GROUNDWATER EXPLORATION IN THE CRETACEOUS BASIN OF THE CENTRAL AFRICAN REPUBLIC

A groundwater investigation was conducted to understand the hydrogeology of the 25,000 km² Cretaceous-aged sedimentary basin in southwestern Central African Republic to support humanitarian water supply challenges. The region consists of rainforest, sparse population density and some of the last pygmy populations in the world. Well drilling efforts have continually been unsuccessful due to difficult drilling conditions and limited aquifers imposed by the vast deposits of fine sands and silts. Exploration for the rare occurrences of coarser grained sedimentary deposits beneath the water table is critical for siting successful wells in the Cretaceous basin.

An alternative, deeper Precambrian crystalline aquifer is known to exist beneath the Cretaceous sediments; however, the thicknesses of the Cretaceous sediments are largely uncharacterized which impede deeper drilling attempts. Most of the country benefits from expansive shallow aquifers located within these fractured Precambrian crystalline igneous formations where Cretaceous sediments do not exist.

The investigation was designed to be cost efficient, practical and focused on improving drilling success and guiding water supply decisions. Tasks included field reconnaissance, analysis of well/boring logs, geomorphic terrain assessment and GIS analysis to map the extent of the Cretaceous basin as a hydrogeologic unit and identify areas that have the greatest possibility of producing a coarser-grained aquifer medium to support wells. Hydrogeologic mapping and modeling was also performed to provide a regional-scale analysis showing estimated drilling depths required to reach water tables within the Cretaceous sediments. The work is on going and additional phases are planned to apply geophysical technologies and field studies to map the thickness of the Cretaceous sediments and further characterize Cretaceous hydrogeology in critical population areas.