Spatial and seasonal variability of groundwater hydrochemistry in the Senegal North Littoral aquifer using multivariate approach

The Senegal North littoral aquifer bears considerable groundwater resources that face both natural and anthropogenic constraints due to proximity of the ocean and numerous socio-economic activities in this semi-arid region. The present study aims to understand spatial and seasonal hydrochemistry variability of this groundwater. Chemical data were analyzed with statistics methods such as Principal Component Analysis and Hierarchical Classification Analysis to classify water types and to decipher geochemical processes. A set of 32 wells was repeatedly sampled in four campaigns between 2010 and 2012 and chemically analyzed in addition to rainwater samples. Results exhibited five water types viz Ca–SO₄, Na–Cl/SO₄, mixed-Cl/SO₄, mixed-HCO₃/Cl and Ca–HCO₃ geographically discriminated in the study region. The last two water types dominate; they occur mostly in the central and eastern part of the aquifer. Whereas the first two highly mineralized classes occur in the narrow coastal strip zone. Natural processes such as dissolution of evaporite minerals (gypsum, halite), carbonate minerals (calcite, dolomite), exchange reaction in addition to anthropogenic sulphate (from mining industry) and nitrate (from agriculture) inputs are the likely dominant processes that control the water chemistry. Chemical changes proceed according to seasonal variability as infiltrating waters reach the water table. The mean global mineralization which is higher in October in post-rainy season (818 µs/cm) and lower in February in the mid of dry season (640 µs/cm) suggests dilution process through delayed recharge. The environmental isotope (δ¹⁸O, δ²H, ³H) measurements further revealed that groundwater in the system is relatively well-mixed as shown by the measured range of values. However, deviation from the rainwater signature indicates combined effect of evaporation processes which may occur at the top surface and in the unsaturated zone during infiltration and also mixing with anthropogenic contribution to the aquifer. Tritium contents in the system compared to those of rainwater confirm differences in infiltration rates where the northern central zone with low ³H contents suggests water prior to thermonuclear bumb test era between 1952 and 1953.