Hydrogeological characteristics of the Albertine Graben, Uganda: Evidence from surface geophysics and hydraulic testing

Abstract Current understanding of the hydrogeological characteristics of riftmargin environments is limited and not well reconciled to large-scale conceptual models of rift evolution. In East Africa where there is substantial dependence on groundwater for rural water supplies, it is unclear how global change including localised development of oil impacts the quality and quantity of available groundwater resources. Hydrogeological characteristics of the Albertine Graben (Uganda) were explored primarily using surface resistivity surveys and hydraulic testing and supported by evidence from lithological logs, structural data and drainage maps. Interpretation of the vertical electrical soundings and lithological data reveal a heterogeneous subsurface with three to five (geo-electric) layers in areas spatially underlain by the Precambrian basement and sedimentary formations. A hydrogeological conceptual model is developed that details the influence of the morphology and geological structures on the hydrogeology of the studied area. It is revealed that the two aquifer environments: (1) transmissive, unconsolidated sediments (Sedimentary formation with mean T = 45 m 2 .day -1 averaging 27 m thick) of predominantly Miocene to Pleistocene age within the trough below the escarpment and, (2) deeply weathered Precambrian basement rocks (Bunyoro formation with mean T of 6 m 2 .day -1 averaging 13 m thick and Karuma formation with mean T of 28 m 2 .day -1 averaging 38 m thick) of lower transmissivity within the Lake Albert basin above the escarpment occur. The results suggest, localised groundwater flow in the metamorphic aquifers towards surface drainage that exhibit a dendritic drainage pattern, and also have a regional flow system that is greatly influenced by NE-SW trending regional structural lineaments. In the Sedimentary formation, a dendritic drainage pattern is also exhibited and groundwater flow is dominantly westwards to Lake Albert that forms the main constant head boundary.