The key role for groundwater in urban water-supply security

During the 20th century, there was an enormous boom in waterwell construction for urban water supply, as well as for agricultural irrigation and industrial processing. Major advances in waterwell drilling, pumping technology, energy access and geological knowledge allowed deep boreholes to be drilled relatively quickly and extract large volumes. Shallow waterwells using affordable technology were developed for private users and community supplies. Groundwater thus became a key natural resource supporting human well-being and economic development – but still all too widely a resource misunderstood, undervalued, poorly managed and inadequately protected (IAH 2015, 2019).

The groundwater dependence of innumerable urban areas appears to be intensifying, such that nearly 50% of the urban population today is believed to be supplied from waterwell sources (IAH 2015). In the cases of the EU and the USA, groundwater provides the public water supply for 310 and 105 million people, respectively. The present drivers of urban groundwater use are widely accelerating rates of urbanisation, increasing per capita water use, higher ambient temperatures and reduced river-intake security due to water pollution and climate change and the relatively low cost of waterwell construction and operation (Foster et al. 2010a). The social value of groundwater should not, however, be gauged solely by volumetric withdrawals. This is because groundwater use brings major economic and health benefits because of local availability, scaling to demand, high drought reliability and generally good quality requiring minimal treatment (Foster & Vairavamoorthy 2013).

Comprehensive national statistics on groundwater pumping for urban supply are not available, and recourse has to be made to data on selected cities to illustrate what are believed to be the most important global trends. Groundwater withdrawals are currently highest over large areas of India, China, Pakistan and Bangladesh, and in parts of North America, Southern Europe, North Africa and the Middle East. A list of major cities with major dependence on groundwater includes Europe (Copenhagen, Odense, Paris, Caen, Limoges, Hamburg, Berlin, Munich, Hannover, Budapest, Rome, Milan, Torino, Utrecht, Den Haag, Warsaw, Wroclaw, Krakow, Portsmouth, Hull, Cambridge and Brighton), Americas (Natal, Ribeirao Preto, Saō Luis Maranhăo, Santiago, Coquimbo, San Jose, Mexico DF, Merida, San Luis Potosi, Leon, Asuncion, Miami, Tampa and Phoenix), Africa (Addis Ababa, Abidjan, Mombasa, Dakar, Dodoma, Arusha, Lusaka and Kabwe), and Asia (Lucknow, Chennai, Indore, Islamabad, Lahore, Rawalpindi, Tianjin, Beijing, Handan, Shenyang, Jakarta, Semarang, Yogyakarta, Ho Chi Minh, Hanoi, Dhaka and Chittagong). South Asia and sub-Saharan Africa in particular are currently experiencing unprecedented rates of urban population expansion, with over 500 African cities growing at an annual rate of 3.9% or higher and 8 million new urban dwellers added annually.

Groundwater exhibits numerous benefits for water-supply development by public utilities (Table 1). Its normally excellent natural quality, requiring only precautionary disinfection before supply into main distribution systems, has long made groundwater the preferred source. Urban centres underlain and/or surrounded by high-yielding aquifers usually have better water mains service levels and lower water prices because of the potential to expand water-supply production incrementally in response to rising demand at modest cost (Howard 2007; Taniguchi et al. 2009; Foster et al. 2010a).

Normally, sufficient groundwater resources are not available within the limits of larger cities to meet fast-growing urban water demand sustainably (Figure 1). Where, high-yielding aquifers are present in the hinterland of cities, the development of ‘external wellfields’ is an attractive option compared to the long-distant import of surface-water resources. But groundwater also remains widely exploited by water utilities through individual waterwells scattered around urbanised areas, many without specific protection measures.

Looking to the future, groundwater resources should allow the rapid development of utility waterwells as the ‘hub’ of new decentralised systems of water service provision for the fast-developing outer urban districts with populations of 20,000–50,000, minimising infrastructure costs, energy use and water losses (Foster & Varaivamoorthy 2013). These would also value wastewater as a resource (with urine recovery as a fertiliser and faeces reuse for energy generation). But it will be necessary to put special efforts into on-the-ground control of other forms of groundwater contamination (such as petrol stations, small-scale motor shops, garages and dry-cleaning laundries) to reduce the risk of pollution of important waterwell sources.

