Introduction

The Arabian Peninsula faces significant water scarcity, which has been exacerbated by population growth, urbanization, and climate change. The region experiences arid and semi-arid climatic conditions, where groundwater resources are the primary source of freshwater. However, groundwater depletion is a major concern as the water demand continues to surpass replenishment rates.

The Upper Mega Aquifer System is one of the major aquifer systems in the Arabian Peninsula, which spans across Iraq, Kuwait, Saudi Arabia, Bahrain, Qatar, the United Arab Emirates (UAE), Oman, and Yemen. It is a critical resource for these countries as it provides freshwater for domestic use, agricultural production, and industrial activities. Understanding the geology and hydrogeology of the Upper Mega Aquifer System is essential for its sustainable management and conservation.

In this article, the geological and hydrogeological features of the Upper Mega Aquifer System are discussed. Furthermore, an examination of the current state of the aquifer system, the challenges faced in managing the resource, and potential solutions to ensure its sustainable use are described. Given the importance of the Upper Mega Aquifer System, it is crucial to explore ways to preserve this vital water resource for the well-being of communities, economies, and the environment.

Geological and hydrogeological overview

Two main geological units can be distinguished on the Arabian Peninsula (Barth and Schliephake 1998; Thompson 2000; Grainger 2007). These are the Arabian Shield and the Arabian Platform (Fig. 1). The geologically oldest unit is the Arabian Shield in the west of the Arabian Peninsula. More recent is the Arabian Platform, which occupies the center and east of the Arabian Peninsula.

Fig. 1
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Simplified geological map of the Arabian Peninsula with locations of cross-sections A–C

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The Arabian Shield consists of igneous and metamorphic rocks of Precambrian age as well as volcanic rocks (so-called “harrats”) of Tertiary and younger age (Brown et al. 1989). Alluvial fillings can be found in the valleys. There are no regionally significant groundwater resources on the Arabian Shield. The igneous and metamorphic rocks generally have a low yield. Medium yields are found in the “harrats” and in the alluvial of the valleys; however, due to the limited extent, these groundwater resources are only of local importance.

The Arabian Platform is characterized by a thick sedimentary succession deposited on the basement from the Cambrian to the end of the Tertiary (Steineke and Bramkamp 1952; Powers et al. 1966; Sharland et al. 2001; Ziegler 2001). The rocks consist of carbonates, evaporites, mudstones, marls and sandstones forming a cuesta landscape (Rausch et al. 2014a). Deposition occurred during several transgressive-regressive cycles. Tectonic movements and cyclic sea level fluctuations led to repeated deposition of marine and terrestrial sediments. In the lower part of the sequence clastic sediments such as sandstones predominate, while in the upper part, biochemical and chemical sediments such as limestones, dolomites and evaporites predominate.

The Arabian Peninsula is tectonically not stressed very much (Frisch and Meschede 2007). It is bordered to the south by the rift zones in the Red Sea and to the west by the Jordan Rift Transform Fault. To the north and east, the Arabian Peninsula is tectonically bounded by the subduction zone in southern Iran and the transform fault in the Arabian Sea. The rifting in the Red Sea leads to the tilting of the Arabian Peninsula; the Phanerozoic sediments of the Arabian Platform dip at ~1° to the NE. The constant dip is interrupted on the Arabian Platform by north–south-trending anticlines and synclines, as well as salt diapirs in the Persian Gulf area. The main faults on the Arabian Shield strike in a northwest–southeast direction.

The tectonic stability throughout the Phanerozoic led to relatively even sedimentation conditions on the Arabian Platform, and ultimately to the development of enormous oil and gas deposits and the formation of one of the largest aquifer systems on earth (De Sitter 1947).

The aquifer system consists of several principal and secondary aquifers that are imperfectly connected hydraulically to each other (Ministry of Agriculture and Water 1984; Pike 1985; Al Sharan et al. 2001; Wagner 2011; Rausch 2016). Groundwater-bearing sandstones and karstified limestones alternate with claystones and evaporites (gypsum and anhydrite) of lower hydraulic conductivity and storage capacity. In general, the aquifers are very productive and the groundwater has a good quality. Due to their wide extent, these aquifers are of supra-regional importance (Margat and Van der Gun 2013; Van der Gun 2022).

