Effect of Subgrid Heterogeneity on Scaling Geochemical and Biogeochemical Reactions: A Case of U(VI) Desorption

The effect of subgrid heterogeneity in sediment properties on the rate of uranyl­[U­(VI)] desorption was investigated using a sediment collected from the U.S. Department of Energy Hanford site. The sediment was sieved into 7 grain size fractions that each exhibited different U­(VI) desorption properties. Six columns were assembled using the sediment with its grain size fractions arranged in different spatial configurations to mimic subgrid heterogeneity in reactive transport properties. The apparent rate of U­(VI) desorption varied significantly in the columns. Those columns with sediment structures leading to preferential transport had much lower rates of U­(VI) desorption than those with relatively homogeneous transport. Modeling analysis indicated that the U­(VI) desorption model and parameters characterized from well-mixed reactors significantly overpredicted the measured U­(VI) desorption in the columns with preferential transport. A dual domain model, which operationally separates reactive transport properties into two subgrid domains, improved the predictions significantly. A similar effect of subgrid heterogeneity, albeit to a lesser degree, was observed for denitrification, which also occurred in the columns. The results imply that subgrid heterogeneity is an important consideration in extrapolating reaction rates from the laboratory to field