Introduction

Desert lakes are a vital component of the arid environment serving as a crucial surface water resource and an integral part of the biodiversity. They can be found in many arid regions over the world, for example the Badain Jaran desert, the Strzelecki Desert, Sahara Desert, Kumtag Desert, North America Desert, Tengger Desert etc.1,2,3,4. These lakes have fragile ecosystems that are highly susceptible to environmental changes5,6. Studying their water origins is essential for a better understanding of the hydrological cycle in arid regions. Desert lakes depend on a balance of water inputs and outflows, and their typical water sources include local precipitation, surface runoff, and groundwater4,6,7. The Rub’ Al Khali (RAK) or “Empty Quarter” extends on ~ 640000 km2, covers 22% of the Arabian Peninsula8 and is the largest continue sand sea on Earth (Fig. 1).

Fig. 1
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Study site with the sampling points location (satellite imageries were downloaded from Google Earth web version (https://earth.google.com/web/)).

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According to satellite imagery, the primary topographic feature in RAK, other than rolling sand sheets, is sand dunes, which occur in a wide variety of shapes (linear, transverse, solitary, etc.) and sizes. Some of the sand dune mountains (so-called “uruq”) in the southwestern part can achieve a height of 200–350 m and extend up to several hundred kilometers9. Most of them have formed during wetter phases of the Pleistocene, or probably even Late Pliocene10. The interdune corridors (2–6 km in width) separate the dunes and define some lakes and wetlands location10,11,12.

RAK current climate is hyper-arid and characterized by very low precipitation (averaging 54.6 mm/yr13) with actual maximum summer temperatures from 47 to 50 °C or more14,15. Rains are of the typical desert torrential type and are usually concentrated during the winter season. Very light, rare local showers can occur during summer, especially in the southeast. Rainfall throughout the interior of RAK is very meager, some areas are reported by Bedouins to have had no rainfall for 20–100 years. The rate of evaporation in the study area is near 3.5 m/y16, thus, evapotranspiration significantly prevails over atmospheric precipitation. The extremely hard arid conditions and an abundance of dunes in RAK desert make it an unlikely environment for rivers and lakes. However, despite the harsh climate, there are several natural surface types of water manifestations in RAK such as lakes, wetlands (sabkha) and widyans12,17,18,19. Some of them are permanent, some are non-permanent (temporary).

Widyans are temporary, non-perennial watercourses that flow only seasonally, in some cases after strong rains, but quite infrequently, they may flow only at intervals of years apart. Today, in the interdune area in RAK after heavy rains, no-permanent lakes may occasionally appear. They have no connection with shallow groundwater, or any other source of continuous replenishment, and dry up within 1–2 years. In the past, there were many late Pleistocene and early-middle Holocene relict lakes in the interdune zone mainly distributed along an east–west axis across the RAK center, covering a distance of some 1200 km15,20,21,22,23. According to radiocarbon dating, they existed for a few centuries during two periods: about 35000–17000 and 5000–1000 years ago10,18,20,24,25. These lakes were created by cataclysmic paleorainfall (monsoons), had no links with rivers, groundwater or any other source of continuous replenishment, and their bed sediments present no evidence of regular refilling10,19,24. Today, the migrating dunes may cover entire paleolakes, or parts of them, further hampering the mapping of any type of lacustrine facies.

In the northeastern part of RAK, the interdunal areas between the megabarchans are characterized by the presence of wet aeolian sabkhas (Figs. 1, 2d). These sabkhas are formed due to the shallow groundwater level, which is typically less than 2 m below the surface9,12,17. In this area, typical sabkhas are recharged by groundwater from a Neogene or Paleocene aquifer9,12. As a result of evaporation, the sabkha pore water composition becomes increasingly saline, leading to the precipitation of gypsum and anhydrite near the surface forming a crystals mat and desert roses. In contrast, towards the southwest, the shallow groundwater table is deeper, and only several inactive dry paleo-sabkhas, also known as “mamlahahs” or inland salt flats, are present12. They are usually underlain by sand with minor silt and clay-size fractions and are floored with salt and gypsum and in some areas with a marl with high lime content.

