrations of other geogenic contaminants (e.g., As and F) are low. These results are comparable with the study of [19], but several samples collected in the study have significant concentrations of Na, SO₄, and Cl.

4.2. Stable Isotopes

The groundwater samples from the Sulaimani–Warmawa Sub-basin for stable isotopic compositions is summarized in

Table 4. Stable isotopes in groundwater samples from the Sulaimani–Warmawa Sub-basin, Kurdistan Region, Iraq, sampling in April 2021.

Samples	δ2H (‰)	δ18O (‰)	Deuterium Excess
WS06	−26.2	−5.4	17.1
WS08	−24.0	−4.9	15.6
WS09	−24.4	−4.6	12.6
WS10	−27.0	−5.0	13.4
WS12	−27.5	−5.1	12.9
WS16	−38.2	−6.7	15.2
WS18	−30.2	−5.5	13.6
WS21	−30.7	−5.4	12.6
WS25	−24.0	−4.5	11.6
WS26	−27.4	−5.6	17.7

and Figure 5. Several factors such as air temperature and humidity, geographic location (longitude and latitude), altitude, and distance from the ocean are influencing the isotopic composition of local rainwater [39,40,41]. Groundwater samples from the study area strongly differ from the global meteoric water line (GMWL) (Figure 5). It has been interpreted in detailed atmospheric studies as the mixing of precipitation from the Eastern Mediterranean, dominating in Spring, and the Persian Gulf, with a possible later contribution of vapour from the highly evaporated Caspian Sea [19,42]. This interpretation is based on that the local groundwater line (GWL) for this study (δ²H (‰) = 5.1245*δ¹⁸O + 0.1265‰) is not very close to the Mediterranean Meteoric Water Line (δ²H‰ = 8 × δ¹⁸O + 22‰) [43].

The isotopic composition ranges from −6.7‰ to −4.5‰ for δ¹⁸O and from −38.2‰ to −24.0‰ for δ²H in the samples collected around Sulaimani City and the surrounding area. The most positive δ¹⁸O value in our dataset is −4.5‰ (Table 4). A previous study reported a more negative value (δ¹⁸O = −7.0‰) for a sample collected at the highest altitude (1224 m a.s.l.) in the recharge area, while the most positive value (δ¹⁸O = −5.5‰) was found close to Sulaimani [19]. The groundwater line (GWL) for this study is δ²H (‰) = 5.1245*δ¹⁸O + 0.1265‰ (Figure 5). This groundwater line also has a lower slope and intercept than the global meteoric water line (GMWL) (Figure 5) and a lower slope than the local groundwater line (LGWL) determined in the previous study, i.e., 5.124 compared to 6.605 [19]. The local GWL for the groundwater samples in the study area also has a lower slope and intercept than the meteoric water lines (LMWLs) for winter precipitation in Zagros, slope 6.67 [42], and yearly LMWL for Zagros with a slope of 6.80 [44]. It might indicate some evaporation impact during infiltration. In relation to the LMWL created by [45] with regression equation δ²H‰ = 7.9*δ¹⁸O + 20, in the north of the study area, it can be concluded that precipitation that recharges the aquifers in the study area originates from different vapour masses.

The stable isotopes serve as a spatio-temporal tracer, reflecting both relief and time changes [46]. In this regard, the altitude effect was evaluated using the local isotopic gradient (LIG). This was based on the relation between the elevation of the water samples and δ¹⁸O and δ²H (Figure 6). The relation shows depletion of δ¹⁸O and δ²H in groundwater with altitude, with gradients of −0.29‰ and −1.5‰ per 100 m, respectively, but R² values are relatively low. The most depleted samples are SA24 and SA03 reflecting the higher altitudes and winter recharge. In contrast, WS25 and WS09 are the least depleted groundwater samples reflecting probably a lower altitude and a warmer period of recharge.

Deuterium excess values in this study range from 11.6 to 17.7, suggesting infiltration of precipitation under relatively more humid conditions in winter months. They are smaller compared to the previous study values from the same region where they were in the range from 13.9 to 22.1, suggesting more contribution of the water vapour from the Mediterranean region [19].

