The theory of residence time distributions is widely used for understanding turnover times and simulating transport and reactions but there are some major limitations to the commonly applied portions of the theory. Hydrologic systems are inherently transient (changing over time) but most of the applications assume that a steady-state approximation is an acceptable simplification without any concrete evidence to support the assumption. This ongoing work is investigating the circumstances under which transience in residence time distributions can be ignored. So far we have learned that it is crucial to include transience in shallow systems that experience connection and disconnection of flow paths (wetting and drying) [Engdahl et al., 2016] but also that steady-state approximations are generally reasonable in confined systems [Engdahl, 2017].
Large colloids generally fall outside the current colloid filtration theory so we’re using numerical simulation tools as virtual hypothesis testing laboratories to understand the processes that affect their motion. These fibers are up to a few millimeters in length and are difficult to study because they can’t be monitored easily, making the numerical tools a logical choice. We’re working on laboratory experiments to help develop the parameters and processes that describe fiber motion in the environment. Details of the simulation approach can be found here, but this animation is probably what you’re really wanting to see. This shows one of our example simulations of fibers moving through a coarse gravel and the length scales are in millimeters.