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Modeling Radionuclide Transport and Uptake in an Integrated Lysimeter Experiment: II. Application to Sodium-22

Dep. of Civil and Environmental Eng., Imperial College, London SW7 2BU, UK.

^* Corresponding author (a.butler{at}ic.ac.uk).

ABSTRACT

A conceptual model of an integrated system of lysimeters is used to investigate the behavior of ²²Na in soils and plants. The application of the model requires three stages of calibration relating to water fluxes, soil sorption, and root uptake. The water fluxes are calibrated against water level changes in a storage reservoir common to eight lysimeters used to support a winter wheat (Triticum aestivum L. cv. Pastiche) crop. The reservoir is part of a control system, which maintains a fixed water table in each of the lysimeters, thereby supplying contaminated water to the wheat crop. Measurements of total ²²Na in soil are four orders of magnitude greater than in the harvested crop. This allows the equilibrium soil sorption coefficient, K_d, to be identified separately from a root uptake coefficient, ', which characterizes the uptake of ²²Na per unit area of root. The model was able to successfully simulate the time-dependent behavior of ²²Na concentrations in the various components of the water table control system, the lysimeter soil profiles, and the harvested crop. In particular, crop uptakes spanning nearly three orders of magnitude over five crop seasons were reproduced reasonably well. The model, therefore, represents a more physically realistic approach to soil-to-plant transfer from contaminated water tables than the traditional method of transfer factors. It also shows, for mobile radionuclides, the importance of characterizing correctly the soil water fluxes when using such an approach.