Groundwater supplies 50% of the world’s drinking water and 25% of the water used globally in agriculture. Locally, especially in arid regions of Africa, the Middle East, and elsewhere, these figures can be far higher, making groundwater a critical resource supporting the lives and livelihoods of hundreds of millions of people.
Groundwater is typically sourced from relatively shallow depths belowground, primarily because these sources have historically been easier to locate and access. (The meaning of “shallow” groundwater can vary regionally; here we consider it to mean water within about 400 meters of the ground surface.) But two key factors are contributing to these conventional groundwater resources becoming increasingly unreliable.
The global water crisis threatens access to clean water and food production around the world—and it has scientists, policymakers, and others searching for sustainable solutions.
First, contamination from natural processes and human activities often compromises water quality and can spread waterborne diseases and hazardous pollutants, especially in near-surface resources [Li et al., 2021]. Second, droughts and overdevelopment result in overuse that places additional stress on these reserves in many parts of the world, further limiting their availability.
These issues, together with similar problems affecting surface freshwater resources, are fueling a growing global water crisis. This crisis threatens access to clean water and food production around the world—and it has scientists, policymakers, and others searching for sustainable solutions.
Below the ground, other sources of water—what we call deep aquifers, at depths between 400 and a few thousand meters—remain largely unexplored and untapped. Despite being less widely known, such deep aquifers can hold significant reserves of water, as they do beneath parts of Africa, Arabia, Australia, and the Americas.
Recent studies revealed additional large amounts of deep freshwater in fractured and karstified rocks and in consolidated or nonconsolidated porous aquifers. Some of these aquifers even extend offshore in regions such as the Horn of Africa, Sicily, and Tunisia [Quiroga et al., 2023; Chiacchieri et al., 2024; Bachtouli et al., 2023]. This research suggests that deep groundwater may be more common than previously thought, indicating potential new resources in areas where other solutions to the escalating water crisis may not be economically or technically feasible.
What We Know About Deep Aquifers
Under the right conditions of subsurface hydraulic conductivity, surface water recharge, and hydrological gradients within aquifers, groundwater can travel great distances underground and can reside there for long periods. The age of most of the water in the Al Kufrah Basin in Libya, for example, has been estimated to be 30,000 or more years [Wright et al., 1982]. Groundwater can also percolate down well below the surface, sometimes maintaining low salinity (i.e., remaining fresh) as it reaches depths much greater than those of typical water wells.
The quality of this deep groundwater is influenced by the composition of the rocks it flows through, the velocity at which it flows, and its residence time underground. In some cases, natural pollutants from the rock, such as arsenic and radioactive elements, contaminate the water, or salts dissolved from rock add salinity. However, in most documented cases of deep-aquifer utilization, the water is safe for drinking and requires minimal treatment. Furthermore, risks of anthropogenically introduced contamination or cross flow are minimized with common modern drilling practices.
Examples of known deep aquifers include those in the Nubian Sandstone Aquifer System, comprising numerous sandstone aquifers holding several thousand cubic kilometers of water down to 3,500-meter depths in northern Africa [Ruden, 2016], and the Upper Mega Aquifer System on the Arabian platform [Abotalib et al., 2019], the Great Artesian Basin in Australia [Fensham et al., 2021], and the unconsolidated Kimbiji Aquifer in Tanzania. Deep aquifers have also been tapped in parts of South America and the United States.
Deep aquifer systems with long residence times are virtually immune to pollution and drought compared with surface water and shallow groundwater.
These deep systems with long residence times are virtually immune to pollution and drought compared with surface water and shallow groundwater. The Guarani Aquifer System in South America and the Great Artesian Basin in Australia, for example, have been key sources for drinking water, irrigation, and industrial use for decades to centuries.
Without proper planning, though, aquifers like the Nubian (which should last several centuries with abstraction rates on the order of 1,000 cubic meters per second) can face overexploitation as extraction exceeds natural replenishment [Ruden, 2016]. In contrast, the example of the Guarani System demonstrates that sustainable management is possible. Shared by Brazil, Argentina, Paraguay, and Uruguay, the Guarani aquifers are managed collaboratively through monitoring, conservation, and contamination prevention, keeping water use within recharge limits and serving as a model for transboundary aquifer management.
Repurposing Existing Data and Technologies
Despite the examples above and other recent discoveries, only a handful of deep freshwater aquifers have been identified worldwide. The reason is largely that deep-freshwater exploration has never been pursued systematically on a regional scale.
This oversight can be attributed to the fact that most water-scarce countries lack the necessary resources to investigate the subsurface for water and other natural resources. Meanwhile, wealthier nations facing water shortages have focused on more energy-intensive solutions, such as desalination, and countries without water scarcity have primarily directed their investments toward energy exploration, even in low-income, water-scarce regions.
Exploring for deep-groundwater resources today still requires significant technical abilities and investments in data gathering and infrastructure. However, the costs of exploration could be substantially reduced—and the benefits expanded to more people—if existing oil and gas data and technologies were repurposed and shared widely. The process of searching for water is akin to oil and gas exploration, even if the targets are often the inverse: Those searching for groundwater focus, for example, on exploratory boreholes deemed “dry” and unsuccessful for oil and gas production.
A unique opportunity exists at present to broaden the search for deep groundwater.
A unique opportunity exists at present to broaden the search for deep groundwater, driven by a data revolution spearheaded by open-access policies from governments and academia and by the increased availability of new satellite data and decommissioned oil and gas data. With such data now more accessible than ever, deep-groundwater studies are increasingly feasible [e.g., Quiroga et al., 2023; Lipparini et al., 2023].
