Variable-Density Flow and Solute Transport in Stratified Salt Marshes

Xiaojing Wu,Chengji Shen,Zhongwei Zhao,Yuansheng Wang

doi:10.3389/fmars.2021.804526

Abstract

Subsurface hydrodynamics underpin the eco-functions of salt marshes. Many studies have investigated these processes under various conditions. However, the impact of soil stratification (a low-permeability mud layer overlying a high-permeability sand layer) on the variable-density groundwater flow (particularly unstable flow) and solute transport in regularly tide-flooded marshes remains poorly understood. The present study numerically explored this question based on a 2D cross-creek section of salt marshes, by comparing cases with and without stratification. Results show that, the low-permeability mud layer delays the initiation of unstable flow and leads to smaller and denser salt fingers. Consequently, solute plume stays in the marsh soil for a longer time and spreads more widely than that in the homogeneous case. Also, soil stratigraphy extends the duration and shrinks the zone of solute discharge across the tidal creek. Sensitivity analysis was conducted based on three key controlling variables: hydraulic conductivity contrast between mud layer and sand layer (Kmud/Ksand), salinity contrast between surface water and groundwater (Csea/Cpore), and mud layer thickness (Dmud). The results demonstrate that the residence time of solute plume in a two-layered salt marsh is less sensitive to Csea/Cpore than to Kmud/Ksand and Dmud. Moreover, the commencement and duration of solute discharge are more sensitive to Kmud/Ksand and Dmud than to Csea/Cpore. While the location of solute discharge zone is highly sensitive to Dmud and slightly influenced by Kmud/Ksand and Csea/Cpore. Findings from this study would facilitate a deeper understanding of the eco-functions of salt marshes.

Highlights

Salt marshes distributed at the ocean-land interface are one of the most productive ecosystems that maintain coastal biodiversity, moderate global warming, and buffer deleterious storm impacts (Chapman, 1974; Vernberg, 1993; Valiela et al, 2000; Artigas et al, 2021)
Unstable flow occurs in both cases, with salt fingers initially forming near the inland boundary, where density effect is dominant and the advection process is much weaker
They divided a salt marsh spatially into fingering flow-dominated and circulation-dominated zones, with the former occupying a relatively large area in a poorly channelized marsh. This paradigm would be greatly different in stratified salt marshes, since the less permeable mud layer acts as a barrier to constrain the movement of solute plume in the bottom sand layer, thereby disabling solute discharge from the marsh platform (e.g., Figure 2)

Summary

Introduction

Salt marshes distributed at the ocean-land interface are one of the most productive ecosystems that maintain coastal biodiversity, moderate global warming, and buffer deleterious storm impacts (Chapman, 1974; Vernberg, 1993; Valiela et al, 2000; Artigas et al, 2021). The asymmetrical pore water dynamics at the two stages lead to a net circulation toward the creek over one tidal cycle (Xin et al, 2012). The tidally averaged near-creek circulation is ecologically significant for salt marshes, as it underpins the “nutrient outwelling” hypothesis, which refers to the net export of nutrients from salt marshes to the sea (Teal, 1962). Subsurface flow affects nutrient cycling between marsh sediments and adjacent surface water, because pore water draining out of salt marsh sediments at low tide is enriched in nutrients compared to surface water (Whiting et al, 1985; Velinsky et al, 2017; Schiebel et al, 2018). The net circulation provides a rapid exchange between the marsh sediments and tidal creeks, thereby supporting the observed nutrient outwelling (Gardner, 2005; Wilson and Gardner, 2006; Peterson et al, 2019)

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