Fig. 2
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Photos of Umm Al Heesh area (see Fig. 1) (a) Small Lakes, (b) Large Lake; (c) Small Lake 1; (d) Sabkha plate near the lake.

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Natural permanent lakes are rare in the Kingdom of Saudi Arabia . One of the largest well-studied permanent water bodies in the Arabian Gulf region is Al Hassa lakes (al-Asfar and Al Uyoun) located between the sand dunes in al-Ahsa Governorate in the Eastern Province. The lakes formed over the years from seasonal rains and spring irrigation water in addition to treated and untreated sewage water from Al-Hafoof water treatment Plant26,27. Several studies have been conducted to investigate the environment of Al Asfar and Al Uyoun lakes, with a focus on assessing the potential for water reuse in agricultural activities in Al-Hassa Oasis. These studies have employed a range of methods, including vegetation ecology, remote sensing, hydrogeological analysis, chemical studies of surface water and soils, and geographic information systems (GIS). The goal of this research was to gain a better understanding of the lakes’ environment and to identify opportunities for sustainable water management in the region27,28,29,30,31.

RAK region is home to several permanent lakes, which are maintained by the natural discharge of groundwater from shallow aquifers. However, due to limited research in the area, there is a scarcity of published scientific information about these permanent lakes.

The Umm Al Heesh lake (“abundance of wild plants”) is one of the largest permanent lake systems consisting of several lakes and located in the interdune corridors of the Hamidan region of RAK (Figs. 1, 2). It is an oasis area in the heart of RAK desert supporting the growth of a wide variety of vegetation (Fig. 2). Umm Al Heesh is located on the East Africa West Asia bird migration flyway32 and can be a key site for supporting migratory birds to provide them with food, vital resting stopover and shelter during their long-distance journeys across arid landscapes.

Despite being mentioned in a few published papers (e.g.,15,22,33), the Umm Al Heesh lake remains poorly studied. There is a noticeable lack of comprehensive research on its hydrology, geochemistry, ecology, and other aspects. Only one published paper34 provide the lake water composition based on a single sample without any discussion regarding the water lake nature and geochemistry.

Our research is the first detailed study aimed to determine the water origin in the lake and identify the primary geochemical processes forming water salinity and quality. The detailed study of arid-region lakes in RAK and the identification of their origin help in their protection especially in the face of climate change. Moreover, arid-region lakes are important for the sustainability of desert ecosystem. Finally, interaction between surface water and groundwater is essential in the understanding of groundwater discharge, a precious resource in a changing climate word.

For the present study, fieldwork was first carried out, including both the collection of lake water, sediment samples, and groundwater samples, specifically targeting the shallow aquifer in the study area. Subsequently, a comprehensive analysis was carried out incorporating past and recent findings, while considering regional geological and hydrogeological setting along with historical satellite images spanning nearly six decades from 1967 through 2025. This multifaceted integrated approach is sought to pinpoint the exact source aquifer responsible for forming the Umm Al Heesh lake in RAK.

Study area

Geology

The Umm Al Heesh lake is located in the southeastern part of the Arabian plate near the Hamidan structure (Figs. 1, 3, 4). This part of the plate within the Kingdom of Saudi Arabia comprises the crystalline Precambrian basement (about 870 Million years B.P–550 Million years B.P), and an overlying succession of younger sedimentary rocks, which belong to the Paleozoic (540 Million years B.P–250 Million years B.P), Mesozoic (250 Million years B.P–65 Million years B.P), and Cenozoic Eras (65 Million years B.P to present) eras35,36. Thick succession of younger sedimentary rocks lies unconformably on the Precambrian basement. They mainly consist of carbonates and clastic, and to some extent of much less important evaporites. The thickness of this succession reaches up to 12 km35. Younger deposits of Quaternary age are also present, which include limestones, unconsolidated silts, sands, and gravels, sabkhas, and wadi sediments.

