Influence of local influx on non-local tracer transport in a variably saturated system

This study examines how different water and tracer influx magnitudes affect non-local transport behavior in porous media, spanning from surface infiltration through a partially saturated zone to a fully saturated region. Two discharge scenarios were tested in a laboratory flow cell to understand how short-term infiltration events, occurring on centimeter-hour scales, influence transport dynamics over decimeter-day scales. The results showed that lower influx rates led to shorter breakthrough times and sharper concentration peaks at the cell-outflow as the tracer plume reached deeper, faster-flowing regions. In contrast, higher influx rates produced slower, more dispersed breakthroughs in the outflow due to the confinement of the plume to the near-surface, lower-velocity zones. Numerical modeling using the Richards and advection–dispersion equations captured the transport behavior of the higher discharge case but were unable to reproduce the breakthrough pattern observed at the outflow for the lower discharge case, suggesting a misrepresentation of the local scale plume distribution in the model simulations. Alternatively, the infiltration of the tracer solution and the system-scale tracer transport were split into two different processes. Setting hypothesized bounding cases for the distribution of the infiltrated tracer as initial conditions for a particle tracking model at the system-scale better captured the eventual tracer breakthrough for both discharge scenarios. The findings highlight the significance of local influx dynamics in shaping non-local, larger-scale transport processes and underscore the challenges of accurately modeling transport phenomena across scales in porous media systems.