Hydrochemical and isotopic investigation of the groundwater composition of an alluvial aquifer, Cap Bon Peninsula, Tunisia

In the Grombalia region, Cap Bon Peninsula, northeastern Tunisia, increasing population and development of agricultural activity during the last few decades have engendered large expansion of groundwater pumping from the shallow, unconfined aquifer. Recently, the water table has displayed some signs of overexploitation such as decline in the water level in boreholes and considerable deterioration of water quality. Hydrochemical and isotopic data were used in conjunction with hydrogeological characteristics to investigate the groundwater composition in this aquifer. It has been demonstrated that groundwaters, recharged mainly in the surrounding foothills, acquire their mineralization principally by water-rock interaction, i.e., dissolution of evaporites and reverse cation exchange, and secondarily by return flow of irrigation waters. The isotopic signature of groundwaters permits identification of two different types. Water with depleted delta O-18 and delta H-2 contents is interpreted as recharge by non-evaporated rainfall, originating from a mixture of Atlantic and Mediterranean air masses. However, water with relatively enriched delta O-18 and delta H-2 contents is thought to reflect contamination by return flow of irrigation waters. Tritium contents, ranging between 1.2 and 4.5 TU indicate that groundwaters in an unconfined aquifer derive from post-nuclear recharge or contain a significant component of post-1950s water. The recent origin of these groundwaters is confirmed by the high carbon-14 activities, exceeding sometimes 100%, indicating the existence of a significant fraction of organic C-14. This younger source of radiocarbon is, however, highlighted by the depleted delta C-13 contents, corresponding to a system where CO2 of C4 plants predominates.