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

• Published in: Hydrogeology journal 2000-06, Vol.8 (3), p.279-294
• Main Authors: Taylor, Richard, Howard, Ken
• Format: Article
• Language: English
• Subjects: Aquifer systems; Aquifers; Bedrock; Crystalline rocks; Geology; Geomorphology; Heterogeneity; Hydrogeology; Mantle; Mathematical models; Meteoric water; Public waters; Pumping tests; Rock; Transmissivity; Water supply; Weathering
• ISSN: 1431-2174; 1435-0157
• DOI: 10.1007/s100400000069

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 yields. Aquifers occur in the bedrock and overlying weathered mantle and are the products of geomorphic activity of meteoric water, principally deep weathering and stripping. The fundamental relationship between the hydrogeology and geomorphology of these terrains has, however, remained unresolved. This study demonstrates the ability of a recently developed tectono-geomorphic model of landscape evolution in Uganda to explain the hydrogeological characteristics of two basins, as determined using a combination of textural analysis, slug tests, packer tests, and pumping tests. The geopetal imprint of long-term deep weathering and erosional unloading is identified in the vertical heterogeneity of the fractured-bedrock and weathered-mantle aquifers; horizontal heterogeneity is lithologically controlled. The two units form an integrated aquifer system in which the more transmissive (5-20m super(2)/d) and porous weathered mantle provides storage to underlying bedrock fractures (transmissivity, T, approximately 1m super(2)/d). The thickness and extent of the more productive weathered-mantle aquifer are functions of contemporary geomorphic processes. The utility of the tectono-geomorphic model, applicable to deeply weathered environments, is that it coherently describes the basin-scale hydrogeological characteristics of these complex terrains.Original Abstract: Les roches cristallines profondement alterees constituent d'importants aquiferes captes pour l'eau potable dans les regions de basse latitude d'Afrique australe, d'Amerique du Sud et d'Asie, mais ces aquiferes possedent une heterogeneite considerable et le rendement des forages y est faible. Ces aquiferes se developpent dans le substratum et dans sa couverture d'alteration; ils resultent des actions geomorphologiques de l'eau meteorique, principalement l'alteration profonde et le decapage. La relation fondamentale entre l'hydrogeologie et la geomorphologie de ces terrains reste cependant non resolue. Cette etude demontre les possibilites d'un modele tectono-geomorphologique recemment developpe de l'evolution des paysages en Ouganda pour expliquer les caracteristiques hydrogeologiques de deux bassins, definies en utilisant une combinaison de l'analyse texturale, de slug tests, d'essais entr