OR/17/056 Introduction

Lapworth, D J, Stuart, M E, Pedley, S, Nkhuwa, D C W, and Tijani, M N. 2017. A review of urban groundwater use and water quality challenges in Sub-Saharan Africa. British Geological Survey Internal Report, OR/17/056.

Urban population growth and water supply in Africa

This review examines the quality of groundwater beneath peri-urban areas in SSA and the factors that impact upon the quality. Peri-urban is not a well-defined concept and can change depending upon the situation (WHO, 2006). Webster’s Dictionary defines peri-urban as ‘of or relating to an area immediately surrounding a city or town’; effectively the boundary between urban and rural areas. Nyambe et al. (2007) expand the definition of peri-urban to include ‘unplanned urban settlements developed due to the rapidly increasing population’.

Urban and peri-urban settlements are expanding rapidly across large parts of Sub-Saharan Africa (SSA), often at a rate that exceeds the capacity of countries to supply basic services. Over the last three decades there have been several initiatives intended to increase access to improved water supply and sanitation across SSA (Bartram and Cairncross, 2010). A recent example is the UN Millennium Development Goals, which had the target of halving by 2015 the population without sustainable access to improved sanitation from the baseline level of 1990 (JMP, 2008). These have recently been succeeded by the UN Sustainable Development Goals (SDGs), which are intended to promote the continued progress in a broad range of development goals including: Goal 6 to ‘Ensure availability and sustainable management of water and sanitation for all’; Goal 9 to ‘Build resilient infrastructure…’; Goal 11 to ‘Make cities and human settlements inclusive, safe, resilient and sustainable’. Groundwater has a pivotal role to play in achieving progress towards the SDGs.

Groundwater is considered a centrepiece of improved drinking water, and the provision ‘safe-supply’ in many parts of SSA, and many urban centres in SSA are dependant partly or wholly on groundwater (Showers, 2002). Within the urban and peri-urban context the important role of groundwater has resulted in widespread development of groundwater resources beneath, and in close proximity to, urban centres. However, the high population densities found in urban areas has also led to the proliferation of unimproved and improved sanitation provision largely through the use of pit latrines, often positioned in very close proximity to wells and springs used for drinking water. Furthermore, the absence of adequate management of household and industrial waste in many urban centres is putting additional pressure on groundwater in terms of water quality and water availability.

Future trajectories of growth, both spatially and demographically, of urban centres in SSA and increased demands on groundwater resources mean that urban centres are likely to be more dependent on groundwater in the future. Per-capita usage is also predicted to rise in line with prosperity, putting additional stress on available groundwater resources. Together, these factors challenge the sustainability of current management of water resources in urban areas across SSA. The present situation is compounded by a limited evidence base regarding groundwater quality studies in urban centres across SSA with which to inform long term policy on the use of urban groundwater resources.

Background

Urban and peri-urban populations in SSA are expanding at increasing rates. A study by the UN population fund (UNPF, 2007) estimates that between 2000 and 2030 Africa’s urban population will double and become the majority compared to the rural population. Currently, 30% of Africa’s population are urbanised (a greater proportion compared to South Asia) and average urban growth rates are approximately 5% (UN, 2005). Although inward migration from rural areas will continue to increase the population or urban areas, the UN predicts that the main growth will come from natural increases in urban populations (UN, 2005). The urban population is also predicted to be distributed in a large number of smaller towns (<200 000) rather than concentrated in large cities. While smaller urban centres may be viewed as more adaptable, from a geographical and policy making standpoint, they often lack the skilled personnel, resources and infrastructure needed to develop in a sustainable way. Current assessments suggest that levels of urban poverty are comparable with those in rural areas, and the proportion of the urban population living in slums in SSA is over 50%, the highest proportion by a factor of two compared to other regions globally (UN-Habitat, 2003). The Millennium Development Goals (MDGs) originally set out to increase access to safe water supplies, but water safety is difficult and expensive to monitor. To create a viable alternative to safety, the MDG for water was revised to assess the type of facility that was used to supply the water and whether or not it might be considered to be ‘improved’. This has been further refined under the current SDGs. Under the MDG for the purpose of monitoring, an improved source was considered to be safe. Recent work, however, has shown that ‘improved’ and ‘safe’ are not synonymous and that many improved water sources are not safe water sources (Linkov and Seager, 2011). Under the recent SDGs a new definition of ‘safe supply’ which includes water quality and access criteria has been introduced. SDGs 6, specifically target 6.1, have defined ‘safe managed supply’ as drinking water sources free of microbiological and chemical contamination, with no seasonality and which are supplied at the household level, a ‘basic supply’ is defined as an improved source within a 30 min round trip. This is a huge subject in its own right and will not be considered further in this review.

