Tdem study of the variation of freshwater lens morphology between rainy and dry season in the coastal shallow aquifer of south Benin

The study was realized with the aim of showing the variation of the freshwater lens morphology in a unconfined coastal aquifer on a site of 360.000m2 between the Atlantic Ocean and the coastal lagoon. For this study 115Time Domain Electro Magnetism (TDEM) soundings were realized in rainy season and in dry season. For two various seasons, the static levels and the electric conductivities of waters in 17 wells were measured. A porosity of 34 % of the aquifer was determined using Archie’s law. That has allowed determining the limit of resistivity value of ground containing freshwater lens. At each season 12 TDEM resistivity maps and 7 TDEM vertical sections are been built for mapping the extent of freshwater lens. The obtained results show that under the pressure of sea water intrusion the freshwater lens moves towards the lagoon in dry season. The consequence of this movement is that few wells inside the freshwater lens in rainy season are found outside the freshwater lens in dry season and are thus contaminated. This TDEM study for monitoring the variation of freshwater lens morphology between rainy and dry season can contribute to the choice of wells with sustainability sanitation.