The provision of secure water-supplies for the rapidly expanding cities of sub-Saharan Africa experiencing climate-change stress will be one of the great infrastructure and environmental challenges of the next 20–50 years. Most African cities are blessed with usable groundwater, and some with the presence of major aquifers, but urban water utilities will need to take a more proactive approach to groundwater resource management and quality protection if the opportunity of a secure water supply is to be sustainably secured. Among the key policy issues that need more attention are rationalising utility groundwater use, prioritising installation of mains sewerage to reduce groundwater pollution risk, promoting enhanced groundwater recharge to improve resource sustainability, using groundwater in ‘decentralised closed-loop water-service systems’ to meet the demands of new outer urban districts, and implementing a consistent policy response to the ‘boom’ in private self-supply from waterwells. The consequences of non-action in terms of much increased exposure to water-supply crises, potentially hazardous water-supply pollution incidents, and irrational public and private investment in water-supply access are highlighted.
Unprecedented urban growth in sub-Saharan Africa
Sub-Saharan Africa is experiencing unprecedented rates of urban population growth (Saghir & Santoro, 2018) – no part of our planet is urbanising faster. The current urban population is estimated to be 472 million, and predicted to double by about 2040, with 528 cities currently growing at a rate of 3.9% p.a. (Figure 1). Certain cities (such as Ougadougo-Burkina Faso and Abuja-Nigeria) are growing even faster (at more than 4.5% p.a.), and Lagos and Kinshasa have already reached megacity status (with population exceeding 10 million).
Moreover, from 2015, some eight million new dwellers are predicted to be added annually to cities of less than 0.3 million inhabitants. Such rates of population growth are, in turn, generating unprecedented growth in demand for water supply and creating an enormous challenge for urban planning, in general, and sanitation services, in particular. While megacities are certainly the ‘hot spots’ of population growth, many medium-sized cities (which often have less capacity for urban planning and water-service provision) are growing faster.
Systematic surveys of the sources of water supply used by the urban population of Africa have been conducted by the Africa Infrastructure Country Diagnostic (AICD) Programme (Foster & Briceño-Garmendia, 2010; Banerjee et al., 2017), and these conclude that rapid urban population growth compounded by decreasing household size is resulting in an average growth of 5.2% per year in dwellings requiring water-supply services, and a steady decrease in the proportion of households whose main source of water is the urban utility (declining from 50% in 1995 to 36% in 2015).
Benefits and challenges of groundwater use for urban water supply
Groundwater development normally has relatively low capital and operational costs, which can be staged with increasing demand. This is a consequence of groundwater being, for the most part, of good quality and not requiring advanced treatment (Foster et al., 2012; Foster & Varaivamoorty, 2013). A further potential advantage is that in many cities groundwater is widely distributed, making it possible to develop utility waterwells cheaply and rapidly as the ‘hub’ of new decentralised closed-loop systems of water-service provision for rapidly developing outer districts with populations of 20,000–50,000.
The presence of major aquifers, or conjunctive use of groundwater from more localised aquifers, will greatly enhance urban water-supply resilience, because they provide a ‘natural buffer’ against variability of riverflows and surface-reservoir levels, as a result of the very large volume of groundwater held in storage. They have ‘water retention times’ ranging from decades to centuries and millennia, and normally at least a few hundred years even for ‘shallow groundwater systems’ (with the notable exception of karstic limestone aquifers), and their stored water is naturally protected from evapotranspiration losses and less vulnerable to pollution than surface water.
Urban groundwater use includes not only withdrawls by water utilities, but also by many categories of private user (Grönwall et al., 2010; Foster et al., 2012; Grönwall, 2016; Healy et al., 2017). In-situ residential self-supply from groundwater is a ‘booming phenomenon’ in numerous sub-Saharan Africa cities and widely represents a significant proportion of the water actually received by users (estimated by AICD to be 26% on average in 2015). This practice is primarily the solution of those with higher incomes, but can have serious implications for:
planning and investment in public water supplies, and in particular can seriously decrease the income of water-service utilities, impacting their ability to recover investment in improving infrastructure and to subsidise the so-called ‘social tariff’;
public health where private waterwells are inadequately constructed and/or sited with respect to in-situ sanitation units or other potential-pollution sources.
