Groundwater is a valuable source of freshwater in coastal areas. The groundwater flux in coastal aquifers generally occurs in two processes: seawater intrusion (SWI) and submarine groundwater discharge (SGD) (Robinson et al., 2018). SWI, the subsurface movement of seawater into freshwater aquifers, is a natural phenomenon in coastal areas (Werner et al., 2013). The hydraulic head difference caused by the density difference between seawater and freshwater drives the landward movement of seawater, thus forming the configuration of a saltwater wedge (SW) (Glover, 1959;Zhang et al., 2019;Zheng et al., 2020). Above the SW, inland fresh groundwater discharges to the sea. Freshwater mixes with saline water at the interface of the SW due to hydrodynamic dispersion. The salinity gradient in this zone drives the convective circulation inside the SW, that is density-driven circulation (Kohout, 1960). The total efflux (including fresh groundwater and circulating seawater) to the sea is commonly termed SGD. Hydrological behaviors in the two processes are not independent, but interrelated. Previous research has shown that increasing fresh groundwater output in coastal aquifers leads to a seaward shift of the SW. The movement of the SW also affects the density-driven circulation flux (Werner et al., 2013). The salinity of groundwater in aquifers can increase due to SWI, thereby reducing the availability of freshwater in coastal areas. Studies have identified SGD as an important source of freshwater, nutrients, metals, and carbon to the ocean, thus impacting coastal water quality and ecosystems (Moore, 2010;Robinson et al., 2018). Research on the hydrological behaviors in the two processes of coastal aquifers is beneficial for the sustainable management of marine and groundwater resources in coastal areas.Most coastlines worldwide experience tidal oscillations that change groundwater flow and solute transport processes in coastal aquifers (Figure 1). Seawater infiltration into coastal aquifers occurs on the rising tide,

No abstract

The fluctuation in sea level induced by tides generates an upper saline plume (USP) in the intertidal region, which is closely related to groundwater flow and solute transport processes in coastal aquifers. Thus, a clearer understanding of the USP configuration is needed to accurately predict the extent of seawater intrusion and water chemical fluxes to the ocean. This study experimentally and numerically examined the effect of transient external forcing conditions on the stability of tide‐induced USP, represented as seasonal subsurface inflow. In contrast to earlier studies, the USP was not always stable or unstable, but experienced a dynamic transformation of stability‐instability as the seasonal subsurface inflow fluctuated. The same intensity of subsurface inflow caused a stable USP in the fixed cases but a dynamic USP state in the seasonal cases. The USP responded rapidly to the fluctuation of seasonal subsurface inflow and was drawn to expand into the intertidal zone. These features of the USP contributed to the increased potential for the onset of an unstable flow when the subsurface inflow varied seasonally. Therefore, ignoring the timescale changes on external forcing conditions may underestimate the generation of unstable USPs. The frequency contrast between seasonal subsurface inflow and tide also influenced the dynamic transformation process occurring in the intertidal region. There was an inverse relationship between the frequency contrast and the duration of the unstable flow. These findings provide a new understanding of the complex intertidal environment in coastal aquifers.

No abstract

No abstract