Groundwater salinisation processes in a coastal irrigated plain, Korba El Mida case study Cap Bon (Tunisia)
Résumé
Since the 1960’s the coastal aquifer overlying the irrigated plain of Korba El Mida (East coast of Cap-Bon) has been under intensive exploitation leading to groundwater degradation quality by seawater intrusion and irrigation return flow. Also, the main coastal sabkhas (wetlands) are no longer the natural outlets of the aquifer, due to a reversal of the hydraulic gradient jointly with groundwater depletion. Since 1962 geophysical surveys, use of environmental isotopes and tracers, and groundwater monitoring campaigns for sampling and level measurements of existing piezometers and surrounding wells, have been carried out in order to assess origins of salinity, extent and evolution along time.
Values of Electric Conductivity may range between 0.3 and 10.6 dS/m in the surroundings of Korba and El Mida; most piezometers and some wells had essentially sodium-chloride facies but some wells present a calcium-sulfate facies. High nitrate concentrations (up to 300 mg/L), potassium and sulphate increase due to mineral fertilizers and manure application are observed in some areas of the aquifer, fact supported by the Cl-NO3 diagrams. The high heterogeneity of the salinity distribution, due to geological and hydrodynamic conditions, generates complex groundwater mixing of different origins: seawater intrusion, irrigation return flow, natural recharge besides the existing induced aquifer recharge in the area. The spatial variability of concentrations in the Korba-El Mida aquifer reveals the complexity of the groundwater contamination by salinization and anthropogenic activities.