26. Managed Aquifer Recharge (MAR) to Supply Libreville, a Water-Stressed City (Gabon)

Libreville has been built on a peninsula, where most groundwater is brackish and far from any large surface water resources. Since the 1970s, the city has been supplied with surface water that is collected from 5 small watersheds and treated in a single main plant located in Ntoum, 40 km east of Libreville. There are few additional surface water resources available nearby, and the national water utility is desperately seeking cheap, alternative water resources. From 2007 to 2014, SEURECA and HYDROCONSEIL provided SEEG with hydrogeological expertise to develop groundwater resource near existing facilities. A thick limestone layer (Madiela limestone) had been previously drilled by oil companies. Madiela has very limited outcrops (5 km²), but proved to be highly productive (2000–10,000 m³/day per borehole), and we successfully developed a new well field. As (a) the aquifer extension is limited and (b) there is a risk of seawater intrusion, the well field was developed step-by-step and the water levels and water quality were carefully monitored (i.e., conductivity, which can indicate seawater intrusion). The aquifer storage capacity was measured through the long dry season drawdown as being 220,000 m³ per m. This is sizeable, but insufficient was the aquifer to be mined. For this reason, we proposed a MAR strategy and monitored aquifer recharge during rainfall events. This recharge proved to be highly impressive (>20 million m³/year). Such a large recharge is higher than expected from direct infiltration in such a tiny aquifer outcrop. Additional recharge mechanisms have been investigated through geophysical prospecting, and the hydrogeological model has been updated accordingly. Presently, Ntoum well field produces 55,000 m³/day of good quality water without this having any significant impact on resource quality or quantity (i.e., no significant drawdown of the water table has been recorded). In light of the recharge efficiency, we have developed a MAR strategy that includes a new well field further to the north, with 4 additional deep wells.