Fig. 3
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Geological map of south part of Arabian Peninsula (made using software ArcGIS 3.03).

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Fig. 4
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Stratigraphic cross-section (schematic) of the Hamidan area (modified after37)—blue arrows are evaporation. Location of line A-B is shown in Fig. 1.

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The shallow sedimentary rocks primarily consist of carbonates of the Aruma, Umm Er Radhuma, Rus, and Dammam Formation35,36. In addition to massive evaporite units occurrence, especially within the Rus Formation, intercalations of shales and marls are also reported within these formations. The youngest and shallowest Neogene rocks are dominantly represented by terrestrial sandstones and conglomerates attributed to the Hofuf formations (Fig. 5). All the formations, in general, are dipping gently northeast towards the Arabian Gulf. Outcrops occur in relatively small areas running from north to south along the western border of RAK area and at the southern border of Kingdom of Saudi Arabia (Fig. 3).

Fig. 5
figure 5

2-D seismic section (modified after37) with interpretation of line through the Hamidan structure.

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The Paleogene formations constitute the aquifer system in the Hamidan area. The Neogene is partly absent there, where the Khobar member of the Dammam Formation subcrops beneath a relatively thin veneer of sands over which more recent sand mountains are superimposed37 (Fig. 4).

The Hamidan structure is a structural uplift deformation that has caused the shallow horizons to move upwards forming a domal structure (a 50 Ma age crater with 16 km diameter37) (Figs. 1, 4, 5). Subsequently, post erosion has completely eroded the Dammam and part of Rus aquicludes, leaving a very thin section of Neogene that is currently covered by surface sand dunes and sabkha (Fig. 4).

There are several wells in and near the Hamidan structure including a deep exploration well that penetrates Cambrian strata37. No igneous material, salt intrusion or any other stratigraphic anomalies were recorded in the pre-Cretaceous section of the drilled deep well. However, the lithology and uncertain paleontology of the deep well within the Paleogene section is indeterminate37, while the deeper intervals are well stratigraphically defined. The lithology and paleontology outside the disturbed crater zone are regular (regionally consistent in character and depth) indicating shallow marine to evaporitic Neogene and Paleocene formations37. Based on seismic and deep borehole data, the inward facing extensional faults bound the Hamidan structure and penetrate into Paleozoic strata (Fig. 5).

Neville et al.37 consider that Hamidan is a meteorite impact structure, in which the surrounding locally denser strata has been relatively lithified due to enhanced hydrothermal activity. Note that the hypothesis of impact origin is unproved and require more direct geological evidence in the future such as presence of high-pressure minerals (e.g. coesite), raised rim, enrichment of iridium, etc.

Hydrogeological settings

Groundwater in RAK is primarily contained in limestone, dolomite and sandstone aquifer formations, which are separated by confining shale layers. These aquifers include Paleozoic sandstone, limestone and dolomite aquifers, Mesozoic sandstones and Cenozoic (Paleocene and Neogene) limestone and dolomites34,38,39. In the Hamidan area, the Paleocene aquifer is a shallow, karstified, and fractured aquifer. The sediments were deposited during a major transgression period which led to the deposition of a thick succession of carbonate rocks34. The Paleocene aquifer is confined and its groundwater flow direction is towards the east in RAK and locally within Umm Al Heesh areas, and as such there is a possibility that Paleocene aquifer water may reach the surface and form lakes.

In general, in eastern RAK, a steep horizontal salinity gradient is observed, starting from 5 g/l and reaching as high as 80 g/l towards the UAE borders34. In the study area, the Paleocene aquifer groundwater salinity changes from 5 to 6 g/l, with a chemical composition that is predominantly Cl−-Na+ with pH = 7.1–7.5, temperature of 52–55 °C, and H2S content34,38,39,40.

