Geochemical and isotopic study of surface and groundwaters in Ain Bou Mourra basin, central Tunisia

摘要

A comprehensive study of the hydrogeological framework and groundwater resources using chemical and isotope methods was recently carried out in the Ain Bou Mourra basin (central Tunisia). In this area, surface water in rivers is seasonal, and therefore groundwater is the perennial source of water supply for domestic and agricultural purposes. A great dam and several harvesting features have been built within a national water management strategy in order to increase the overall water availability in the basin. In this study, groundwaters from the Paleogene-Neogene formations in the Ain Bou Mourra together with rain and surface waters have been analyzed in order to investigate the mineralization processes, water origin and recharge sources. Chemical data indicate that surface and groundwater samples show a large spatial variability of chemical facies. Most of the groundwaters are slightly to moderately mineralized and are of Ca-Mg-CI-SO_4 water type, induced mainly by feldspar and silicate weathering, dissolution of evaporitic and carbonate minerals and cation exchange reactions. Data inferred from ~(18)O and deuterium isotopes in groundwater samples indicated recharge with modern rainfall. Water characterized by lower δ~(18)O and δ~2H values is interpreted as recharged by non-evaporated rainfall originating from Mediterranean air masses at higher altitude. However, water with relatively enriched δ~(18)O and δ~2H contents is thought to reflect the occurrence of an evaporation process related to the long term practice of flood irrigation. The investigated surface water samples collected from the dam, river, hill reservoirs and pumped-storage water power plant are affected by a significant evaporation and reveal large seasonal variations which could be controlled by the water volume changes in the open surface reservoirs and the meteorological conditions during evaporation, condensation and precipitation. Considerable tritium concentration in groundwaters allowed qualitative identification of the present day component. It is assumed that this component is supplied by recent precipitation at the Oligocene -Miocene outcrops of the study area. Data revealed the existence of high ~(14)C activities in groundwaters. Reservoir of dissolved carbon is simultaneously depleted in ~(13)C, reflecting the domination of CO_2 produced by C_4 plants in formation of carbonate mineralization of analyzed waters.