The principal hazards confronting urban waterwell use are:
the frequent degradation of groundwater quality from inadequate in-situ sanitation, leaky storage of hydrocarbon fuels and casual disposal of industrial effluents to the ground (Foster & Tyson, 2015; Lapworth et al., 2017) – such pollution normally impacts private self-supply waterwells preferentially, as a result of their generally shallow depth and typical locations, but more persistent contaminants (such as nitrates and some synthetic organic industrial and community chemicals) often persist to greater depths and pollute utility waterwells;
the tendency to over-exploit groundwater resources within urban areas where the water utility is a major abstractor, which can be accompanied by land subsidence impacting the urban infrastructure and saline-water intrusion especially in coastal settings.
Critical role of groundwater in climate-change adaptation
Climate change is widely predicted to impact water resources, causing more frequent droughts, higher evaporation from surface-water bodies, and more intense rainfall events with land flooding and ‘flashy’ streamflow. The geographical distribution of these impacts is still subject to considerable uncertainty, but they are likely to be most severe in semi-arid climatic zones. Making better use of water storage, in one form or another, will be critical for water-supply security and groundwater stored in aquifers offers sustainable, decentralised, cost-effective solutions for climate-change adaptation, at the scale of specific cities and their hinterland catchments (Foster et al., 2018).
The recent Cape Town water-supply crisis (Olivier & Xu, 2019) is a classic example of the type of situation that can arise under climatic stress where a major municipal water-service utility relies exclusively on a large surface-water reservoir, and has not diversified its sources to include local groundwater systems (in this case the Cape Flats, Table Mountain and Atlantis aquifers which offered a potential of at least 200 Ml/d). During the 2017/18 drought, the storage of the largest reservoir fell to below 15% in June 2017 and a domestic supply restriction of 100 lpd/capita was imposed, with subsequent reductions of 80 and 50 lpd/capita in September 2017 and February 2018, and a warning that the situation was fast approaching when the available resource would drop to about 100 Ml/d, all domestic taps would be shut off and the population would have to queue for 25 lpd/capita at 150 collection points.
Four criteria are key to assessing the potential role a given groundwater system can play (in conjunctive use with local surface water) for climate-change adaptation, and the level of management it will require to fulfil this role – storage availability, supply productivity, natural quality and pollution vulnerability. A question arises about the natural resilience of groundwater reserves themselves to climate change, and potential reductions in recharge rate and shallow aquifer storage as a result of current global warming trends. On the one hand, higher ambient temperatures will trigger more intense (but fewer) major rainfall events and increased recharge may result while, on the other hand, fewer but heavier rainfall events could result in soil compaction with increased erosion and reduced infiltration.
Contribution of groundwater to urban poverty reduction
The indirect contribution of groundwater to urban poverty reduction arises because water utilities can develop groundwater sources stepwise at much lower cost than surface-water sources, thus allowing them to offer a lower ‘basic connection charge and social use tariff’, which should make their supply more accessible to the poor. However, most of the poor in sub-Saharan Africa live in peri-urban settlements, many of which are unplanned and lack legal status. Thus, city planners often tend to impede the provision of public infrastructure (power and water) services to such areas, and water utilities are anyway reluctant to extend their piped networks into such settlements because they anticipate high capital cost and low revenue collection.
Concomitantly, private waterwell construction costs (for drilling and equipping) in most hydrogeological settings will be in the range of US$2,000–20,000, depending on the local geology, but are considerably higher (US$30,000–45,000) where deep boreholes (of 200–300 m) are required, as in the case of Nairobi and Addis Ababa, for example. Thus, private waterwell ownership will remain the preserve of the rich living in high-cost housing and apartment blocks, and this will have knock-on implications for utility revenue collection (Foster et al., 2018).
Many peri-urban settlements are not formally planned and lack legal recognition – and thus city planning procedures tend to deter the establishment of utility services and water-supply companies are reluctant to extend their networks because of anticipated high cost and low revenue. Hence, poor peri-urban communities can only gain direct access to groundwater where:
community-based organisations use social capital and political connection to secure funding for non-reticulated waterwells from government programmes;
non-governmental organisations have provided non-reticulated waterwells (sometimes in association with water-utility pro-poor departments) to supply collection standposts;
low-cost dugwells can be constructed to tap an exceptionally shallow water table, although these have the handicap of being much more vulnerable to contamination.
