Seismic Imaging to Constrain Groundwater Models for a Better Understanding and Management of Water Quality in Coastal Benin, West Africa: A Saltwater Intrusion Problem

摘要

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 relies on groundwater derived from the Godomey aquifer for its domestic water supply. The aquifer is undergoing saltwater intrusion due to an increase in pumping to accommodate a growth in population. Hence, there is substantial interest in better characterizing the groundwater system for the purpose of determining appropriate management strategies to ensure sustainability of this freshwater resource.I collected seismic reflection data along 15 transects to characterize the geometry of the Godomey aquifer system. I used standard high-resolution seismic methods to image the upper 200 meters using a sledgehammer source and a 120-channel recording system. Three transects were processed with an iterative updating flow that includes prestack depth migration, residual moveout analysis and reflection tomography, while the remaining 12 transects were processed with routine processing flows and poststack time migration. I identified one unconfined aquifer and three confined aquifers separated by reflective clay layers. Some transects showed areas of truncated reflectors which I interpret as channels that could provide potential high permeability conduits for saltwater flow to the Godomey well field.I updated the aquifer geometry of the existing hydrologic model based on the seismic reflection data and used a three-dimensional finite difference groundwater model to evaluate the impact of the updated geometry on groundwater flow. The updated groundwater model predicts increased recharge from Lake Nokoué, a shallow lake with elevated chloride concentrations that borders the eastern edge of the primary production well field.