Predicting permeability in fractured rock aquifers of Northwestern Uganda at a regional scale

Abstract

Study regionNorthwestern Uganda. Study focusSiting water supply wells in fractured rock aquifers is challenging due to the heterogeneity of fracture systems since fractures must be both connected and abundant for water to be extracted in usable quantities. Two conceptual models explaining the permeability generating mechanisms have been proposed in the literature: the faulting model and the weathering model. To test these hypotheses, we estimated hydraulic conductivity in northwestern Uganda with eight inverse groundwater flow models calibrated to hydraulic heads and river discharge. Each model incorporated different assumptions about the hydraulic conductivity distribution based on age, surface slopes, and faults. New hydrological insights for the regionThe two-layer heterogeneous basement model with erosion and the two-layer homogeneous model with normal faults were shown to have the best fit for these data. Because of model non-uniqueness, it is difficult to conclusively determine if either conceptual model is correct due to limited field observations. However, the results suggest that faulting could play a significant role in the regional groundwater flow field and further study is needed to determine the importance of normal faults in groundwater flow. Additionally, results from the two-layer heterogeneous basement model with erosion suggest that rock age is a poor predictor of weathering extent as spatially and temporally varying rates of erosion prevent a clear trend between age and hydraulic conductivity.