Stable isotope composition in precipitation and groundwater of Shwan Sub-Basin, Kirkuk governorate, northeast of Iraq

Isotopic techniques are often successfully used in applications of hydrological studies. Investigating stable isotopes, particularly δ²H and δ¹⁸O in precipitation, is valuable for hydrological, climatological, and meteorological applications (Harvey & Welker 2000). Isotopic hydrology can provide such information because the spatial and temporal variations in the stable isotope values in precipitation result from the isotopic fractionation occurring during the evaporation of seawater and condensation during the advection of water vapor (Dansgraard 1964; Ichiyanagi et al. 2007). Therefore, the isotopic composition in the local precipitation is primarily controlled by regional-scale processes. It is mainly affected by the origin of wet air masses, the trajectories of the moisture transport over the lands, their possible partial condensation in the continental areas, and mostly the average rainout history of the air masses (Dostika et al. 2010). Many causes and processes influence the fractionations of stable isotopes in precipitation. These are represented by moisture conditions from source areas, moisture transport paths, precipitation histories, weather conditions that lead to precipitation, and sub-cloud processes (Guan et al. 2013). When measured in precipitation over a short term, the isotopic composition is very changeable (monthly, or even daily and event-based) (Guan et al. 2013).

Stable isotopes have been used recently to manage groundwater in semi-arid regions when there is a lack of hydrogeological information required for this purpose. Furthermore, the possible challenges to simulate underground processes with standard techniques have accelerated the trend toward their use (Jassas & Merkel 2015). Besides, the study of stable isotopes in water has been used to investigate the origin of groundwater and the interaction between surface and groundwater in semi-arid and arid regions (Jassas & Merkel 2015; Boschetti et al. 2018; Dar et al. 2021; Krishan et al. 2022). Accordingly, the isotopic composition of groundwater persists until it mixes with entirely different water in isotopic content (Sami 1992). As a result, evaporation will be the most influential factor causing differences in isotopic composition in precipitation and groundwater (Boronina et al. 2005). The increase in salinity in groundwater is caused by evaporation enrichment and salt dissolution. The dissolved salts can be obtained from aquifer salts or surface soil salts that have dissolved and leached down with the recharging water (Krishan et al. 2022).

In the last decades, many studies using stable environmental isotopes and solute concentrations in water have been used to investigate the origin of groundwater and the interaction between surface and groundwater in semi-arid and arid regions (Jassas & Merkel 2015; Dar et al. 2021; Krishan et al. 2022). Stable isotopes in precipitation, surface water, and groundwater are all-powerful tools for studying the behavior, origin and distribution pattern of recharge. Shwan Sub-Basin in northern Iraq is regarded as one of the sub-basins of the foothill zone large aquifer system in northern Iraq, with good quality and quantity of groundwater reserves. Despite its importance, there has not been another stable isotopic detailed study since the one conducted by Sahib et al. (2016). This study presents a quantitative understanding of the different factors that govern isotopic composition in precipitation, such as climatic factors and the source of moisture air masses. Besides, the groundwater isotope composition and their variations are required to establish the basic information for groundwater management and groundwater recharge mechanism in the study area. Therefore, the present study aims to examine: (1) the stable isotope composition in precipitation at the study area, (2) whether isotope composition in precipitation vary consistently concerning moisture source directions, (3) the primary factors controlling stable isotope values in precipitation, (4) the stable isotope composition in groundwater and (5) the amount and source of groundwater feeding by investigating the stable isotope concentration in groundwater.

The study area is located in Iraq's folded zone of the unstable shelf and part of the foothill zone. Since the emergence of the southern neo-Tethys in the Late Jurassic, the unstable shelf has been the most subsiding region of the Arabian Plate (Buday & Jassim 1987). The Miocene-Pliocene continental collision and the Late Cretaceous ophiolite obduction onto the northeast border of the Arabian Plate caused the most subsidence. The structural trends and facies changes on the unstable shelf are thus parallel to the Zagros-Taurus suture belts. The unit's surface folds are a distinguishing feature (Jassem & Goff 2006). The age of Outcrops Formations ranges from Miocene to Holocene (Al-Naqib 1959). The Kany Domalan Mountain series, a part of the Baba Dome series, forms the southwest edge of the area represented by the Kany Domalan Mountain series.

