Upscaling sorption/desorption processes in reactive transport models to describe metal/radionuclide transport: A critical review

Models are a mainstay of the environmental sciences; they allow for both deeper understanding of process knowledge and, to a limited extent, predictive capabilities of current day inputs on the future. Mathematical codes have become increasingly complex with explicit inclusion of many processes that could not be accounted for using simpler solving techniques. And yet, for metal/radionuclide transport in subsurface systems, the inclusion of smaller scale processes in a numerical solver do not always lead to better descriptions of larger scale behavior. The reasons for this are many, but included in this review are the following: unknowable conceptual model errors, discrepancy in the scale of model discretization relative to the scale of the chemical/physical process, and omnipresent chemical and physical heterogeneities. Although it is commonly thought that larger, more complex systems require more complex models to gain insight and predictive capability, there is little to no experimental evidence supporting this thought. Indeed, the evidence points to the fact that larger systems can be well described with simple models. To test this thought and to appreciate the incorporation of scaling behaviors into reactive transport modeling, new experiments are needed that are intermediate in scale between the more traditional bench and field scales.