Data and methods

Remote sensing

The satellite imagery data from 1967 to 2025 were selected from Google Earth web version (https://earth.google.com/web/) to identify the location, shape and size of Umm Al Heesh lakes using mainly visual interpretation.

Sampling

The water samples for chemical analysis were collected from the Umm Al Heesh lakes in July 2003 (S1), April 2022 (S2) and February 2024 (S3, S4 and S5). All water sampling points were located at a distance of 1–2 m from the lake shore, with an average depth of approximately 0.5 m at each sampling point. In 2024, the lake water was sampled for isotope analysis, in addition to sampling the upper part of the beach sediments for further mineral composition analysis. Moreover, rainwater and Paleocene aquifer groundwater samples were collected to study isotopic composition. The polyethylene sampling bottles were rinsed with the collected water at least three times, then filled, covered with hermetically sealed lids, and labeled with the sample name, date and time, depth, measured parameters and exact sampling coordinates before being sent to the laboratory for analysis. No water was allowed to evaporate from the sample during the sampling process, storage, and transportation to the laboratory.

Analytical methods

During the collection of water samples, the temperature, Total Dissolved Solids (TDS) and pH were measured using Ultrameter II Digital Handheld (model 6PFCᴱ, Myron L Company, USA). Additionally, hydrogen sulfide (H2S) was detected using Honeywell BW™ Max XT ll (USA). The HCO3− content was determined using Mettler DL-77 Auto Titrator fitted with a pH glass electrode. Determination of Cl− was performed by potentiometric titration with a standard AgNO3 solution using Mettler DL77 Auto Titrator. Concentrations of S, Ca, K, Mg, Na and Sr in water were determined using the Agilent 5110 inductively-coupled plasma optical emission ICP-OES (SALAM 528). The reliability of the hydrochemical data was assessed by checking electroneutrality, which were found to be better than ± 5%.

Isotopic analysis of δ18O, δ2H, 87Sr/86Sr, and 14C in water samples was conducted using a Picarro L2130-i Analyzer. The δ18O and δD (or δ2H) isotopes data were expressed in delta notation (δ) as parts per thousand (‰) relative to the Vienna Standard Mean Ocean Water (V-SMOW), with a precision of ± 0.2‰ for δ18O and ± 0.8‰ for δD. The abundance of 14C was normalized to dissolved inorganic carbon content (DIC) in the water sample as pmC.

An X-ray mineralogical analysis of two samples of the beach sediments was carried out on randomly oriented powders using a Rigaku ULTIMA IV powder X-Ray diffractometer with copper (Cukα) radiation (40 kV, 40 mA), in the 3°–70° (2θ) interval with a step size of 0.02° increment and a speed of scan of 10.

Results and discussion

Morphology of the Umm Al Heesh lake

The earliest available aerial imagery dates back to 1967 (Fig. 6) and confirms that this lake was present at that time. Later on, the Umm Al Heesh lake continued to appear in subsequent satellite imageries captured periodically, showing its shape changing over time. Until 1995, Umm Al Heesh lake had a water tongue in the south (Fig. 6). Since 1995, a new body of water, previously visible on satellite images as the sabkha salt flats, has begun to form. The large body of water was formed by a topographic spill and simply changed shape over time due to changes in water flow. Since 2017, the body of water has reached its maximum extension (Fig. 6) and remains in place to this day, appearing as a lake. We call this body of water here the Large Lake (Fig. 1). Over the entire observation period, only the central part of the Umm Al Heesh lake has not changed in shape (Fig. 6). This part consists at least of three elongated individual lakes, which are separated from each other by small sand dunes. We refer to these lakes as Small Lake 1, Small Lake 2, and Small Lake 3 (Fig. 1).The recent satellite imagery details a low-lying area of sabkha with recent evaporites overlying the Hamidan structure near the Umm Al Heesh lake (Figs. 1, 2d, 6).

