Factors controlling the evolution of groundwater dynamics and chemistry in the Senegal River Delta

•We investigate hydrochemical conditions in groundwater of the Senegal River Delta.•Groundwater chemistry is governed by interactions with surface water.•Groundwater softening progressively occurs by lateral flush close to rivers.•Irrigate water does not seem to have a strong influence on groundwater salinity. Senegal River Delta. The Senegal River Delta is a strategic region for the development of irrigated agriculture. Despite a Sahelian climatic context, the management of the river with dams ensures water availability throughout the year. With the intensification of agriculture, degradation of cultivated soils is observed, mostly linked to the existence of a shallow salty aquifer. In this context, regional surveys were performed to characterize groundwater–surface water interactions and to identify the impact of artificial river management and agricultural intensification on the evolution of groundwater dynamics and chemistry. Results show that groundwater far away from rivers and outside irrigated plots has evolved from marine water to brines under the influence of evapotranspiration. Near rivers, salinity of groundwater is lower than seawater and groundwater mineralization seems to evolve in the direction of softening through cationic exchanges related to permanent contact with fresh water. Despite large volumes of water used for rice cultivation, groundwater does not show any real softening trend in the cultivated parcels. Results show that the mechanisms that contribute to repel salt water from the sediments correspond to a lateral flush near permanent surface water streams and not to vertical drainage and dilution with rainfall or irrigation water. It is however difficult to estimate the time required to come back to more favorable conditions of groundwater salinity.