4.3. Tritium

The results of the tritium analysis for groundwater samples, one rainfall sample and one water sample from Tanjero River in Sulaimani City, with calculated minimum groundwater ages are shown in

Table 5. Tritium analysis results and calculated minimum age of groundwater using 7.3 TU in rain as initial concentration; N/A—not available.

No.	Samples	pH	EC_µS/cm	Activity TU	Minimum Age (Years)	Aquifer Type
1	WS01	7.5	350	3.5	13.1	Tanjero Aquifer
2	WS03	6.3	560	4.1	10.2	Tanjero Aquifer
3	WS04	8	550	3.9	11.1	Tanjero Aquifer
4	WS05	7.8	340	2.6	18.3	Tanjero Aquifer
5	WS12	8.3	350	3.2	14.7	Quaternary Intergranular Aquifer
6	WS13	7.9	440	2.7	17.7	Karstic-Fissured Aquifer
7	WS14	8.4	2980	≤0.8	40.5	Quaternary Intergranular Aquifer
8	WS15	7.5	370	3.2	14.7	Quaternary Intergranular Aquifer
9	WS18	7.8	990	3.4	13.6	Tanjero Aquifer
10	WS19	7.4	650	4.3	9.4	Tanjero Aquifer
11	WS22	7.4	890	4.9	7.0	Tanjero Aquifer
12	WS23	6.03	490	1.7	25.9	Tanjero Aquifer
13	WS24	8.5	870	0.7	41.7	Aquitard
14	Rainfall	N/A	N/A	7.3	0	N/A
15	Tanjero River	5.4	540	4.4	9.0	N/A

and in Figure 7.

The tritium value of the rain sample in Sulaimani City was 7.3 TU, the tritium for the Tanjero River sample in Sulaimani City was 4.4 TU, and the mean values of the groundwater samples were 3.2 TU for the Quaternary Intergranular Aquifer, 3.55 TU for the Tanjero aquifer, and 2.7 and 0.7 TU for the Karstic-Fissured Aquifer and Aquiclude, respectively, Table 5. The tritium concentration in the water well samples was within less than 0.8 to 4.9 TU. In general, there were higher values close to the catchment boundaries (except for wells WS23 and WS24) and lower values in the lowlands.

The tritium value of the rain sample in Sulaimani City (7.3 TU) was not used as input value for the piston flow model. Instead, a value of 6.5TU (with an average error of 0.9 TU) was estimated by modelling based on IAEA-NUCLEUS isoscape modelling [47].

5. Discussion

The flow system at Sulaimani is a gravity flow system with most recharge in the mountains around the basin with altitudes more than 1200 m [19]. The principal process responsible for the formation of Ca-HCO₃ groundwater chemistry is the dissolution of carbonates, as indicated by increasing δ¹³C(DIC) values and ⁸⁷Sr/⁸⁶Sr ratio values in the direction of flow [18]. However, there are several “pockets” of groundwater with high Na, SO₄, and Cl concentrations, probably caused by limited flushing and the presence of stagnating groundwater.

The new stable isotope data are more variable than the data based on the [19] study, and their lower slope (5.124 compared to 6.605) may indicate some evaporation during recharge.

Motzer [48] suggested that the tritium values in groundwater below 0.8 TU indicate old/sub-modern groundwater (originating before 1950).

In reference to this, only highly mineralized samples WS14 and WS24 show old age originating before 1950, and the others are a mix of modern and old groundwaters. Sample WS14 is from the Quaternary intergranular aquifer, and WS24 is from the aquitard. Tritium concentrations remain almost constant or decrease slightly with depth down to 125 m (4.9 TU) and then decrease with the depth until they reach the value of 1.7 TU at a depth of 450 m (Figure 7). There are several possible ages of groundwater for most of the groundwater samples, from around 10 years up to at least 40 years old. Groundwater samples that are classified as modern are relatively shallow groundwaters (WS03, WS04, WS15, WS18, and WS22), and they are from an unconfined aquifer. On the other hand, samples from higher depths show older ages, and they are supposed to be from confined aquifers, similar to the finding of [49]. The presence of higher tritium concentration at shallower depths suggests that the groundwater was recharged by recent precipitation compared to the samples which show the mixing with the older groundwater in the deeper parts of the flow system [50].