Fig. 6
figure 6

Aerial (1967) and satellite imageries (1986, 2000, 2017) of Umm Al Heesh lakes captured in different years (picture represented 1967 source is Saudi Aramco aerial imagery archive, the satellite imageries represented 1986, 2000 and 2017 were downloaded from Google Earth web version (https://earth.google.com/web/)).

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Analysis of water physiochemical characteristics

Following visual inspections of the Small Lakes in 2022 and 2024, areas of groundwater discharge inside the lake were detected. The water samples were collected as close as possible to these areas. The presence of H2S was detected from the Small Lakes, while no H2S in the area of the Large Lake was found.

In the coastal areas the Large Lake has thin layers of salts and salt crusts covering the sandy beach sediments. The mineralogy of the beach sediment of Large Lake (S4) consists of plagioclase (albite), quartz, calcite, halite, gypsum with small peaks of anhydrite, dolomite, illite/mica and k-feldspars (Table 1, S4 in Figs. 1, 7). The bottom sediments of Small Lake 1 (S3 in Figs. 1, 7,) are loose and contain lower salt content (8.5 wt% of gypsum and no halite), and their mineralogical content is dominated by quartz, plagioclase, potassium feldspars and calcite minerals (Table 1, Fig. 7).

Table 1 Mineral phase quantification of the beach sediment samples (S3, S4 in Fig. 1) (wt%).
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Fig. 7
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The resulting X-ray diffraction spectrum for beach sediment samples from Small Lake S3 (a) and Large Lake S4 (b).

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A notable difference in the mineral composition of bottom sediment samples S3 and S4 is observed. Sample S4 exhibits a higher content of halite (24.4 wt%), gypsum (10.1 wt%) and anhydrite (2.7 wt%) compared to sample S3 (Fig. 7, Table 1). This suggests that the Large Lake water may have undergone a greater degree of evaporation than the Small Lakes water, resulting in the increased concentration of these minerals.

In the study area the sampled Paleocene aquifer groundwater is characterized by a Cl−–SO42−–Na+–Ca2+ water type with TDS 6594 mg/l, pH = 7.2, temperature 53 °C and H2S content (sample S6 in Table 2). The isotopic values of δ18O (− 6.4‰) and δ2H (− 45.6‰) are typical for the Paleocene aquifer (δ18O from − 0.42 to 6.95‰ and δ2H from − 19.56 to − 52.3‰) (Table 3, Fig. 8a). According to radiocarbon dating, the age of the Paleocene aquifer groundwater in Hamidan area is about 22844 years, which indicates an older recharge period during the late Pleistocene when the rains took place and a net of paleo rivers spread widely15,33.

Table 2 Chemical analyses of studied water.
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Table 3 Isotopic (δ18O, δ2H and 87Sr/86Sr) composition of studied water.
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Fig. 8
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The measured values of δ18O and δ2H isotopes (a) and 87Sr/86Sr ratios (b) in studied water.

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The measured water chemical composition of Small Lake 1 (S3), collected in 2024, is predominantly Cl−–SO42−–Na+–Ca2+ with TDS = 6450 mg/l, pH = 7.2, temperature 52 °C and H2S content. This composition is very similar to the water samples composition (S1 and S2) collected from Small Lake 1 in 2003 (pH = 7.3, H2S content, TDS = 6745 mg/l) and 2022 (pH = 6.9, H2S content, TDS = 6896 mg/l) (Table 2) which is comparable to Paleocene aquifer groundwater. The δ18O (− 5.49‰) and δ2H (− 42.1‰) values of Small Lake 1 water also lie in the typical range for Paleocene aquifer groundwater (Table 3, Fig. 8a).

It is observed that chemical composition and temperature values of the Small Lakes water have remained constant over time (Table 2). The water temperature in Small Lakes 1 and 2, according to measurements in June 2003, is near 54 °C and is quite similar to the temperature measured in January 2024, which is 52 °C. This temperature is higher than the annual surface temperature in the study area, which is approximately 33.6 °C12. Considering the geothermal gradient in the study area (about 2.96 °C per 100 m), these temperature values are typical for the lower part of the Paleocene aquifer.

