Geochemical and biogeochemical reaction rates are important variables controlling the fate and reactive transport of contaminants, and remediation efficiency of reactionbased remediation technologies. In subsurface sediments, geochemical and biogeochemical reactions couple with transport processes such as advection and diffusion. The transport processes control the rates of supply of reactants and removal of reaction products, which affect subsequent mass actions for geochemical and biogeochemical reactions, while the geochemical and biogeochemical reactions control chemical speciation, species transformation, contaminant degradation, and aqueous/solid phase re-distribution that subsequently affect transport. Reaction thermodynamics, kinetics, rate expressions and parameters were typically determined in laboratory systems using experimental apparatus such as batch, stirred flow-cell, and column systems. In this presentation we will use a contaminant (uranium) desorption case to demonstrate that the macroscopic reaction rates measured in stirred-flow cell, small column, and large column systems were scale-dependent and decreased over three orders of magnitude with increasing spatial scale. The scale-dependency of the desorption rate was attributed to the dif-
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