Groundwater is often seen as the most important component of an ‘improved’ water supply as it is often more reliable than surface water, less vulnerable to pollution and therefore less expensive to treat, and more resilient to climate variability (MacDonald et al., 2011). Many expanding urban areas in SSA are dependent on groundwater for their water-supply (Adelana et al., 2008; Foster et al., 1999). However, despite government and NGO efforts to increase access to safe groundwater, the most vulnerable people still rely on inadequate sources of deteriorating quality.

The change from rural to urban land use causes a dramatic change in groundwater recharge, as well as in water quality and demand (Kittu et al., 1999; Lerner, 1990). This has important implications for resource sustainability and the balance between water supply and growing urban population demand. Compared to Latin America or Asia (Chilton, 1999; Lawrence et al., 2000), there have been few detailed studies that assess urban groundwater resources in SSA (Howard et al., 2003; Taylor et al., 2009). Consequently, there is a limited evidence base on groundwater quality status in SSA with which to inform policy as well as the necessary institutional capacity, facilities and skills (Starkl et al., 2013). Existing studies largely include only basic chemical and microbiological parameters, and have focused on large cities or slums (Wright et al., 2012; Xu and Usher, 2006). There is also a paucity of robustly designed, i.e. statistically representative, studies to draw conclusions from. Given the large number of smaller but rapidly growing rural, groundwater-dependent towns in SSA, the development of effective strategies to protect the resource from contamination is a priority and effective tools for mapping risk to groundwater are needed by a range of stakeholders.

Developing groundwater resources is attractive from an installation and maintenance standpoint, as well as the reduced costs associated with infrastructure and the often minimal requirements for water treatment. However, urban groundwater is easily contaminated from a range of sources and along with increased abstraction rates, this can lead to locally reduced water quality and falling water levels (Cronin et al., 2006; Morris et al., 2000). As population density increases and urban centres expand the increasing pressures on the groundwater resource have a disproportionate impact on poorer inhabitants (Grönwall et al., 2010), who have a greater dependence on shallow unregulated and untreated groundwater sources. Several factors combine to perpetuate these conditions including, inadequate characterisation of the groundwater resource, limited planning and the inadequate supply of piped-water from official institutions. As well as the need for investment and structural reform to the water sector in SSA, there is clearly a very important role for NGO and community-based organisations to help fill the gap and provide local expertise (Allen and Bell, 2011; Allen et al., 2006a). Maintenance is a critical issue for water availability and quality, both in SSA and elsewhere, and has a direct influence on how communities use and relate to a range of different water sources. There are also important cultural factors that control the use and adoption of improved water and sanitation provision, and these may operate on a longer time scales than those of individual projects or policies.

Several studies have shown that faecal contamination of shallow groundwater can be elevated during periods of intense rainfall, increasing the risk of widespread gastrointestinal disease. These observations have important implications for climate change which is expected to change the intensity and frequency of rainfall events (Howard et al., 2010; Howard et al., 2003; Taylor et al., 2009; Taylor et al., 2008; Taylor et al., 2009). In addition to microbiological contamination, and naturally-occurring water quality problems such as fluoride and arsenic, urban aquifers are also at risk of pollution from a large range of contaminants including nitrate, toxic metals, hydrocarbons and micro-organics (Adelana et al., 2008). This is due to inadequate waste management and regulation in new rapidly growing centres, and because groundwater protection is rarely considered a priority during early development (Howard et al., 2006).

