Aquifers quantity and quality evolution within urbanized coastal areas: insights from piezometric and geochemical analyses of the Deep Continental Terminal Aquifer in Côte d’Ivoire

Coastal groundwater resources can be complex to study, due to temporally variable saline water intrusion associated with sea level variation. In the current climate change context, the challenge of rising sea level in West Africa is compounded by the pressures of a growing coastal population. In the densely coastal African cities, groundwater resources are therefore subject to major impacts on their quantity and quality by both sea-water intrusion, enhanced by aquifer over-exploitation, and pollution due to the lack of sanitation facilities, while freshwater needs are growing.

The aim of this work is to propose an integrated approach by combining hydrodynamic and geochemical studies in order to develop sustainable management strategies of the main coastal aquifer of the Greater Abidjan in Côte d’Ivoire. This area accounts for 36% of the national population and whose main source of drinking water is the Continental Terminal (CT) aquifer.

In 2024, a field campaign conducted on a set of 28 piezometers, reaching depths of up to 300 meters, enabled a 3D analysis of the aquifer. Current piezometric data align with the topography, but temporal data show a continuous decline in water levels. From a geochemical point of view, groundwater has generally a very low to moderate mineralization, with electrical conductivity values ranging from 21 to 2830 µS/cm (average of 288 µS/cm). However, groundwater tends to be more mineralized at greater depths in urbanized areas, where nitrate concentrations are higher. The waters of the CT aquifer are characterized by high acidity, with an average pH value of 5.1, reflecting the silicate nature of the aquifer, amplified by sulfide oxidation and dissolution of high amounts of soil CO₂. Furthermore, some piezometers show relatively high chloride concentrations (600-756 mg/l), combined with isotopic ratios ¹⁸O and ²H similar to those of seawater. These observations suggest the presence of saline intrusion in some coastal deep wells, as well as recharge by ancient waters, particularly in areas covered by Quaternary deposits.