Abdoul Aziz Gning^{1, 2,}, Mansour Gueye¹, Mapathé Ndiaye², Fatou Ngom Diop¹, Raymond Malou¹

The Senegal River delta is located in a semi-arid sahelian zone of West Africa. Due to low pluviometry in the delta, this river is an exceptionally important water resource for the region. The importance of surface water from the Senegal River is elevated more by the fact that much of the groundwater accessible by shallow wells is saline. Salinization in the Senegal River delta originates from marine water invasion, both past and present, into the continent. Marine invasion and even full transgressions have been known to what is now the delta throughout geological history. Evidence of cyclical marine transgressions and regressions go back to the Jurassic, and multiple transgressions have occured in the Quaternary period alone. The main interest of this work is on the hydrogeological aspect of salinization of shallow aquifers in the the Senegal River delta. The objective is to be able to characterize salinization dynamics and the related surface water-groundwater interactions. Identifying mechanisms of groundwater freshening, and to understand the role of surface river water to this end, will be investigated in depth. Irrigated agriculture is not a primary object of study, although it’s presence and impact on groundwater is closely linked and evaluated. To approach the problematic, a combination of multiple monitoring methods was used in the interest of characterizing groundwater salinizationand/River water intrusion in the Senegal river delta. Collection of data for this study was founded on field work. This included hydrochemical monitoring as well as geophysical profiling over multiple selected sites. Field work was performed during the months of February and March 2016, in the middle of the dry season. Analysis of geochemical and geophysical data collected during the course of this study has brought together strong evidence of river water recharge into the shallow aquifer from the Senegal River delta, and to have dimensioned the sub-surface hydraulic intrusion. The magnitude of intrusion seems to be a function at least of the relative difference in head and of the debit of the surface water that is in closest proximity. In other words, even the most simple hydrodynamic parameters play an important role in the phenomenon. Other factors - geologic, climactic, chemical - have their part. Comparing the data of this study with historical context and data that has been collected over at least the past century has aided in constructing such additional factors controlling water dynamics.

ERT, Fieldwork, alluvial groundwater, salinization, irrigation, hydrogeophysics, Senegal River Delta