Présentations Colloques

Oral Presentation
6.01
Session 6.01: Managed Aquifer Recharge
Collignon Bernard
Managed Aquifer Recharge (MAR) as to supply Libreville water-stressed city (Gabon)
Objective- supplying a water-stressed city**Libreville has been implemented on a peninsula, were most groundwater is brackish and far from any large surface water resources. Since the 70’s, the city has been supplied with surface water, collected from 5 small watersheds and treated in a single main plant located in Ntoum, 40 km eastward of Libreville. There are little additional surface water resource in the nearby and SEEG is desperately seeking for alternative cheap water resources.**Design and methodology**In 2007, SEURECA and HYDROCONSEIL provided SEEG with hydrogeological expertise, as to develop groundwater resources in the nearby of existing facilities.**A thick limestone layer had been previously drilled by oil-companies- Madiela limestones whose outcrops are very limited (4 km2).**Madiela proved to be very productive (2,000 - 7000 m3 day per borehole) and we have developed a new well field. As (a) the aquifer extension is limited and (b) sea water intrusion is at risk, the well field was developed step by step, with a careful monitoring of water quality (i.e. conductivity, as a signal of sea water intrusion) and water level. **Data and results**The aquifer storage capacity has been measured through the long dry season drawdown as 6.3 million m3. This is unneglectable, but far to meet SEEG expectations. For this reason, we have also monitored aquifer recharge during rainfalls and it proved to be impressive (> 20 million m3 year). **Such a large recharge is more than direct infiltration in the tiny aquifer outcrop. It led to us to seek for other recharge areas through geophysical prospecting. Ntoum aquifer recharge is more complex than initially expected and the hydrogeological model has been accordingly updated.**Conclusion**Taking into account the recharge efficiency, we have developed a MAR strategy, with additional wells. Presently, Ntoum well field produces 55,000 m3 day of good quality water, without any significant impact on resource quality or quantity (i.e. no significant drawdown of the water table has been recorded).********
France