Usefulness of historical measurements of tritium content in groundwater for recharge assessment in semi-arid regions: application to several aquifers in central Tunisia

Tritium is a well-known tracer, used to estimate the age of modern groundwater (<50 years) and consequently to capture information on the longer-term components of recharge in aquifers. However, its current low concentration in groundwater limits its usefulness for recharge assessment, which is more symptomatic of arid and semi-arid regions where the recharge is characterized by extreme spatial and temporal variability. Regions where groundwater tritium was studied at a time close to the peak of nuclear bomb testing (1963) would be worth studying again today to establish new information. This report describes the application of a radioactivity decay model to a long (up to 50 years) record of tritium in rainfall and the associated recharge to several aquifers in central Tunisia; these aquifers were sampled for tritium in 1967. The results show that the groundwater renewal rate has a large range: 0.06-2.2% of annual mean rainfall for Plio-Quaternary aquifers, 0.1-6.46% for Mio-Pliocene aquifers, and 0.1-1.5% for the upper Oligocene aquifers. A good agreement was found between the recharge estimated in this study and the recharge estimated in previous studies only for the upper Oligocene aquifers. This suggest that the methodology of recharge estimation, described in this report, is reliable only for aquifers of homogeneous lithology with a localized recharge area, but the method is less consistent for detrital aquifers, i.e. composed of lenticular sediments. The results also highlight the usefulness of historical (1950-1970) measurements of tritium in groundwater for the estimation of the groundwater recharge.