The Bai-Hassan Formation (confined aquifer, unconfined aquifer) is a prolific hydrogeological unit in the studied area, consisting of alternating sandstone and gravel layers with clay and conglomerate layers (Al-Tamimi & Al-Shwani 2019; Al-Hayali et al. 2020). All wells in the basin partially penetrate the Bai-Hassan formation. The study area's typical groundwater flow direction is from east to southeast to northwest (Figure 2), and the hydraulic gradient’ (I) average is 0.0138. The number of renewable reservoirs from the annual rainfall in the area to the groundwater was 57.20 × 10⁶ m³/year, the persistent storage was 1,044.5 × 10⁶ m^3, and the amount of water consumed in the shallow wells in the region for general purposes and irrigation was 5.62 × 10⁶ m³/year (Al-Tamimi & Al-Shwani 2019).

Altogether, four rainwater samples were sampled from monthly precipitation at Shwan Sub-Basin for the 2020–2021 hydrological year. The rainwater sampling was during December 2020 and January, February, and March 2021. Thirty-two groundwater samples and one surface water sample (LZR) were collected from Shwan sub-Basin for two seasons (Figure 3). The first season was in November 2020, and the second season was in May 2021. All the samples were immediately stored at 4 °C before being sent to the laboratory for stable isotope analysis (δ²H and δ¹⁸O).

Stable isotope analysis was performed at the Institute for Applied Geosciences, Technische Universität Darmstadt, Germany. Stable water isotopes were analyzed using infrared spectra based on wavelength-scanned CRDS (cavity ring-down spectroscopy) L 2130-i – Picarro Inc., Santa Clara, CA, USA.

Three sequential injections of each sample were measured, and raw data were corrected for sample-to-sample memory. The reported value is the mean value. The data sets were corrected for instrumental drift during the run and normalized to the international standards VSMOW/SLAP2 were measured in each run. External reproducibility based on repeated analyses of a control sample (CS) was better than 0.4‰ and 1.6‰ for δ²H and δ¹⁸O (Van Geldern & Barth 2012). Meteorological data used in this study were obtained from the Iraqi Meteorological Organisation and Seismology Kirkuk Branch of Baghdad. The pan method was dependent on measuring the evaporation.

Given the quality, it can be employed as a baseline after being calculated as a local meteoric water line (LMWL). Due to kinetic fractionation, the isotope ratios fluctuate between places across the research area. Initially, Craig assumed that isotopic enrichments concerning ocean water have a linear relationship across the entire range for waters that haven't been exposed to severe evaporation.

The diagnosing of the origin of air moisture was performed using the back-trajectory analysis that is assumed to approximate the moisture source direction for the four precipitation events at Shwan sub-Basin. The back-trajectories were obtained using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) (Stein et al. 2015). This model has been used in numerous stable isotope studies to track precipitation moisture sources (Crawford et al. 2013; Soderberg et al. 2013; Juhlke et al. 2019). HYSPLIT is the online version (http://ready.arl.noaa.gov/HYSPLIT.php) of the model called READY (Real-Time Environmental Application and Display System). HYSPLIT was initially developed by the NOAA (US National Oceanic and Atmospheric Administration) Air Resources Laboratory in 1982, and this has been dramatically improved since then. The HYSPLIT model can compute simple air parcel trajectories (backward and forwards) and complex simulations involving dispersion and deposition (Stein et al. 2015; NOAA 2021). The NCEP (National Center for Environmental Prediction) and GDAS (Global Data Assimilation System) model data were used as the meteorological data to perform the HYSPLIT model, obtained via the online website included in the HYSPLIT link. The 120-hour backward trajectory analysis was performed for each precipitation event at the study site at two levels above ground (500 m, 1,000 m). Simultaneously, assuming that these heights represent the planetary boundary layer (PBL) from which the rain clouds develop.

Generally, the 10-day backtrace period is the average residence time of moisture residing over the target area in the atmosphere (Trenberth 1999). At two elevation levels of 500 and 1,000 m AGL, the continental and marine effects are expected by the time the air mass is spent over land and the sea, respectively, before entering the study region (using back trajectories derived from the HYSPLIT, see above).

