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Uranium isotopic disequilibrium for groundwater classification: first results on complexe terminal and continental intercalaire aquifers in Southern Tunisia

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Environmental Geology

Abstract

A fractionation of uranium (U) series into parent–daughter pairs – 234U and 238U – always occurs in natural waters, and the disequilibria between these is commonly used as a tracer of groundwater flow. We report here an interpretation of the U-content and U isotope disequilibria in groundwater sampled from the deep Complexe Terminal and Continental Intercalaire aquifers of southern Tunisia. Variations in both the contents of these isotopes (0.006–2.4 ppb) and 234U/238U activity ratios (ARs) (1.7–15.4) were observed. The data could be plotted in two distinct fields of reciprocal U concentration versus 234U/238U AR according to groundwater flow and regional bedrock differences. An initial assessment aimed at verifying whether the results of this investigation support those of previous hydrogeological and isotope studies, thereby suggesting that the disequilibrium between U isotopes in groundwater may represent a useful tool for hydrogeological investigations of deep and fossil groundwater. In addition, the disequilibrium can be used for quantifying the recharge or mixing rates between different formations with the aim of delineating the preferential outflow pattern or determining residence times of waters.

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Chkir, N., Zouari, K. Uranium isotopic disequilibrium for groundwater classification: first results on complexe terminal and continental intercalaire aquifers in Southern Tunisia. Environ Geol 53, 677–685 (2007). https://doi.org/10.1007/s00254-007-0682-1

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