Impact of natural and human processes on the hydrochemical evolution of overexploited coastal groundwater: Case study of the Mornag aquifer refill (South-East Tunis, Tunisia)

The Mornag coastal groundwater underlies the plain of this region. It is in northern Tunisia and 20 km SW of Tunis City. This region is known for its important agricultural activity.This aquifer is the main groundwater resource of the department of this region. To limit the overexploitation, there is a creation of some perimeters in the Naasen and Irrisala localities where wastewater is used. To maintain the hydrostatic groundwater balance, freshwater is injected (artificial refill).In the Nassen area treated wastewater irrigation is slowed and then stopped while the Irrisala perimeter is still irrigated by the same water type. To maintain hydrostatic equilibrium of the aquifer, water from the Méjerda Canal (M.C.) was injected.A geochemical study of the different parts of this groundwater is carried out, in order to clarify the salinisation origin and the hydrochemical evolution of the Mornag groundwater and to estimate the possible relationships between the natural processes (marine water intrusion), the water quality used either for irrigation or for artificial refill, and the geologic environment on the water quality of this aquifer.Its salinity, pH, temperature and electrical conductivity were measured and their respective averages are 3.1 g/l, 7.4, 18.7 °C and 4.4 mS/cm. The concentrations of sodium, calcium, magnesium, bicarbonate, chloride, sulphate, potassium and bromides were measured and their respective averages are 29.0, 15.1, 10.9, 4.8, 26.8, 10.5, 0.3 and 0.0482 mg/l.rMg2+/rCa2+ and rBr−/rCl− ratios were calculated. The results suggest that the Mornag groundwater can be divided into 4 areas where the water chemistry depends on the natural processes and the water quality used for irrigation and artificial recharge:-for the coastal zone, marine water infiltration provokes the increase in salinity of this part of the aquifer;-infiltration of wastewater used for irrigation and run-off water resulted in changing geochemical water quality of these aquifer areas;-stopping of the use of wastewater, irrigation and artificial recharge water with good water quality did improve the aquifer water quality.