Influence of Dynamically Stable‐Unstable Flow on Seawater Intrusion and Submarine Groundwater Discharge Over Tidal and Seasonal Cycles

Abstract

Temporal changes in the external forcing conditions of coastal aquifer systems, such as seasonal inland freshwater input, may disturb the balance between the ocean forces and freshwater hydraulic gradient. An increase or decrease in this imbalance may affect the stability of tide‐induced upper saline plume (USP). In this study, we performed laboratory experiments and numerical simulations to investigate the influence of seasonal inland freshwater input on USP stability and the associated behavior of seawater intrusion (SWI) and submarine groundwater discharge (SGD). In contrast to previous studies, we found that USP was not always stable or unstable in the intertidal zone, but experienced a dynamically stable‐unstable process as the seasonal inland freshwater input varied. The extent of SWI and SGD showed a dual response to the seasonal inland freshwater input. On one hand, seasonal fluctuation in inland freshwater input directly induced responsive changes in SWI and SGD; on the other hand, dynamic stability of the USP caused by the seasonal fluctuation resulted in an additional high‐frequency fluctuation in the extent of SWI and SGD during the unstable flow. Moreover, the response of saline SGD and total SGD to the seasonal fluctuation in the inland freshwater input was nonlinear. This overall increased the SGD compared to the scenarios with a temporally constant inland freshwater input. Therefore, ignoring seasonal changes in the inland freshwater input may underestimate the amount of water exchanges across the aquifer‐sea interface.

A Hydrochemical Study of Groundwater Salinization in Qinzhou Bay, Guangxi, Southern China

“…However, we assumed the natural conditions were stable using Dirichlet boundary conditions at the inland and seaward boundaries and Neumann boundary conditions at the top and bottom (Figure 8). The real conditions might be more complicated than this assumption, for example, the seasonal meteorological phenomena, the variable runoff, and the tidal stream (e.g., Bouwer, 2002; Fang, Zheng, Guo, et al., 2022; Fang, Zheng, Wang, et al., 2022; Hunter et al., 2015; Zheng et al., 2021). Building more explicit models based on the field environment may provide a transparent process of SWI, but meanwhile need, more hydrological tests to determine accurate material properties.…”

“…The material properties of the vadose zone used in this simulation are with reference to Fang, Zheng, Wang, et al. (2022). The permeability settings referred to the pumping tests during 2013–2015 implemented by the Wuhan Center of China Geological Survey, whose values are taken from the ranges shown in Figure 2a.…”

“…The sea level is set as 3 m. More explicit models to account for tidal effects may refer to Fang, Zheng, Guo, et al. (2022) and Fang, Zheng, Wang, et al. (2022).…”

“…In this study, we use a numerical model to simulate the spatio‐temporal evolution of SWI and the contaminant transport due to the mariculture. The SWI process involves the coupling of the buoyant flow, fluid flow, and solute, which needs to account for the variations of water saturation, the mass density of the porous fluid, the hydraulic gradient, and the hydrodynamic advection‐dispersion of solute mass (Cobaner et al., 2012; Fang, Zheng, Guo, et al., 2022; Fang, Zheng, Wang, et al., 2022; Pool et al., 2011). To solve the multi‐physical coupling problem, we applied the finite element software COMSOL Multiphysics 6.0 to perform the numerical simulations.…”

Mega‐Tidal and Surface Flooding Controls on Coastal Groundwater and Saltwater Intrusion Within Agricultural Dikelands

“…The presence and morphology of USPs are strongly dependent on factors including beach morphology, tidal range, density differences, and heterogeneity (Bakhtyar et al., 2013; Evans & Wilson, 2016; Robinson et al., 2018). The USP dimensions and spatial and temporal stability are also dependent on the magnitude and timing of external forcing (Buquet et al., 2016; Fang et al., 2021; Fang, Zheng, Guo, et al., 2022; Fang, Zheng, Wang et al., 2022; Grünenbaum et al., 2023; Kuan et al., 2012; Yu et al., 2019). Despite the relationship between tidal amplitude and USP morphology and dynamics, numerical modeling studies investigating USP dynamics, or more generally aquifer‐ocean interactions, have been limited to micro‐ (and occasionally macro‐) tidal ranges (e.g., Levanon et al., 2017; Yang et al., 2013).…”