HR: 11:20h
AN: V11H-05 INVITED     [PDF]
TI: Modelling of Groundwater Weathering Rates From U- and Th- Series Nuclides
AU: * Reynolds, B C
EM: reynolds@erdw.ethz.ch
AF: ETH Zurich, Departement Erdwissenschaften Sonneggstrasse 5 ETH Zentrum, Zurich, CH-8092 Switzerland
AB: The transport of U, Th, Ra and Rn nuclides of the $^{238}$U- and $^{232}$Th decay series in continental groundwaters can be used to constrain important physico-chemical parameters that occur within the aquifer, principally long-term weathering rates, $\alpha$-recoil effects, and adsorption-desorption characteristics and irreversible precipitation. These naturally occurring radionuclides are ideal for modelling the mobility of pollutant nuclides. In terms of understanding the aquifer system, and to decouple the effects of the vadose zone from the water chemistry evolution within the aquifer, it is important to look at large aquifer systems and waters with older groundwater ages. The simple measurement of U activity ratios gives an estimate of the importance of $\alpha$-recoil effects compared to the bulk dissolution rates within the aquifer system, rather than any age information. This usually leads to observed increases in $\delta ^{234}$U away from the vadose zone, where bulk dissolution rates are highest. Given a constant recoil fraction, that predominately reflects grain size of accessory minerals, $\delta ^{234}$U will be a direct measure of the localised weathering rate. Rn gas concentrations also reflect the $\alpha$-recoil inputs; activities are generally too high to reflect direct recoil from within mineral grains and are mainly derived from a Th enriched surface coating within the aquifer system. This Th enriched layer slowly develops over millions of years within an aquifer system, and can contain over 10 % of the immobile (insoluble) elements within the aquifer. It will also have a $^{230}$Th activity that has reached a steady state value. The surface layer contributes significantly to the Ra activity from the parent Th isotopes incorporated in this layer. If the entire decay series is modelled for an aquifer system it is possible to derive the amount of Th within the surface layer and thus the long-term recoil fraction and weathering rate. In turn, differences between observed and modelled activities (rather than ratios) can be used to deduce localised adsorption co-efficients for U and Ra. Unfortunately, the model estimates the relative adsorption rather than the effective distribution co-efficient needed to evaluate retardation factors and more effectively model the mobility of pollutant nuclides within the natural environment.
DE: 1045 Low-temperature geochemistry
DE: 1815 Erosion and sedimentation
DE: 1886 Weathering (1625)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting