The United Nations World Water Development Report 2022 on groundwater, a synthesis

ABSTRACT This paper is a synthesis of the 2022 250-page United Nations Report on groundwater (https://unesdoc.unesco.org/ark:/48223/pf0000380721), which accounts for 99% of liquid fresh water on Earth and is the source of one-quarter of all the water used by humans. Many of the world’s largest cities, and numerous smaller cities and towns, rely on groundwater. The report presents an overview of the regional presence and use of, and demand for, groundwater. It relies on a synthesis of 700 bibliographic references. Very useful figures are gathered on water resources and uses. The report barely touches on the role of geology, and hydrogeological expertise in preparing the report would have been helpful. More explanations of various hydrogeological aspects would have been useful, such as underground 20 watersheds and basins, runoff, flow rates, the water cycle (large and small), vertical water exchanges, and groundwater flow inertia which helps populations to adapt to annual and seasonal fluctuations. The variability in atmospheric components (e.g. meteorological variability) is more often dominated by the climate change question. This intellectual approach puts groundwater under the influence of atmospheric effects, which leads to an alarmist discussion of multiple aspects: abnormal water table drawdown or increase, exposure to surficial pollutions, and many negative aspects of saltwater intrusion in coastal aquifers. the bibliographical references in the report can be found following the page numbers of the document, cited in parentheses here. The very numerous bibliographical sources are responsible for some contradictions or errors. Some economic elements are almost ignored, such as bottled water and thermalism in 35 relation with groundwater, and North–South cooperation pre-1990, although many positive results related to this period are depicted, which have greatly helped to improve methodologies.


Introduction
Groundwater participates in numerous natural cycles and processes, and it plays an important role in sustaining human health, livelihoods, economic development and ecosystems. Awareness of these interlinkages has led to the widely shared view that groundwater development and management should take place within integrated approaches. This does not detract from the critical need to properly understand the specific facets of groundwater and of the processes it is involved in. The United Nations report discussed in this article is intended to contribute to this understanding (24).
Humans fulfil their need for good-quality water from subterranean sources. Springs, the surface manifestation of underground water, played a key role in social development, and the first water wells were sunk initially in parts of Asia, the Middle East and Ethiopia to depths of up to 50 m. During the twentieth century (and even the nineteenth century: the Grenelle borehole in Paris was drilled in 1841), there was a major boom in water well construction for urban water supply. Major advances in water well drilling, pumping technology, energy access and geological knowledge allowed for faster drilling of deep boreholes, and for the extraction of larger quantities of water. Shallow wells, installed with affordable technology and fitted with hand pumps, were developed for community supply in rural areas. Groundwater thus became a key natural resource supporting human well-being and economic development -but one that was still widely misunderstood, undervalued, poorly managed, and inadequately protected (60).
The Intergovernmental Hydrological Programme (IHP)'s major achievements with special focus on groundwater include the worldwide promotion of hydrogeological mapping, the establishment of a global initiative on transboundary aquifers (the Internationally Shared Aquifer Resources Management initiative), the World-Wide Hydrogeological Mapping and Assessment Program and the establishment of the International Groundwater Resources Assessment Centre (IGRAC; ref. 3) (Figure 1). The main objective of IHP's eighth phase was to translate scientific knowledge into the action that is required for water security (32).
Global groundwater withdrawals were estimated to have exceeded 900 km 3 /year by 2010, with water wells and springheads providing some 36% of potable water supply (60). Groundwater, the world's largest distributed store of fresh water, is naturally well placed to play a vital role in enabling societies to adapt to intermittent and sustained water shortages caused by climate change. It is essential to satisfy the increased demand for water in order to realise many of the United Nations' Sustainable Development Goals, including zero hunger, water for all and climate change action. Groundwater, the world's largest distributed store of fresh water, is naturally well placed to play a vital role in enabling societies to adapt to intermittent and sustained water shortages caused by climate change. Rapid economic and population growth compounded with poor planning and inadequate governance has resulted in overexploitation and water quality degradation in certain areas, threatening the lives and livelihoods of populations that depend on this vital resource (133).

Data
Although often relatively expensive, monitoring is a wise investment. Identifying problems at an early stage can be highly cost-effective, allowing mitigation measures to be introduced before serious deterioration of the resource takes place. Conventional monitoring programmes can be augmented by citizen science initiatives, which can also promote the integration of local knowledge into hydrogeological characterisation and groundwater system assessments. Remote sensing techniques have also been used by the scientific community for the past 25 years to improve monitoring and estimation of groundwater resources (7). But, to date, the focus of data collection has been on the spatial extent of open water with no focus on groundwater or on the differentiation or mapping of aquifers. Their quality (natural or altered by human activities) and their temperature are essential (97).
Advocacy for open data is growing and online infrastructure is being developed to support the sharing of groundwater data and information. Also, the number of national and international portals with access to groundwater data and information is steadily growing. Conventional monitoring programmes can be augmented by citizen science initiatives, where volunteers take additional measurements/samples. Manual sampling can be supported with new technologies, such as smartphone apps for data collection, which make paper forms obsolete and hence decrease errors in data handling. Citizen science goes beyond just taking measurements: engagement of the public (e.g. via semi-structured interviews) and capacitybuilding for in situ measurements can help ensure the integration of local know-how into hydrogeological assessments. In doing so, one-way communication from the scientific community towards the civil society can be avoided (150).

Observation and understanding of natural hydrological mechanisms
In many low-and middle-income countries, hydrogeological capacity is missing, even when groundwater makes up the largest part of their managed water resources. This often comprises both technical and institutional capacity (8).
Statistics relating to water abstraction and use in industry are notably scarce. Industry and energy account for 19% of global fresh water withdrawals. This number refers to self-supplied water, which includes groundwater. The data also point to large geographic differences, with industrial withdrawal varying from 5% in Africa to 57% in Europe. However, Aquastat data are not parsed out into industrial groundwater withdrawals. Such data can only be found for some higher-income, industrialised countries. The United States Geological Survey shows that for self-supplied industrial use in the USA the total amount of water withdrawn decreased significantly from 1985 to 2015, while surface water is still the main source. According to another estimate, groundwater contributes 27% of the water withdrawn globally for manufacturing (74).
Remote sensing (in the form of airborne and satellite observations) is widely used to study and predict hydrological processes. Since changes in surface water bodies can be detected directly from remote observations, remote sensing has found a broad application in hydrological science and the management of rivers and lakes (150). Although satellites do not give direct indications of groundwater quality, they can provide information on land use and geology, which can be related to groundwater quality and vulnerability to pollution. Moreover, remote sensing results can be used as additional variables to support predictive modelling. For instance, predictions involving certain contaminants can be derived from information about anthropogenic activities, such as soil salinity problems arising from intensive irrigation (151).

Water resources inventory
The fresh water volume is irregularly distributed over the continents, which is explained partly by differences in the size of the continents and partly by differences in the mean volume of fresh water per unit of area. At the scale of countries and smaller territories, the spatial variation in equivalent water depth is much more pronounced, with values ranging from 0 to almost 2000 m (12).
The weakest point of the report is its neglect of geology, the three-dimensional analysis of aquifers, the vertical properties concerning protection and seepage, the actual properties of sedimentary aquifers or basement rock aquifers, and the reservoir role of saprolite, which would have permitted an objective representation and a proper explanation of the worldwide distribution of groundwater resources. The report focuses on simple observations, and provides poor explanations of them.

Groundwater and interrelations with surficial ecosystems
Aquatic groundwater-dependent ecosystems can be found across a variety of landscapes, ranging from high mountain valleys to the bottom of the ocean and even in deserts. Possibly the most obvious groundwater-dependent ecosystems are springs: highly diverse, endemic and abundant ecosystems found in over 2.5 million locations around the world, including caves, oases, fountains, geysers and seepages. Though small, spring habitats are exceptionally biodiverse. A study in northern Arizona detected 20% of the flora of an entire forest in springs that represented <0.001% of the landscape (93).
Wetlands are referred to here in accordance with their definition under Article 1.1 of the Ramsar Convention, as areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters. (93) Groundwater discharging to marine environments is a ubiquitous phenomenon along coastlines. It creates unique ecosystems where saltwater and fresh water mix and can be a substantial nutrient source for coastal and estuarine waters (93).
Another tool widely used to protect aquatic ecosystems is the monitoring of environmental flows, which are the quantity, timing and quality of freshwater flows and levels necessary to sustain aquatic ecosystems that, in turn, support the cultures, economies, sustainable livelihoods and well-being of humans. For example, the European Union Water Framework Directive establishes groundwater quantity and quality thresholds. In this case, minimum environmental flows should not be considered "ecological demands", but restrictions to various uses in order to prevent environmental flows from competing with other human water demands. There are many environmental flow methods, but very few explicitly consider or quantify the groundwater contribution to environmental flows (99).
Vegetation impacts have not yet been thoroughly researched or integrated in the management of groundwater recharge. In the Sahel, groundwater levels have increased over the past few decades, primarily as a result of changes in vegetation from deeprooted natural vegetation to shallow, less water-consuming crops. Water holes in arid environments are often purely groundwater-fed, and thus groundwater is crucial to sustaining the complex food webs of arid landscapes, such as savannahs. Wildlife-dug waterholes can be life-supporting, and demonstrate the intricate links among groundwater, ecosystem support and biodiversity (94).
In more humid areas, riparian forests and wetlands can purify nitrogen-rich runoff and drainage from agricultural and livestock farming activities, reducing the nutrient loading. Conversely, in more arid areas, seasonal flooding may enhance groundwater recharge in floodplains, while sedimentation may provide significant nutrient and soil-improving amendments (94).
Subterranean ecosystems are ubiquitous but generally poorly understood. Organisms and microorganisms are found in varying composition and abundance in all aquifers. These subterranean ecosystems often help purify water and impact aquifer storage, sometimes increasing storage through bioturbation and feeding on biofilms and other times decreasing storage through clogging pore space (94).