In many cities such access to groundwater allows provision of a basic water supply that is cheaper and safer than the potential alternatives (like water-tanker distribution). However, while community-driven groundwater development is unquestionably beneficial in the short term, in the urban environment it can quickly spiral out of control, with major problems arising from groundwater pollution in particular.
Examples of current urban groundwater dependency
There are many sub-Saharan African cities and innumerable towns that have major dependency on groundwater for water supply, although until now resource development by the water utility and private users has been rather disjointed and lacking strategic coordination, both in respect of meeting water demand and dealing with sanitation. Three examples are selected here (and presented in ‘case profile’ Boxes 1–3 below) to illustrate the current position – Lusaka-Zambia, Mombasa-Kenya and Douala-Cameroon.
In all these cases absolute water-supply security has unquestionably been enhanced by the fact that the corresponding water utility has developed groundwater but, variously, the general lack of resource management and pollution protection has eroded somewhat the natural security of these sources.
Lusaka, the Zambian capital, has grown rapidly from a population of 0.5 million in 1978 to 2.8 million in 2018, and is predicted to reach three million by 2021. It has long been dependent on local groundwater for its public water-supply, as are commercial and industrial uses. In November 2018, the Lusaka Water & Sewerage Company (LWSC) operated 228 waterwells in the public-supply network and these provided about 140 Ml/d or about 60% of their total supply (with an intake and treatment works on the Kafue River providing a further 80 Ml/d). LWSC still has almost 50% ‘unaccounted-for water’ and poor revenue collection, but has taken an important ‘pro-poor’ initiative by drilling stand-alone boreholes to supply water kiosks with a subsidised tariff (US$0.25/m3 compared to US$0.35–0.60/m3 for the mains supply).
In addition, there are several thousand private waterwells (with a total abstraction in the range 80–340 Ml/d according to season), but in low-income peri-urban areas (where 70% of the population reside) most households still rely on shallow dugwells where the water table is at less than 3 m depth.
Most waterwells are located within the built-up area with very little protection of their recharge capture areas, and the dolomitic-limestone formations they tap (while generally high-yielding) are very vulnerable to pollution from urban wastewater, industrial effluents and agricultural cultivation (Kangomba & Bāumle, 2013). In the peri-urban areas pit latrines are the predominant form of sanitation, with less than 20% of faecal matter being ‘safely managed’ and the rest constituting a serious hazard for groundwater quality and the cause of frequent cholera outbreaks (Kappauf et al., 2018). Some large-scale projects to extend the main sewer network and wastewater treatment capacity are underway, but in the unplanned peri-urban slums these are difficult and costly to implement. In addition to the economic and technical constraints, improved governance and better ways to integrate key stakeholders in urban water-supply and sanitation are urgently required.
Douala, the Cameroon capital, situated on the Gulf of Guinea, has grown very rapidly from a population of about 1.0 million in 1995 to 3.8 million in 2015, with urban expansion from the Wouri estuary onto the mainland, and very marked increases in urban population density from less than 100 to over 2,500 person/km2. The Doula Basin contains thick sedimentary formations, including a semi-confined coarse sandy Mio-Pliocene aquifer (up to 220 m thickness) and the consolidated deeper Continental Terminal aquifer. The climate is hyper-humid (with a precipitation of around 4,000 mm/a and a limited ‘dry season’ during December–February), resulting in major diffuse recharge of approaching 1,000 mm/a.
The Cameroon Water Utilities (CAMWater) provide a public supply to about 40% of the urban population, deriving 50% from waterwells (25% tapping deeper horizons of the Mio-Pliocene and 25% the underlying Continental Terminal), with the balance from a river intake and treatment works. The rate of groundwater abstraction increased from about 55 Ml/d in 1990 to 175 Ml/d in 2010. Access to domestic water-supply remains a problem, and the demand is met by private self-supply from shallow waterwells and purchase from water tankers. More than 70% of the urban population is served by latrine sanitation.
There is concern about anthropogenic impacts and groundwater quality degradation in the upper flow horizons of the Mio-Pliocene aquifer, which exhibits a patchy quality with EC reaching 500–2,000 μS/cm (compared to <200 μS/cm in the deeper horizons) and very different stable-isotope composition (Nlend et al., 2018), but the generally reducing conditions result in the auto-elimination of NO3 and SO4. In general terms, the hydrogeologic conditions are very favourable for developing new public water-supply wellfields upstream of the city in either the deeper horizons of the Mio-Pliocene and/or the Continental Terminal aquifers, and taking measures to protect their recharge capture areas from intensive agriculture, high-density urbanisation and industrial effluents.
