Effects of Multiconstituent Tides on a Subterranean Estuary With Fixed-Head Inland Boundary

Abstract

While tides of multiple constituents are common in coastal areas, their effects on submarine groundwater discharge (SGD) and salinity distributions in unconfined coastal aquifers are rarely examined, with the exception of a recent study that explored such effects on unconfined aquifers with fixed inland freshwater input. For a large proportion of the global coastline, the inland areas of coastal aquifers are topography-limited and controlled by constant heads. Based on numerical simulations, this article examines the variation of SGD and salinity distributions in coastal unconfined aquifers with fixed-head inland boundaries at different distances from the shoreline (i.e., 50, 100, 150, and 200 m). The results showed that the fluctuation intensity of freshwater input was enhanced as the inland aquifer extent decreased, e.g., the range of tide-induced fluctuations in freshwater input increased by around 5 times as the inland aquifer extent decreased from 200 to 50 m. The frequency spectra of the fluctuations of SGD and salinity distributions showed that the coastal aquifer of a shorter inland aquifer extent smoothed out fewer high-frequency tidal constituents but enhanced interaction among different tidal constituents. The interaction among tidal constituents generated new low-frequency signals in the freshwater input and salinity distributions. Regressions based on functional data analysis demonstrated that the inland freshwater input and salinity distributions at any given moment were related to the antecedent (previous) tidal conditions weighted using the probability density function of the Gamma distribution. The influence of the antecedent tidal conditions depended on the inland aquifer extent.

Effects of Future Increases in Tidal Flooding on Salinity and Groundwater Dynamics in Coastal Aquifers

“…Interacting tidal constituents at the coastline drive the exchange of water and solutes across the aquifer‐ocean interface over a range of temporal scales. Previous studies have demonstrated that di‐ to quarter‐diurnal tidal periods and spring‐neap cycles lead to dynamic intertidal pore water salinities and increase fluid and chemical exchanges across the ocean‐aquifer interface (Abarca et al., 2013; Geng & Boufadel, 2017b; Heiss & Michael, 2014; Robinson, Gibbes, et al., 2007; Yu et al., 2021). Previous studies that incorporated multi‐constituent tides in numerical models generally considered four to five constituents: S 2 , M 2 , S 1 , O 1 , and K 1 (Abarca et al., 2013; Geng & Boufadel, 2015; Heiss & Michael, 2014; Robinson, Gibbes, et al., 2007; Yu et al., 2019, 2021).…”

“…Interacting tidal constituents at the coastline drive the exchange of water and solutes across the aquifer-ocean interface over a range of temporal scales. Previous studies have demonstrated that di-to quarter-diurnal tidal periods and spring-neap cycles lead to dynamic intertidal pore water salinities and increase fluid and chemical exchanges across the ocean-aquifer interface (Abarca et al, 2013;Geng & Boufadel, 2017b;Heiss & Michael, 2014;Robinson, Gibbes, et al, 2007;Yu et al, 2021). Previous studies that incorporated multi-constituent tides in numerical models generally considered four to five constituents: S 2 , M 2 , S 1 , O 1 , and K 1 (Abarca et al, 2013;Geng & Boufadel, 2015;Heiss & Michael, 2014;Robinson, Gibbes, et al, 2007;Yu et al, 2019Yu et al, , 2021.…”