Water abstraction and use
After inspection of time series of annual records, it appears that groundwater withdrawal rates in most European countries have stabilised or are even slightly declining. This seems to be the case in North America also (i.e. Canada, USA and Greenland) and in South and East Asia (19). While the rate of increase in fresh water use has levelled off in most developed countries, it continues to grow in most of the emerging economies, as well as in middle-and lower-income countries. Globally, water use is expected to grow by roughly 1% per year over the next 30 years, driven by increasing demand in the industry as well as by municipal and domestic uses, in combination with population growth (15).
Approximately 70% of global groundwater withdrawals is used in the agricultural production of food, fibres, livestock and industrial crops. An estimated 38% of the land equipped for irrigation is serviced by groundwater. In the broader context, irrigated agriculture still accounts for 70% of freshwater withdrawals, and an estimated 90% of all water evaporation. Water use for food processing is also significant: up to 5% of global water use. These numbers highlight the overall water-intensive character of food production (48) (Figure 1).
The area of land equipped for irrigation (including full control, equipped wetlands, and spate irrigation) globally has more than doubled since the 1960s, from 139.0 Mha in 1961 to 325.1 Mha in 2013. Asia accounts for 72% of the global area equipped for irrigation, predominantly in South and East Asia, and 41% of its cultivated area is irrigated. Sub-Saharan Africa has the least irrigation development: the irrigated area accounts for 3.4% of the regional cultivation, compared with 41% in western Asia (49).
The countries with the largest area under irrigation are China (73 Mha), India (70 Mha), the USA (27 Mha) and Pakistan (20 Mha). The proportion of total groundwater abstraction used for irrigation varies significantly among these countries. India, as the largest groundwater user globally, at an estimated 251 km 3 per year abstracted, uses 89% of its groundwater abstraction for irrigation. China is relatively less reliant on groundwater, with an estimated 54% of total groundwater abstraction going into irrigation on average, but with significant geographic disparities, with the North China Plain being more critically reliant on groundwater compared to the southern regions. Other countries, such as Bangladesh, Iran, Mexico, Pakistan, Saudi Arabia and the USA, are also heavily reliant on groundwater for irrigation, with amounts of groundwater abstraction for irrigation ranging from 71 to 94%. The global economic contribution of groundwater to agriculture is estimated at roughly 210-230 billion dollars per year, with a productivity of US$0.23 to US$0.26 per cubic metre produced (50).
Groundwater has a significant impact on engineering and construction. Underground construction, for example of tunnels, frequently requires either temporary or permanent dewatering. Deep excavations and buildings with large underground areas, such as basements and underground parking lots, face the same challenges, frequently exacerbated by the large volumes requiring removal and also by the water pressures that are the result of high local or regional heads. Unlike mining, which mainly occurs in more remote areas, often with relatively untouched groundwater, construction is commonly carried out in urban areas, where the groundwater may already be polluted, requiring treatment upon dewatering and before discharge (78).
Groundwater abstraction has played a major role in accelerating food production from the 1970s onwards, especially in semi-arid and arid areas with limited precipitation and surface water. At the same time, it has sustained local to regional economies that are dependent on groundwater for livelihoods, economic growth and food security. In order to meet global water and agricultural demands by 2050, including an estimated 50% increase in food, feed and biofuel relative to 2012 levels, it is of critical importance to increase agricultural productivity through the sustainable intensification of groundwater abstraction, while decreasing water and environmental footprints of agricultural production, which can be achieved, for example, through agro-ecology and better food policy and economic instruments (48).
To understand the diverse and dynamic impacts of agricultural groundwater use across the globe, four types of socio-ecologies can be pointed out: • arid agricultural systems, such as in the Middle East and North Africa, where groundwater is also increasingly in demand for higher-value nonagricultural uses; • industrial agricultural systems, such as those in Australia, Europe and the western USA, where groundwater supports commercial precision agriculture and attracts relatively high financial resources for its management; • smallholder farming systems, such as in South and (increasingly) Southeast Asia as well as the North China Plain, where groundwater irrigation is the mainstay of 1-1.2 billion, mostly poor farmers; • groundwater-supported extensive pastoralism, as in much of Sub-Saharan Africa and Latin America (48).
Groundwater presents a local water resource that is perennial, relatively easy to access and adapted to agriculture use, and allows the improvement of prosperity, food security and general quality of life. Data recorded in Asia 20 years ago show that compared with large projects of irrigation by channels, the multiple uses of groundwater were favourable, with better equity between individuals, social classes, gender, and regions. In Africa, Asia and South America, when poor farmers try to improve their livelihood by small-scale exploitation or cattle breeding, they often resort to groundwater and small pumps. This is limited by the cost implications associated with groundwater exploration and construction, and difficulties in financing. Only 3-5% of total cultivated land in Sub-Saharan Africa is under irrigation, with the majority concentrated in three countries: Madagascar, South Africa and Sudan. Agriculture is, however, the main source of livelihood for many people. The agricultural sector accounts for about 30% of the gross domestic product in Sub-Saharan Africa but employs about 65% of the population, the majority of whom are women, yet crops are produced almost entirely under rainfed conditions. Given the importance of the agricultural sector in Africa, any improvement in the sector has the potential to transform the living conditions of the population. The development of groundwater could act as a catalyst for economic growth by increasing the extent of irrigated areas, thereby improving agricultural yields and crop diversity, and ultimately transforming the entire value chain (118). Various industrial processes use water, namely production, treatment, cleaning, dilution, refreshing and transportation. They are used in foundries, oil refineries, chemistry, and food and paper production. Many industrial processes use groundwater where surface water is limited in quantity but also where quality is important. Groundwater is often less contaminated than surface water and requires less treatment. Industries like textiles and garments, leather, and pulp and paper have a high specific water consumption. For example, the wet processing of 1 kg of cotton fabric needs 250-350 L of water. The tanning industry has a specific water use of 170-550 L per hide. Water withdrawal in European paper production for pulp, paper and board production was around 3700 million m 3 in 2012, of which 90% came from surface water, and 8.5% came from groundwater sources (76).
The textile industry is a large user of groundwater. In Bangladesh, this sector supplies itself with groundwater for the different units of wet processing facilities, and therefore is in desperate need of efficient water management. Almost all dyes, speciality chemicals and finishing chemicals are applied to textile substrates from water baths/using wet processes. In addition, most fabric preparation steps, including desizing, scouring, bleaching, and mercerising, use aqueous systems and groundwater (76).
The beverage, bottled and mineral water sectors are unique in that groundwater is a raw material that becomes the product. According to market research, the industry is expected to grow 8% annually. The sources of mineral water used for human consumption need special attention, as these watersheds and aquifers need to be protected against any type of microbiological and chemical pollution (77). Large international food and beverage companies are in increasing competition and in dispute with local communities and municipalities over the amount that can be withdrawn without depleting local groundwater resources and affecting domestic and other supply (78).

Water resources management and modelling
Deterministic models are powerful tools, based on physical and/or chemical properties of the environment to first copy then predict an aquifer's steady state and its evolution. Yet models are a simplification of the real world, and they come with a level of uncertainty, which depends on several factors, including the number and the complexity of physical and chemical processes simulated, the heterogeneity of the subsurface, the quality and quantity of input data, and the model calibration. With the advances in computational capabilities and algorithms, it is highly recommended to carry out uncertainty analyses, whereby the level of confidence of model predictions can be estimated (151). Data collection is the first step of modelling and scenario analysis.
The scientific knowledge of hydrogeology and the methods and tools available are sufficient to address most groundwater management issues, like siting wells, optimising abstraction and predicting its effects at the local and regional scale, preventing contamination, etc. The challenge lies more in the scarcity of reliable data for area-specific groundwater assessments and scenario analyses, especially in low-income countries, and in the limited dissemination of data, information and knowledge among researchers, practitioners and decision makers (144). Groundwater models nowadays are also used as one component in much more complex hydro-economic modelling frameworks, where scenario analyses encompass the outcomes of various models, addressing a diversity of topics and issues (152).
Modelling is, however, a weak element in the report. It does not enter into the successive and necessary steps of creating and calibrating a model, data collecting in sufficient number, hydrological model performance. Models are very often used one time only, to solve a single question, thus losing the intellectual investment needed to create the model. National actions -or actions within water basinsbegin to hatch for capitalising the efforts and create model libraries, widely accessible, and widening the toolbox for water management.