The analytical results of the Large Lake water (samples S4 and S5) indicate that the pH is 8.3 and is slightly alkaline. The measured temperature range of 35–38 °C and the TDS (14403–15919 mg/l) is 2.5 times higher than that of the Small Lakes and the Paleocene aquifer groundwater (Table 2). The water chemical composition is predominantly Cl-–SO42-–Na+–Ca2+. No H2S is found in the Large Lake water, but black sulfide silt and salt minerals like gypsum, halite and anhydride are detected at the lake bed (Table 1). The isotopic composition of the Large Lake water samples (δ18O from 1.97 to 3.32‰ and δ2H from − 2.51 to 1.97‰) are heavier than in the Small Lake 1 water and the Paleocene aquifer groundwater (Table 3). Note that the Large Lake water stable isotopic signature lies in the isotopic values range of typical sabkha waters (δ18O from 4.21 to 9.27‰ and δ2H from 16.9 to 34.7‰) (Fig. 8a) formed due to evaporation, salts precipitation and dissolution processes.

The 87Sr/86Sr ratios from the three lake water samples (S3, S4 and S5) show similar values (0.707719, 0.707735 and 0.707735‰), with minor variations in the 5th decimal place and are in the known range of Paleocene seawater and Paleocene aquifer groundwater of the RAK basin41 (Table 3, Fig. 8b). Minor fluctuations in 87Sr/86Sr values may be due to the influence of geochemical processes, such as precipitation and dissolution during the life of the Large Lake.

Water origin discussion

The long life of the Umm Al Heesh lake under hard-arid conditions, where evapotranspiration significantly prevails over atmospheric precipitation, indicates that the water in the lake is preserved through continuous groundwater recharge to maintain the volume, water level and constant water quality. Otherwise, the lake would dry up, have higher salinity, and exhibit heavier isotopic compositions.

The Hamidan area has a long history of groundwater flowing to the surface34 and the sabkha is a major discharge area of groundwater, separating the down-gradient discharge from the up-gradient recharge areas. Conduits for upwards seeping of groundwater to the surface are comprised of numerous faults and fractures associated with local structural deformation (Fig. 4). Faults connected with palaeochannels can serve as pathways for the upward transport of deep-seated water to the land surface33 and also control abnormally high discharge to form lakes42.

The continuous interaction between groundwater and lake water is a fundamental determinant of the Umm Al Heesh life and protection. The recharge and discharge relationship between groundwater and surface water is governed by a combination of geological, geomorphological, hydrogeological and climatological factors.

The Neogene and part of the Eocene deposits are eroded due to the geological uplift in the Hamidan area. The local faulting and fracturing associated with the Hamidan structure created many breaches and windows in the aquiclude of the Eocene layer above the Paleocene aquifer (Fig. 4). The most viable mechanism that explains the circular Hamidan structure is apparently a meteorite impact; however, this hypothesis is still ambiguous and unproven37. According to the seismic data37, the lake area is located on the flanks of the structure (Figs. 4, 5), but is still located in fault zones that allow groundwater to rise to surface. There are many impact crater lakes on Earth, some of which have a ring-like shape, while others have different, irregular forms (for example, lake Karakul in Tajikistan; lake Manicouagan in Quebec, Canada; lake Siljan in Dalarna, Sweden; lake Bosumtwi in Ghana etc.)43,44,45 (https://impact.uwo.ca/).

In the Hamidan area, the Paleocene aquifer is the first shallow aquifer under the surface, which is confined and in flowing conditions, with surface pressures in the order of 150 psi34. Consequently, groundwater from the Paleocene aquifer can rise to the surface through fractures and faults and form lakes.