The World Health Organization (WHO) Guidelines for Drinking-Water Quality are published with the purpose of protecting public health, however, ‘Safe drinking-water, as defined by the Guidelines, does not represent any significant risk to health over a lifetime of consumption, including different sensitivities that may occur between life stages’ (WHO, 2006). Using the WHO Guidelines as a reference many countries in SSA have drafted national standards for drinking water quality. National standards take into account local factors such as geology, geography, social as well as economic factors and as such may differ from WHO guideline values significantly. While standards for drinking water quality may be published at the national level, there are many different approaches across SSA to implementing the standards including, frequently, minimal or no enforcement. The amount and quality of groundwater quality data currently available across SSA is highly variable, and this, to some extent, is a consequence of inadequate implementation and regulation of drinking water quality standards in this region.

Structure and scope of review

This report comprises a review of the state of current understanding of groundwater quality beneath urban centres and peri-urban areas in SSA and the issues that contribute to changes in groundwater quality. The report will cover published and grey literature. There is a wealth of information available for large cities but the small rural towns and the rapidly expanding peri-urban zones are much less well characterised. This report provides a much needed synthesis of published groundwater quality studies carried out in urban and growing urban centres in SSA. We will also make a broad assessment of likely future water quality pressures on this critical resource bringing expertise from across SSA and include socio-economic aspects of water supply degradation.

This report reviews and synthesises existing understanding of water quality in urban centres and per-urban centres (including case studies from Southern, East and West Africa) to provide an authoritative assessment of the current state of knowledge of urban groundwater quality in SSA. This document provides a) an overview of the key issues related to groundwater development in an urban context and an entry point for reader new to this topic, b) a systematic review of empirical studies assessing groundwater degradation in urban groundwater in SSA, c) identifies gaps in the current evidence base regarding groundwater quality degradation in urban groundwater. It will also include a review of novel methods of assessing the sources of groundwater pollutant.

Published information was sought predominantly from documents available on the internet on groundwater quality, pollution assessments and supporting data from a wide range of African countries (Figure 1.1). As part of this review a database of literature were compiled of articles, books and reports using Web of Science, PubMed, Google Scholar, ScienceDirect, as well as reference lists available in published documents. Grey literature, available in the BGS Groundwater Archive at Wallingford, UK, and in-country documents were used to inform the case studies from Nigeria, Zambia and elsewhere. The geographical extent of the review covers a large part of SSA and includes the southern parts of Mali, Niger, as well as Ethiopia and South Africa.

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Figure 1.1    Countries in Sub-Saharan Africa covered by this review. Source ArcGIS 10.1 data sets (Zhang et al., 1995).

Figure 1.2 shows the major cities coloured by population underlain by quantitative groundwater storage maps based on porosity and saturated aquifer thickness (MacDonald et al., 2012). Urban centres with populations >500 000 are displayed and these include most of the locations covered in this review. The population density in Nigeria stands out and Lagos and Kinshasa are the largest cities, although Nigeria also has significant areas of urban sprawl such as Ibadan. It is clear that there is a large concentration of urban centres in coastal regions, and the most densely populated regions are in West Africa and selected parts of East Africa. Major urban centres are underlain by aquifers with a range of estimated aquifer productivity, including many that have low-moderate productivity (0.5–1 l/s) or less. Several large urban centres on the coast of West Africa, from Luanda round to Nouackchott intersect with moderate-high productivity alluvial aquifers; however, these tend not to be regionally extensive with the exception of the Niger Delta aquifer and the Senegal-Mauritania sedimentary basin. The largest and most highly productive aquifers are found in the interior of Africa, in northern Niger, Chad, Mali, Angola and DRC, all of which have low population densities. Figure 1.3 shows the population density and growth in Africa including hot-spots of urban growth in West and East Africa.

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Figure 1.2    Major cities in SSA by population and relationship with groundwater storage Population data from ArcGIS 10.1 data sets (Zhang et al., 1995), aquifer productivity from MacDonald et al. (2012).

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Figure 1.3    Population density and growth hot-spots in Africa (data source: UNEP).