Governance and legislation
A groundwater policy includes fundamental standards and basic guiding principles. Sustainability; efficiency; equity; the precautionary principle; the "polluter pays" principle; conjunctive management; demand, supply and maintenance management; and integrated water resource management are critical to inform future decisions (158). Groundwater governance and management both address abstraction and allocation, use efficiency, and quality protection. While often used interchangeably, this report distinguishes between the two concepts. Groundwater governance processes set the conditions for and enable groundwater management, planning and policy implementation. Principles for "good" water governance include equitable access, accountability, transparency, stakeholder participation, inclusiveness, etc. Groundwater management is action-oriented: focusing on practical implementation activities and the "nitty-gritty" of day-to-day operations, it emphasises the results of decisions (40).
Legislation regarding groundwater resources defines binding and enforceable entitlements and identifies rights and obligations that are subsequently operationalised through management decisions, including monitoring and enforcement. For instance, the European Union's Water Framework Directive and its Groundwater Directive have triggered many management activities (42). It is essential that countries commit themselves to developing an adequate and effective framework for groundwater governance. This requires that governments take the lead and assume responsibility to set up and maintain a fully operational governance structure, including the knowledge base, institutional capacity, laws, regulations and their enforcement, policy and planning, stakeholder participation, and appropriate financing. It is also incumbent upon countries to ensure that their policies and plans are fully implemented. It is imperative that governments assume their role as resource custodians in view of the common-good aspects of groundwater and ensure that access to (and profit from) groundwater is distributed equitably and that the resource remains available for future generations (10).
Conjunctive use of groundwater and surface water in agriculture is significant. It typically supports intensification within existing surface water irrigation areas where it enables perennial cropping and salinity control. The evolution of conjunctive use is typically not managed or planned, but rather a coping mechanism for farmers when established surface water systems fail to secure perennial fresh water access (50). Mining can use the same technologies available to other industrial sectors to manage water, decrease its use and improve efficiency. In some cases, poorer-quality water can be used. For example, saline water can be improved with some separation processes. Groundwater management can be incorporated into regulations covering an approach, from abstraction to optimized use of water (81).
In order to access and use groundwater resources sustainably, cooperation, sharing and partnerships with the many other groundwater stakeholders is necessary for overall management of the resource -in other words, stewardship. This approach values water in many ways, from economic accounting and its monetary value through environmental worth, to sociocultural values, such as recreational, cultural and spiritual values (86). Industry and energy have more control, through their ownership and organisational structures, over how much groundwater they use. As a result, they can act more nimbly and quickly than governments. Water reuse and recycling, and zero-discharge initiatives, are all focused on using less water.
These activities in turn form part of the shift to greener industry, environmental and social governance, and industry and energy water stewardship. They can dovetail into industry improvements and larger societal and government plans and activities, such as integrated water resources management, and move towards circular economies. Even the financial sector is now exerting its considerable influence over sustainable investing, and this will have a knock-on effect, favouring clients, including those in industry and energy who need financing and use groundwater sustainably, and encouraging others to do so (88).
In recent years, economic tools have been developed more broadly. Financial institutions help companies to better consider hydrologic hazards which can also influence groundwater. Hydrologic hazards can threaten the reputation, earnings, and financial stability of companies that ignore them. Financial institutions require them to institute production consistent with water resource protection.
In the majority of jurisdictions today, public or government ownership of groundwater is the norm, and groundwater extraction and use are based on administrative entitlements such as individual permits, licences or concessions that, in many jurisdictions, are time-bound and qualified in terms of volumes and rates of extraction. However, in some jurisdictions with sizeable populations, such as India, Pakistan, the Philippines and more than half of the states in the USA, groundwater rights are tied to land ownership and groundwater is regarded as private property (43). An additional factor is commercial and political pressures to over-exploit groundwater, alongside the overall political situation and the position of the government in the eyes of the local population (including mutual trust or the lack thereof) (45).
The nature of groundwater makes it highly suited to dispersed water supply for populations living in rural areas, and it is often the most cost-effective way of providing a secure water supply to villages. This is especially the case in Sub-Saharan Africa and South Asia, where the rural population is large but dispersed. Groundwater will continue to be the predominant source of household water for the rural population in developing nations. Community hand-pump water wells began to be adopted in the 1980s, during the United Nations International Drinking Water and Sanitation Decade, and led to a steady increase in access to safe water for rural people in low-income countries (67).
[In the 1970s and 1980s, the village hydraulic programmes in western Africa and the bilateral cooperation projects between local authorities and competent organisations faced this objective and developed advanced programmes for the characterisation of groundwater resources in difficult environments, subtropical zones and crystalline bedrock. Both parties profit from both local development and improved methodologies in their specific context. This cooperation seemed to be ignored in the report.] Private water wells also play an important role in providing for domestic use around the world. For example, Mali has over 170,000 private traditional family water wells, and it is estimated that in Ethiopia, Malawi and Zambia, more than 85% of households rely on private water wells for their drinking water supply (68). Rural community groundwater supply is not without its challenges. Recent research in Ethiopia, Malawi and Uganda showed that less than 50% of water boreholes were functioning reliably and about 25% were contaminated with pathogens. The reasons are complex and include engineering and design issues, along with deficient long-term maintenance and management of the water service (68). This is the problem of the small cycle of water in rural areas, which is not discussed in the report.
Solving the question relies on the national agencies for water management, the Ministries, responsible for groundwater resources and water supply. The national agencies have a responsibility to ensure: • the basic regulation of groundwater abstraction. • adequate coordination on groundwater between national actors, basin agencies, local groundwater user organisations and aid/relief organisations, as appropriate; • effective mechanisms for groundwater monitoring and regulation enforcement; • horizontal coordination with other departments on groundwater; and • support for operational arrangements in transboundary aquifers.
Local municipal agencies will need to: • ensure that the local water permit system is functioning; • stress the need for attention to the operation and maintenance of water wells; • coordinate solid-waste and wastewater management to protect groundwater; • consult and support local groups that work on sanitation and waste management; • communicate to farmers the need to prevent groundwater pollution; and • encourage education/vocational training institutes (including youth groups) to include water supply and groundwater management in their curricula (71).
Several obstacles must be faced, including the current failure to clearly differentiate types of water supply sources in national and international databases, and the fact that private water well abstraction often falls outside the radar of public databases (71). Groundwater is available to provide a very useful and often underused resource for industry, but it has to be sustainably managed in concert with other stakeholders. It is a key resource for many industries and contributes in this way to employment and economic growth. Industries that withdraw groundwater include manufacturing, mining, oil and gas, energy generation, engineering, and construction. Industries with a high groundwater dependency via supply chains include the apparel and food and beverage sectors. Their combined withdrawals can lead to stronger competition/interactions between the different industries, as well as with other sectors, communities and the natural environment (74).
The further development of groundwater in Sub-Saharan Africa is currently limited not by a lack of groundwater, but rather by a lack of investment. There is a pressing need to find ways to unlock the potential of groundwater, to help develop sustainable livelihoods and achieve equitable growth. This involves investments in infrastructure, institutions, trained professionals and knowledge of the resource. Technological advancements (such as Earth observation, renewable energy and advanced drilling methods) can support the development of groundwater but must be accompanied by a strong professional groundwater community, to get the best from the technologies (121).
The WFD ([European] Water Framework Directive) requires identifying and characterising groundwater bodies and -in conjunction with monitoring dataassessing the impacts of human pressure on groundwater. It also addresses the risks of failing to meet environmental objectives and establishes measures for achieving and keeping good quantitative and chemical status. The WFD has contributed to harmonising approaches to delineating and assessing groundwater bodies, also in the EU's neighbourhood (123).
During the second cycle of River Basin Management Plans (2016)(2017)(2018)(2019)(2020)(2021) in the EU, the status of groundwater bodies was assessed, showing that a good chemical status has been achieved for 74% of groundwater bodies, while good quantitative status had been achieved for 89% of the groundwater bodies. In Canada, the constitution decrees that provinces and territories have primary legal jurisdiction over water and groundwater, while the federal government has powers to manage groundwater on federal lands, including national parks (125). Elaborating and implementing groundwater management plans for priority aquifers is the ultimate test of adequacy for governance provisions, and involves the following stepwise sequence of actions in each adaptive management cycle: • identification and characterisation of groundwater management units; • assessment of resource status, opportunities and risks; • reaching consensus on required aquifer services and plan objectives; • drawing up the management strategy (including specific measures, monitoring needs and associated finance); • planning implementation over a specified period, with systematic monitoring, review of effectiveness, and adjustment of the next cycle (159).
China's take on groundwater policy and planning shows how the two are sometimes indistinguishable. The 1988 Water Law lists planning in an independent chapter to emphasise its importance and legal status. It states that integrated water planning should be centred on watersheds rather than on administrative boundaries, with the regional planning complying with watershed planning, and based on a comprehensive scientific survey, investigation and assessment at relevant administrative levels. However, the stipulated segmentation in managing water quantity and quality inevitably hinders effective integration. A Plan of Groundwater Pollution Control and Remediation and a National Plan for Land Subsidence Prevention and Control provided official directives for groundwater management up to 2020. These and other plans apply in parallel with the "Three Red Lines" policy of 2012, which sets targets on total water use, efficiency improvement and water quality improvement. Further scientific planning should protect soils and groundwater to meet the 14th Five-Year Plan. Moreover, the Water Pollution Prevention and Control Action Plan aims to control groundwater quality (162).

