Hydrogeology of an urban weathered basement aquifer in Kampala, Uganda | Hydrogéologie d’un aquifère de socle altéré en contexte urbain à Kampala, Ouganda Hidrogeología de un acuífero urbano en un basamento meteorizado en Kampala, Uganda 乌干达坎帕拉城市风化基岩含水层的水文地质 Hidrogeologia de um aquífero urbano de embasamento alterado em Kampala, Uganda
2022
Nyenje, Philip M. | Ocoromac, Denis | Tumwesige, Stephen | Ascott, Matt J. | Sorensen, James P. R. | Newell, Andrew J. | Macdonald, David M. J. | Gooddy, Daren C. | Tindimugaya, Callist | Kulabako, Robinah N. | Lapworth, Dan J. | Foppen, Jan Willem
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 20–50-m 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.
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