Tindimugaya, Callist; (2008) Groundwater flow and storage in weathered crystalline rock aquifer systems of Uganda. Doctoral thesis , UCL (University College London).

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Abstract

Groundwater is widely developed for town water supplies in weathered crystalline rocks in sub-Saharan Africa but the sustainability of this abstraction is unknown. Groundwater flow and storage in aquifers underlying two towns in central (Wobulenzi) and southwestern (Rukungiri) Uganda are assessed using environmental tracers and aquifer responses to hydraulic stress. Stable isotope ratios ( H: H, O: O) in precipitation and groundwater, the timely response of groundwater levels to bimodal rainfall, and short groundwater residence times of less than 22 years and big proportions of modern groundwater (5 to 100 %), derived from reconstructed atmospheric inputs of H and anthopogenic gases (CFC-113, CFC-12, CFC-11), clearly indicate active rainfall-fed, groundwater recharge. Diagnostic plots (s versus t/r , log-log, log-linear, derivative, flow dimension) of drawdown responses are used to inform conceptual models of groundwater flow. In Wobulenzi, linear flow through individual bedrock fractures at early pumping times (between 600 and 1500 minutes) is succeeded by radial flow through interconnected fractures that induce vertical flow in a thick weathered (regolith) aquifer at late pumping times (approximately 1800 minutes). Groundwater abstraction from bedrock fractures of 12 m h" per borehole, has not significantly affected groundwater storage over the last 9 years due to vertical leakage from overlying weathered aquifers that is commensurate to rainfall-fed recharge. In Rukungiri, a highly productive aquifer comprising coarse-grained, fluvial sediments, is identified in palaeochannels of former westerly flowing river networks. Fluvial sediments can feature significant thicknesses in palaeochannels of major river networks truncated by Miocene to Pleistocene rifting but subsequent erosion in the intra-arch basin, draining to the downfaulted rift floor significantly constrains the extent and thickness of the aquifers and, hence, the sustainability of groundwater abstraction. Depletion of groundwater storage over the last 8 years as a result of abstraction (12 m h" per borehole), is indicated by water-level declines of 2.5 m a". The identification of the palaeochannel aquifer provides new insight into the understanding of the relationship between the geomorphology and hydrogeology of deeply weathered environments and a new target for groundwater development in the humid tropics.

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