The amount of precipitation varied from 3.56 mm to 17.02 mm during the precipitation events. The mean annual precipitation for the hydrological sampling year was 100.62 mm. after comparing this mean annual precipitation value with the mean annual precipitation for the last four decades (325.43 mm). we infer the sampling year was dry (Figure 3). Measured values of the precipitation isotopes δ²H and δ¹⁸O in this study are highly variable, ranging from −83.38 ‰ to 38.34 ‰ and from −11.24 ‰ to 4.37 ‰ for δ²H and δ¹⁸O, respectively. The results also show that the most precipitation depleted in isotopic composition was in the months of December and February.

Locally, d-excess in precipitation of the study area ranges from 3.3679 ‰ to 21.2285‰, with a mean of 11.16‰ (Table 1). The highest value of d-excess was detected on 23 March 2021, while the lowest d-excess value was on 20 January 2021(Table 1 & Figure 5). The sub-cloud processes of moisture exchange and raindrop re-evaporation comprise most of the small-scale spatial variability in precipitation isotopes (Guan et al. 2009). Sub-cloud evaporation and moisture exchange supported the lowering of d-excess in residual rainwater (Figure 5).

The LMWL of Shwan Sub-Basin was plotted along with the GMWL (GMWL) and East Mediterranean water line (EMWL). The LMWL (Figure 6) had empirical formula δ²H=8.114 δ¹⁸O+11.53. The LMWL is relatively close to the GMWL and the EMWL with the empirical formula of δ²H=7.4 δ¹⁸O+14.5 (IAEA 2005).

Moreover, LMWL compared with the Iraqi meteoric water line (IMWL) with the empirical formula δ²H=7.5 δ¹⁸O+13.8 (Ali et al. 2015) and Sulaimaniya meteoric water line (SMWL) (Hamamin & Ali 2013) with an empirical formula δ²H=7.7 δ¹⁸O+14.4, here the isotope results also go with these two meteorological lines (GMWL and EMWL), as well. The high d-excess in precipitation is typical of the EMWL. However, the LMWL is on the right-hand side of the EMWL. Furthermore, the d-excess value can vary locally due to the air mass source regions (Jassas & Merkel 2015). Furthermore, LMWL compared with the IMWL with the empirical formula δ²H=7.5 δ¹⁸O+13.8 (Ali et al. 2015) and SMWL (Hamamin & Ali 2013) with an empirical formula δ²H=7.7 δ¹⁸O+14.4, here the isotope results also go with these two meteorological lines (GMWL and EMWL), as well. The high d-excess in precipitation is typical of the EMWL. However, the LMWL is on the right-hand side of the EMWL. Furthermore, the d-excess value can vary locally due to the air mass source regions corrected to (Jassas & Merkel 2015).

By sub-cloud evaporation and moisture exchange. Moisture exchange may cause an increase in such (Clark & Fritz 1997). The continental effect shows itself as a decrease in the isotopic composition of cloud water, is one example of such an influence (and the resulting rain). As shown in Figure 7, the isotopic composition of precipitation between December 2020 and February 2021 had the most depleted value for this investigation, prioritizing the continental influence (a & c). Before arriving in the research region, the moisture in the air mass traveled a long continental trip. In addition, there were multiple sources of moisture air masses for the December precipitation event (Figure 7(a)), with moisture air masses coming from the continental tropics, Mediterranean Sea, and Arabian Gulf. For February, the sources vary from the Red Sea to the Arabian Gulf. Whereas the precipitation events with the highest values of isotopes composition (January 2021 and March 2021). For these months, the moisture sources were directly from the Mediterranean Sea, and the continental path was relatively not that long (Figure 7(b) and 7(d)). Here we can estimate the increase of the oceanic effluence, then continental. The surface RH was generally between 67 and 83 percent (Table 1, Figure 5). While the RH in the sub-clouds ranged from 83 to 94 percent, the moisture source for precipitation in occurrences (Figure 7) tends to be variable, originating from several sources. The surface RH was generally between 67 and 83 percent (Table 1, Figure 5). Simultaneously, RH in the sub-clouds ranged from 83 to 94 percent of the moisture source for precipitation in these instances (Figure 7). D-excess in rainwater is decreased by sub-cloud evaporation and moisture exchange. Moisture exchange may cause an increase in d-excess in the resultant precipitation when the d-excess in the sub-cloud layer is greater than that of raindrops.

