Investigation of the elevation of saltwater wedge due to subsurface dams

Water Resources Research

Chang

et al. 2021

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Countermeasures against saltwater intrusion (SWI) are critical to prevent coastal groundwater deterioration. Among different measures to prevent SWI, subsurface dams have shown to be an effective approach, but it is likely to produce residual saltwater behind the dam in a landward aquifer. This study investigated the influences of subsurface dam design and aquifer properties on the dynamic characteristics of residual saltwater in a field‐scale aquifer and for the first time revealed the desalinization mechanism of residual saltwater behind the dams from the point of mixing zone. It was found that the low‐concentration mixing zone (LCMZ) (for the area between 10% and 50% of seawater salinity) was a major channel for the saltwater to flow over the dam to the ocean boundary while the residual salt was continuously dispersed to the LCMZ from the high‐concentration mixing zone (HCMZ) (for the area between 50% and 90% of seawater salinity) under high‐concentration gradients. Moreover, we developed two formulas of the reduction rate of saltwater wedge length (RSWL*) and the removal rate of total residual salt mass (RTSM*) to evaluate the desalination effectiveness of high‐ and low‐concentration residual saltwater, respectively. The results showed that it took much longer time for a taller dam and a dam at a closer position to the sea boundary to desalinize the high‐concentration residual saltwater in the upstream aquifer, more than 50 years for the cases of dam height beyond 16 m. On the contrary, only a slightly shorter time was needed to remove the low‐concentration saltwater behind the dams with the decrease of the distance from the sea boundary. Aquifer properties including the hydraulic gradient, hydraulic conductivity, and dispersivity strongly altered the desalinization time of the residual saltwater. The dispersivity was found to be the most critical factor influencing the removal effectiveness of saltwater retained in the landward aquifer. Increase of dispersivity from 1 to 3 m can dramatically reduce the desalinization time from more than 30 to 4 years.

Section: Resultsmentioning

confidence: 99%

Timescale and Effectiveness of Residual Saltwater Desalinization Behind Subsurface Dams in an Unconfined Aquifer

Water Resources Research

Chang

et al. 2021

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“…The seawater was prepared by dissolving 34.5 g of sodium chloride (NaCl, AR grade) and 1.5 g of red food dye (Sinopharm Chemical Reagent, Co., Ltd) per liter of deionized water. Previous studies have examined that food dyes can permeate synchronously with saltwater in a sand flume and have an inert relationship (adsorption and reaction) with the porous medium (Chang et al, 2019(Chang et al, , 2020Goswami & Clement, 2007). The seawater and freshwater densities were measured using a densitometer (AlfaMirage SD-200L) to be 1.025 g/mL and 1.0 g/mL, respectively.…”

Section: Methodsmentioning

confidence: 99%

Influence of Tide‐Induced Unstable Flow on Seawater Intrusion and Submarine Groundwater Discharge

Fang

Water Resources Research

et al. 2021

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Groundwater is a valuable source of freshwater in coastal areas. The groundwater flux in coastal aquifers generally occurs in two processes: seawater intrusion (SWI) and submarine groundwater discharge (SGD) (Robinson et al., 2018). SWI, the subsurface movement of seawater into freshwater aquifers, is a natural phenomenon in coastal areas (Werner et al., 2013). The hydraulic head difference caused by the density difference between seawater and freshwater drives the landward movement of seawater, thus forming the configuration of a saltwater wedge (SW) (Glover, 1959;Zhang et al., 2019;Zheng et al., 2020). Above the SW, inland fresh groundwater discharges to the sea. Freshwater mixes with saline water at the interface of the SW due to hydrodynamic dispersion. The salinity gradient in this zone drives the convective circulation inside the SW, that is density-driven circulation (Kohout, 1960). The total efflux (including fresh groundwater and circulating seawater) to the sea is commonly termed SGD. Hydrological behaviors in the two processes are not independent, but interrelated. Previous research has shown that increasing fresh groundwater output in coastal aquifers leads to a seaward shift of the SW. The movement of the SW also affects the density-driven circulation flux (Werner et al., 2013). The salinity of groundwater in aquifers can increase due to SWI, thereby reducing the availability of freshwater in coastal areas. Studies have identified SGD as an important source of freshwater, nutrients, metals, and carbon to the ocean, thus impacting coastal water quality and ecosystems (Moore, 2010;Robinson et al., 2018). Research on the hydrological behaviors in the two processes of coastal aquifers is beneficial for the sustainable management of marine and groundwater resources in coastal areas.Most coastlines worldwide experience tidal oscillations that change groundwater flow and solute transport processes in coastal aquifers (Figure 1). Seawater infiltration into coastal aquifers occurs on the rising tide,

“…In the model, the isoline of 50% seawater salinity was located by visualizing the 18 000 mg/L isoline. While in the experiment, the 50% isoline was located by directly observing the shape of the saltwater wedge (Chang et al, 2020; Luyun Jr. et al, 2011). In order to reduce the error caused by this method, the experimental conditions should be kept stable and the angle should be consistent when observing the position of the saltwater wedge toe.…”

Section: Methodsmentioning

confidence: 99%

“…The average hydraulic conductivity (K) of the porous media measured by Darcy's law was 0.0048 m/s and the porosity (n) measured by the volume method was 0.4. The longitudinal dispersivity (α L ) of porous media obtained by fitting the breakthrough curves in an onedimensional column experiment was 0.15 cm, which represents a value common for laboratory-scale experiments (Chang et al, 2020;Sun et al, 2019). The transverse dispersivity (α T ) was set to be 1/10 of the longitudinal dispersivity, as same as the previous studies (Lu et al, 2013;Luyun Jr. et al, 2011;Walther et al, 2017).…”

Section: Experimental Methodsmentioning

confidence: 99%

“…The underground physical barrier can also change the local groundwater environment (Ishida et al, 2011;Shen et al, 2020). For example, although full-section barriers (or high subsurface dams) prevent SWI effectively, they can block the groundwater drainage channel and lead to the accumulation of salt and land-based pollutants, thereby deteriorating groundwater quality (Chang et al, 2020;Senthilkumar & Elango, 2011). To utilize and manage groundwater resources sustainably, Jamali et al (2013) is closely related to groundwater quality and aquifers with a small groundwater discharge are more susceptible to salinization (Qu et al, 2017;Zhou et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Effects of mixed physical barrier on residual saltwater removal and groundwater discharge in coastal aquifers

Gao

Chang

et al. 2021

Hydrological Processes

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