Pollution
There are many sources of anthropogenic groundwater pollution. Most of them are located at or near the land surface: agriculture, households, sewerage, landfills, industries and other urban sources, storage tanks, roads, canals, etc. (35). In the South America and Caribbean area, there are shortcomings in the protection and monitoring of groundwater, giving way to its intensive exploitation and/or contamination, ultimately endangering its sustainability as well as its accessibility to the most vulnerable populations, who depend on these groundwater sources for their drinking water supply (6).
It is estimated that agricultural pollution has overtaken contamination from settlements and industries as the major factor in the degradation of inland and coastal waters. The principal pollutants from agriculture are nutrients, pesticides, salts, sediments, organic carbon, pathogens, metals and drug residues. Nitrate, from chemical and organic fertilisers, is the most prevalent anthropogenic contaminant in groundwater globally, notably leading to eutrophication of surface waters. In the European Union, 38% of water bodies are under significant pressure from agricultural pollution; in the USA, agriculture is the principal source of pollution of rivers; and in China, agriculture is responsible for a large proportion of surface and groundwater pollution by nitrogen (54).
Types of pesticides commonly used in agriculture include insecticides, herbicides and fungicides. When improperly applied or disposed of, they can pollute soil and water resources with carcinogens and other toxic substances, while their degradation products can be hazardous to the terrestrial and aquatic biosphere, as well as to human health. The global market in pesticides is worth more than US $35 billion per year. Contamination with organic micro-pollutants, like pesticides, in agricultural areas is less documented in emerging economies. However, where the issues have been investigated in vulnerable socio-economic environments with intensive agriculture, results have shown the presence of contaminants in excessive concentrations, indicating an emerging environmental and health hazard of critical concern (55).
Among the pollutants produced by households and found in sewerage, microbiological compounds and so-called "emerging micro-pollutants" are distinctive (35). The use of antibiotics for intensive livestock farming has increased with the growing global demand for meat. It is largely unregulated in developing countries, with China as the leading registered producer and consumer of antibiotics within livestock farming. While antibiotics protect animals from infections, they also generate antibiotic-resistant bacteria that can be pathogenic to humans and that are very difficult to treat. They are typically transmitted to the environment, including groundwater, through animal waste. A widespread prevalence of antibiotic-resistant bacteria has been documented at the global level, with groundwater contamination reported in China, Kenya, South Africa and the USA (56).
A considerable number of aquifers in arid and semiarid areas as well as in coastal plains are impacted by geogenic contaminants such as salinity and fluoride. Groundwater in the East African Rift often has high concentrations of fluoride: a study in Ethiopia found that more than 40% of boreholes in the Ethiopia Rift Valley have concentrations above the guidelines established by the World Health Organization (WHO) and therefore constitute a major risk to health (117).
Anthropogenic groundwater quality deterioration is also on the rise, caused by factors such as mining activities (e.g. South Africa), poor irrigation practices (e.g. in the Nile Valley and the Senegal River basin) and urbanisation (e.g. Nairobi, Accra, Maputo, etc.). A recent survey across Ethiopia, Malawi and Uganda reveals that nearly 20% of water wells exceeded the WHO standard for bacteriological quality (118).
The global environmental and social costs of agriculture-derived surface and groundwater pollution are estimated to exceed billions of dollars annually. In the USA, pesticide contamination of groundwater and eutrophication of fresh water are estimated to cost US$1.6-2 billion and US$1.5-2.2 billion per year, respectively. The global annual cost of salt-induced land degradation in irrigated areas is estimated at US $27.3 billion through the loss of crop production. Groundwater pollution from agriculture has direct negative impacts on human health. For instance, high levels of nitrates in water can cause methaemoglobinemia (blue-baby syndrome) in infants (56).
Evidence suggests that laws and regulations to prevent or limit diffuse groundwater pollution from agriculture, and especially their enforcement, are generally weak. There has been more progress in groundwater laws and regulation than in effective implementation and enforcement, which represents a significant obstacle to sustainable groundwater management. In many countries, regulations are poor or non-compliance is pervasive, with groundwater pollution continuing largely unchecked. Attempts to regulate diffuse pollution through pollution fines have not worked, as it is difficult to identify polluters. Economic instruments for pollution control of surface and groundwater are increasingly employed. These include taxes, "setasides" (the conversion of agricultural land to natural uses) and payments to limit production or the intensity of land use. Taxes include polluter payments, dedicated environmental taxes, and taxes on technologies, products and inputs that have adverse ecological consequences (e.g. pesticides). Well-known approaches for reducing pollution, such as "the polluter pays principles" are possible, through green taxes on pesticides and fertilisers, for example, but they are not often applied, are priced too low to act as deterrents, or have unintended distributional impacts, as poor farmers will be hit harder by such taxes (56).
Rural electrification has been a principal driver for groundwater development in India. Concentration of groundwater development is notable when rural power grids are extended into areas that would otherwise rely on diesel generation or wind energy, such as evidenced in Ethiopia, Kenya and South Africa (57). In 2015, the US Environmental Protection Agency finalised a federal regulation for the disposal of coal ash: the Coal Ash Rule, which established requirements for groundwater monitoring. The 2018 data from 4600 groundwater monitoring wells included over 550 coal ash ponds and landfills, representing over 75% of the coal plants in the USA. The data showed that the groundwater beneath almost all coal plants (91%) is contaminated, sometimes to significant levels. Arsenic and lithium are commonly found and the majority of plants have unsafe levels of at least four toxic chemicals that can seep from coal ash. Moreover, the issue is compounded by older closed coal ash dumps that are not covered by the regulation (85).

Groundwater protection
The most sustainable and cost-effective approach to managing groundwater quality is to ensure its adequate protection, thus avoiding contamination. This can be achieved through vulnerability mapping, development of groundwater protection zones and land-use planning (8). Benchmarking can promote behavioural change among farmers by showing them how they perform in comparison to other farmers, in terms of the application of fertilisers and pesticides. promoting corporate social responsibility within the private sector is also advocated (57).
The industrial and mining sectors have a strong potential for increasing water use efficiency, stimulating water recycling and reuse, and limiting water pollution. To reduce or avoid negative impacts of industrial groundwater use, the techniques and methods of resource efficiency and cleaner production, and the employment of eco-industrial parks, will be needed to reach the Sustainable Development Goal Target 12.4 on sustainable production and consumption. Resource-efficient value chains, using the circular economy approach, will minimise the consumption of raw materials, as well as water and energy (79).
To prevent wastewater discharge and its negative impacts, the aim is to treat effluent to recover it as clean water and reuse it in the industrial process, reducing water consumption to near-zero levels. As such, water recycling to control water pollution is achieved in stages, first by making the effluent fit for treatment, through conventional physicochemical treatment, reverse osmosis and/or biological treatment. Thereafter, a series of post-treatment steps remove hardness, silt, turbidity and organics to a level where fouling of membranes does not occur. The Indian government has imposed this process on its textile and garment manufacturing industry by legislation, starting in 2006 in Tamil Nadu. Many factories were shut down by the state's high court, due to their inability to meet compliance requirements. The policy has been expanded to nine states in the Ganges River basin and applied to five industrial sectors: textile, pulp and paper, distilleries, tanneries, and sugar. In recent research and development it has been replaced with the concept of "minimal" liquid discharge that enables up to 95% liquid discharge recovery. This takes into account that attaining the final 3-5% of liquid elimination can nearly double the treatment cost (80).
Biological, chemical and physical treatment technologies are employed and a combination of technologies is often utilised. Biological treatment techniques include bio-augmentation, bioventing, biosparging, bioslurping and phytoremediation. Chemical treatment techniques include ozone and oxygen gas injection, chemical precipitation, membrane separation, ion exchange, carbon absorption, aqueous chemical oxidation, and surfactant-enhanced recovery -others may be implemented using nanomaterials. Common physical treatment techniques include pump and treat, air sparging, dual phase extraction, and membrane techniques like reverse osmosis (80).
Nature-based solutions were designed and implemented in the catchment area of the Sulejów Reservoir (Poland), an area characterised by cyanobacterial blooms due to heavy pollution of groundwater with nitrates and phosphorus from nonpoint-source pollution. A subsurface zone of pine sawdust mixed with soil or limestone led to a reduction in phosphate and nitrate concentrations in the groundwater by 58% and 85%, respectively (100).