In addition, in some instances (e.g. Lima – Peru and Delhi – India), construction work has been undertaken in the riverbed to enhance surface water recharge to groundwater or to direct excess monsoon river flow to groundwater recharge basins, which are other forms of conjunctive use.

The term ‘self-supply’ refers to water-supply investments that are financed by users themselves (Oluwasanya et al. 2011). In developing economies, most self-suppliers use groundwater and share their supply with neighbours. Self-supply from groundwater provides a rapid solution in areas where it is technically feasible and affordable. The proximity to the home of a private waterwell, combined with low recurrent costs and, in some cases, low initial investment costs, makes private waterwells an attractive option. Such investments unlock significant finance for water-supply access and are a fast-growing phenomenon (Foster et al. 2010a; Grönwall et al. 2010). Nevertheless, private groundwater use tends to pass under the radar of official national water-supply statistics (Danert & Healey 2021) or the phenomenon is not recognised at all by the government (Foster & Hirata 2011).

The use of private waterwells for urban self-supply has ‘mushroomed’ in recent years, especially in South Asia, Latin America and sub-Saharan Africa (Foster et al. 2010a; Grönwall 2016). For example, some 125 million people in sub-Saharan Africa and 340 million in India depend primarily on this type of source. The practice usually commences as a ‘coping strategy’ for irregular or inadequate piped water supplies but then continues in perpetuity as a ‘cost-reduction strategy’ to avoid paying higher water tariffs.

Private waterwell construction costs in most hydrogeological settings will be in the range of US$2,000–20,000 but are considerably higher (US$30,000–45,000) where deep boreholes (of 200–300 m) are required. Private waterwell ownership will thus remain mainly the preserve of the wealthy, and it is not a ‘pro-poor’ phenomenon. While the practice reduces the pressure on water utility supplies, it can also have serious impacts on their cash flows and investment cycles (Foster et al. 2018, 2020a). There is obviously a clear need for some regulation of urban waterwell self-supply because, without regular quality monitoring (and if necessary, water treatment), it will be a ‘risky business’, especially from shallow aquifers as a result of the risk of their significant pollution.

Research into urban self-supply from groundwater has revealed the following:

• In India, an estimated 340 million dwellers depend primarily on self-supply sources, and many medium-sized cities are highly dependent on domestic self-supply from groundwater, which can amount to 40–60% of water-supply provision (Alam & Foster 2019).

• In Brazil, domestic self-supply from groundwater amounts to about 35% of the total drought water supply to Sāo Paulo (despite the city not being underlain by a major aquifer) and nationally, there are at least 2.5 million private urban waterwells, which represent 6–7 times the annual investment in water supply by government agencies (Foster & Hirata 2011).

• In Nigeria, the level of dependency on private waterwells is extremely high, having reached some 38–43 million of the total urban population (75–80 million) by 2009 despite the coverage of public mains water supply having expanded, with only about 20% of the Lagos population (18–20 million) being served by piped utility water supply, 50% owning private boreholes and another 30% obtaining water from these sources.

In global climate circles, one hears the expression ‘water security’ used with ever-increasing frequency, together with declarations about the urgency to increase water security in these times of unprecedented global change. Global warming is likely to increase the frequency of serious droughts in many settings and create increased anthropogenic pressures that impact the water security landscape, resulting in aggressive political positioning and increased potential for local conflict or even violence. The water security concept has a very broad scope (Grey & Sadoff 2007; Cook & Bakker 2012) but at its heart is a societal issue and, thus, a political concern.

‘Water security’ was defined by UN-Water in 2013 as ‘the capacity of a population to safeguard sustainable access to adequate quantities of acceptable quality water for sustaining livelihoods, human well-being, and socio-economic development for ensuring protection against water-borne pollution and water-related disasters, and for preserving ecosystems in a climate of peace and political stability’. The related concept of ‘water scarcity’ has evolved considerably since the definition of the ‘Falkenmark (1989) water-stress index’ in the 1980s, which led to ‘absolute water scarcity’ being defined as situations where local water resources are equivalent to less than 1,000 m³/year/person of water availability.

