Tracer transport experiments in well–type flows through a strongly heterogeneous porous medium: Results and analysis

The analysis of transport at large scales in radial flows, typically induced by pumping wells, is often complicated by the heterogeneity of formations, ultimately leading to a non–Fickian behavior. The latter remains only partly understood, especially under high levels of spatial disorder in the hydraulic properties: an issue that field–investigations struggle to capture. To advance current understanding, we set–up an artificial, highly heterogeneous porous medium with given heterogeneity statistical structure of the hydraulic conductivity. Then, several transport experiments have been conducted and analyzed by means of temporal moments, that were used, among the others, to derive apparent (upscaled) parameters to be employed in the context of the so called equivalent homogeneous medium (EHM). The breakthrough curves (BTCs), recovered at the pumping well, showed huge variability, spatial dependence, incomplete mass recovery and other anomalous features, such as pronounced asymmetry, early peaks, and heavy tails. Our results suggest a pre-peak phase (i.e. the rising limb of the BTCs), during which solutes move mainly through highly conducting inclusions and a post-peak phase (i.e. the descending limb of the BTCs), in which transport is strongly influenced by retention through zones of low conductivity. The comparison of the BTCs pertaining to two different tracers underscores the pivotal role of diffusion within poorly conducting inclusions. Overall, nonetheless the numerous limitations, the EHM framework proved to be a simple and valuable tool for preliminarily assessment of transport processes through strongly heterogeneous porous formations.