The aquifer system can be further subdivided into a Lower and an Upper Mega Aquifer System. Both systems are completely hydraulically separated from each other over large areas. The separating layer is formed by the anhydrites of the Upper Jurassic Hith formation.

The Upper Mega Aquifer System

The Upper Mega Aquifer System stretches from the Euphrates-Tigris Basin in Iraq, through Kuwait, Saudi Arabia, Bahrain, Qatar, the United Arab Emirates, Oman and Yemen. Its total area is ~1,860,000 km2. The morphology of the area is characterized by wide plains broken up by escarpments and a few small hills. In the south, the Rub’ Al Khali (“Empty Quarter”) desert, the largest sand desert in the world with an area of about 640,000 km2, determines the landscape. The geodetic heights range from sea level along the coast of the Persian Gulf to ~1,200 m above sea level (asl) in the Jawl Plateau in the south. The area lies almost entirely in the region of the tropic deserts of the northern hemisphere, whose climate is characterized by arid to hyperarid conditions (Köppen and Geiger 1936; Glennie 2005; Edgell 2006).

The Upper Mega Aquifer System comprises a succession of Cretaceous and Tertiary sediments. Hydrogeological cross-sections through the aquifer system are shown in Fig. 2. A brief description of the aquifer system with information on the lithology and hydraulic parameters is given in Fig. 3 (Kalbus et al. 2011; Hermes et al. 2014; Abdulrazzak and Elbaraasi 2017; Awadh et al. 2021; Ouda et al. 2017).

Fig. 2
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Geological-hydrogeological cross-sections through the Arabian Platform (for location of cross-sections A-A', B-B', and C-C' refer to Fig. 1)

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Fig. 3
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Geological, lithological and hydrostratigraphical units of the Upper Mega Aquifer System. The corresponding hydraulic properties (hydraulic conductivity K, storage coefficient S, specific yield Sy), their representative average values and ranges are shown on the right side

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In the lower part, the strata consist mainly of sandstones, while the upper part is composed of carbonates (limestone and dolomite) and evaporites (gypsum and anhydrite). The principal aquifers are the Wasia-Biyadh Aquifer (Deckelmann et al. 2012), the Umm Er Radhuma Aquifer (Bakiewicz et al. 1982; Dirks et al. 2018; Alfy et al. 2019) and the Dammam Aquifer. Secondary aquifers are the Lower Cretaceous limestones of the Sulaiy-Yamama-Buwaib (Wolpert et al. 2015), the Aruma Aquifer and the Neogene Aquifer. These aquifers are separated from each other by several aquitards, the Shu’aiba and Aruma mudstones and the Rus.

The sandstones of the Wasia-Biyadh Aquifer are only slightly consolidated and have a high primary porosity. The Wasia-Biyadh Aquifer is the most productive aquifer within the Upper Mega Aquifer System and contains the largest groundwater resources in storage. The upper part of the Umm Er Radhuma Aquifer is intensively karstified; it has only a slightly lower yield than the Wasia-Biyadh. Large amounts of groundwater are also stored in the Umm Er Radhuma Aquifer.

The hydraulic separation by the aquitards is not perfect. Cross formation flow is enabled by zones of higher vertical conductivity in the aquitards (hydraulic windows). These hydraulic windows are aligned to anticline structures. Due to the tectonic stress, the fissures are more intensive there and thus the permeability in the vertical direction is increased. In the case of gypsum and limestone, there is also an intensification of karstification, which further increases permeability (Dirks and Rausch 2008; Rausch et al. 2012).

The aquifer base is formed over large areas by the thick anhydrite layers of the Hith formation. The Hith is missing in the north and extreme south, where the Upper Mega Aquifer System lies directly on top of the Lower Mega Aquifer System.

Groundwater dynamics

The general groundwater flow direction on the Arabian Platform follows the dip of the geological strata from southwest to northeast. This applies to all aquifers within the aquifer system, although there are large differences in the piezometric heads between the individual aquifers. The discharge areas are the Persian Gulf, coastal and inland sabkhas and springs in the east of the Arabian Platform.

