Modeling the vulnerability of the Continental Terminal water table of the Togo coastal sedimentary basin under the combined effects of anthropogenic disturbances and climate change

Kokou Aimé DJONGON
GEOPS & GEOTOP
Year of Phd ( 2^nd)
kokou-aime.djongon@universite-paris-saclay.fr

Thesis Directors : Élisabeth GIBERT- BRUNET¹, Florent BARBECOT² , Viorel HOROϲ

1. UMR8148-GEOPS, CNRS-UPS, Université Paris-Saclay, Bâtiment 504, 91400 Orsay, France
2. GEOTOP, Université du Québec à Montréal, 201, av. du Président-Kennedy, Montréal, QC H2X 3Y7

Thematics : Hydrogeology, GIS, Geochemistry
Keywords : Vulnerability, DRASTIC, isotope, groundwater, Continental Terminal 

In Togo, the coastal sedimentary basin represents 6% of the country’s surface. It provides the water needs of 40% of the population and 90% of the industries from a phreatic aquifer of the Continental Terminal (CT) and two confined aquifers (the Paleocene and the Maestrichtian). The strong demographic growth of this area increases the demand for water. However, the drinking water supply network is not accessible to all the population. As a result, more than 65% of the urban population and 53% of the rural population use water from shallow wells and boreholes that tap into the Continental Terminal aquifer.

However, this precious water resource of the Continental Terminal is threatened by pollution and the question arises as to what factors are degrading the quality of the water in this aquifer and what measures can be taken to limit this degradation?

The objective of this project is to evaluate the vulnerability of the Continental Terminal aquifer through hydrogeochemical balance studies and to develop a decision support tool to assess the risks of degradation of the water of this aquifer in the face of anthropogenic actions and in a context of climate change.

To achieve the objectives of this project, first, the spatial recharge index method of Galvão et al., 2018 will be used to characterize the recharge areas of the Continental Terminal aquifer on the one hand, and the analysis of stable isotopes of the water molecule of 300 tap water samples collected in localities located in the coastal sedimentary basin to assess the waters that cause this recharge on the other hand. Then, the vulnerability will be estimated with the DRASTIC method of Aller et al., 1987 and verified from the water quality map. Finally, a decision support tool will be developed to assess the risks of water quality degradation by combining the vulnerability of this aquifer and the threats or sources of pollution that exist at the surface.

This work will allow i) the improvement of knowledge on the recharge areas on the one hand, and the origins of the water that recharges the terminal continental aquifer on the other hand, ii) the establishment of an isoscape map of the groundwater for the evaluation of the origin of the withdrawal water, iii) the evaluation of the vulnerability zones of the phreatic aquifer and iv) the development of a decision support tool to evaluate the risks of degradation of the water quality in order to allow the managers to propose measures of protection of the groundwater in the coastal sedimentary basin.