Implications of the change in confinement status of a heterogeneous aquifer for scale-dependent dispersion and mass-transfer processes.

A series of experimental tracer tests were performed to explore the implications of the change in thepressure status of a heterogeneous bimodal aquifer for scale-dependent dispersion and mass-transfer processes.The sandbox was filled with sands and gravel channels and patches to form an alluvial-like bimodalaquifer. We performed multiple injections of a conservative tracer from 26 different locations of the sandboxand interpreted the resulting depth-integrated breakthrough curves (BTCs) at the central pumpingwell to obtain a scale-dependent distribution of local and field-integrated apparent longitudinal dispersivity(respectively, alocL and aappL ). We repeated the experiments under confined (CS) and unconfined (UNS)pressure status, keeping the same heterogeneous configuration. Results showed that alocL (associated withtransport through gravel zones) was poorly influenced by the change in aquifer pressure and the presenceof channels. Instead, aappL (i.e. macrodispersion) strongly increased when changing from CS to UNS. In specific,we found aappL≈ 0.03r for the CS and aappL≈ 0.15r for the UNS (being r the distance from the well).Second-to-fourth-order temporal moments showed strong spatial dependence in the UNS and no spatialdependence in the CS. These results seem consistent with a “vadose-zone-driven” kinetic mass-transfer processoccurring in the UNS but not in the CS. The vadose zone enhances vertical flow due to the presenceof free surface and large contrasts in hydraulic conductivity triggered by the desaturation of gravel channelsnearby the pumping well. The vadose zone enhances vertical mixing between gravel and sands andgenerates BTC tailing. In the CS vertical mixing is negligible and anomalous transport is not observed.