Présentations Colloques

    Oral Presentation
    Session 8.03: Geometrical structure and hydrogeological properties of Hard-Rock aquifers.
    Bon André Firmin
    Evaluation of the groundwater potentialities of the Yaounde (Cameroon) fissured hard rock aquifers
    Surface water nowadays constitutes the main potable water supply for the Yaounde city and its suburbs (2.10 million inhabitants). This resource is not sufficient to satisfy the population needs of this large city built on a weathered Precambrian basement, cut by N-S, NW-SE and NE-SW fractures. This basement is constituted from metasedimentary and metaplutonic rocks which hydrogeological knowledge can contribute to partly answer a growing water demand. The present research, based on a statistical analysis of the data from 86 borewells covering an about 300 km2 area, shows a similar range of weathering cover (saprolite) both for the paragneisses (7 to 47 m) and the orthogneisses (2 to 40 m). For these two types of geological formations, the blowing discharges are also similar (respectively between 0.1 and 10 m3 h and between 0.2 and 12 m3 h). The most represented discharge class, whatever the lithology, is the one between 1 and 5 m3 h. From the hard rock aquifers literature, such a productivity is well representative of aquifers with open and connected fissures networks. Statistically, the pervious fissured zone extends from 10 to 60 m below the base of the soft weathered rocks (saprolite), with a higher density between 10 and 25 m and a second zone, more represented, between 30 and 50 m. The ratio discharge depth measured from the saprolite’s base (Q B), a representative indicator in such a context, ranges from 0.003 to 0.36 m3 h m, and was mapped over the whole study area. On this basis, the links between morphology, weathering and Q B, as well as with the location of the borewells was investigated.


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