Numerical Study of Tide‐Induced Airflow and Salt–Fresh Water Dynamics in Unconfined Aquifers

Abstract

This study employed a coupled water‐air two‐phase flow and salt water transport model to analyze the behaviors of generated airflow in unsaturated zones and the fluctuations of salinity at the salt–fresh water interface in a two‐layered unconfined aquifer with a sloping beach surface subjected to tidal oscillations. The simulation results show that as the new dynamic steady state including effects of tidal fluctuations is reached through multiple tidal cycles, the dispersion zone in the lower salt water wedge is broadened because fresh water/salt water therein flows continuously landward or seaward during tidal cycles. The upper salt–fresh water interface exhibits more vulnerable to the tidal fluctuations, and the variation of salinity therein is periodic, which is irrelevant to the hydraulic head but is influenced by the direction and velocity of surrounding water‐flow. With the tidal level fluctuating, airflow is mainly concentrated in the lower permeable layer due to the restraint of the upper semi‐permeable layer, and the time‐lag between the pore‐air pressure and the tidal level increases with distance from the coastline. The effect of airflow in unsaturated zones can be transmitted downward, causing both the magnitude of salinity and its amplitude in the upper salt–fresh water interface to be smaller for the case with airflow than without airflow due to the resistance of airflow to water‐flow. Sensitivity analysis reveal that distributions of airflow in unsaturated zones are affected by the permeability of the upper/lower layer and the van Genuchten parameter of the lower layer, not by the van Genuchten parameter of the upper layer, whereas the salinity fluctuations in the salt–fresh water interface are affected only by soil parameters of the lower layer.

Effects of Water Retention Curves and Permeability Equations on the Prediction of Relative Air Permeability

“…The simulation of air movement in unsaturated porous media needs the accurate estimate of relative air permeability (RAP) as a function of the water content (Springer et al., 1995). The RAP, in fact, is a required parameter for the numerical simulation of subsurface two‐phase flow under isothermal (e.g., Kueper & Frind, 1991; Zang et al., 2019) and thermal conditions (e.g., Mohanty & Yang, 2013; Vanderborght et al., 2017), which has significant implications for many geophysical issues such as the investigation of soil aeration and evapotranspiration (e.g., Ben‐Noah & Friedman, 2018), the determination of transport of organic contaminants in the underground (e.g., Essaid et al., 2015), the simulation of reservoir CO 2 injection (e.g., Celia et al., 2015; Oostrom et al., 2016), and the study of hydraulic fracturing fluid migration in the subsurface (e.g., Birdsell et al., 2015a, 2015b).…”