Transience of seawater intrusion and retreat in response to incremental water-level variations

This paper provides for the first time an experimental study where the impact of sea-level fluctuations and inland boundary head-level variations on freshwater–saltwater interface toe motion and transition zone dynamics was quantitatively analysed under transient conditions. The experiments were conducted in a laboratory flow tank where various (inland and coastal) head changes were imposed to the system and the response of the key seawater intrusion parameters was analysed with high spatial and temporal resolution. Two homogeneous aquifer systems of different grain size were tested. The numerical code SEAWAT was used for the validation. The results show that in cases of sea-level variations, the intruding wedge required up to twice longer time to reach a new steady-state condition than the receding wedge, which thereby extend the theory of timescale asymmetry between saltwater intrusion and retreat processes in scenarios involving sea-level fluctuations. The intruding and receding rates of the saltwater wedge were respectively similar in the scenario involving sea-level and the freshwater-level changes, despite change in transmissivity. The results show that, during the intrusion phase, the transition zone remains relatively insensitive, regardless of where the boundary head change occurs (i.e., freshwater drop or sea-level rise) or its magnitude. By contrast, a substantial widening of the transition zone was observed during the receding phase, with almost similar amplitude in the scenario involving a rise of the freshwater level compared with that caused by a drop of the saltwater level, provided that an equivalent absolute head change magnitude was used. This transition zone widening (occurring during saltwater retreat) was greater and extended over longer period in the low hydraulic conductivity aquifer, for both freshwater-level rise and sea-level drop scenarios. The concentration maps revealed that the widening mechanism was also enhanced by the presence of some freshwater sliding and into the wedge during saltwater retreat, which was thereafter sucked upward towards the interface because of density difference effects.