Hydrogeology of an urban weathered basement aquifer in Kampala, Uganda

Hydrogeology Journal https://doi.org/10.1007/s10040-022-02474-9 REPORT Hydrogeology of an urban weathered basement aquifer in Kampala, Uganda Philip M. Nyenje 1 & Denis Ocoromac 2 & Stephen Tumwesige 1 & Matt J. Ascott 3 & James P. R. Sorensen 3 & Andrew J. Newell 3 & David M. J. Macdonald 3 & Daren C. Gooddy 3 & Callist Tindimugaya 4 & Robinah N. Kulabako 1 & Dan J. Lapworth 3 & Jan Willem Foppen 2,5 Received: 24 July 2021 / Accepted: 2 March 2022 # British Geological Survey, Makerere University, Uganda Ministry of Water and Environment and IHE Delft Institute for Water Education 2022 Abstract Weathered basement aquifers are vital sources of drinking water in Africa. In order to better understand their role in the urban water balance, in a weathered basement aquifer in Kampala, Uganda, this study installed a transect of monitoring piezometers, carried out spring flow and high-frequency groundwater level monitoring, slug tests and hydrochemical analyses, including stable isotopes and groundwater residence time indicators. Findings showed a typical weathered basement aquifer with a 20–50m thickness. Groundwater recharge was 3–50 mm/year, occurring during sustained rainfall. Recharge to a deep groundwater system within the saprock was slow and prolonged, while recharge to the springs on the valley slopes was quick and episodic, responding rapidly to precipitation. Springs discharged shallow groundwater, mixed with wastewater infiltrating from onsite sanitation practices and contributions from the deeper aquifer and were characterised by low flow rates (< 0.001 m3/s), low pH (<5), high nitrate values (61–190 mg/L as NO3), and residence times of <30 years. The deeper groundwater system occurred in the saprolite/saprock, had low transmissivity (< 1 × 10−5 m2/s), lower nitrate values (<20 mg/L as NO3), pH 6–6.5 and longer residence times (40–60 years). Confined groundwater conditions in the valleys were created by the presence of clay-rich alluvium and gave rise to artesian conditions where groundwater had lower nitrate concentrations. The findings provide new insights into weathered basement aquifers in the urban tropics and show that small-scale abstractions are more sustainable in the deeper groundwater system in the valleys, where confined conditions are present. Keywords Basement aquifer . Groundwater flow . Recharge . Urban . Uganda Introduction * Philip M. Nyenje * Dan J. Lapworth * Jan Willem Foppen 1 Department of Civil and Environmental Engineering, Makerere University, P.O. Box 7062, Kampala, Uganda 2 IHE Delft Institute for Water Education, P.O. Box 3015, 2601 DA Delft, The Netherlands 3 British Geological Survey, Wallingford, Oxfordshire OX10 8BB, UK 4 Directorate of Water Resources Management, Ministry of Water and Environment, P. O. Box 20026, Kampala, Uganda 5 Department of Water Management, Delft University of Technology, P.O. Box 5048, 2601 DA Delft, The Netherlands From 1990 to 2018, the urban population in sub-Saharan Africa (SSA) grew from 135 to 423 million (UN-DESA 2019), while the percentage of the population living in urban informal settlements or slums remained constant at around 55–60%. By 2050, in SSA, it is predicted that there will be 1.25 billion people living in urban areas of which 723 million people will live in slums (UN-DESA 2019). This growth will continue to lead to a rise in the demand for potable water (Carter and Parker 2009; Foster and Sage 2017; Parnell and Walawege 2011; Taylor et al. 2004). In many situations, water supply in these areas is met by groundwater self-supply (e.g. Silva et al. 2020; Sutton and Butterworth 2021) from protected springs, shallow wells and deep boreholes (Carter and Parker 2009; Gaye and Tindimugaya 2019; Grönwall 2016; Komakech and de Bont 2018; Lutterodt et al. 2018; MacDonald et al. 2012; Okotto et al. 2015; Okotto-Okotto Hydrogeology Journal et al. 2015; Olago 2019; Taylor and Barrett 1999; WaterAid 2010). In Kampala, Uganda, groundwater is an important part of the urban water cycle, not only for water supply (Flynn et al. 2012; Howard et al. 2003; Kulabako et al. 2007, 2008, 2010; Nabasirye et al. 2011; Nastar et al. 2019; Silvestri et al. 2018), but also in relation to on-site sanitation and wastewater infiltration to groundwater (Fuhrimann et al. 2016; Katukiza et al. 2015; Katukiza et al. 2014; Lutterodt et al. 2014; Lutterodt et al. 2012; Ronoh et al. 2020; Sakomoto et al. 2020; Tumwebaze et al. 2019), and groundwater input to wetlands in and around the city (e.g. Kansiime et al. 2007; Kyambadde et al. 2004; Nyenje et al. 2014a; Were et al. 2021, 2020). The hydrogeology and flow systems of the uppermost 2–3 m of the subsurface in some of the lowland parts of Kampala (Bwaise) has been studied in some detail (Kulabako et al. 2007; Nyenje et al. 2013, 2014a). Although, the hydrogeology (i.e. evolution, texture and hydraulic properties) of the deeper weathered basement has also been studied (see Guma et al. 2019; Taylor and Howard 1999, 2000), there is still limited understanding of its linkage with groundwater flow systems and the rest of the urban water cycle. Crystalline basement rocks with a thick mantle of in situ weathered material underlie approximately 34% of Africa’s land surface (Bianchi et al. 2020; Macdonald et al. 2008, 2012; MacDonald and Calow 2009, Lachassagne et al. 2011; Taylor and Howard 1998; Vouillamoz et al. 2005; Wright 1992). The established hydrogeological conceptual model of the weathered profile of crystalline basement rocks in tropical Africa includes (Bianchi et al. 2020): a few meters of residual soil (RS) that often comprise ferralitic soils with laterite pisoliths; an upper saprolite (US) layer composed of mainly secondary clay minerals, with thicknesses ranging from a few to tens of meters; a lower saprolite (LS) layer, similar to the US in thickness but with a much higher proportion of primary minerals; and a fractured layer, up to tens of meters thick, referred to as the saprock (SR), transitioning with depth into the underlying unweathered basement rock (Acworth 1981; Chilton and Foster 1995; Dewandel et al. 2006; Jones 1985; Wyns et al. 2004; Dewandel et al. 2006; Maréchal et al. 2004; Courtois et al. 2010; Lachassagne et al. 2011, 2014, 2021; Belle et al. 2019; Alle et al. 2018; Bianchi et al. 2020). The residual soil and saprolite layers form a largely unconsolidated part of the profile, also known as the regolith (Bianchi et al. 2020). While the fissures are often assumed to be the result of tectonic or lithostatic decompression processes, it has also been suggested that they are related to the weathering process with the degradation of bedrock driven by the swelling of minerals such as biotite (Lachassagne et al. 2011). The fissured and fractured saprock typically represents the most transmissive zone within the weathering profile (Lachassagne 2008). Basement aquifers (or hard-rock aquifers) are therefore able to develop within the weathe (...truncated)