The coastal city of Cotonou in Bénin, West Africa, is a large population center that is facing a serious threat to the sustainability of its fresh water supply. Cotonou is Bénin's largest city with approximately 1.5 – 2.0 million people. It relies on the Godomey aquifer for domestic water supply. The aquifer is undergoing saltwater intrusion and this problem is likely to worsen without significant steps to improve management of the water supply. Lake Nokoué is a nearby water body that has high salinity levels throughout much of the year and is thought to be the primary source of salinity in the aquifer. Within Lake Nokoué is Ganvié (a city of greater than 30,000 inhabitants built entirely on stilts in the lake). The presence of this lake city and the fact that the lake is heavily relied upon for fishing has resulted in severe manipulation of the lake for waste disposal, navigation and fish farming. The continuity of the aquifer and saltwater flow paths are poorly understood but this information is critical to ensure sustainable access to fresh water in this growing urban center.
Shallow seismic reflection is a geophysical technique that is well suited to mapping the aquifer system, and when coupled with electrical resistivity measurements, can be a powerful tool in identifying flow pathways for saline water contamination. To map the hydrogeologic units of the Godomey aquifer system, we will acquire seismic reflection and resistivity surveys over two field campaigns. The first field season will be focused on land seismic acquisition and coincident resistivity data in the area west of Lake Nokoué and into the Godomey well field. The second field season will be focused on marine seismic acquisition on Lake Nokoué and repeated electrical resistivity measurements. All seismic profiles will be tied to existing well logs with the primary objective of mapping the water bearing units and locating saltwater flow paths. The aquifer map produced through seismic reflection analysis will provide critical information to the local water management agency and measured geometry of geologic units will be used to improve hydrologic models of the aquifer.
This work will have a number of important benefits, both direct and indirect. These include: (i) Development of a high-quality aquifer map for use by the water agencies of Benin in the management of the domestic water supply for the city of Cotonou, (ii) Opportunity for collaboration among colleagues from Gonzaga University, Boise State, the national university in Benin and the Benin government water agency, (iii) Integrating undergraduate and graduate students directly into international field work, thus allowing U.S. students from Notre Dame and Boise State to work directly with their peers and faculty from Benin, (iv) Technology exchange among the three partner schools and the Benin government agency, (v) training on modern geophysical equipment and state-of-the-art data processing and analysis software for students from Benin, (vi) and direct integration of the results of this work into coursework at Boise State and Gonzaga University.