Effects of Unsaturated Flow on Salt Distributions in Tidally Influenced Coastal Unconfined Aquifers

Abstract

Around 40% of the world's population resides within 100 km of the coastline, while coastal populations are expected to continue growing rapidly (Barragán & de Andrés, 2015). To meet the considerable demand for freshwater, coastal groundwater is often over-exploited (mainly by intensive pumping), shifting the dynamic interface between fresh groundwater and seawater landward (Jasechko et al., 2020;Michael et al., 2017). As a result, severe seawater intrusion occurs in many coastal zones around the world, often involving complex salt distributions within coastal aquifers, especially in the presence of tides. The distribution of salinity in coastal unconfined aquifers plays an important role in controlling fresh groundwater discharge and land-sourced chemical loadings to the ocean, thereby affecting the availability of fresh groundwater and the quality of nearshore marine systems (Robinson et al., 2018;Werner et al., 2013).Over recent decades, substantial effort was devoted to investigating salt distributions in coastal aquifers, including through field measurements (e.g.,

SUPHRE: A Reactive Transport Model With Unsaturated and Density‐Dependent Flow

“…Neglecting unsaturated flow effects leads to an expansion of the USP and a seaward movement of the saltwater wedge (Figure 7e). This is consistent with previous findings regarding unsaturated flow effects on salt distributions in tidally influenced coastal unconfined aquifers (Luo, Kong, Yu, et al, 2023). As a result of the USP expansion, O 2 infiltrated to a deeper location within the USP when ignoring unsaturated flow effects (Figure 7f).…”

“…Figure 7 presents the phase-averaged (average over a tidal period) salt and O 2 concentration distributions predicted by SUPHRE. Regardless of considering unsaturated flow or not, another seawater circulation cell (upper saline plume, USP) appeared above the saltwater wedge (SW) in the intertidal zone because of seawater infiltration during the high tide and subsequent discharge during the ebb tide (Figures 7a and 7c) (Heiss et al, 2017;Luo, Kong, Yu, et al, 2023;Robinson et al, 2006Robinson et al, , 2007Xin et al, 2010). Similar salt and O 2 distributions can be anticipated while ignoring oxygen-consumption within the aquifer (i.e., treating oxygen as a conservative species like salt).…”

“…10.1029/2023MS003975 biogeochemical reactions. As mentioned previously, unsaturated flow affects groundwater flow and conservative solute transport in coastal unconfined aquifers (Kong et al, 2015;Luo, Kong, Yao, et al, 2023;Luo, Kong, Yu, et al, 2023). However, its effects on reactive transport remain poorly understood.…”

“…Like SUTRA, unsaturated flow in SUTRA-MS cannot be switched off directly when the unsaturated zone develops in the model domain. By comparing SUTRA with a saturated flow only model, Luo, Kong, Yu, et al (2023) indicated that unsaturated flow effects in SUTRA can be largely removed by assigning a large α value. Following this approach, α was assigned a value of 55 m 1 (a capillary fringe height of around 0.018 m, other parameters remained unchanged) to remove unsaturated flow effects.…”

“…Unsaturated flow is a fundamental aspect of groundwater flow and solute transport (e.g., Barry, Barry, & Parlange, 1996;Geng et al, 2017Geng et al, , 2020Kong et al, 2015;Luo et al, 2018;Luo, Kong, Yao, et al, 2023;Luo, Kong, Yu, et al, 2023;Parlange & Brutsaert, 1987). Šimůnek et al (2006) accounted for unsaturated flow effects in a reactive transport model HP1.…”