Groundwater Flow Modelling: A Decision-making Tool for Water Resource Management in Coastal Areas - Case Study of the Oussouye Plateau (South Senegal)

Hydrogeological and hydrochemical investigations were used to develop a conceptual model of the Continental Terminal (CT) aquifer functioning in the Oussouye plateau (South Senegal). Two field campaigns were carried out in June and October 2017 to measure physicochemical parameters and groundwater sampling. The geometry of the CT was established using geophysical technics (electrical methods) and the drilling logs from previous studies carried out in Oussouye region. These investigations led to build the mathematical model under the Visual modflow interface with the Modflow-2000 code developed by USGS. The results show a general trend of groundwater flow towards the Casamance River and its tributaries from piezometric mounds in the central area of the plateau which represent the potential recharge zones. Regarding chemical quality, 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. The analysis of water samples shows an excellent groundwater quality. Major ions contents do not exceed WHO standards except Iron (Fe) which are relatively high in some wells. The mathematical model was calibrated in steady state. The average difference between simulated and observed head is 0.009 m and the root mean squared is less than 0.2m. 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 7 m for 300 m3/d. This significant drawdown should be avoided for this type of piezometric configuration where the maximum hydraulic head is around 5 m. However, the model revealed a sustainable groundwater potential for the needs of local and neighboring populations by 200m3/d.