A tectono-geomorphic model of the hydrogeology of deeply weathered crystalline rock: Evidence from Uganda

Deeply weathered crystalline rock forms important aquifers for public water supply throughout low-latitude regions of Africa, South America, and Asia, but these aquifers have considerable heterogeneity and produce low well yi elds. Aquifers occur in the bedrock and overlying weathered mantle and are the products of geomorphic activity of meteoric water, principally deep wea thering and stripping. The fundamental relationship between the hydrogeolog y and geomorphology of these terrains has, however, remained unresolved. Th is study demonstrates the ability of a recently developed tectono-geomorphi c model of landscape evolution in Uganda to explain the hydrogeological cha racteristics of two basins, as determined using a combination of textural a nalysis, slug tests, packer tests, and pumping tests. The geopetal imprint of long-term deep weathering and erosional unloading is identified in the v ertical heterogeneity of the fractured-bedrock and weathered-mantle aquifer s; horizontal heterogeneity is lithologically controlled. The two units for m an integrated aquifer system in which the more transmissive (5-20 m(2)/d) and porous weathered mantle provides storage to underlying bedrock fractur es (transmissivity, T, approximate to 1 m(2)/d). The thickness and extent o f the more productive weathered-mantle aquifer are functions of contemporar y geomorphic processes. The utility of the tectono-geomorphic model, applic able to deeply weathered environments, is that it coherently describes the basin-scale hydrogeological characteristics of these complex terrains.