Active management and recycling
Groundwater active management is producing an advantage for water resource exploitation through a deliberate enhancement of natural physical or chemical properties of soils and aquifers. It is possible to temporarily overexploit a resource during exceptional dry periods, when other resources are insufficient (Société Hydrotechnique de France, 2007, ref. 4). 1 Recent inventories have demonstrated that offshore fresh or brackish groundwater occurs in many parts of the world, either in submarine discharge zones of aquifer systems that are recharged on the neighbouring land (37) [and a long-time actual project in France in Port Miou, an offshore karstic spring exploited for domestic use]. A recent project in Delhi captures excess monsoon river flow to recharge the aquifer supplying drinking water to the city, which is another form of conjunctive use (64). In Brazil, cities only 1 On page 32 it is explained that groundwater overabstraction will send large volumes to the sea and will contribute to sea level rise and coastal flooding. Extra abstraction would certainly be used for agricultural purposes, which will send most of the water to the atmosphere, by evapotranspiration. If the world groundwater annual abstraction were doubled and if the total amount (e.g. 959 km 3 -2017 data) reached the sea, as the ocean surface has an area of 379 million km 2 , the sea level rise would amount to 2.5 mm.
supplied with surface water were almost two times more exposed to the consequences of the 2013-2017 drought than cities using significant amounts of groundwater. [On Jeju island (South Korea), rainwater is collected on the very large areas of roofs covering greenhouses, exploited at medium altitude. The water is preserved from any pollution and infiltrated into the volcanic fissured bedrock. It percolates slowly downwards, towards the coastal touristic areas, and is used precisely during the touristic season (Pointet, 2020, ref. 5)].
Windhoek (Namibia's capital) receives an annual rainfall of 360 mm, making it one of the driest capital cities on Earth. In the early 1990s, Windhoek's existing water sources (three dams and a groundwater wellfield) began to struggle to meet growing water demands (121). City planners responded with an innovative set of solutions: during times of surplus, treated water was stored underground in aquifers, so that it was protected from evaporation and could be used during times of shortage. Windhoek also began to reuse a proportion of its wastewater, treating it to drinking water standards at a new treatment plant.
Windhoek then began to operate a "dual pipe" water supply in some areas: semi-purified sewage from an old water treatment plant was distributed to sports fields, parks and cemeteries for irrigation, further saving potable water. Windhoek's scheme and other water management actions have proved far less expensive than other water supply solutions, and made Windhoek a world leader in the sustainable use of reclaimed water (132).
[In Spain, in coastal areas (30 places or more) wastewater is treated to a point where it is harmless for the environment. It is then injected in the ground alongside the coastline. Reaching the aquifer, it creates a piezometric dome that slows the groundwater seepage to the sea, which reduces the saltwater intrusion in the aquifer and maximises the available fresh water resource] (ref. 5).
Rajasthan, India's most arid state, is prone to frequent droughts and is highly dependent on groundwater to meet both irrigation and drinking needs. With erratic rainfall, overexploitation of groundwater and the highest evaporation losses in the country, agricultural communities in the region face increasing challenges in meeting their water needs. In 2016, the government of Rajasthan launched a programme to help rural communities become self-reliant in meeting water needs. The programme constructed irrigation tanks, dams, trenches and other water-harvesting structures to capture runoff. It also promoted water conservation through micro-irrigation and improved the watershed by planting trees in barren wastelands and developing pastures. Interim results after two monsoons showed that out of 21 non-desert districts, 16 districts saw an increase of groundwater levels by an average of 1.4 m. Internal impact assessments also reported that participating villages reduced water transportation (i.e. water tankers) by 56% compared to non-participating villages (136).
In Qatar, three types of projects are being implemented. The first one consists of recharge through wells located in depressed areas where rainwater accumulates naturally; this is implemented in nonurban areas to recharge the groundwater basins. The second type uses recycled water, mainly treated wastewater, to recharge deep boreholes in the Doha basin. The third type collects and treats urban stormwater and mixes it with shallow groundwater to recharge deep wells in the Doha aquifer in order to reduce salinity (141).

Geothermal energy
Geothermal electricity production typically requires deep drilling to access high temperatures, and significant permeability at such depths to allow for the free circulation of fluids. The fluids used may be natural groundwaters within deep sedimentary aquifers (e.g. in Italy and California, USA) or igneous complexes (e.g. in El Salvador, Iceland, Kenya). Alternatively, where rocks have limited permeability, they can be artificially stimulated or hydraulically fractured to allow for the circulation of introduced fluids, forming an enhanced geothermal system (e.g. Soultz-sous-Forêts, France). Generation of electricity conventionally requires production of steam at the surface to drive turbines. Electricity can, however, be generated at lower temperatures (<180°C) in binary cycle systems, with the hot water produced used to vaporise organic fluids (e.g. high-pressure butane or pentane) that drive turbines (110) (Figure 2).
By 2020, around 30 countries were generating a total of 95 TWhe of geothermal electricity per year, with a total installed capacity of 16.0 GWe. This marks an increase of 3.7 GWe over 2015 at an estimated cost of US$10.4 billion. The largest producing nations (in order of total installed capacity) are the USA, Indonesia, the Philippines, Turkey and Kenya, all of which are known for their active geothermal and volcanic provinces. The relative growth in wind and solar energy has in recent years outstripped that of geothermal electricity, reflecting the lower cost and perceived risk of the former, and their shorter payback periods. However, geothermal power plants are, contrary to wind and solar energy plants, well suited to producing an electrical base load. Installed capacity is projected to grow by ~20% between 2020 and 2025 (112).
Large modern buildings (offices, data centres, hospitals, etc.) have a large cooling requirement, even in winter and in temperate climates. Many industrial processes also have cooling requirements, and the need for low-carbon cooling will likely increase as climate change progresses. Cool shallow groundwater (e.g. 10-12°C in many parts of the United Kingdom) is well suited for receiving surplus heat and effecting cooling, via a well doublet arrangement. Cool shallow groundwater can also be used for heating via ground source heat pumps. A heat pump is an electrically powered refrigerant device that transfers heat from a cold medium (e.g. groundwater at 10°C) to a warm medium (e.g. a central heating system at 45°C). Although wind and solar technologies can generate low-carbon electricity, relatively few technologies exist to provide low-carbon heating. The heat pump is a key technology that utilises electricity highly efficiently to deliver heating and cooling. It may be able to deliver 3.5 kW of heat to a building for every 1 kW of electrical power consumed, resulting in dramatic reductions in cost and CO 2 emissions. As of 2020, around 6.5 million geothermal heat pumps are thought to be installed worldwide, representing the fastest growing part of the geothermal sector (112).
One of the main opportunities provided by lowerenthalpy geothermal energy is its contribution to decarbonising domestic, commercial and industrial heating and cooling, which accounts for at least 40% of global energy consumption and CO 2 emissions. The installed geothermal capacity for direct thermal supply in 2020 was almost 108 GW, marking a growth rate of ~9% per annum, with 284 TWht per year being supplied. Leading nations include (in order of installed capacity) China, the USA and Sweden, with Scandinavian nations having a high per capita uptake. Of the installed capacity, 78 GW (72%) was provided by geothermal heat pumps (112).
Use of shallow (low-enthalpy) geothermal technology for heating and cooling is especially attractive in temperate continental climates, where there is a large seasonal air temperature "swing" and where groundwater temperatures are not only much warmer than winter air temperatures but also much cooler than summer air temperatures. Here, surplus heat from cooling processes, injected into the ground during summer, can be stored in the aquifer and recovered to be used during winter. This is termed aquifer thermal energy storage. In pioneering nations, such as the Netherlands and Sweden, the ground/groundwater is increasingly seen as just one component (a seasonal source, sink or thermal "buffer") in flexible fifth-generation district heating and cooling networks (113).