Groundwater samples classified as modern are relatively shallow (Figure 8). Samples of modern groundwater ages are in proximity to the outcrop area and in the shallowest part of the aquifer system. For groundwater samples with mixed ages, most of the sampling sites are relatively shallow and located near the outcrop area. The two mixed samples that are farthest from the outcrop are from relatively shallow wells. Some wells near the outcrop are classified as pre-modern, but groundwater ages can be pre-modern near the outcrop where the aquifer is confined.

The older water sample (WS14), with the minimum tritium concentration of ≤0.8 TU, is characterized by very high mineralization of Na-SO₄-Cl groundwater type, which can be interpreted as a consequence of limited flushing in a stagnating zone of relatively low permeability (Figure 9). Based on the observed good fit for the piston model, it can be assumed that the recharged water is transported mostly by advection with only a limited impact of dispersion [44,51].

To evaluate temporal variation in tritium values, previous studies on tritium value ranges within the studied basin have been addressed. Hamamin and Ali [52] examined tritium values in the groundwater of the Basara basin, which is located west of the study area. Their findings indicate that tritium concentrations in spring and well samples during 2013 ranged from 5.5 to 7.0 TU, which is more than the tritium values in the current study. Another study was conducted by [53] in the Halabja Basin southeast of the study area in 2018, and it was found that the tritium concentration in the spring and the well samples ranged from 2.28 to 4.28 TU, and there was 4.8 TU in precipitation. It is noticeable that the temporal trend of tritium concentration in the groundwater of the area is decreasing with time, which is also revealed globally in numerous studies [54,55].

Velocities calculated on the basis of tritium data and residence times were selected on profiles compared with velocities calculated on the basis of the hydraulic data in Table 2 and typical carbonate porosities reported in [56]. All velocities based on the hydraulic data were lower compared to the velocities based on the tritium data, i.e., on profile WS01-WS23 64 m/y compared to 390 m/y, on profile WS22-WS18 160 m/y compared to 333 m/y, and on profile WS19-WS12 50 m/y compared to 1600 m/y. A possible explanation may lie in the presence of preferential flow zones within the karstified rocks, and in the fact that the hydraulic data in Table 2 are derived from pumping tests, which do not necessarily capture the heterogeneous nature of the aquifer [56,57]. For this reason, we consider velocities based on tritium data more reliable. Maximum difference is on the profile WS19-WS12 in the axis of the valley where a high degree of karstification can be expected.

Tritium and δ¹⁸O values show only a limited positive correlation in different groundwaters (Figure 10). Enriched waters in terms of δ¹⁸O correspond to modern waters with higher concentrations of tritium. The outcropping of the aquifer in some parts of the study area [29,30] may lead to some evaporation of recharged water and its isotopic enrichment [58]. The water samples can be divided into two major groups, Group I: recharged by recent infiltrations of precipitation and group II: recharged in the past. Most measured samples belong to group I, except samples WS14 and WS24 which belong to group II. Sample WS24 is from aquitard and reflects older recharge in limited flow zones [30].

6. Conclusions

This study provides new insights into groundwater recharge mechanisms, flow dynamics, and vulnerability of groundwater in the Sulaimani–Warmawa sub-basin. Hydrochemical and isotopic data demonstrate that recharge predominantly occurs in high-elevation karstified zones, with groundwater evolving from the Ca–HCO₃ type to the Mg–Ca–HCO₃ type along the flow path as a result of progressive water–rock interaction. The observed altitude-related depletion in stable isotopes and low-slope groundwater line confirm winter-dominated recharge under semi-arid climatic conditions, with limited influence from evaporation.

Tritium concentrations distinguish recently recharged groundwater from older more mineralized groundwater stored in lower-permeability zones. The good fit for the piston flow model results, based on tritium concentrations, indicate limited mixing and preferential flow through heterogeneous karstic formation. Importantly, residence times are relatively short (less than a decade in recharge-proximal areas), highlighting rapid flow pathways and elevated vulnerability of drinking-water sources around Sulaimani to surface-derived contamination.

These findings emphasize the need for active groundwater protection strategies focused on recharge-zone land-use control, nitrate monitoring, and managed groundwater abstraction. Future work should include multilevel sampling and establishment of a regional tritium precipitation record to refine age estimates and improve understanding of vertical flow components. Long-term isotope monitoring is recommended to detect hydrochemical shifts associated with climate variability and increasing anthropogenic pressures.