In reality, the conceptualisation of water scarcity needs to consider not only the ratio of ‘fresh water withdrawals to renewable resources’ but also the local availability of natural groundwater storage, which can completely transform the water resource situation at times of surface-water stress. Another significant factor is the seasonality of the local rainfall regime. It is also most important to distinguish ‘physical water scarcity’ from ‘economic water scarcity’. Nowhere is this more apparent than across the African continent (Dankaer & Taylor 2017), where only the northern and southern extremes exhibit physical water scarcity but much of the tropics currently suffer economic water scarcity due to a lack of investment in rational water development. There is also evidence of a direct correlation between drought proneness and persistent poverty in Sub-Saharan Africa (Grey & Sadoff 2007), further increasing pressure to find reliable water sources with sufficient storage to maintain supply through drought periods.

Water scarcity indicators need to define water resource availability levels below which serious allocation issues and environmental risks arise. These can have an impact at different levels – individuals, groups, states and countries – and relate to different geographic scales and socio-economic sectors, most notably the human population, environment conservation, food production and industrial activities. From a water resource perspective, the water security concept would be more rational if applied at the scale of a specific city and if it more clearly distinguished the differing impacts of drought, flood and pollution. However, to date, the concept has been principally applied at the national scale, using parameter indexation to arrive at a ‘security score’, whose primary objective is to provide an incentive for national governments to invest in water resource management measures.

Globally, groundwater is the most abundant source of freshwater resources (representing 97% of non-frozen water), and one that can be accessed by waterwells and from springs. Aquifers hold very large volumes of freshwater naturally in storage, and their presence much improves local water security during prolonged droughts for all types of water users (Foster et al. 2018, 2020a). Groundwater storage plays such a fundamental role in shaping water security that it should be taken more prominently into account in water security evaluations, rather than considering only built reservoir infrastructure. The presence of major aquifers in the vicinity of cities can enhance urban water-supply resilience, because they provide a ‘natural buffer’ against the variability of river flows and surface-reservoir levels, as a result of the very large volume of groundwater held in storage. Aquifers have ‘water retention times’ ranging from decades to millennia (Foster & Vairavamoorthy 2013) and normally at least a few hundred years even for ‘shallow groundwater systems’ (with the notable exception of karstic limestones).

So what are the specific facets of groundwater systems that need to be taken systematically into consideration when evaluating urban water security? Research on groundwater and droughts (Villholth et al. 2013) can be helpful here. Incorporating groundwater considerations into the evaluation of water security will require indexation of the following:

• groundwater storage availability: an indicator of ‘buffer capacity’ to support water-supply abstraction in extended drought but one which will be constrained by current resource status (aquifer water level and salinisation trends) and connectivity to surface water;

• groundwater supply productivity: a measure of how easy it is to extract groundwater from an aquifer, which relates to its depth and the aquifer transmissivity, and any evidence of reducing productivity due to falling water levels;

• groundwater pollution protection: in terms of the interaction between aquifer pollution vulnerability (Foster et al. 2013), pollution pressure, the effectiveness of pollution control and aquifer protection measures, and any evidence of deteriorating quality trends.

The recent water-supply crises in São Paul – Brazil in 2016–2019 (Box 1), Cape Town – South Africa in 2016–2018 (Box 2) and Chennai – India in 2017–2019 (Box 3) have highlighted the vulnerability of major cities to surface-water drought and the critical importance of using (not ignoring) ground­water resources so as to provide a more secure water-supply element.

Groundwater systems, for the most part, exhibit exceptional resilience to drought impacts and are well positioned to enhance water security for a wide range of users. However, to perform this role sustainably, they require

• good hydrogeological data and understanding to guide groundwater management including the occurrence and use of non-renewable resources,

• careful administration of resource abstraction,

• effective protection against pollution.

Given the numerous uncertainties and continuously evolving groundwater conditions in urban areas, it is wise for an ‘adaptive approach’ to groundwater resource management to be adopted – this should be based upon continuous monitoring of groundwater levels and quality trends (Box 4) and guided by transient numerical aquifer modelling (Foster 2020). This will permit the evaluation of future groundwater abstraction scenarios and lead to the definition of more robust and sustainable solutions to municipal water supply.