CO 2 sequestration
Carbon capture and sequestration (CCS) is the process of storing carbon in deep aquifers to curb the accumulation of carbon dioxide in the atmosphere. It is undertaken because natural carbon dioxide (CO 2 ) sinks (i.e. forests, oceans and soils) are considered unable to accommodate the increasing amounts emitted by humans and to mitigate their consequences for climate change. CCS reduces CO 2 emissions from point sources such as industrial processes or power generation through the chemical capture of emitted CO 2 . This CO 2 is then compressed and injected into subsurface strata at depths in excess of 800 m where prevailing pressures and temperatures are sufficient to convert CO 2 gas into a liquid. Suitable geologic sites are deep aquifers and emptied hydrocarbon reservoirs, protected by overlying impervious strata (114). CO 2 from single sources is stored at pilot sites for CCS research -e.g. Ketzin, Germany; Lacq, France; In Salah, Algeria; Aquistore, Canada -and operational facilities -e.g. Sleipner and Snøhvit, Norway; Decatur, USA; Gorgon, Australia. Projects are also planned at industrial sites, where many emitters of CO 2 can use the same storage site or sites (Porthos, Netherlands; Northern Lights, Norway; Teesside, UK) (114). Largescale geological storage of CO 2 (i.e. projects on the order of 1 Mt of CO 2 per year) include the Sleipner and Snøhvit projects in the North Sea and the Quest Project in Canada (Government of Alberta, 2019). At each of these sites, ~1 Mt of CO 2 that would otherwise be released to the atmosphere is captured and permanently stored annually. Extensive saline aquifer formations, both onshore and offshore, have a theoretical capacity to store billions of tons of CO 2 , although the practical useable capacity will be lower. As sites are often far from large emission sources and intercontinental transport of CO 2 incurs substantial costs, the economic CCS storage potential is country-and region-specific. In most regions, storage capacities themselves do not pose a constraint to CCS use, but government subsidies are still required to cover the costs. CCS is considered an important tool to reduce emissions from fossil fuels from the industrial sector and, when combined with biomass combustion and direct air capture, to achieve net negative emissions (114).

Groundwater and international perspectives
The General Assembly of the United Nations and the Human Rights Council recognise that equitable access to safe and clean drinking water and sanitation are human rights. As such, groundwater resources need to be protected as part of the human right to a safe, clean, healthy and sustainable environment, which was recently recognised by the Human Rights Council (42).
River basin organisations seldom contemplate groundwater, partly due to a lack of knowledge and capacity in aquifer assessment and partly because of a historical institutional separation of surface water and groundwater. As a result, river basin planning becomes incomplete. In several parts of the world, though, cooperation has started and this suggests some emerging best practice, modelled on approaches used in transboundary river basin management (46).
The Asia-Pacific region is the largest groundwater abstractor in the world, containing seven of the 10 largest groundwater-extracting countries: Bangladesh, China, India, Indonesia, Iran, Pakistan and Turkey. These countries alone account for roughly 60% of the world's total groundwater withdrawal. The critical driver of groundwater development in the region is rising demand for water due to growing populations, rapid economic development and improving living standards. Utilisation of groundwater resources has provided numerous benefits for irrigation, industrial activity, domestic use, drought resilience and livelihood enhancement (133).
In several regions of the world there are also aquifers that are still exploited at very low rates, far below the maximum sustainable rates. These aquifers harbour an unexploited groundwater potential, available to be tapped and used. A global inventory of such aquifers is not yet available, but available information suggests that many of them can be found in sparsely populated regions of Sub-Saharan Africa, the northern half of South America, the Russian Federation and Canada, among others. The rates of withdrawal are in parts of these regions constrained by insufficient financial means for appropriate technical infrastructure rather than by low water demands or low availability of groundwater (37).
In much of Africa and Asia, customary water rights are intrinsically linked to land and embedded in land tenure systems. However, customary rules relating to water resources may be unfair or even discriminatory, and may work against the interests of women, children and minorities (43). Lessons on participatory planning can also be drawn from Gujarat and Rajasthan (India). Here, researchers engaged villagers to create ownership and behavioural change around groundwater overdraft. End users learned to monitor rainwater, operate automatic weather stations and put data into a repository app. These enabled calculations of the water balance recharge and assessing how much irrigation could be allowed (162).
Research into urban self-supply from groundwater has revealed that: • in India, an estimated 340 million dwellers depend primarily on self-supply sources from groundwater, and many medium-sized cities are highly dependent on domestic self-supply from groundwater, which can amount to 40-60% of water-supply provision; • domestic self-supply from groundwater in Brazil amounts to about 35% of total supply to Sāo Paulo in drought conditions (despite not being recognised by the authorities and the city not being underlain by a major aquifer) and nationally there are at least 2.5 million private water wells which represent 6-7 times the annual investment in water supply by government agencies (65). The case of Nigeria is particularly significant in view of its very large and rapidly growing urban population and high levels of self-supply. By 2009, some 38-43 million of the total urban population (75-80 million) were estimated to be dependent on private water wells, despite the fact that the coverage of public water supply also expanded. In the city of Lagos alone, about 20% of the population of 18-20 million is served by utility water supply, with about 50% owning private boreholes and another 30% also obtaining water from these sources (66).
Groundwater provides the only feasible and affordable way to extend basic water access to unserved rural populations in much of the world. Still, 11% of the global population lacks access to basic water services, and delivering sustainable groundwater services to these people is a major priority. The shallow borehole equipped with a shallow handpump still has a major role to play in the rapid upscaling of village water services and needs to be accompanied by improved attention to maintenance. The ultimate objective is household access to water, which would see a gradual move from community handpumps to reticulated systems, but again mainly based around groundwater. The use of solar energy for pumping has multiple potential benefits in terms of water security and net zero emissions. But this shift relies on boreholes being able to supply higher yields (>100 m 3 / d) sustainably, which will require substantial investment in understanding hydrogeology for appropriate borehole siting (68).
According to the Groundwater Governance Project (2016c) (Food and Agriculture Organization of the United Nations (FAO)), the vision for a "Global Framework for Action" involves effective institutions with the capacity to look ahead and plan, to be inclusive and legitimate in the eyes of stakeholders, and to come to credible and verifiable commitments, with the following components: • sound organisational design with adequate capacity for policymaking and public administration of resource use and pollution protection; • mechanisms for permanent stakeholder engagement and participation to foster socially responsible attitudes and actions on groundwater as a common pool resource; • procedures for cross-sector coordination and comanagement to allow groundwater issues to be adequately addressed in the policies and practices of linked sectors; and • institutions for the management of groundwater resources that traverse intranational and international boundaries.
Institutions, by themselves, are not enough to properly govern intra-and international groundwater/aquifers. They need to be accompanied by national (and sometimes subnational) policies and laws to guide these institutions in their work (46). In California (USA), the Sustainable Groundwater Management Act of 2014 tasks local agencies with regulating pumping in relation to aquifer recharge. These agencies have mandates to track and monitor abstraction and are required to map aquifer recharge areas. The act requires planning of land use to achieve sustainability with transparency and stakeholder engagement and learning within and between basins (162).

