Upscaling colloidal transport from the column to the field: The value of a large gravel column experiment as an intermediate step

Due to public health or environmental concerns, examining the effects of preferential flow processes on the transport of pathogenic microorganisms or contaminants of emerging concern must be studied in the laboratory. However, the resulting transport parameters cannot be directly applied to field-scale groundwater models. This research explores how an upscaling relationship, with removal as a function of distance, can be found using B. subtilis spores and microspheres as colloidal tracers in saturated flow-through experiments at three different scales. The study investigates transport processes in a 4-m-tall undisturbed gravel column and compares the results with previously published data. Results showed that colloidal removal (log-removal, attachment rate/efficiency) in heterogeneous porous media follows a power law as a function of travel distance, rather than an exponential relationship, which is normally assumed in removal equations. It was found that by using a power function, it was possible to decrease the difference between the attachment coefficients so that the meso-scale value was closer to the small-scale value. To the contrary, using a dual permeability model increased the difference between attachment rates at these two scales. Groundwater transport modeling may benefit from taking this power law relationship into account, instead of using a constant first-order removal rate, as most tracer tests are performed at a smaller scale than the scale that is being modeled. The meso-scale column provides insights into upscaling processes by incorporating an intermediate step when comparing groundwater transport at the column scale to the field scale.