Mombasa is a major coastal city, whose population has grown rapidly from 0.4 million in 1989 to 1.5 million in 2018, and is set in a metropolitan area with a population of well over three million. The total water demand is estimated to be in excess of 250 Ml/d, but the currently available production capacity of the Mombasa Water Company (MWC) is only capable of meeting 20–25% of this demand.
Groundwater provides most of the MWC supply with the main sources being: (a) the Baricho-Sabaki wellfield, 100 km distant, and commissioned in 1981 with a design capacity of 95 Ml/d, but only delivering 30 Ml/d to Mombasa due to offtake by other towns; (b) the Mzima springs, via a 200 km pipeline, commissioned in 1957 with a design capacity of 35 Ml/d but currently delivering 20 Ml/d; (c) the much nearer Tiwi waterwells, commissioned during 1971–2002, with a production of up to 10 Ml/d; and (d) Marere springs, providing another 5 Ml/d.
Although not adequately quantified and assessed, the very substantial urban water-supply deficit is met by large-scale water tankering and by large numbers of uncontrolled shallow private waterwells (mainly to 25 m depth in sands requiring well-screens), whose water quality is widely compromised by both wastewater percolation and saline-water encroachment, and is often virtually brackish in character (Mwaguni et al., 2013).
Groundwater resource potential and sustainability
Spatial variation of groundwater resource potential in urban areas
Groundwater resource potential will vary quite widely with the type and scale of aquifer system occurring in the proximity of a given city. A preliminary estimate (based on the BGS Groundwater Productivity Dataset of 2011) suggests that about 35% of cities in sub-Saharan Africa are located in areas with ‘highly-productive aquifers’ (Figure 2), but there will also be cities where local aquifers do not have sufficient ‘production potential’ to support water-utility waterwells. It is thus useful to classify groundwater systems into the following categories:
major aquifers capable of providing high waterwell yields and of supporting large abstraction;
intermediate aquifers allowing some urban conjunctive use or supply of specific districts;
minor aquifers only supporting small waterwell yields for off-grid private supply.
The category into which the available groundwater resources fall will dictate the need for different levels of proactive policy engagement by the water utilities.
Key urban groundwater management challenges
Intensive urban groundwater resource development both by water utilities coupled with a boom in private waterwell use (while bringing many local benefits) presents significant sustainability challenges at city or aquifer scale, since completely uncontrolled abstraction can lead to resource depletion, quality degradation and destabilise water-utility business models.
Groundwater resource depletion is likely to occur if waterwell construction and abstraction are not regulated, since there is unlikely to be sufficient recharge within urban limits to balance groundwater demand by water-utility and private users. If groundwater systems are to perform a potentially critical role in climate-change adaptation, they will require (like any other ‘infrastructure’) proper management and protection. The key challenges and actions in this respect are:
rationalising utility groundwater use, such that (where possible) it is used conjunctively with surface water to improve drought water-supply security and obtained from wellfields outside the urbanised area with protected groundwater capture areas to assure consistent supply quality and avoid the need for advanced water treatment;
demand-side management to ensure that groundwater withdrawls are revised in alignment with realistic assessments of average renewable resources, after taking into account the need to conserve some environmental discharge;
promoting groundwater use in ‘decentralised closed-loop water-service systems’ to respond to the demands of rapidly developing new urban districts, as a means to reducing dependence on more expensive centralised linear water-service systems;
supply-side management by promoting recharge enhancement measures (such as widespread use of drainage soakaways and permeable pavements by municipal engineering departments), taking into account changes in rainfall patterns and the need to ensure adequate water quality;
promoting effective protection against groundwater pollution by prioritising installation of mains sewerage in urbanised areas underlain by high-yielding aquifers to reduce groundwater pollution risk (from nitrates and community synthetic chemicals), declaring protection zones around important waterwell and spring sources, and combining forces with the environmental regulator to ensure that industrial waste disposal is properly controlled;
defining a more integrated approach to private self-supply waterwells (in coordination with municipal government and national ministries) to reduce their health risk, since ‘off-grid solutions’ to water-supply access come at a ‘questionable social price’ and greatly complicate logical planning of urban water-supply and sanitation services.