In the outcrop of the aquifers, i.e. in the areas with groundwater recharge, one observes a downward groundwater movement, while further to the east in the direction of the discharge areas there is an upward groundwater movement. This flow pattern corresponds very well to the model of a regional aquifer system developed by Tóth (1963).

For the aquifers of the Wasia-Biyadh, the Umm Er Radhuma, and the Dammam, the piezometric head distributions and the corresponding groundwater flow directions are shown in Figs. 4, 5 and 6. The piezometric head differences between the Umm Er Radhuma, Dammam and Neogene aquifers are small, which is true for the outcrop areas of Umm Er Radhuma and Wasia-Biyadh aquifers; the difference between the two aquifers reaches more than 200 m in the Persian Gulf area. There, the Aruma is developed in claystone facies forming an effective aquitard.

Fig. 4
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Piezometric head distribution, general groundwater flow direction, and total dissolved solids (TDS) in Wasia-Biyadh Aquifer

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Fig. 5
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Piezometric head distribution, general groundwater flow direction, and total dissolved solids (TDS) in Umm Er Radhuma Aquifer

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Fig. 6
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Piezometric head distribution, general groundwater flow direction, and total dissolved solids (TDS) in Dammam Aquifer

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The low rainfall on the outcrops of the Upper Mega Aquifer System results in a small amount of groundwater recharge. The Hith Aquiclude generally prevents inflow from the underlying Lower Mega Aquifer System. This is shown, among other things, by the large difference in the piezometric heads of the Upper and Lower Mega Aquifer Systems: the piezometric heads of the underlying Dhruma Aquifer are more than 100 m higher over large areas of the Arabian Platform than those of the Wasia-Biyadh aquifer. A small inflow from the Lower into the Upper Mega Aquifer System exists to the north of the Arabian Platform, where the Hith Aquiclude is not developed.

Wadis, which cut through the Jurassic escarpment landscape in a west-east direction, bring further inflow and thus connect the outcrops of the Lower Mega Aquifer System (Palaeozoic – Jurassic) with those of the Upper Mega Aquifer System. This “bypass” of the Hith Aquiclude via Quaternary wadi sediments is also the only outflow from the Lower Mega Aquifer System. The wadis transfer groundwater into the upper aquifers.

Isotope investigations of the groundwater showed maximum residence times of 20,000–30,000 years. The age of the groundwater increases in the direction of groundwater flow, with the highest ages measured on the coast of the Persian Gulf, in Qatar and in Bahrain. However, tritium is also found in the groundwater in the outcrop area, which proves recent, however small, groundwater recharge (Shampine et al. 1979; Wagner and Geyh 1999). Studies of stable isotopes (oxygen-18 and deuterium) show that groundwater was essentially formed when the climate was wetter than today. This means that the water stored in the aquifers is mostly “fossil groundwater” (Faulkner 1994; Michelsen et al. 2015).

For ~5,000 years, after the end of the last pluvial period, the aquifer system has been depleting. Groundwater recharge is still less than the natural outflow from the aquifer system. The aquifer depletion is documented by the reduction in the artesian head of natural springs in Bahrain (Rausch et al. 2014a). Archaeological investigations of the historical irrigation system showed that in the last 4,000 years it has decreased from 12 to 2 m asl (Larsen 1983). On the mainland, former lakes and sabkahs document paleogroundwater levels.

Hydrochemistry and groundwater quality

The groundwater quality in the Upper Mega Aquifer System generally develops from west to east according to the Chebotarev-Sequence (Chebotarev 1955a, b, c). The salinity increases continuously from the areas of groundwater recharge (1,000–2,000 mg/L) to the outflow into the Persian Gulf (5,000–10,000 mg/L). Even in the outcrop area, the groundwater is mostly carbonate-saturated, with calcium and hydrogen carbonate being the dominant ions. Along the flow path, the solution of evaporites becomes more and more important, with increasing solution content, calcium and sulfate become the most common ions. Even after gypsum saturation has been reached, ion exchange leads to a further increase in salinity, and the groundwater develops into sodium-chloride water in the east of the Arabian Platform (White 1965; Dirks 2007, Al-Sofyani 2018).