Even with accessible oil and gas data, scientists face challenges in repurposing them for groundwater research. One example is the difference in how the oil and gas industry measures water salinity compared with how measurements are done using hydrogeological methods. The industry typically uses indirect methods, such as resistivity measurements and chloride titrations, to obtain generalized salinity estimates sufficient for evaluating hydrocarbon reservoirs. In contrast, hydrogeologists often use direct laboratory techniques that provide detailed information about the broad range of ions and elements in the water, offering more insights into its composition and quality. As a result, additional validations or adjustments of oil and gas data are often required before they are used in groundwater research, making the process more complex.
For oil and gas data to be repurposed effectively in groundwater exploration, such methodological differences and other challenges—which are not widely recognized among water scientists—must be overcome through extensive cross-disciplinary work with oil and gas geoscientists and engineers. The water salinity measurement issue, for example, could be addressed by scientists working together to standardize an additional laboratory-based salinity measurement for samples from future oil and gas boreholes. Such measurements could use methods similar to those applied in water research to ensure the data are more accurate and useful for hydrogeological applications.
Renewability and Sustainability
Each promising discovery of a deep aquifer comes with a responsibility: Before it’s tapped as a water resource, we must first assess its volume and quality, determine whether it is connected to surface waters, and understand both recent and historical recharge processes. We also must gauge its potential to continue serving as a water source in decades, centuries, and millennia to come.
Deep aquifers receive large quantities of water during prolonged wet periods but very little during dry stretches. The current climate conditions of the African Sahara, for example, offer little water to the deep subsurface. In contrast, this region had a much wetter landscape between 11,000 and 5,000 years ago that provided more water to deep aquifers.
Assessments of sustainability must consider renewability on human (i.e., century) timescales.
Our concept of the renewability of these aquifers can thus vary greatly depending on the timescale considered. Although current recharge rates may be low or even negligible for many deep aquifers, anticipated climatic changes—resulting partly from Earth’s orbital changes, which affect the amount and distribution of sunlight reaching the planet—may replenish them over thousands of years. So viewed with a longer perspective, some deep aquifers can indeed be considered renewable. However, assessments of sustainability must consider renewability on human (i.e., century) timescales.
Investigating deep-aquifer renewability over varying timescales requires using high-capacity computing to process extensive regional climate, topographic, geological, and hydrogeological databases that help scientists understand and model key aquifer characteristics. These characteristics include water provenance, infiltration rates, and recharge mechanisms, as well as the climate dependence of these factors; aquifer storage capacity and geometry; climatic conditions at recharge points over extended periods; and the locations of discharge and extraction points relative to recharge areas, along with the time it takes water to travel from source to extraction points.
Renewability is not the only factor determining deep-aquifer sustainability, though. Sustainability is also intrinsically related to usage and fundamentally depends on four key parameters: effective recharge, storage capacity [Cuthbert et al., 2023], discharge, and extraction. If storage capacity is reasonably assumed to be constant on relevant timescales—that is, we disregard potential effects on capacity from changes in rock porosity and sea level fluctuations (which can, e.g., increase or decrease seawater infiltration into coastal aquifers)—only recharge, discharge, and extraction remain relevant. Natural recharge, as discussed, is highly variable over decades to millennia, and discharge can fluctuate accordingly. Managing aquifer sustainability thus depends crucially on controlling extraction in relation to these two variable parameters.
Research to Support Resource Decisions
Every aquifer is unique, and its feasibility and sustainability as a water resource must be assessed individually within its own local context. Ultimately, decisions about whether to tap deep aquifers may reflect the balance between the need for water and the energy used to acquire it, which involves weighing political, socioeconomic, and regulatory issues, as well as technical considerations.
Energy requirements may vary significantly. Under certain conditions, water from deep aquifers can exhibit artesian flow, naturally reaching the surface without the need for pumping. In other cases, as with shallow groundwater, extracting deep groundwater may require energy for pumping.
Amid rising global water demand, deep groundwater remains underrepresented in hydrogeological research and government policies.
Although drawing shallow groundwater may seem like it would always require less energy, the energy needed for extracting deep groundwater can be comparable to, or even less than, that required for shallow sources, especially considering that yield capacities of deep boreholes can exceed those of shallow boreholes by more than 20 times [Godfrey et al., 2019; Ruden, 2007]. In addition, in deep boreholes, pumps are typically installed at relatively shallow depths, similar to those in shallow boreholes, because the pressure within the aquifer helps push water toward the surface.
Amid rising global water demand, deep groundwater remains underrepresented in hydrogeological research and government policies. However, some African nations, including Namibia, Somalia, and Tanzania, are taking steps to incorporate deep-groundwater exploration into their water resources plans, either through official policies or with pilot drilling projects.
Given the potential scale of, and return from, deep-groundwater sources—and the scope of growing concerns over water security globally—we suggest that further exploration efforts are needed. These efforts demand interdisciplinary approaches and cross-sectoral collaboration among academic scientists, the oil and gas sector, and governments. With such work, we not only can better understand the resources deep beneath us but also better assess their sustainability and manage them responsibly.
References
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Author Information
Claudia Bertoni ([email protected]), Earth Sciences Department, University of Oxford, Oxford, U.K.; and Fridtjov Ruden, Elizabeth Quiroga Jordan, and Helene Ruden, Ruden AS, Oslo