Policy needs for secure and efficient groundwater use
The elaboration and implementation of sustainable urban groundwater policy carries with it a number of critical institutional requirements including:
cross-sectoral involvement of municipal authorities with planning water infrastructure and land-use at right scales;
water-utility engagement in groundwater management and protection;
community mobilisation on urban water infrastructure development.
All of the above measures will require significant financial investment, since at present the level of funding allocated for ‘managing natural infrastructure’ in most cases remains totally inadequate. In this context, it will be absolutely essential for urban water utilities to participate proactively as the major stakeholder of groundwater as a drinking-water source, and their formal participation in various roles at different scales is contemplated.
It will also be necessary to establish, maintain and improve groundwater level and quality monitoring networks (Foster et al., 2019), since the information they provide will be essential for making ‘adaptive adjustments’ to water resource policy and land-use management to ensure groundwater sustainability. However, few countries have, as yet, fully embraced the principles of conjunctive use, integrated water resources management and adaptive groundwater management at the practical operational level, and as a result, there have been numerous recent examples of urban water-supply crises (e.g., Cape Town).
Factors facilitating or impeding water-utility involvement
Urban water-utility engineers are too often beset by day-to-day problems, and tend to look for operationally simple arrangements (such as a single major water-supply source and a large water-treatment works) rather than to more secure and robust conjunctive solutions. A ‘resource culture’ will thus need to be fostered within water utilities of developing cities to promote a more balanced approach between short-term operational efficiency and long-term water-supply security.
It must be recognised that, to date, the response and contribution of urban water utilities to sustainable groundwater management has been very patchy (with a few notable exceptions). This necessitates an urgent institutional diagnostic of the reasons why. Do water utilities have adequate professional awareness about (and training on) the potential role of groundwater resources and what is involved in their management and protection? Do governance factors, such as variable water-utility remit and regulation, and their access to financial investment, for groundwater development and management tend to constrain their approach?
Among the impediments to a more proactive position would appear to be:
an assumption (perhaps reinforced by legislation) that groundwater resources (and land-use change affecting them), are the sole responsibility of another national or local organisation, such as the environment agency, water resources ministry or basin authority;
the prevailing regulatory regime requires the water utility to provide a ‘wholesome safe drinking-water supply’ and the only way this can be guaranteed is by advanced water treatment (whose cost can be charged to water users), and that more economic ‘groundwater capture area protection’ cannot be relied upon in this regard;
the water utility is under a time-limited and action-specific operating concession (e.g., for mains leakage reduction) to a public body, such as municipal government department or national ministry, meaning that the development of new groundwater sources and their protection through land-husbandry agreements is outside its remit;
the water utility has to conform with defined local geo-political boundaries in its operations (as prescribed under its municipal concession), and these seriously constrain the approach it can take to wellfield construction, groundwater management and protection;
the operational size and the area of the water utility are too small to allow it to contribute realistically to groundwater resource management and protection, unless it gets involved with costly private sector land purchase.
What does appear likely is that urban water utilities that have responsibility for both water-supply and mains sewerage/drainage services are often more able to act effectively to protect potable groundwater by prioritising:
the installation of mains sewerage and elimination of high-density in-situ sanitation in areas with good quality shallow groundwater;
the use of drainage soakaways for stormwater runoff from roofs and paved areas (and the use of permeable pavement to reduce land-surface impermeabilisation) so as to enhance aquifer recharge;
the promotion of urban wastewater use for managed aquifer recharge with careful attention being paid to its chemical and biological quality.
Groundwater development will, in the majority of cases, provide a much higher level of urban water-supply resilience, as long as water utilities also invest in the management and protection of their waterwell sources. The consequences of non-action in this respect will manifest themselves in terms of much increased exposure to water-supply crises, potentially hazardous water-supply pollution incidents, and irrational public and private investment in water-supply access.
The authors would variously like to acknowledge their colleagues in Lusaka, Doula and Mombasa, who undertook essential work on field data collection. Special thanks are also due to Alan MacDonald and Craig Woodward of the British Geological Survey–Edinburgh Office for creating Figure 2 and to Helene Celle-Jeanton of the Université de Franche-Comté Chrono-Environnement Laboratoire for her advice on the sampling strategy in Douala. Stephen Foster, Michael Eichholz and Julia Gathu are all members of the IWA Groundwater Management Specialist Group Steering Committee.