EFFECT OF SUBGRID HETEROGENEITY ON GEOCHEMICAL REACTION RATES, AN EXAMPLE OF URANYL DESORPTION

The subsurface sediment heterogeneity affects the transport, mixing, and interactions of reactants that affect local and overall reaction rates, leading to orders of magnitude changes in the observed geochemical reaction rates in heterogeneous subsurface porous media as compared to their corresponding intrinsic reaction rates. This rate change poses a significant challenge to scale geochemical reactions from laboratory to field as reaction rates or rate constants become scale-dependent. In this presentation, we will present an investigation on the effect of subgrid heterogeneity in sediment properties on the rate of geochemical reaction rates using uranyl[U(VI)] desorption as an example. The subgrid heterogeneity, which is often below spatial resolution of characterization, is the source of the most uncertainty in reaction rate extrapolation. A U(VI)-contaminated sediment collected from US Department of Energy’s Hanford site was sieved into 7 grain size fractions that have distinct reaction properties. Six columns were assembled using the same sediment; but its grain size fractions were arranged differently to mimic field subgrid heterogeneity in both physical and geochemical properties. The apparent rate of U(VI) desorption varied significantly in the columns. Those columns with sediment structures leading to heterogeneous reactive transport had much lower rates of U(VI) desorption than those with relatively homogeneous reactive transport. Modeling analysis indicated that a model of U(VI) desorption characterized from well-mixed reactors significantly over-predicted the measured U(VI) desorption in the columns with heterogeneous reactive transport. A triple domain model, which operationally separates reactive transport properties into three subgrid domains improved the predictions significantly. Similar effect of the subgrid heterogeneity was also observed, with a less degree, for the denitrification process. The results indicate that the subgrid heterogeneity is an important consideration in extrapolating reaction rates from laboratory to field, and multi-domain approaches can significantly minimize the extrapolation uncertainty.