Hydrogeological characterization and hydrodynamic behaviour of the overexploited Diass aquifer system (Senegal) inferred from long term groundwater level monitoring

Dakar, the Capital city of Senegal concentrates about 23.2% (about 3 millions inhabitants) of the total population and a large proportion of the industrial activities. Water supply is ensured by surface water pumped and piped from the Guiers Lake (250 km distant from the capital) and from groundwater resources. Among these latter, the Diass aquifer system contributes to a substantial proportion (31% in 2019) of the total water supply distribution due to growing demand induced by the rapid demographic growth (about 2.5%). The Diass horst aquifer system located 50 km east of Dakar (Senegal) is exploited with two main aquifers covered by a sandy superficial aquifer: the confined/unconfined Palaeocene karstic limestone and the confined Maastrichtian sandstone aquifer underneath. This system has experienced intensive groundwater abstraction during the last 60 years to meet the increasing water demand. Abstraction for urban drinking water occurs in nine pumping fields with a rate reaching 174,000 m3/d in 2019. This high yield together with the drought conditions since the 1970s is likely to affect groundwater imbalance and change the flow regime. The objective of the study is to improve our understanding of the system dynamic with regards to the high pumping rate in order to build a conceptual scheme for further hydrogeological modeling of the system. In this study, we use monitored pumping rates, piezometric level from 1960s to 2019 and rainfall data from 1931 to December 2016 together with the hydrogeological configuration to infer the dynamics of the aquifer system. The high abstraction rate during the period 1958-2019 which vary from 16,000 to 174,000 m3/d has caused a continuous groundwater level decline (up to 30 m), a modification of the flow patterns and to some extent a quality deterioration through salinization processes as shown in a few boreholes in Sébikotane and Mbour. The piezometric levels which were above the sea level prior 1959 exhibit now negative values and can even reach -40 m in the vicinity of the pumping fields creating therefore piezometric depressions and convergent flow pattern. The hydrodynamic of the system derived from the results show that the reservoir acts as a multilayer aquifer system with interconnected compartments by faults that allow flux exchanges except the confining Ponty and Sébikotane faults. Overexploitation inducing important drawdown has induced an increase of the drainance fluxes between those different compartments. In order to foster more appropriate and sustainable groundwater abstraction in the complex hydrogeological system with regards to demand and water quality conservation, it is important to assess the main system behavior.