Modeling the Recharge and the Renewal Rate Based on 3H and 14C Isotopes in the Coastal Aquifer of El Haouaria, Northern Tunisia

Estimation of groundwater recharge in a semi-arid area is difficult due to the low amount and variability of recharge. In the El Haouaria region (N.E. Tunisia), this hydrodynamic survey revealed a decrease in unconfined groundwater reserves of approximately 11% over the last 30 years due to the abstraction of water. Over that same time period, the groundwater table displayed a natural decrease of about 10 m in depth over an area of 230 km2. Hydrogeological investigations have shown that two aquifers which vertically integrate all of the permeable zones in the region can be distinguished: a shallow aquifer formed by filling the Quaternary and another deeper one in the Pliocene. These two aquifers are separated by a semi-permeable layer through which vertical exchange by seepage is possible. Two analytical models, taking into account the long term decrease of the water table, were used to interpret 3H and 14C content in groundwater. Forty groundwater wells were sampled to obtain additional information on isotopic characteristics of the groundwater defined in previous studies. The median annual renewal rate (recharge as a fraction of saturated aquifer volume) varies between 0.2 and 0.3%. For representative characteristics of the aquifer (30 m of saturated thickness, porosity 14%) this implies a recharge of 10 to 12 mm/yr. Which is lower than estimates of 18 to 36 mm/y obtained using hydrodynamic methods.