Solute transport influenced by unstable flow in beach aquifers

Abstract

Recent studies have shown that, in beach aquifers, tide-induced upper saline plume (USP) may become unstable in the form of moving salt fingers under certain hydrogeological conditions. However, to what extent solute transport in beach aquifers is affected by the unstable flow associated with such salt fingers has not been quantitatively examined, despite the implications inferred by these studies. This study numerically investigated this problem by considering a land-sourced conservative solute plume in a conceptual 2-D beach aquifer. The results demonstrate that unstable flow leads to salt fingers that periodically develop from the USP and then move seaward across the intertidal zone. Compared to the quasi-steady USP case, unstable flow considerably modifies the migration path of land-sourced solute plume towards the sea, moving it deeper into the aquifer until reaching the base. Also, unstable flow increases the residence time of the land-sourced solute plume in the beach aquifer and causes the solute discharge zone to vary over time and shift from the mean shoreline (in the quasi-steady USP case) to the low-tide mark. The temporal variations of total solute efflux pattern are also altered by unstable flow, changing from a unimodal to a bimodal pattern. Sensitivity analysis based on dispersivity, hydraulic conductivity, tidal amplitude and inland freshwater flux reveals that the transport path of solute plume is more sensitive to hydraulic conductivity and tidal amplitude, while the residence time and plume spreading are more sensitive to inland freshwater flux.