Solute transport affected by unstable flow in coastal beach aquifers

Previous studies have examined solute transport in subterranean estuaries affected by steady tide-induced seawater circulation with a stable upper saline plume formed in the intertidal zone. Based on numerical simulations, we investigated the solute transport behaviour in a subterranean estuary under unstable flow conditions caused by tides and density gradients. The flow instability led to the formation of salt fingers that penetrated and moved across the intertidal zone. Therefore, the migration path of a landward solute plume towards the sea changed significantly, moving deeper into the aquifer until reaching the base. The residence time of solute in the nearshore aquifer was increased by 500 days under the simulated condition. The solute discharge zone varied over time and shifted from the mean shoreline to low-tide mark. The temporal variations of the total solute efflux were also modified significantly by the unstable flow, showing a change of solute discharge from a unimodal to bimodal mode. The sensitivity analysis reveals that the transport path of the plume is more sensitive to the hydraulic conductivity and tidal amplitude, while the residence time and plume spreading are more sensitive to the inland freshwater flux. These findings have important implications for the understanding of flow, solute transport and biogeochemical processes in subterranean estuaries.