Abstract
The well-investigated aquifer in southern Tunisia have been selected
from the literature to test the potential use of uranium isotopic
compositions as a groundwater dating method. This is the Senonian
carbonate aquifer of the Nefzaoua basin
(https://doi.org/10.1016/j.quaint.2020.01.024). For the this aquifer, an
increase in U 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 U concentrations with a decrease in 14C values. For this
aquifer, positive results were obtained on uranium-isotope dating. It
was found that the groundwater residence time in the aquifer increases
from 440 to 11,300 years from the recharge area along the generalized
flow path (Figure). Uranium age correction model under oxidizing
conditions in the aquifer is given in “Malov, 2018. Evolution of the
groundwater chemistry in the coastal aquifers of the south-eastern White
Sea area (NW Russia) using 14C and 234U-238U dating. Science of the
Total Environment. https://doi.org/10.1016/j.scitotenv.2017.10.197” It
is shown that for uranium-isotope dating of groundwater, three main
conditions must be met: 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. The author
understands the reality of the fact that groundwater dating methods are
under development and improvement. Nevertheless, analysis of the
evolution of the chemical and isotopic composition of groundwater,
analysis of the geological and hydrogeological history of the region,
hydraulic estimates of groundwater velocities in aquifers, hydrodynamic
and balance justifications for the formation of groundwater, and
analysis of the composition of stable isotopes can increase the
reliability of dating. In the future, it is necessary to continue
studies to assess the retardation factor and recoil loss factor in order
to improve the uranium-isotope method for dating groundwater under
oxidizing conditions for uranium. This work was supported by the RFBR
(project no. 20-05-00045_A)