Abstract
AbstractThis 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.
Highlights
For populations living in coastal zones, which represents about 70% of the world's population (Webb & Howard, 2011), groundwater represents the main, in some cases the only, source of water supply
The expansion of the transition zone (TZ) did not occur during the intruding phase of saltwater wedge whether it resulted from freshwater‐level drop or saltwater‐level rise, regardless of the magnitude of the head change
This finding gives initial indications that in coastal aquifer systems, the widening of the TZ width may not be substantial during the landward displacement of freshwater–saltwater interface, whether it occurs as a result of water table drop or sea‐level rises (SLRs), regardless of the magnitude of the boundary head change
Summary
For populations living in coastal zones, which represents about 70% of the world's population (Webb & Howard, 2011), groundwater represents the main, in some cases the only, source of water supply In such areas, groundwater is susceptible to degradation given its proximity with seawater as well as the intense water demand associated with ever‐increasing population densities. The freshwater–saltwater interface is a specific characteristic feature of coastal aquifers that occurs as a transition zone (TZ) where the salt concentration varies gradually from that of the freshwater to that of the saltwater in the seaward direction These parameters outline the wedge‐like shape of the plume that the intruding saltwater tend to form while encroaching homogeneous isotropic coastal aquifers, albeit aquifer heterogeneity may, to various degrees, severely affect this highly idealized shape
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