The geomorphological conditions, such as dune altitude and morphology as well as climatologic condition (wind direction, seasonal precipitation etc.), have an effect on the Umm Al Heesh lake size and shape, water chemistry and, partly, on the recharge and discharge relationship between groundwater and surface water. The lake has undergone changes in its shape and configuration over time. Before 2017, a large secondary lake existed in the northern part of the Small Lakes (Fig. 6), but now it has completely dried up, exposing the sabkha surface area (Fig. 1). On the other hand, since 1995, a dry, isolated saline basin in the southern part of the Small Lakes has started to fill with water from a Small Lake 1 source. Along the Large Lake margins, salt-tolerant shrubs and bushes are quite rare due to the high salinity of the water and the variable hydrological regime (Fig. 1b). In contrast, typical salt-tolerant shrubs, reed and bushes grow around the Small Lakes’ bodies forming green oases. Due to a protection by reed belts, no effect was detected on the large parts of the Small Lakes’ shoreline caused by wind water waves.

According the aerial and satellite images the Umm Al Heesh lake water surface area has decreased since 1967 (Fig. 6). A general decrease in the average terrestrial water storage for RAK region was estimated -0.32 ± 0.0107 cm/yr13. This means that Umm Al Heesh has lost at least 18.56 cm of water depth since 1967 which reflected on its shape. Such significant changes under a hyper-arid condition can be a result of a hydrological imbalances caused by global warming: decreased water inflows (groundwater runoff, precipitation) and/or increased water outflows (evaporation).

According to a data analysis since 197946,47, the Arabian Peninsula experienced an average temperature increase of 2.1 °C (2.5 °C in summer), almost three times the global average, making the region a global hotspot for climate change. Climate projections indicate that the region is expected to experience significant warming by the end of the twenty-first century, with temperature increases ranging from 2.6 °C ± 0.57 °C and 7.6 °C ± 1.53 °C, depending on the level of greenhouse gas emissions scenarios46,47,48. A higher air temperature trend will cause the subsequent lake water temperature increase which is expected to be greater than the global average trend of 0.34 °C per decade5,49. The same applies to the increase in evaporation in the peninsula, which may be greater than the projected annual average of 16% under hyper-arid conditions5. As a result, the amount of water lost from Umm Al Heesh water surface through evaporation can be expected to increase, which may lead to an even greater reduction in the lake size and shape over time and subsequent changes in water quality.

According to climate projections50,51, precipitation is expected to increase over most of the Arabian Peninsula by the end of the century. However, it is essential to note that the current precipitation levels in the region are extremely low, averaging 54.6 mm/yr13. Even if precipitation increase by 20% by the end of the twenty-first century50,51, it would not be sufficient to offset the increasing evaporation rates. Furthermore, data from 2012 onwards show a negative trend in groundwater storage in RAK region, with an average annual change of -0.32 ± 0.104 cm/yr13. The decline in groundwater storage indicates a decrease in aquifer head, resulting in reduced water discharge into Umm Al Heesh, which contributes to the observed decline in lake surface water levels. Although long-term increases in precipitation may potentially recharge the aquifer in outcrop areas along the western border of the RAK and the southern border of the Kingdom of Saudi Arabia, this effect is unlikely to significantly impact the lake water level for at least 20000 years, based on the 14C results.

Our comprehensive analysis of the hydrogeochemical data and geological conditions of the area shows that Umm Al Heesh consists of several lakes with different chemical and isotopic compositions of water and is a dual closed lakes system, located in the interdune zone.

The first system (Small Lakes) is a group of permanent groundwater (spring) lakes (Fig. 1) formed by the natural discharges of Paleocene aquifer groundwater to the surface through natural cracks, sinkholes or caves (Figs. 2a,c, 4). The water of the spring Small Lakes is consistently renewed by fresh portions of Paleocene aquifer groundwater, which allows maintaining a constant water temperature, chemical and isotopes composition over time (Tables 2, 3).