The two uppermost aquifers are excluded from the trend of mineralization rising from west to east: In the Neogene and Dammam aquifers, sodium chloride water with a high salinity (5,000–10,000 mg/L) occurs almost exclusively; a Chebotarev-Sequence has not developed. In the Neogene aquifer, this is due to the heterogeneous lithology and the widely distributed groundwater recharge. The Dammam Aquifer, on the other hand, lacks outcrops in the west of the Arabian Platform; thus, groundwater recharges directly through precipitation. Groundwater inflow into the Dammam Aquifer occurs only through underlying and overlying aquifers. An exception to this is the Qatar Peninsula, where widespread Dammam outcrops occur and local groundwater recharge due to precipitation exists. The infiltrated rainwater forms freshwater lenses on the highly saline fossil groundwater.

Figures 4, 5 and 6 show the development of mineralization in the principal aquifers. The subdivision is made into classes of total dissolved solids (TDS) in freshwater (0–1,000 mg/L), slightly saline water (1,000–2,000 mg/L), moderately saline water (2,000–5,000 mg/L), saline water (5,000–18,000 mg/L) and highly saline water (>18,000mg/L).

In the other limestone aquifers (Umm Er Radhuma, Aruma) and sandstone aquifers (Wasia, Biyadh), the Chebotarev sequence is well developed. With a similar evolution of the overall mineralization, there are aquifer-specific features of water type and mineral content, mainly caused by variations in lithology and preferential groundwater flow paths along tectonic fault lines. In the Umm Er Radhuma Aquifer, for example, a tongue of freshwater (TDS < 2,000 mg/L) extends along the Ghawar anticline to Al Hassa. In the area around Hafar Al Batin groundwater shows a higher mineralization (2,000–5,000 mg/L) already close to the outcrop, since evaporite layers occur towards the top of Umm Er Radhuma (Al Mussawi 2014).

The hydrochemical development of the groundwater in the Upper Mega Aquifer System is overlaid to the east by physical processes: the high hydrostatic pressure in the aquifers leads to density differentiation and natural reverse osmosis (White 1965). The result is a strong mineralization of the groundwater up to 200,000 mg/L. The physical processes play a greater role in the lower aquifers (Wasia, Biyadh) than in the upper ones. In the Umm Er Radhuma Aquifer, for example, the high salinities are only reached below the Persian Gulf, while in Wasia and Biyadh similarly high mineralization already occurs below the mainland of Saudi Arabia. Besides physical processes, also chemical reactions in a hydrothermal environment may contribute to the observed high salinities (Ghomaa and Al Bassam 2021).

At the beginning of the Pleistocene and in the interglacial periods, the sea level was up to 120 m higher than today. During these times, parts of the Arabian mainland were flooded, in particular the sea penetrated large parts of the Rub’ Al Khali desert. At sea level highs there was a sea connection from the Persian Gulf to the Arabian Sea not only through the Strait of Hormuz but also through the Rub’ Al Khali desert. Oman was then cut off from the Arabian Peninsula and formed an island (Glennie and Singhvi 2002). The seawater intruded the aquifers and is possibly responsible for the sudden increase in salinity that can be observed today in the Rub’ Al Khali Desert along the Saudi Arabia-UAE border. However, the measured salinity of up to 200,000 mg/L is a multiple of the salinity of the sea; thus, additional processes such as concentration through evaporation or via the hydrophysical processes described previously must be assumed.

The sandstone aquifers (Wasia, Biyadh) are characterized by a low hydraulic gradient, resulting in a low groundwater flow rate, and the absence of a true discharge area like the Persian Gulf. The outflow occurs into the overlying aquifers with flow through the aquitards. It is therefore assumed that there is still connate water in the eastern part of these aquifers.

Hydraulic windows lead to groundwater exchange between the aquifers, which can be detected by anomalies in groundwater age and hydrochemical composition, e.g. in the lanthanide signature of the groundwater (Siebert et al. 2012). The dissolved proportions of the various lanthanides differ depending on whether the water, for example, has flowed through sandstone or limestone formation. These proportions no longer change once the solubility limit has been reached. In the Upper Mega Aquifer System, proven hydraulic windows exist in the Al Hassa and Yabrin areas where groundwater from the Umm Er Radhuma Aquifer enters the overlying aquifers and eventually exits to the surface (Al Tokhais and Rausch 2008; Rausch 2009). A number of springs existed in Al Hassa before the lowering of the groundwater level by anthropogenic extraction caused them to dry up (Al Tokhais et al. 2012). At Yabrin, the pressure level of the Umm Er Radhuma reaches just below the ground level; a large inland sabkha has formed.

