On the potential joint use of uranium and carbon isotopes for groundwater dating

Abstract:

The well-investigated aquifers have been selected from the literature to test the potential use of uranium isotopic compositions as a groundwater dating method. These are the Senonian carbonate aquifer of the Nefzaoua basin and Mio-Pliocene sandy aquifer of the Redjime Mâatoug, Tozeur, and Chott El Charsa Plain basins, Tunisia. For the Senonian aquifer, an increase in uranium concentrations along the generalized flow path is observed in proportion to an increase in total dissolved solids, which may indicate the predominance of dissolution processes over alpha-recoil processes under oxidizing conditions for uranium. There is also an increase in uranium concentrations with a decrease in ¹⁴C values, although three determinations of ¹⁴C are not enough. At the same time, for this aquifer, positive results were obtained on uranium-isotope dating. It was found that the groundwater residence time in the aquifer increases from 443 to 11281 years from the recharge area along the generalized flow path. In Mio-Pliocene multi-layer sandy aquifers, groundwater sampling was carried out in a very wide interval of depths from 40 to 1174 meters, in connection with which a very wide range of distribution of values of the redox potential of groundwater, retardation factor and recoil loss factor is assumed. That is, the necessary requirements for using the proposed method of uranium-isotope dating of groundwater, namely: i) oxidizing conditions for uranium in the aquifer, ii) an increase in uranium concentrations with a decrease in ¹⁴C activities and iii) the homogeneity of the aquifer in terms of hydraulic conductivity and lithological composition are not met.