The Continental Terminal (CT) aquifer in Casamance (Southern Senegal) is one of the main sources of fresh water for usual activities. This shallow aquifer especially in the Oussouye plateau, bounded largely by salt brackish water (locally called bolongs) and the Casamance River is very vulnerable to intensive abstractions that could influence saline intrusion advance. The increase in water demand and the lack of boreholes there are leading the authorities to set up a rational management system for this aquifer. The aim of this study is to set up a management and decision-making tool through a groundwater modelling for an efficient and sustainable exploitation of this vulnerable aquifer.
The methodological approach particularly geophysical, hydrogeological and hydrochemical investigations aim to develop a conceptual model of the CT aquifer functioning. Two field campaigns were carried out (June and October 2017) to measure physicochemical parameters and groundwater sampling. All the sampling points were then surveyed by differential GPS to define the piezometric map and thus the groundwater flow. The geometry and hydrodynamic parameters (conductivity and transmissivity) of the CT were approached using geophysical technics (electrical methods) but also the drilling logs from previous studies carried out in Oussouye region. These investigations made it possible to define the conceptual model of the aquifer and to build the mathematical model under the Visual modflow interface with the Modflow-2000 code developed by USGS. This model takes into account the abstractions of current boreholes and dug wells where pumping rates were investigated by statistical survey.
The results show a general groundwater flow towards the bolongs and the Casamance River from two domes located in the Nord and in the center of the plateau which represent the recharge zones. Electrical conductivity varies from 28 to 1314 µs/cm with high variance and standard deviation values reflecting variable sources, geochemical and dilution processes occurring in the plateau. Despite physiochemical data, the analysis of water samples show an excellent groundwater quality. Major elements contents meet WHO standards except the Iron contents (Fe) which are relatively high in some wells due to the nature of the aquifer formation. The mathematical model was calibrated in steady state. The average difference between simulated and observed head is 0.08 m, while the average absolute difference is 0.24 m. Simulations under transient conditions showed that the groundwater is vulnerable to high pumping rate due to the drawdowns at the catchment wells, which can reach 7m for 75 m3/h. This significant drawdown should be avoided for this type of piezometric configuration where the maximum hydraulic head is 6m. However, the model revealed a sustainable groundwater potential for the needs of local and neighboring populations by transfer at 50m3/h with a steady state regime system reached after 10 years.