Abstract Throughout Africa, many communities rely on fractured crystalline basement rocks of Precambrian age as the primary source of water supply. The hydrogeology of these aquifers is often poorly understood, and boreholes are frequently sited and designed with little appreciation of the local fracture system. This is especially true in Uganda, where the role of fractures in determining well yield and relative importance of deep and shallow fracture zones have been a continous source of debate since a major national well drilling program was initiated over ten years ago. Recent studies in Uganda have attempted to resolve outstanding hydrogeological questions through the implementation of one of the most comprehensive borehole fracture studies ever undertaken in continental Africa. During the study, 233 packer tests were carried out at 22 sites. Six of the sites, together with a further 16, were subjected to constant yield recovery tests. One well was tested before and after the deployment of yield-enhancing hydraulic fracturing techniques to investigate its effectiveness. Study results reveal the presence of an extremely poor aquifer with little transmissive capability. Across the study area, the hydraulic conductivity of the bedrock average 10-7 m/s; the majority of transmissivity values are less than 1 m2/d. Packer tests show that fractures occur throughout the depth of well, but are usually very few in number and often exhibit a relatively low permeability. Most wells relied on a single permeable zone for most of their water. In approximately half of the boreholes tested, the most permeable zone was found to occur at the top of the uncased section, just below the regolith. No relationship was found between the distribution of boreholes with shallow, intermediate or deep, high yielding fracture zones and physiographic features. Neither was any relationship observed between hydraulic conductivity and depth within the upper 60 meters of rock tested. Notably, however, no fracture zones more permeable than 10-5 m/s were found below a depth 57m, and it is suggested that a depth of approximately 60m below ground level may represent the effective base of aquifer. Packer tests performed in a hydraulically fractured borehole confirmed increases in borehole transmissivity by a factor of about 3. The tests were also able to show where improvement in hydraulic conductivity was being achieved.
Rsum. Estimation de la permabilit de fractures dans les aquifres du socle cristallin en Ouganda. Partout en Afrique, les ressources en eau de nombreuses communauts proviennent pour lessentiel des roches cristallines fractures dge prcambrien. Lhydrogologie de ces aquifres est souvent mal connue; frquemment, limplantation des forages sappuie sur une connaissance bien faible du dispositif local de fracturation. Cest particulirement vrai en Ouganda, o le rle de la fracturation dans la dtermination du dbit des puits et limportance relative des zones de fractures superficielle et profonde sont le sujet de discussions permanentes depuis le lancement, il y a 10 ans, du programme de forages. Les tudes rcentes en Ouganda ont tent de rpondre aux principales questions concernant lhydrogologie en ralisant lune des tudes les plus compltes sur la fracturation dans les forages jamais entreprises en Afrique centrale. Cette tude sappuie sur 233 tests avec packer mens sur 22 sites. Six de ces sites, puis 16 autres ont t soumis des tests de rabattement dbit constant, puis ltude de la remonte. Un puits a t test avant et aprs un dveloppement par fracturation hydraulique pour dterminer lefficacit de la mthode. Les rsultats obtenus montrent lexistence dun aquifre trs rdiut faible transmissivit. Dans la rgion tudie, la conductivit hydraulique du socle est en moyenne infriur 107 m/s; la majorit des valeurs de transmissivit sont infrieures 1 m2/j. Les tests avec packers montrent que les fractures sont prsentes tout au long du puits, mais quelles sont en gnral trs peu nombreuses et prsentent une faible permabilit. Lessentiel de la ressource de la plupart des puits provient dune zone permable unique. Dans prs de la moiti des puits tests, la zone la plus permable a t recoupe au sommet de la partie non tube, juste sous le rgolite. Il nexiste aucune relation entre la physiographie et la rpartition des puits selon la position superficielle, intermdiaire ou profonde des fractures productrices. Il na pas t non plus observ de relation entre la conductivit hydraulique et la profondeur, dans la tranche de roche teste des 60 m suprieurs. Toutefois, aucune zone fracture prsentant une permabilit meilleure que 105 m/s na t trouve au-dessous de 57 m, ce qui permet de supposer que la base relle de laquifre est environ 60 m sous la surface du sol. Les tests avec packers effectus dans le forage dvelopp par fracturation hydraulique montrent une amlioration de la transmissivit dun facteur 3. Les tests ont aussi montr o stait produite lamlioration de la conductivit hydraulique.