The behaviour of short-lived radiogenic lead isotopes (²¹⁴Pb and ²¹²Pb) in groundwaters and laboratory leaching experiments

Abstract

The concentrations of ²¹⁴Pb (half-life=26.4minutes) and ²²Rn (half-life=3.84days) have been measured in deep groundwaters of Gujarat, India. The results show that the abundance of214Pb in the water is only not, vert, similar 25% of that expected from its production through the radioactive decay of dissolved ²²²Rn. This deficiency if modelled in terms of a first-order removal, yields a residence time of ~10 minutes for ²¹⁴Pb in these waters. The estimated residence time for ²¹⁴Pb is the shortest observed for any nuclide in natural water systems and suggests that reactive nuclides lead like could be removed from aqueous phases to adjoining solid surfaces on extremely short time scales. Results of laboratory experiments using the ²¹²Pb-²²⁴Ra pair are compatible with the observed fast removal of214Pb from groundwaters. Re-evaluation of ²³⁴Th residence times in these waters using a model with a recoil flux of ²³⁴Th into aqueous phase, the same as that of ²²²Rn, yields values in the range of 23 to <176 minutes, very similar to that of ²¹⁴Pb. This "concordancy" in the residence times seems to suggest that the geochemical behaviour of ²³⁴Th and ²¹⁴Pb in these waters is quite similar.