Using archive hydrogeological data to enhance the hydrodynamic knowledge of hardrock aquifers in Western Africa

Abstract

In the Kara River Watershed (KRW, northern Togo), drinking water is mainly supplied by groundwater flowing through fissures in the metamorphic formations of the Dahomeyides belt. The study was based on the use and valorisation of hydrogeological archive dataset from 1970s to 2021. The database comprises 1,389 boreholes, but only 710 are considered after pre-treatment, and provides information mainly on total depth, weathering thickness, discharge at drilling, piezometric level, specific discharge, transmissivity, and the nature of the lithology tapped. The methodological approach involved statistical analysis of data, characterization of the fissured horizon, establishment of relationships between hydrodynamic parameters and satellite image processing. The results showed that the aquifers structure is close to those observed worldwide in hardrock context and they provide operational details on the hydrogeological functioning of these environments in the West African particular context. From the surface downwards, aquifers show a layer of saprolite (thickness between 0.2 and 37.3 m) acting as a storage level, a fissured layer whose permeability depends on the number and connectivity of the fissures, and then a level of very low permeability, unfissured bedrock. In the study area, the depth of the useful fissured medium is 52 m with an average useful discharge of 7.1 m3.h-1. The coefficients of variation for specific discharge and transmissivity are greater than 100%, reflecting the structural heterogeneity of the study area. Analysis of the discharges measured in the boreholes as a function of lithology shows that the metasediments are more productive than other geological formations. Finally, all the geological, hydrogeological, and hydrodynamical data have been used to propose a preliminary conceptual model of the watershed's hardrock aquifers. These results will serve as decision-making tools for water managers and will facilitate the policy of integrated management of groundwater resources at the scale of the KRW. The developed methodology also shows how archive data should be used to achieve optimized management of aquifers without major investments.