Transboundary aquifers
Transboundary areas need, for their long-term protection and management, an international cooperation devoted to shared aquifers. The shared well-being of groundwater, ecosystems and humans may be enhanced by groundwater management, conjunctive water and land management, nature-based solutions, and improved ecosystem protection (97, ref. 3) (Figure 3).
Cooperative management of transborder aquifers can be complex due to obstacles in aquifer-sharing countries, which may include: • a lack of understanding of the transboundary character among the authorities, managers and concerned populations; • the absence of a specific legal and institutional framework; • different management and governance approaches and priorities; • a lack of political will for cooperation and implementation of long-term management (174).
In 2000, United Nations Educational, Scientific and Cultural Organization IHP launched the Internationally Shared Aquifer Resource Management initiative, aimed at preparing a global inventory of transboundary aquifers and developing and supporting cooperation between countries through the improvement of knowledge of transboundary aquifers. The initiative carried out regional studies designed to delineate the aquifers, as well as to assess and analyse hydrogeological, legal, socio-economic, institutional and environmental aspects. The regional inventories revealed that some of the most important aquifers in Africa and in Latin America are transboundary. The initiative contributed towards building the knowledge base and provided guidance for countries' cooperation on trans-boundary aquifers. Substantial advancement has also been achieved with regards to the legal component. UNESCO-IHP assisted the International Law Commission in the preparation of a set of 19 draft articles on the Law of Transboundary Aquifers that are annexed and mentioned in several resolutions of the General Assembly of the United Nations. As a result of International Shared Aquifer Resources Management activities, projects have been initiated in different regions to help countries establish cooperative mechanisms for the management of transborder aquifers (173).
For the 42 countries sharing waters in Europe and Northern America, legal and institutional frameworks for transboundary cooperation increasingly cover aquifers. Out of the 36 countries sharing transboundary aquifers in these region, 24 have reported that operational arrangements cover 70% or more of their transboundary aquifer area. The institutional and legal frameworks, notably the Convention on the Protection and Use of Transboundary Watercourses and International Lakes, have strengthened cooperation in the region (125).
There are very few cases worldwide of interstate agreements regarding transboundary aquifers in force: the Genevese aquifer (France, Switzerland), the Northwestern Sahara Aquifer System (Algeria, Libya, Tunisia), the Nubian Sandstone Aquifer System (Chad, Egypt, Libya, Sudan), the Guarani Aquifer (Argentina, Brazil, Paraguay, Uruguay), the Saq-Disi Aquifer (Jordan, Saudi Arabia), and the Carboniferous Limestones (Belgium, France). Frequently, transboundary aquifers are part of a broader water cooperation agreement developed for transboundary river basins (176).
Cooperative management of transboundary aquifers can be complex due to obstacles in aquifer-sharing countries, which may include: • a lack of understanding of the transboundary character among the authorities, managers and concerned populations; • the absence of a specific legal and institutional framework; • different management and governance approaches and priorities; • a lack of political will for cooperation and implementation of long-term management; • tensions between countries, unequal resource partitioning, groundwater quantity and quality decline, and different management capacities within the social, economic and environmental contexts of aquifer-sharing countries; • fragmented knowledge of the aquifers; • precise data not being shared; • insufficient financing; • a lack of knowledge and capacity for developing and executing scientific/technical studies, and for setting up formal institutions; and • different languages spoken, or different cultural or political orientations, on either side of the border (174).
The European Union Water Initiative Plus for Eastern Partnership Countries (EUWI+) supported the implementation of the Water Framework Directive's integrated management principles to surface water and groundwater in the Eastern Partnership countries: Armenia, Azerbaijan, Belarus, Georgia, Republic of Moldova and Ukraine. In particular for groundwater, it was a challenging change, switching from the earlier, traditional approach focusing on individual, local problems and describing the hydrogeological situation and the chemistry of groundwater only, to a more holistic perspective of delineating groundwater bodies and also considering the relevant human pressures and their impacts on groundwater quantity and chemistry. With the support of EUWI+, in total 117 groundwater bodies were delineated (including 42 transboundary ones). Cooperation between Belarus and Ukraine, both taking steps to apply the WFD, to identify transboundary groundwater bodies demonstrates the process and underlines the need to harmonise across borders (125) (157).
In Southern and Central America, several countries, including parts of Argentina, Brazil, Mexico, Paraguay and Peru, face significant overexploitation and contamination of their groundwater. Mexico has tried multiple approaches to improve the management of its overexploited aquifers. Throughout the region, the most common groundwater quality problems are associated with unwanted elements of natural origin (mainly arsenic and fluoride), anthropogenic pollutants (nitrates, faecal pollutants, pesticides), various compounds of industrial origin (mining by-products, organochlorine solvents, hydrocarbons, phenolic compounds, etc.), and emerging pollutants, such as cosmetics, antibiotics, hormones and nanomaterials. For example, in Bolivia, the quality of groundwater is being threatened by industrial, agricultural and domestic pollution, while in Honduras, the high demand for water in urban areas threatens the future availability of this resource (129). In 2010, the four countries sharing the Guarani Aquifer Agreement (Argentina, Brazil, Paraguay and Uruguay) decided to negotiate a treaty (the first of its kind) focusing on the management of the Guarani Aquifer, which is also notable given the absence of serious conflicts over the natural resource. The negotiations went on for a decade and the Agreement on the Guarani Aquifer was ratified by all parties and entered into force on 26 November 2020. The GAA sets out a transboundary aquifer governance framework. It contains the general rules of international law applicable to transboundary water resources (both surface and groundwater). Countries are sovereign but are at the same time committed to cooperate and to not cause significant harm to neighbouring states (132).
Some of the countries sharing the Dinaric Karst Transboundary Aquifer System (Albania, Bosnia and Herzegovina, Croatia, and Montenegro) initiated in 2010 a collaborative effort to facilitate the equitable and sustainable management of the aquifer system, and to protect the unique ecosystems that depend on it. The project improved the knowledge of karst aquifers in the area and the coordination among countries, agencies and other stakeholders. Being the first major project globally to address transboundary karst aquifers, it has been used as an opportunity to introduce new, integrated management principles in shared karst aquifers of such magnitude. The project identified regional management actions, such as measures regarding policy and legislation, monitoring and data management, training and awareness-raising, as well as necessary investments (177).
Ministers from The Gambia, Guinea-Bissau, Mauritania and Senegal signed, in September 2021, a declaration on the establishment of institutional transboundary cooperation around the Senegal-Mauritanian Aquifer Basin. The ministers also agreed to begin talks on the creation of a mechanism to ensure the concerted and sustainable management of their shared groundwater resources (179).
A Philosophiae doctor was highlighted in 2021 by the Société Hydrotechnique de France, and the author, Jeanne Perrier, received the Henri Massé award. It analyses the state building process and development projects in Palestine through the water and agriculture questions, focusing on governance, negociation and appropriation at a local scale. The violence of them is weakening the state-building process (Perrier, 2020, ref. 7).

Demography and regional projects
Very high rates of urban population growth are generating unprecedented demand for water supply and sanitation, creating an enormous challenge for urban planning (60). About 400 million people in Sub-Saharan Africa do not have access to even basic water services. The majority of these people live in rural areas. Even cities, where household connections are more common, suffer from supply outages and unreliable flow due to high and increasing demand. Therefore, the overwhelming priority for most countries is to improve access, first to basic services and eventually to safely managed household supply (116).
Extensive hydrogeological investigation across the continent reveals that Africa possesses large groundwater resources. MacDonald et al. (2012), who produced the first quantitative groundwater map of Africa, estimated the total groundwater storage in Africa to be 0.66 million km 3 (0.36 to 1.75 million km 3 ). Not all of this groundwater storage is available for abstraction, but the estimated volume is more than 100 times that of the estimated annual renewal of fresh water resources in Africa (116). Though not all of this reserve is usable, one may assume that its volume exceeds 100 times the yearly renewal of water resources of the region.
Regarding climate change, future climate evolution could affect Africa's surface water supply but might not decrease groundwater resources due to the dependence of recharge on intense rainfall events, which are forecast to increase in the future. Therefore, groundwater is expected to be increasingly used as source of reliable water supply throughout Africa (116). Several countries, particularly (but not exclusively) in North Africa, have considerable water security when groundwater storage is taken into consideration. This storage provides a significant buffer before abstraction will impact the regional groundwater system (117).
Despite the great potential for groundwater development in the Sub-Saharan region, several factors hamper an increased use of the resource. Most of these challenges are common across countries. The main governance challenge is to overcome inertia in the institutional setup. Up until the last decade, groundwater has received little attention from policymakers. For instance, while Southeast Asian countries tapped their groundwater resources to transform agricultural activity in the 1970s and 1980s, there was no such effort in Africa. Africa has also missed many other landmark global groundwater development trends. Regular monitoring of groundwater levels or quality, the first step to groundwater management, is restricted to only a few countries.
Globally, in the 1980s, many countries started building groundwater databases and developing hydrogeological maps. There are few universities that teach groundwater as a subject, and few professional bodies for hydrogeologists or drillers. Data-and information-sharing is still in its infancy, despite the rapid growth and availability of data capturing tools. Regulatory frameworks to protect and safeguard groundwater at national levels are either weak or not enforced. There are signs that groundwater is beginning to be taken more seriously, possibly due to the realisation of the major role that groundwater plays in achieving water supply targets (119). For example, several continental-, regional-and national-level initiatives have recently been established. These include the South African Development Community's Groundwater Management Institute, and the Groundwater Desk Office and its associated groundwater programme within the African Ministerial Council on Water. Finance continues to be a critical issue for developing groundwater resources. The funding gap between current spending and what is required to achieve the Sustainable Development Goals is highest for Sub-Saharan Africa, where the achievement of universal water supply would require 10 times the current level of investment of US$13.2 billion. A large proportion of this amount is required for operation, maintenance and rehabilitation of existing systems, which often fail to attract funding. Qualified personnel with the capacity to conduct hydrogeological and geophysical studies is rare. Therefore, work is often carried out by semiskilled personnel, which often results in poor-quality construction or inappropriately sited boreholes, leading to long-term problems with functionality. To site and construct the higher-yielding boreholes required for large-scale irrigation or town supply, the complex hydrogeological environment found in much of Africa demands considerable expertise, which -again -is difficult to find. The general shortage of groundwater professionals impacts the staffing of institutions and of local and national government offices in many countries, hampering emerging initiatives to oversee effective groundwater monitoring, planning and development (119).
Several countries in East Africa are underlain by volcanic rock aquifers, which have limited storage and complex flow paths. The large sedimentary aquifers of Northern Africa are generally located far from the point of need. Groundwater quality in East Africa can be a challenge due to the elevated fluoride levels, and the anthropogenic contamination in urban areas is often compounded by poor sanitation. Widespread contamination from nitrates or pesticides has not yet been identified, although this may follow the patterns observed in other parts of the world as agricultural practices intensify. Although still often cheaper to treat than surface water, costs for treating groundwater are likely to rise (119). An example is Cape Town and the measures the city took to ensure that the taps continued to flow. Historically, Cape Town depended on surface water sources to meet its water needs. However, an increase in population coupled with climate change caused a water shortage crisis, and groundwater became part of the solution (120).
The North China Plain has among the lowest water resource availability per capita, in both China and the world. Rapid economic development over the past 40 years was sustained by groundwater exploitation, resulting in severe groundwater level decline, limiting further development in the region. In recent years, multiple water management plans have been implemented to address this issue. Actions include harvesting rainwater, diverting river water from the south, promoting water-saving irrigation technologies, and subsidising drought-resistant crops and "Grain for Green" projects. As a result of these and other measures, the rate of groundwater decline appears to have been reduced in Beijing and part of Hebei province (137).
In Morocco, aquifer contracts have been introduced as a new participatory groundwater management measure to enhance sustainability based on local needs. Traditional knowledge also continues to be applied, such as rhetaras [= fogaras = laoumnia], which are ancient tunnels used to convey water by gravity for irrigation -mostly from groundwater sources (141). In 2006 the government adopted a new management approach that issues aquifer contracts to all groundwater consumers in a designated aquifer region. Under this participatory framework, agreements are forged among local stakeholders, including governmental organisations, public institutions, agricultural water users' associations and research institutions, to identify needs and secure mutual benefits in order to improve groundwater management and availability. This was a direct departure from centralised management arrangements administered at the national level (142).