Water budget

In recent decades, humans have massively intervened in the water budget of the Upper Mega Aquifer System. Above all, groundwater extraction for agricultural irrigation as well as the injection of groundwater into deeper oil-bearing layers to maintain oil production, have made a major impact on the water budget. It therefore makes sense to differentiate between the historical situation (before the start of industrialization, around 1950) and the current situation when considering the water budget. Before the discovery of the oil and natural gas resources in the Middle East, neither the financial nor the technical possibilities existed for exploiting deeper aquifers. In historical times, anthropogenic groundwater abstraction was negligibly small and limited to the use of natural springs and shallow wells. At least since the 1970s there has been intensive pumping of the groundwater, whereby the main user has been agriculture. Due to the intensive use of groundwater, a depletion of the groundwater resources on the Arabian Platform can be observed.

The inflow and outflow components for the Upper Mega Aquifer System are:

Inflow components:

  • Groundwater recharge from precipitation

  • Inflow through wadis from the Arabian Shield

  • Inflow of groundwater from underlying aquifers

Outflow components:

  • Evaporation from inland and coastal sabkhas

  • Groundwater discharge from springs (only historical)

  • Inflow into the Persian Gulf

  • Anthropogenic groundwater abstraction for agricultural irrigation, industry and drinking water supply (only today).

The individual water budget components are described in the following. The associated rates and an estimate of the relative uncertainties in their determination (Lloyd 2007; Rausch et al. 2010b) are listed in Table 1. The uncertainties describe the inaccuracy in the quantification of the respective water budget terms. Together with the actual amount, it provides a benchmark for the sensitivity of the water budget in relation to the individual component. It also shows where further research and investigation is useful to improve quantification.

Table 1 Water budget, components of the water budget and uncertainty of the budget components for predevelopment and present state. The unit MCM/a stands for million cubic meters per year. Totals and changes are in italic
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Groundwater recharge from precipitation

In the outcrop area of the Upper Mega Aquifer System, annual precipitation ranges from 180 to <60 mm/a (Dirks et al. 2012a). The lowest rainfall falls in the south in the Rub’ Al Khali desert and the temporal distribution of precipitation is limited to sporadic events. Due to the high temperature, most of the precipitation evaporates; thus, only a small part contributes to groundwater recharge with surface runoff occuring, but generally only for a short time. Perennial watercourses do not exist. Own field and laboratory measurements (Schüth et al. 2010; Kallioras et al. 2010, 2011, 2012; Schulz et al. 2015a) and hydrologic calculations (Keim et al. 2008, 2012) as well as the results of other studies (Al Sharan et al. 2001) result in annual groundwater recharge rates of 0.5 mm/a to a maximum of 20 mm/a. The highest rates of groundwater recharge are found in the outcrop areas of the karstified limestones of the Umm Er Radhuma and the Dammam. The regional mean value for the Upper Mega Aquifer System is 2.2 mm/a. The mean annual groundwater recharge for the entire area is 129.9 m3/s—4,092 million cubic meters (MCM/a). It can be assumed that the groundwater recharge has changed little over the last 80 years, so that the groundwater recharge is almost the same for the time before large-scale groundwater abstraction and today.

In the geological past, precipitation and thus groundwater recharge were significantly higher. Climatic conditions have changed over the past 10,000 years from weather conditions after the end of the last Ice Age to arid conditions that have persisted for ~5,000 years. During the “Pluvial Period”, which lasted from ~9,500 to 5,000 years, the average annual precipitation was between 350 and 450 mm/a. It can be assumed that at this time the groundwater recharge was 25–45 mm/a (Hötzl and Zötl 1978; Engelhardt et al. 2010, 2013a, b). Further humid phases occurred during the late Pleistocene, which had their maximum ~80,000, 100,000 and 125,000 years ago (Fleitmann et al. 2003, 2004; Beineke 2006; Rosenberg et al. 2011; Lüning and Vahrenholt 2019).