Groundwater systems in urban areas usually have complex dynamics evolving with time because of the influence of various anthropogenic recharge sources, including water mains leakage and soakaways from road-and-roof drainage (both of good quality), and leakage from sewerage systems and infiltration from in situ sanitation (both of poor quality). This recharge usually more than compensates for the reduction of rainfall recharge due to land-surface impermeabilisation. However, the total rate of groundwater recharge in urban areas is rarely sufficient to meet fast-growing water demand and resource overexploitation with continuously falling water table occurs widely (Foster et al. 2010a).

This needs to be managed by both controls of private waterwell abstraction and redistribution of water utility withdrawals to allow the long-term use of groundwater. There are some recent examples from the developing world of successful management of urban groundwater abstraction, notably Lima – Peru (Box 5) and Bangkok – Thailand (Box 6).

The sustainability of urban groundwater use is strongly influenced by a complex array of local developmental decisions, which are rarely viewed in an integrated fashion because the production and distribution of water supplies is by water service utilities, urbanisation and land-use planning is by municipal government offices, and installation of sewered sanitation, and disposition of liquid effluents and solid wastes are by environmental authorities and public health departments. A more integrated approach is required to avoid costly problems, where local aquifers are providing an important component of municipal water supply (Foster 2020), which should include such measures as:

• defining areas with critical levels of resource exploitation as a basis for restricting further abstraction,

• providing clear criteria by area for issuing waterwell permits in terms of safe separation and maximum pumping rates,

• controlling municipal and private groundwater abstraction on the basis of the defined areas, including the relocation of municipal waterwells, and increased resource-use fees,

• pursuing opportunities to enhance aquifer recharge through rainwater harvesting and infiltration soakaways.

For most large cities, the long-term sustainable groundwater resource is limited by the aquifer geographical extension. In many such cases, it may prove more rational to use the large groundwater storage of nearby aquifers as a strategic reserve for conjunctive use with surface-water sources rather than for the traditional use of groundwater for base-load municipal water supply in selected districts (Foster et al. 2010b). But most conjunctive use presently encountered in the developing world amounts to a ‘piecemeal coping strategy’ in which waterwells have been drilled by water utilities on an ad hoc basis in newly constructed suburbs to meet their water demand at the lowest possible capital cost, and surface water has been recently imported from a major new distant source to reduce dependency on waterwells. The most common impediment to conjunctive use is the fact that urban engineers (usually pressed by day-to-day problems that require urgent attention) are forced to look for operationally simple set-ups, such as a major surface water source and large treatment works, rather than more secure and robust conjunctive use solutions (Foster 2020). Such impediments need to be confronted through capacity building and by water resource agencies engaging closely with water utilities on resource development strategy.

In general terms, urbanisation has a direct interaction with shallow groundwater underlying cities through increasing contaminant load, as a result of in situ sanitation (especially important in the developing nation context) and to a lesser degree, sewer leakage, and also inadequate storage and handling of ‘community’ and industrial chemicals, and disposal of liquid effluents and solid wastes (Taniguchi et al. 2009; Foster et al. 2010a).

In situ sanitation of major urban areas presents a significant groundwater quality hazard, which needs to be recognised and managed. This problem is further accentuated by the fact that self-supply from groundwater is generally more intensive where access is easiest – namely in the presence of shallow unconfined aquifers, which are the more vulnerable to pollution. In most aquifer types, except the extremely vulnerable, there will be sufficient natural groundwater protection to eliminate faecal pathogens in percolating wastewater from in situ sanitation, but there are exceptions. However, elevated (and often troublesome) concentrations of N compounds (usually nitrate) and dissolved organic carbon (DOC) in groundwater will also be present and can penetrate to considerable depths (Taniguchi et al. 2009; Foster et al. 2010a; Lapworth et al. 2017). Such groundwater pollution can persist for years after the source of contamination is removed by the installation of main sewerage or other alternative sanitation. The most cost-effective way for water utilities to deal with excessive nitrate in groundwater is by dilution through mixing from a carefully protected source, such as an ‘external well field’.

