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

Xiayang Yu,Chengji Shen,Ling Li,Pei Xin

doi:10.3389/fenvs.2020.599041

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.

Highlights

Ocean tides are identified as an important driving force that controls the hydrodynamic and biogeochemical processes in coastal aquifers (Werner et al, 2013; Robinson et al, 2018)
We focused on the effects of multiconstituent tides on submarine groundwater discharge (SGD) and salinity distributions in unconfined aquifers with fixed-head inland boundaries at different landward positions, under the assumption that the yearly averaged inland freshwater input is the same in aquifers of alternative landward extent
We focused on four cases with the fixed-head inland boundary located at different locations as follows: (1) Fixed-head and LI 50 m (LI is the inland aquifer extent, which measures the horizotal distance from the inland boundary to the shoreline), with the seaward aquifer extent LS 50 m, the seaward aquifer thickness HS 27 m, and the inland aquifer thickness HI 33 m

Summary

Introduction

Ocean tides are identified as an important driving force that controls the hydrodynamic and biogeochemical processes in coastal aquifers (Werner et al, 2013; Robinson et al, 2018). We focused on the effects of multiconstituent tides on SGD and salinity distributions in unconfined aquifers with fixed-head inland boundaries at different landward positions, under the assumption that the yearly averaged inland freshwater input is the same in aquifers of alternative landward extent.

Results

Conclusion