A conceptual and empirical model for the combined influence of oxidation/reduction reactions and transport processes on the fate of selenium contaminated sediments in the Central Valley has been developed and evaluated. Interestingly, in the presence of a fluctuating water table, the combined effects of these processes enable a net transport of selenium in the opposite direction of the net flux of water. From a macroscopic perspective, the processes are now well understood and more than decade of field monitoring data is available to both calibrate and test this model. When examined however from a more fundamental point of view many outstanding issues remain. Chief among these, and critical to a satisfactory understanding of the problem, are the factors that control the kinetics of selenium oxidation/reduction reactions and how and why these vary in space and time.
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