Groundwater Dating with 3H - 3He
The earlier discussion leaves us with the impression that our ability
to produce quantitative tritium ages for groundwaters is fading as fast
as the thermonuclear bomb peak. However, by measuring 3H
together with its daughter 3He, true
ages can be determined through calculations that do not rely on the complicated
tritium input function. The drawback to this approach is that 3He
is not a routinely sampled nor measured isotope (Clarke et al., 1976).
The method was first introduced to the field of hydrogeology in 1979 (Torgersen
et al., 1979). Schlosser et al. (1988) provide an excellent review and
application and Ekwurzel et al. (1994) compare this technique with dating
based on chlorofluorocarbons (CFCs) and 85Kr .
Helium-tritium systematics
The decay of tritium from an initial concentration 3Ho
after some time t is predicted by:
3Ht = 3Ho e–lt
However, determining t requires that we know 3Ho.
The decay of tritium leads to an ingrowth of 3He, which would
then be:
3Het = 3Ho(1– e–lt)
By combining both equations, we can cancel the dependency on the input
concentration of tritium 3Ho:
3Het = 3Ht(e–lt
–1)
The helium concentration at time t, 3Het, is expressed
in TU (1 3He per 1018 H). The measured 3He
must be corrected for atmospheric 3He that is dissolved
at the time of recharge. Note that atmospheric He is dominantly 4He.
This input is assumed to be at equilibrium with the atmosphere, and considers
the following points:
-
Atmospheric 4He concentration is 5.24 ppmv (Glueckauf, 1946)
-
Atmospheric 3He/4He ratio is 1.384 · 10–6
(Clarke et al., 1976)
-
The solubility of atmospheric helium is temperature dependent, and for
10°C is
4.75 · 10–8 cm3 STP/cm3 H2O
(Fig. 7-11).
-
4He is slightly more soluble in water, with a fractionation
factor, a ~ 0.983 (Benson and Krouse, 1980)
Fig. 7-11 Solubility of noble gases in water (Andrews, 1992).
The measured value of 3He, corrected for atmospheric 3He,
represents 3He ingrown from 3H decay, and is then
used in the dating equation:
t = 12.43/ln2 · ln(1 + [3Het]/[3Ht])
where [3Het]/[3Ht] is the concentration
ratio of these two isotopes expressed in tritium units.