During the first season, the stable isotope composition in groundwater samples ranged from −33.16‰ to −21.06‰ with a mean value of −24.43‰ and from −5.85‰ to −4.01‰ with a mean value of −4.64‰ for δ²H and δ¹⁸O, respectively. In the second season, the ranges for δ²H and δ¹⁸O are −30.38‰ to −21.63‰ with a mean value of −24.66‰ and −5.63‰ to −4.02‰ with a mean value of −4.59‰, respectively (Table 2). The δ²H and δ¹⁸O plots (Figure 6) show close results of isotopic composition in groundwater samples, most of the groundwater samples within the exact location in the plot.

The stable isotope concentrations in groundwater may define the mixing of groundwater from different recharge origins, such as precipitation and direct infiltration of surface water (Herczeg et al. 1997). The δ²H and δ¹⁸O values in the groundwater and the surface water (values measured in LZR water samples from the present study and from a previous study by Sahib et al. 2016 see Table 2). The stable isotope in groundwater of the present study lies between EMWL and GMWL and is close to the LMWL see Figure 6(a) and 6(b) and Table 2, indicating the direct hydrologic link among the three types of water systems (rainwater, LZR, and groundwater). Figure 6(a) and 6(b) show the interaction between groundwater and surface water in the study area. The stable isotope values of surface water are similar to the stable isotope values of groundwater and lie within the exact location (Figures 8,⁹¹⁰–11). Consequently, the wells close to the LZR were feeding on the river during the seasons with a low amount of precipitation.

The average weighted oxygen isotopes (δ¹⁸OW.Av) in rainwater are calculated, and the results are listed in Table 3. The comparison between the results δ¹⁸OW.Av in rainwater and isotopic composition in groundwater of Shwan sub-Basin indicates that the values do not match (Table 3). This conveys that the recharge from rainwater during this hydrological sampling year is shallow. This may explain the evaporation before recharge, especially with the low rainfall. Similar results were obtained after recalculating δ¹⁸OW. Av in rainwater by excluding the low precipitation values. By applying Equation (3), the estimated recharge in Shwan Sub-Basin for the sampling year is about 9.2% of annual rainfall, equal to about 9.3 mm of total annual rainfall.

The high isotope composition (δ²H and δ¹⁸O) in precipitation of the study area gives good agreement with the high values observed in eastern tropical Africa and the Arabian Peninsula in previous studies (Leng 2006; Michelsen et al. 2015; Heydarizad et al. 2021). This indicates the climate conditions in these regions have a great influence on stable isotopes values in precipitation.

The moisture source for the precipitation events came from the Mediterranean Sea during the precipitation events with enriched isotope values, the moisture flux over the study area from the west during the precipitation events with depleted isotope values, and moisture transported from more than one source during the precipitation events with depleted isotope values.

The fluctuation in stable isotope values in the research area's groundwater is produced by the mixing of rainwater, surface water, and groundwater, as well as the variable in stable isotope content in precipitation and the effect of evaporation during the precipitation path aquifers. Groundwater isotopes results showed that the samples for two sampling seasons lying between EMWL and GMWL are very close to the LMWL, indicating that the groundwater recharge is mainly of precipitation origin from direct infiltration through the soil, on the other hand, during dry periods, an indirect recharge process from the LZR recharges groundwater in the northwest part of the study area. The latter hypothesis is supported by isotopic resemblances between stable isotope values in river water and groundwater, and the high permeability of the geological formations in the foothill zone (quaternary river terraces and slope sediments) are mainly gravel and sand sediments. The stable isotope values in groundwater with the flow direction decreased downstream and upstream due to the preference flow pattern. We infer from this study that the sampling year was dry by comparing the hydrological year 2020–2021 meteorological data to the long-term meteorological data (the previous four decades). Consequently, anticipating a low recharge rate and this assumption is supported by low groundwater recharge gained from the method of weighted oxygen isotopes. Such findings could be of importance for future studies that are going to use stable isotopes in water-related sciences and climatical studies in the nearby areas, e.g. studying the surface, groundwater and comprising processes mainly for recharge and developing more reliable water management strategies. However, this study was done for one hydrological year only and it would be ideal to investigate such data over periods of at least two years and obtain much precise implications.

The authors gratefully acknowledge the hydrogeology Lab staff in the Institute for Applied Geosciences, Technische Universität Darmstadt, Germany, for their help in performing the analysis of stable isotopes. The authors also thank the NOAA Air Resources Laboratory (ARL) for providing the HYSPLIT transport and dispersion model and the READY website (https://www.ready.noaa.gov) used in this publication.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.