The second system is a temporary puddle lake, which is formed mainly due to surface flow along the relief elevation from the source spring lake to the sabkha plate (Figs. 1, 2b, 4). It is impossible to exclude a presence of low-flow Paleocene aquifer groundwater springs discharged at the bottom of Large Lake, but additional research is required to identify their presence and location. In any case, today the inflow of water into the secondary Large Lake is low and insufficient to maintain constant water quality over time, as in the Small Lakes. No H2S was found in the water of the Large Lake due to its release and oxidation. Note that the water composition of the Large Lake, as a closed hydrological system, depends mainly on labile sabkha lithologies that are leached and altered by inflow waters in the drainage basin, as well as on surface water evaporation. As a result, the water of the Large Lake changed its initial chemical composition and became more concentrated and saline—TDS concentration in the Large Lake increased to approximately 2.5 times that in both the spring lakes and Paleocene aquifer groundwater (Table 2). Salts such as gypsum, halite and anhydrite are precipitated from the saturated water in bottom sediment and on the shore (Table 1, Fig. 7). Additionally, the isotopic δ18O and δ2H composition has become heavier and more typical for sabkha water due to various physical and chemical transformation, including water evaporation, salt precipitation and dissolution of sabkha salts (Table 3, Fig. 8a). However, the 87Sr/86Sr ratio of the Large Lake water stays constant and similar to the Paleocene aquifer (Table 3, Fig. 8b), which shows that the primary water of the Large Lake is the Paleocene aquifer groundwater.

Under current circumstances, the Small Lakes are not expected to dry up and disappear. However, their disappearance could occur if there is a further decrease in the Paleocene aquifer pressure, causing it to transition into an unconfined aquifer due to anthropogenic or climate warming effects. Under current conditions, a decline in the natural flow rate of the Paleocene aquifer groundwater is not expected to occur anytime soon, and certainly not anticipated within coming several decades. It is worth noting that the secondary Large Lake may change its location and shift in response to evolving dune topography, much like it had done previously before 2017 (Fig. 6).

Conclusions

In this study, we used an integrated approach that combined geochemical and isotopic analysis, geological data, field surveys, aerial and satellite imagery to determine the origin of the Umm Al Heesh lake water and identify key processes that dictate its water quality.

Our results indicate that Umm Al Heesh is a dual lake system developed in the same RAK basin, under similar climatic conditions but with different hydrology and geochemistry. The primary source of water in the lakes is Paleocene aquifer groundwater with TDS 6594 mg/l, pH = 7.2, and isotopic values of 87Sr/86Sr 0.707717‰, δ18O and δ2H— − 6.4‰ and − 45.6‰ respectively which is continuously replenished through fractures and sinkholes in Small Lakes water and maintain a constant chemical and isotopic composition there (TDS 6450–6896 mg/l, pH = 6.9–7.3, 87Sr/86Sr 0.707719‰, δ18O − 5.49‰ and δ2H − 42.1‰). Surface flow along the interdune topography causes water from the Small Lakes to spread into the sabkha depression to form a secondary Large Lake. Its surface water chemistry responds to processes of increasing salinity up to 14403–15919 mg/l due to evaporation and dissolution of sabkha minerals. As a result, the chemical composition and isotopic signature of δ18O (1.97–3.32‰) and δ2H (− 2.51– + 5.87‰) of the secondary Large Lake water have undergone significant changes compared to the source.

The study also found evidence of climate change affecting the lake’s water level, which has decreased by at least 18.56 cm since 1967. This has impacted on the Umm Al Heesh shape and hydrological balance. Further study of Umm Al Heesh lake is recommended for understanding the hydrology of the RAK desert, the sustainability of groundwater systems in extreme arid conditions, and even planetary science, as similar craters on Mars or the Moon may have once contained water. In addition, Umm Al Heesh lake serves as an important ecological refuge, supporting unique biological life and species adapted to the desert environment. It is important to protect and preserve the unique Umm Al Heesh lake, as it may be fragile and vulnerable to climate change and human activities.