Inflow through wadis

Several wadis whose drainage basins include large parts of the Arabian Shield and subordinate older strata of the Arabian Platform drain into the drainage basin of the Upper Mega Aquifer System. From north to south, the four most important wadis are Wadi Ar Rimah and its eastern continuation Wadi Al Batin, Wadi Hanifah and its continuation Wadi Sabha, Wadi Ad Dawasir, and Wadi Najran. The alluvial valley fillings of the wadis consist of unconsolidated gravel, sandy and clayey sediments and form local porous aquifers (Loschko and Rausch 2022). The groundwater inflow through these valley aquifers infiltrates into the underlying hard rock and contributes to groundwater recharge. The total groundwater flow is estimated at 2 m3/s (63 MCM/a) for the predevelopment state. Due to today’s intensive use of the alluvial aquifers, the inflow rate has been reduced to 1.5 m3/s (47 MCM/a).

Groundwater inflow from underlying aquifers

The Hith Aquiclude is not developed in the north and south of the study area, i.e. the separating layer between the Lower and Upper Mega Aquifer System is missing and an exchange of groundwater can occur there. The piezometric heads of the underlying aquifers are usually higher than those of the overlying layers; therefore, the groundwater flows from the underlying aquifers into the upper mega aquifer. One exception would be the immediate outcrop areas, where there is an inflow from the Upper Mega Aquifer System downwards into the underlying aquifers. Overall, the inflow from the underlying aquifers outweighs the outflow into them. The net groundwater flow from the underlying aquifers into the Upper Mega Aquifer System can be estimated at 1.6 m3/s (50 MCM/a).

Outflow from inland and coastal sabkhas

Evaporation of rising groundwater from the sabkhas accounts for most of the natural outflow. It should be noted that when estimating evaporation from sabkhas, a distinction must be made between active and inactive sabkhas. Not all of today’s sabkha areas are still active. Quantifications of the evaporation rates vary between 100 and 520 mm/a (Schott et al. 2008, 2012; Schulz et al. 2015a). Evaporation from sabkhas is estimated at 103.3 m3/s (3,255 MCM/a) for the predevelopment time, and 96.7 m3/s (3,047 MCM/a) for the present. The lowering of the groundwater level as a result of the large groundwater withdrawals leads to a reduction in the active subsurface areas and thus to a reduction in the amount of water that evaporates.

Spring discharge

In the predevelopment state, prior to any major human activities and before the introduction of motor pumps, artesian springs used to discharge at oases in the east of the study area and from undersea springs in the Persian Gulf. This state lasted until the 1960s. The largest oases in the study area are Al Hassa Oasis, Al Qatif Oasis and oases in the north of Bahrain Island. In the predevelopment state, the Al Hassa springs had a total discharge of about 10 m3/s (315 MCM/a; Al Tokhais and Rausch 2008; Al Tokhais et al. 2012; Rausch 2009). In the Al Qatif oasis, the discharge rate was ~1.6 m3/s (50 MCM/a). The springs of Bahrain (Rausch et al. 2008, 2010b, 2011, 2014b; Dirks et al. 2012b; Di Fusco 2021) had a discharge rate of 2.8 m3/s (88.3 MCM/a). In contrast, the discharge from the submarine springs was low (Groundwater Development Consultants, “Umm Er Radhuma Study”, unpublished report for the Ministry of Agriculture and Water, Riyadh, 1980). The discharge rate was ~0.01 m3/s (0.3 MCM/a). Taking into account smaller spring discharges not mentioned here, the total discharge from the springs can be estimated at 15 m3/s (473 MCM/a). Today, due to the heavy extraction of groundwater since the middle of the last century, all land springs have dried up. Only a few submarine springs can still be found in the Persian Gulf (Kortarba-Morley et al. 2010).

Discharge to the Persian Gulf

The Persian Gulf borders the aquifer system to the east. Flow to the Persian Gulf is estimated at 12 m3/s (378 MCM/a) for the predevelopment state. Due to the high groundwater abstraction rate in the inflow area, the current discharge rate has reduced to 10 m3/s (315 MCM/a).

