Study of hard rock aquifer productivity and a statistical approach for the determination of potential productive depths: case of the Bongouanou region, eastern Ivory Coast
In the department of Bongouanou in the Ivory Coast, the potable water supply for the population is mainly assured by groundwater that is continuous in a hard rock aquifer of granite and schist. Unfortunately, more than 20% of bore holes that are drilled prove to be negative, because of their too weak productivity. The objective of this work was to improve the knowledge of this typical aquifer and to estimate its potential as a source of groundwater. To this end, statistical analyses of hydrogeological parameters from 230 drillings realized in the department have been carried out. For an annual average recharge of the aquifer of 145 mm, the average flow of drillings is 4.34 m3•h‐1 and the average transmissivity is 1.5•10‐4 m2•s‐1. The hard rock aquifers of the Bongouanou region are thus modestly productive, as is typical of weathered hard rock. The statistical results reveal that schist, which represents more than 80% of the territory, is more productive than granite. Generally, the drillings that are established on the slope and the bottom of valley in this region yield more important flows than those on the plateau. The influence of weathering on the productivity of drillings is evident from the apparent relationship between the thickness of the weathered material (saprolite) and the productivity of the aquifer. For weathered thicknesses of less than 15 m (indicative of the absence of a permeable fissured horizon) and greater than 60 m (interpreted as an indication that the drill hole intersects a weathered subsurface discontinuity), the flows are very weak. We also observed that the first 30 m of the fissured horizon, situated just underneath the weathered base, are more permeable and thus the most productive; beyond this limit, the permeability diminishes markedly with depth. From the ground surface, the optimal depths that are the most productive of groundwater are found between 40 and 70 m for granite, and between 40 and 80 m for schist. Finally, this work allowed us to propose a conceptual model of the structure and the hydrodynamic properties of the hard rock aquifers in this Ivory Coast region.