In situ long-term chloride transport through a layered, nonsaturated subsoil. 2. Effect of layering on solute transport processes

We analyze observed Cl- transport during a large-scale in situ unsaturated infiltration experiment in terms of the observation scale of the transport process and the physical stratification of the Tertiary sedimentary flow domain. By comparing piston flow and local velocity profiles, we show that velocity variations cannot be explained by water content changes alone. We also demonstrate that by comparing a layered convection-dispersion (CD) model with the observations, the dispersivity profile cannot be explained with the model, which produced unrealistic local dispersivity values. These discrepancies were partially explained by nonrepresentative sampling using porous cup solution samplers (PCS). We hypothesize that fingering flow or convergence phenomena below sand-clay interfaces leads to nonrepresentative artificially high dispersivity values. Velocity and dispersivity values immediately above the clay layers, however, seem more reliable due to convergence and more lateral mixing induced by a larger water content. Following the criteria derived from the Chuoke equation, we show that the subsoil can be subjected to fingering flow. We found that this process likely persisted for some 17 yr. Therefore, we conclude that the fine-textured clayey layers regulated the flow rate through time, approaching a quasi-steady-state flow condition. Overall, solute transport processes in the unsaturated layered subsoil appeared to be very strongly influenced by stratification of the flow domain. An apparent highly variable flow field was induced by the clay layers interbedded in a sandy deposit, while the local PCS sampling devices were likely too small to properly assess the mixing regime at this large scale.