Groundwater pollution can be substantially reduced by deploying so-called dry or eco-sanitation units, in which urine is separated from faeces and not discharged to the ground. While such installations are highly recommended for new urban areas overlying significant shallow groundwater resources, their deployment as a universal solution to the groundwater contamination problem has absolute limitations since retro-installation in large numbers of existing properties is impractical, and it is unsuitable for certain cultural groups who use water for anal cleansing. It is thus usually more practical to prioritise recently urbanised areas for coverage by mains sewerage to protect good groundwater quality against progressive degradation, limit the density of new urbanisation with in situ sanitation to contain potential contamination and establish utility waterwell protection zones around all sources favourably located to take advantage of parkland or low-density housing areas.

A separate question is leakage from mains sewerage systems, which is always negative from the point of view of groundwater resource quality. It rarely receives attention from water service utilities since they do not see investment having any direct return by increasing water available for sale to consumers. Without adequate maintenance to reduce leakage losses, sewerage systems may not afford the protection to groundwater which is often assumed.

There are basically two institutional approaches to protect groundwater quality: specific regulatory codes and planning consultation (Table 2), but the challenge of implementation in the developing world is greater because of rapid population growth and limited public awareness.

The establishment of water utility wellfields outside cities, with their capture areas being declared as drinking water protection zones, must be promoted as ‘best engineering practice’ (Foster et al. 2010a; Grolleau & McCann 2012; Barataud et al. 2014; Thomsen et al. 2014). But in the developing world, their promotion often encounters administrative impediments related to fragmented powers over land use between the municipalities that comprise ‘metropolitan areas’. Incentives need to be established for the groundwater resource interests of a given urban municipality to be assumed by a neighbouring rural municipality, such that adequate protection can be offered for the capture area of ‘external municipal wellfields’ providing water supply to the main urban area.

In many countries, there is a lack of clear incentive for water utilities to take action on groundwater quality protection through land-use controls because of uncertainty about the success of the approach and the time taken for the beneficial impacts of controls to be realised. It is often perceived by utilities as more secure to achieve drinking water quality goals by complex water treatment, which is completely under water utility control and whose capital and operating costs can be charged to water users.

Groundwater is widely far more significant in urban water supply than is commonly appreciated and is also often the ‘invisible link’ between various facets of the urban infrastructure. It is a fundamental component of the urban water cycle, and there is always a need for groundwater to be integrated when making decisions on infrastructure planning and investment, whatever its use status.

In most developing cities, population growth precedes the construction of mains sewerage and wastewater treatment facilities, and in the meantime, shallow groundwater can become contaminated from inadequate in situ sanitation. It may be years before the full extent of pollution becomes apparent, and thus, it is critically important to recognise the incipient signs of pollution and put in place groundwater protection measures. Polluted groundwater not only has implications in terms of increased public health risk but also water-supply treatment costs. Advanced treatment of groundwater to achieve drinking water quality also greatly increases the carbon footprint of water-supply operations.

A major challenge as regards groundwater quality protection is promoting the acceptance of differential land management for important recharge areas in the interest of groundwater recharge quality. The impediments that have to be overcome include the often fragmented responsibility for land-use control and providing incentives for rural municipalities to protect the wellfields of neighbouring metropolitan areas.

Urban groundwater tends to affect everybody but is often the responsibility of ‘nobody’. Municipal, provincial/state and national governments must find the political will and the practical means to constrain water demand and limit groundwater abstraction to avoid aquifer depletion; encourage water utilities to spread groundwater abstraction over larger areas and protect wellfield investments through the declaration of special protection areas, and plan urban sanitation and regulate the handling and storage of industrial chemicals and effluents adequately. For this to happen, it will be essential to appraise the roles of water resource/environment agencies, municipal water service utilities and municipal government offices, and the avenues of consultation between them.

The author generously acknowledges the excellent dialogue on this topic maintained during 2018–2022 with members of the IWA-Groundwater Management Group Steering Committee, of which he is the Chair, a dialogue that has enriched the content of this paper.

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

The authors declare there is no conflict.