Groundwater abstraction for agricultural irrigation, industry and drinking water supply

Historically, groundwater abstraction was limited to shallow wells. A withdrawal of 6.1 m3/s (190 MCM/a) for agriculture and 1.0 m3/s (32 MCM/a) for drinking water is assumed; however, the withdrawals to date have risen drastically. The net agricultural groundwater abstraction from the Upper Mega Aquifer System is now at 67.1 m3/s (2,113 MCM/a), and 32 m3/s (1,009 MCM/a) are abstracted for industry and drinking water. Total agricultural abstraction is even higher: irrigation return flow is ~10% of total agricultural abstraction and, for budget considerations, the net agricultural abstractions need to be reduced. Return flows from sewage have not been considered. Agriculture has mostly settled in the places of historical oases, and due to the extensive use of the aquifers, it causes large-scale groundwater lowering of up to 150 m (Al Tokhais et al. 2012). The other consequences are obvious: the deterioration of the groundwater quality due to the rise of saline deep water or seawater intrusions along the coast; an increase in production costs due to the need to drill deeper wells; and cases of land subsidence where groundwater is withdrawn from shallow aquifers. The destruction of the environment and the old oasis landscapes goes hand in hand with a loss of quality of life.

Summary water budget

Due to the increasing scarcity of the groundwater resources, the conflict between the users also increases. Agriculture, industry and drinking water supply compete today for the same groundwater resources. Furthermore, the water supply for future generations must also be considered; today’s unsustainable consumption of groundwater resources is destroying the basis of life for future generations (Rausch 2021).

Table 1 lists all inflow and outflow components of the water budget for the predevelopment and present states. The uncertainties with which the respective values can be determined are also listed. High uncertainties are associated with components that cannot be directly measured—groundwater recharge, inflow from the underlying aquifer, and outflows to the Persian Gulf. Low uncertainties refer to components that could be verified by measurements. For historical spring discharge, measured values are documented in the literature, and agricultural abstraction was verified on selected sites by field visits and then extrapolated by a remote sensing approach. Components with moderate uncertainties were estimated with the help of laboratory and field investigations (sabkha evaporation) and hydrogeological flow estimations (wadi inflow). An agricultural groundwater abstraction estimate is the most important component in the water budget. This estimate can be determined relatively precisely and forms a reliable parameter in the water budget together with the withdrawals for domestic and industrial water consumption. Another important balance item is the groundwater recharge from precipitation; however, it is associated with a high degree of uncertainty. Various infiltration processes take place and numerous parameters must be estimated. A focus of future research should therefore be to develop improved methods for more accurate quantification of groundwater recharge.

Considering the Pleistocene climate history of the Arabian Peninsula, it can be assumed that the Upper Mega Aquifer System stored a maximum amount of groundwater at the end of the last Pluvial ~5,000 years ago. Since that time, the aquifer system has been running dry, and an equilibrium has not yet been achieved between the groundwater levels and the groundwater recharge, which is greatly reduced under arid conditions (Schulz et al. 2017). In the water budget, this is expressed by the fact that the budget is negative even in the predevelopment state (–120 MCM/a). That means that the outflows from the system are higher than the inflows; however, it must be noted that the amount is small compared to the other water budget components, especially compared to the anthropogenic withdrawals. These abstractions increased continuously since the 1950s, making the water budget for today (2022) negative (–2,295 MCM/a). The large difference between inflow and outflow is at the expense of the stored groundwater volume and leads to a local groundwater drawdown of up to 150 m. The water budget clearly shows that the unsustainable use of fossil groundwater resources leads to “groundwater mining” (Rausch and Dirks 2019).

Conclusions and outlook

The Upper Mega Aquifer System is one of the largest aquifer systems of the world. It is built up by several bedrock aquifers. Principal aquifers are the Wasia-Biyadh, the Umm Er Radhuma, and the Dammam. The stored groundwater is mainly fossil. Due to the good water quality and high yield, the aquifers are intensively exploited, which leads to depletion of the groundwater resources. The presented hydrogeological overview is the result of a decade-long investigation of the aquifer system. Its data are the basis for a hydrogeological conceptual model of the Upper Mega Aquifer System and several mathematical groundwater models of the aquifer system (Engelhard et al. 2010, 2013b, Schulz et al. 2017). These models could serve as a tool for integrated groundwater management of the Upper Mega Aquifer System (Al Saud and Rausch 2012; Siebert et al. 2016).