Abstract

The rapid increase in the population of Conakry, the capital of Guinea, has put an increased pressure on the city's water supply. The larger part of the population, and particularly the economically underprivileged, resort to the contaminated waters of wells drilled with traditional, artisan methods. This situation is at the origin of water-borne diseases and a high incidence of child mortality in the poorer sections of the city. An economical and efficient solution to the problem would be the use of waters in nonpolluted alternative areas. The Kaloum peninsula, on which Conakry is situated, is insufficiently prospected from a hydrological point of view. This project will apply modern prospecting methods for the discovery of usable underground water layers. It will contribute to the strengthening of the research capacity of the "Direction nationale de hydrologie de Guinée".

Post-Project Summary

Researchers used 1:50 000 scale aerial photos of the city of Conakry to identify fracture zones and high-permeability zones in the basement. Geophysical surveys were conducted to see whether intensely fractured lattices (fracture nodes) caused pans corresponding to depressions in the dunitic substratum. They used piezometry and isotopic hydrogeology to follow the surface variations in the water table in the dry season and the rainy season, and to sample underground water to determine the variation in its quality at the bottom of the water table. Observations covered 152 water points, including 143 wells and springs, and 9 boreholes. A total of 500 samples were analyzed over 3 consecutive years (1992-1994). FLOWPATH software was used to develop two preliminary models for underground water flow in the Kaloum region. The observations confirmed the ubiquity of nitrates in the water table. About 38% of samples had more than 45 mg-NO3/l, the limit for drinking water adopted by many countries. The maximum contamination zone (45 to 220 mg-NO3/l) stretched from the east to the extreme west of Conakry near the airport, more or less following the population density, which is higher on the southern slope and the Niger road. The sources of contamination coincided with septic tanks, garbage dumps and wastewater outlets. Nitrate distribution maps showed more acute contamination during the rainy season, in terms of both concentration and extent. Nitrates tended to diminish as depth of installations increased. A number of potential sites in the basement were identified for boreholes. However, it was recommended that local hydrogeological studies be carried out before siting any facility considered a potential source of nitrate contamination, including garbage dumps and sewage treatment plants.

Recipient Institution(s)