Effects of Tidally Varying Salinity on Groundwater Flow and Solute Transport: Insights From Modelling an Idealized Creek Marsh Aquifer

Abstract

Most existing numerical research on tide‐induced groundwater dynamics assumes a constant surface water salinity on the seaward boundary (constant salinity case). Few studies have investigated the influence of tidally varying salinity on shallow groundwater dynamics in coastal aquifers (tidal salinity case). We compiled field observations of tidally varying salinity in multiple estuaries across the eastern coast of China and a tidal creek in North Inlet‐Winyah Bay, United States. Numerical simulations were then conducted to explore the effect of tidally varying salinity on groundwater flow and salt transport in an idealized creek marsh aquifer. Results showed that the upper saline plume and classical saltwater wedge appeared in all cases, but the salinity in the saltwater wedge was diluted in the tidal salinity cases. Notably, groundwater transit times were shorter in the tidal salinity case than in the constant salinity case, especially under the creek bottom. Quantitative analyses indicated that tidally varying salinity significantly enhanced surface water‐groundwater exchange, increasing submarine groundwater discharge by 10% and the total inflow of surface water across the water‐sediment interface by 7%. As the density of groundwater differs from that of the overlying surface water, fingered saltwater flow formed in sediments under the creek bottom, leading to some small local water circulation cells. These small cells reduced groundwater transit times and almost doubled the water exchange rate. Coupling the density‐dependent flow to a simplified nitrogen reaction network revealed that the tidally varying salinity may have the potential to influence nitrogen biogeochemical transformations that modify nitrogen loads prior to discharge.

Submarine Groundwater Discharge in a Coastal Bay: Evidence from Radon Investigations

“…Approaches to evaluate SGD mainly include seepage meters [21][22][23], hydrological models [24][25][26], and geochemical tracers [27,28]. In particular, naturally occurring geochemical tracers, such as radium as radium ( 223,224,226,228 Ra) and radon ( 222 Rn) isotopes, have been widely used in tracing SGD and water mixing processes in coastal systems [14,[29][30][31][32].…”

Variable-Density Flow and Solute Transport in Stratified Salt Marshes

“…To understand how soil stratigraphy would affect the variabledensity flow and solute movement in salt marshes, the current study set up Case H without soil stratigraphy and Case L with a mud layer overlying a sand layer. Following Xiao et al (2019a), who established the numerical model based on investigations of the Crabhaul creek in North inlet-Winyah Bay National Estuarine Research Reserve system, Case H considers homogeneous sandy marsh sediments, with the K s and φ set to 1 × 10 −3 m s −1 and 0.36. Case L uses a layered soil structure of a 2-m mud layer overlying a 3-m sand layer, with K s and φ being 1 × 10 −5 m s −1 and 0.65 for mud.…”

“…The interplay of soil stratigraphy and salinity contrasts has been rarely investigated. One exception is the recent work of Xiao et al (2019a), which examined the combined effects of tidally varying salinity and soil heterogeneity on subsurface hydrology in salt marshes. However, the findings of Xiao et al (2019a) are applicable to micro-tidal marshes only, because they did not consider tidal inundation.…”

“…One exception is the recent work of Xiao et al (2019a), which examined the combined effects of tidally varying salinity and soil heterogeneity on subsurface hydrology in salt marshes. However, the findings of Xiao et al (2019a) are applicable to micro-tidal marshes only, because they did not consider tidal inundation. When the marsh platform is not flooded, the exchange between tidal water and pore water is driven only by the filling and draining through the tidal creek.…”

Tidal‐Driven Submarine Groundwater Discharge and Its Influences on the Carbonate System of a Coastal Coral Reef in the Northern South China Sea

“…The SGD‐associated carbon fluxes are found to be significant in regional and global carbon budgets (Cai et al., 2003; Chen & Borges, 2009; Moore, 2010; Moore et al., 2006; Santos et al., 2009), which have great impacts on the carbonate system in coastal ocean (Atkins et al., 2013; Kim et al., 2011; Liu et al., 2012). Previous studies have also demonstrated that SGD provides an important source of nutrients (Moore et al., 2006; Rodellas et al., 2015; Santos et al., 2009; Slomp & Van Cappellen, 2004; Wang et al., 2018, 2020; Zhang, Santos, Li, Wang, Xiao, Guo, & Wang, 2020) and metals (Beck et al., 2007; Bone et al., 2007; Johannesson & Burdige, 2007; Windom et al., 2006) to salt marshes (Cai et al., 2000), estuaries (Moore et al., 2006; Santos et al., 2009), coral reefs (Paytan et al., 2006; Santos et al., 2010), bay systems (Wang et al., 2018, 2020; Zhang, Wang, Li & Song, 2020), and other ecosystems on the continental shelf (Cai et al., 2020; Moore, 2010; Xiao et al., 2019). Therefore, a mechanistic understanding of SGD input is crucial to improve our knowledge on the coastal biogeochemical dynamics.…”