Impact of Time‐Dependent Velocity Fields on the Continuum‐Scale Transport of Conservative Chemicals

Abstract

AbstractGroundwater input in natural systems can vary over a wide range of time scales due to different natural phenomena and anthropogenic activities. These variations give rise to time‐dependent velocity fields, which in turn may influence the dynamics of a migrating chemical plume relative to migration in a constant‐velocity domain. Anomalous transport, which is ubiquitous in many groundwater systems and generally yields longer (non‐Fickian) tails of migrating chemical plumes relative to those subject to Fickian dispersive processes, is of particular interest in this context. Herein, transport of conservative chemical species in macroscopically 1D columns and 2D flow cells was analyzed via laboratory experiments and corresponding numerical simulations. Different time‐dependent velocity field magnitude conditions were compared to study how the transient water input affects the resulting tracer breakthrough curves. A stochastic‐based numerical model was employed to interpret the results and simulate conditions that extend beyond the laboratory scale. The laboratory measurements show that different time‐dependent velocity magnitude conditions, which preserve the average discharge of a comparable constant‐velocity system, yield similar, long‐tailed breakthrough curves compared to those of the constant‐velocity case. The breakthrough curves are then shown to be quantified with the continuous time random walk (CTRW) framework. Appropriate choice of particle transition times and distances at the moment of velocity change enables matching of the CTRW results to the experimental results obtained in the experiments. The negligible impact of time‐dependent velocity field magnitudes is shown to be consistent for 1D and 2D flow fields, homogeneous and heterogeneous media, and different flow scenarios.