HR: 10:45h
AN: H42A-02 INVITED    [Abstracts]
TI: Using U/Th Series Nuclide Systematics for Modelling Subsurface Radionuclide Transport
AU: * Porcelli, D
EM: don.porcelli@earth.ox.ac.uk
AF: Oxford University Dept Earth Sciences, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Strekopytov, S
EM: Stanislav.Strekopytov@earth.ox.ac.uk
AF: Oxford University Dept Earth Sciences, Parks Road, Oxford, OX1 3PR, United Kingdom
AU: Shaw, S
EM: Sam.Shaw@earth.ox.ac.uk
AF: University of Leeds School of Earth and Environment, Environment Building, Leeds, LS2 9JT,
AU: Hilton, D
EM: drhilton@ucsd.edu
AF: Scripps Inst. Oceanography Univ California San Diego, 9500 Gilman Drive, La Jolla, CA 92093, United States
AB: U- and Th- series nuclides have provided essential tools for studying weathering and subsurface element transport processes. The radionuclides U, Th, Ra, Rn, and Pb have a range of half-lives and contrasting chemical behaviours, and their distribution between subsurface solids and water can be used to quantify rates of soil formation, chemical and mechanical weathering of watersheds, and potentially, subsurface water flow rates. Decay systematics clearly connect the different isotopes, although transfer between different phases and through subsurface systems are generally defined through a series of assumptions that have not been experimentally substantiated. Modification of these assumptions may have significant effects on the choice of models and the conclusions of U/Th series studies. Two key areas of uncertainty are: Nuclide inputs. Comparing the different isotopes requires relating the input rates from recoil and weathering. The common assumption is that nuclides are released at similar rates by recoil and congruently by weathering. Various theories have been proposed for preferential release of Rn and leaching of radionuclides, although such effects, as well as possible radionuclide fractionation during weathering release, have not been substantiated. Surface interaction mechanisms. It is generally assumed that radionuclides are removed from water onto surfaces only by reversible adsorption. However, coprecipitation, incorporation into aging secondary mineral structures, and different binding mechanisms can inhibit isotope exchange with nuclides in solution. Data from the unconfined Mojave River Basin aquifer, with a reasonably well-defined flow pattern and groundwater ages of up to ~40,000 years illustrates the response of the U/Th series nuclides to extended water- rock interaction. Measurements of 222Rn find relatively uniform recoil rates throughout the aquifer. If this rate is applied to other daughter nuclides, then the rate of 234Th release can be obtained This then defines the input of 234U, which is expected to increase with groundwater age, regardless of the extent of reversible adsorption. The narrow range of 234U/238U ratios implies that there is continuing weathering release of 238U as well. However, measured U concentrations are actually relatively constant, indicating that either the recoil of 222Rn is actually much higher than that of other elements, or U is irreversibly incorporated into the aquifer solids at the same rate as it is released. Sequential leaching experiments with aquifer materials suggests that released daughter nuclides are produced within Fe oxyhydroxides surface coatings, compatible with similar loss rates for recoiled daughter nuclides. Mechanisms for incorporation of U are therefore required here, and may be important elsewhere for greatly limiting trace elements, including low level contaminants, in the environment. Incorporation of such processes in radionuclide transport models will strongly affect inferred transport rates.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1806 Chemistry of fresh water
DE: 1832 Groundwater transport
SC: Hydrology [H]
MN: 2007 Joint Assembly