Permeability of the weathered bedrock aquifers in Uganda: evidence from a large pumping test dataset and its implications for rural water supply

Weathered crystalline-rock aquifers underlie around 40% of sub-Saharan Africa providing water supplies for half of its rural population; they are also prevalent across the tropics. The hydrogeology of these aquifers is complex and better understanding of the controls to aquifer transmissivity, alongside accurate borehole siting and appropriate design, is pivotal to the long-term performance and sustainability of water services in many countries. This study examines a substantial new dataset (n = 655) of aquifer transmissivity values across Uganda derived from previously unanalyzed pumping-test data. These data provide important new insights: weathered crystalline-rock aquifers are distinguished by pervasively low transmissivities (median <2 m2/d) in comparison to other areas in the tropics; highest transmissivities are observed in boreholes <50 m deep, implying that drilling deeper boreholes does not typically improve supply success or sustainability in Uganda; and increased groundwater abstraction through the use of higher-yielding pump technologies will prove challenging in most areas of Uganda and limit their use for piped water supplies. This study highlights the scientific value of data held in drillers’ records, which remain unanalyzed in many countries but may provide useful insight and understanding of aquifer characteristics.