ABSTRACT
ABSTRACT: This study of recharge to the Continental Terminal and Quaternary aquifers in the southern Ivory Coast was carried out using both hydrochemical and isotopic methods. Solutes in the groundwater in these aquifers originate mostly from atmospheric contributions and the hydrolysis of silicate minerals. In places, the water quality has deteriorated by nitrate pollution and/or salt-water intrusion from the lagoons. The comparison of the isotopic composition in rain waters with the groundwaters indicates that infiltration by rainwater constitutes the main source of groundwater recharge in the greater Abidjan area. Recharge to the aquifers can occur all year round, not just in the rainy season during which runoff exceeds infiltration. Lack of temporal and spatial variability of heavy isotopes of groundwater is indicative of a good mixing of waters from different recharge episodes and ambient groundwater which is consistent with the porous nature of the aquifer. The isotopic balance of 13C composition of Total Dissolved Inorganic Carbon (ranging from −26.7 to −11.4‰ PDB) and soil CO2 (from −25.3 to −11.6‰ PDB) indicates that biogenic gas produced under forest cover in the area of Abidjan (mean δ13C =−25.0‰ PDB) and under Savanna around Dabou (δ13C =−11.6‰ PDB) plays an important role in the carbon mineralization of the groundwaters of the Continental Terminal and the Quaternary aquifers. High 14C values (between 92.7 and 114.7 pmC) and the 3H content (∼8.4 TU) show that recharge to the Continental Terminal and the Quaternary aquifers is a recent phenomenon. The residence times computed from 14C activities and tritium content vary from modern to about five hundred years.
