A model for the groundwater transport of naturally occurring U, Th, Ra, and
Rn nuclides in the U-238 and Th-232 decay series is discussed. The model d
eveloped here takes into account transport by advection and the physico-che
mical processes of weathering, decay, alpha-recoil, and sorption at the wat
er-rock interface. It describes the evolution along a flowline of the activ
ities of the. U-238 and Th-232 decay series nuclides in groundwater. Simple
sets of relationships governing the activities of the various species in s
olution are derived, and these can be used both to calculate effective reta
rdation factors and to interpret groundwater data. For the activities of ea
ch nuclide, a general solution to the transport equation has been obtained,
which shows that the activities reach a constant value after a distance (x
) over bar(i), characteristic of each nuclide. Where (x) over bar(i) is muc
h longer than the aquifer length, (for U-238, U-234, and Th-232), the activ
ities grow linearly with distance. Where (x) over bar(i) is short compared
to the aquifer length, (for Th-234, Th-230, Th-228, Ra-228, and Ra-224), th
e activities rapidly reach a constant or quasi-constant activity value. For
Ra-226 and Rn-222, the limiting activity is reached after 1 km.
High delta(234)U values (proportional to the ratio epsilon 234(Th)/w238(U))
can be obtained through high recoil fraction and/or low weathering rates.
The activity ratios Th-230/Th-232, Ra-228/Ra-226 and Ra-224/Ra-226 have bee
n considered in the cases where either weathering or recoil is the predomin
ant process of input from the mineral grain. Typical values for weathering
rates and recoil fractions for a sandy aquifer indicate that recoil is the
dominant process for Th isotopic ratios in the water. Measured data for Ra
isotope activity ratios indicate that recoil is the process generally contr
olling the Ra isotopic composition in water. Higher isotopic ratios can be
explained by different desorption kinetics of Ra. However, the model does n
ot provide an explanation for Ra-228/Ra-226 and Ra-224/Ra-226 activity rati
os less than unity.
From the model, the highest Rn-222 emanation equals 2 epsilon. This is in a
greement with the hypothesis that Rn-222 activity can be used as a first ap
proximation for input by recoil (Krishnaswami et al 1982). However, high Rn
-222 emanation cannot be explained by production from the surface layer as
formulated in the model. Other possibilities involve models including surfa
ce precipitation, where the surface layer is not in steady-state.