Climate and groundwater
Climate change influences groundwater systems directly through changes in the water balance at the Earth's surface, and indirectly through changes in groundwater withdrawals as societies respond to shifts in fresh water availability (102). Climate and land cover largely determine rates of precipitation and evapotranspiration, whereas the underlying soil and geology dictate whether a water surplus can be transmitted to an underlying aquifer (102). One observed and widespread impact of climate change influencing groundwater replenishment is the intensification of precipitation. In areas with inadequate sanitation provision, heavy rainfall events can flush faecal microbial pathogens and chemicals through shallow soils to the water table (5).
The exceptionally wide occurrence of large volumes of groundwater, combined with the resource's unique buffer function, offers great potential for water supply security in climate change adaptation. This provides widespread easy access and the possibility of reliable use when surface water sources fail (e.g. during prolonged periods of drought). The ability of groundwater resources to buffer short-term changes and shocks can also help mitigate the impacts of anthropogenic and natural disasters and emergencies, such as industrial accidents, droughts, floods, earthquakes and landslides, when surface water supply systems are directly affected (38).
Observations show that other regions -for instance, Northern Europe -under current climate scenarios are potentially facing net groundwater accumulation, seasonally or over several years, through longer periods of sustained higher-than-normal rainfall (54).
One observed and widespread impact of climate change influencing groundwater replenishment is the intensification of precipitation. As warmer air holds more moisture, greater ET is required to reach condensation points in a warming world (104). This transition results in fewer light precipitation events and more frequent heavy precipitation. This "intensification" of precipitation is strongest in the tropics, where the majority of the world's population is projected to live by 2050 (104). But the report ignores dissimilarities between intense rainfalls and subsequent flash floods, and abundant but slow rainfalls -especially in cold seasons -which favour groundwater recharge. The transition towards fewer but heavier rainfalls is expected to enhance groundwater recharge in many environments. Major aquifers in France were replenished to a maximum in March 2018 and May 2019 (ref. 5). Heavy rainfall has been shown to contribute disproportionately to groundwater recharge in locations across the tropics, including drylands, where extreme (heavy) rainfall creates ephemeral surface water bodies that generate focused recharge. The disproportionately greater contribution of heavy rainfalls to groundwater recharge has similarly been noted in drylands outside of the tropics in Australia and the south-western USA. Episodic increases in groundwater storage on arid zones over the world are explained by exceptional yearly rainfalls (104).
Across continental northern latitudes, as well as in mountainous and polar regions, global warming alters meltwater flow regimes from ice and snow, impacting groundwater recharge. In temperate regions, warming results in less snow accumulation and earlier snowmelt, as well as more winter precipitation falling as rain and an increased frequency of rain-on-snow events. The aggregate impact of these effects is a reduced seasonal duration and magnitude of recharge, which lowers water storage in catchments and amplifies severe summer low flows, so that streamflow becomes inadequate to meet domestic and agricultural water requirements and to maintain ecological functions (105).
The impacts of receding alpine glaciers on groundwater systems are not well understood. As glaciers recede due to climate change, meltwater production initially increases to a maximum, known as "peak water", before dropping off as the glaciers continue to retreat; approximately half of the world's glacierised drainage basins are considered to have passed peak water. In the tropical Andes of Peru, glacier meltwater flows steadily decrease after peak water but during the dry season groundwater continues to discharge to streams, maintaining baseflow during the waterstressed dry season. Similarly, recent analyses highlight increases in focused recharge due to increased meltwater contributions to streamflow in glacierised drylands. Over the longer term under climate change, a reduction in recharge occurs due to increasing evapotranspiration, which may reduce meltwater contributions that generate focused recharge from summer low flows (106).
The seasonal freezing of soils that affects ~50% of exposed land in the Northern Hemisphere is an important control on snowmelt infiltration and strongly influences the amount and timing of winter and spring runoff in cold regions (106).
Aquifer systems in continental northern latitudes or alpine and polar regions where long-term recharge and discharge are impacted by changing meltwater regimes (e.g. Rocky Mountains, Indus basin) and a thawing permafrost (e.g. Canada, Russia) that increases hydrologic connectivity and risks of contamination. Aquifers in rapidly expanding low-income cities (e.g. Dakar, Lucknow, Lusaka) and large displaced and informal communities (e.g. in Bangladesh, Kenya, Lebanon) reliant on on-site sanitation provision (e.g. pit latrines, septic tanks), where the increased frequency of extreme rainfall can amplify leaching of surface and near-surface contaminants (109).
Low-intensity abstraction for domestic water supply from low-storage, weathered crystalline rock aquifers receiving recharge annually across humid equatorial Africa is generally resilient to groundwater depletion. Abstraction of largely "fossil" groundwater from regional-scale sedimentary aquifer systems (e.g. Nubian sandstone, Kalahari sands) in African drylands is climateresilient but ultimately unsustainable and controlled by the prevailing available groundwater storage (109).
Intensive pumping of aquifer systems for groundwater-fed irrigation in drylands (e.g. in north-west India; the California Central Valley and central High Plains, USA; the Souss aquifer, Morocco; the North China Plains) where there is high consumptive use of groundwater and reductions in recharge under climate change could threaten the continued viability of irrigated agriculture; intensive pumping of aquifers for dryland cities (e.g. Lahore; San Antonio) facing potential reductions in recharge under climate change could threaten the continued viability of public water supply, given that other perennial sources of water are either limited or do not exist (109).
In tropical Africa there are growing calls to draw from groundwater storage to improve the climate resilience of water and food supply. Adaptations to climate-driven shortages in water supply to cities such as Dar es Salaam (Tanzania) in 1997 and Cape Town (South Africa) in 2017 involved not only reductions in fresh water demand but also supply-side strategies that increasingly used groundwater as a climate-resilient source of fresh water that can be used conjunctively with surface water resources. Further, improved community hygiene and sanitation provision can enhance the resilience of groundwater-fed water supply to climate change in densely populated, lowincome communities by reducing risks of faecal contamination.
Climate change is explored in detail in the report; the report is, on the other hand, relatively poor on demographic evolution, a role that is of major importance compared to meteorological effects, especially in developing countries.

Conclusion
The dissemination of scientific data about groundwater increasingly occurs in open-access publications, including journal articles, textbooks and manuals. A notable initiative is the so-called Groundwater Project 41, which is promoting free access to groundwater knowledge, through online books and other educational materials. It is important to share scientific knowledge with all, particularly in low-income countries where the price of books and subscriptions to scientific journals can be a barrier to accessing scientific information (153).
Given the increasing relevance of groundwater resources in the context of global change, groundwater specialists should not only add to the knowledge base, but also help in developing policies and participate in decision-making. However, their potential contribution is often not recognised. Organisations engaged in groundwater management, such as water companies; water, sanitation and hygiene agencies; river basin organisations; and environmental agencies would benefit from engaging hydrogeologists in the development of their activities. In many low-and middle-income countries, hydrogeological capacity is missing, even when groundwater makes up the largest part of their managed water resources. This lack of capacity often comprises both human capacity as well as institutional capacity. Weak institutional groundwater governance and management, in turn, undermine associated water security. Well-founded capacity-building schemes are, therefore, an essential building block to move from a vicious circle of groundwater overexploitation and environmental degradation to a virtuous circle of creating local groundwater champions who eventually foster sustainable management practices and strengthen institutional capacity.
• data, information and knowledge are indispensable guides towards proper groundwater development and management; • strong institutions are key to progress in groundwater management; • stakeholders have a diversity of interests and should not be ignored; • legal provisions clarify agreed rights and rules of the game regarding groundwater; • transboundary aquifers call for cooperation; and • groundwater policy and planning provide road maps for concerted action.

Funding
This work was supported by the

Disclosure statement
No potential conflict of interest was reported by the authors.