How does layered heterogeneity affect the ability of subsurface dams to clean up coastal aquifers contaminated with seawater intrusion?

Abdoulhalik, Antoifi; Ahmed, Ashraf

doi:10.1016/j.jhydrol.2017.08.044

Cited by 53 publications

(27 citation statements)

References 21 publications

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“…The subsurface dam was widely used to mitigate SWI, and normally it should be taller than the thickness of saltwater wedge at a specified position (Chang et al., 2019, 2020; Zhang et al., 2019). The thickness of saltwater wedge can be determined with field monitoring wells as well as experimental and numerical modeling methods (Abdoulhalik & Ahmed, 2017a, 2017b; Chang et al., 2019). In the following studies, the reference values of L dam , d h /d L , K f , α L , and various H dam were used to analyze the influences of dam height on the removal of residual saltwater trapped in the upstream aquifer (see Table 1).…”

Section: Resultsmentioning

confidence: 99%

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

Zheng

Chang

et al. 2021

Water Resources Research

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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.

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Section: Resultsmentioning

confidence: 99%

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

Zheng

Chang

et al. 2021

Water Resources Research

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“…It is often difficult to derive a clear understanding of the mechanisms affecting SWI directly from field‐based investigation (Werner et al, ). The challenge of measuring and quantifying coastal aquifer hydrodynamics and SWI in field sites has promoted the use of laboratory and numerical modelling tools to gain a valuable insight into SWI response to various geological and/or hydrological stresses, such as (a) change in seaward freshwater discharge resulting from fluctuations at the inland head boundary (Abdoulhalik & Ahmed, ; Abdoulhalik & Ahmed, ; Abdoulhalik, Ahmed, & Hamill, ; Goswami & Clement, ; Lu & Werner, ; Robinson, Ahmed, & Hamill, ; Robinson, Hamill, & Ahmed, ) in head‐controlled systems or from variations of the regional freshwater flux (Chang & Clement, ; Stoeckl & Houben, ; Stoeckl, Houben, & Dose, ) in flux‐controlled systems and (b) SLR (Hussain & Javadi, ; Morgan, Bakker, & Werner, ; Morgan, Stoeckl, Werner, & Post, ).…”

Section: Introductionmentioning

confidence: 99%

Transience of seawater intrusion and retreat in response to incremental water‐level variations

Abdoulhalik

Ahmed

2018

Hydrological Processes

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This paper provides for the first time an experimental study where the impact of sea‐level fluctuations and inland boundary head‐level variations on freshwater–saltwater interface toe motion and transition zone dynamics was quantitatively analysed under transient conditions. The experiments were conducted in a laboratory flow tank where various (inland and coastal) head changes were imposed to the system and the response of the key seawater intrusion parameters was analysed with high spatial and temporal resolution. Two homogeneous aquifer systems of different grain size were tested. The numerical code SEAWAT was used for the validation. The results show that in cases of sea‐level variations, the intruding wedge required up to twice longer time to reach a new steady‐state condition than the receding wedge, which thereby extend the theory of timescale asymmetry between saltwater intrusion and retreat processes in scenarios involving sea‐level fluctuations. The intruding and receding rates of the saltwater wedge were respectively similar in the scenario involving sea‐level and the freshwater‐level changes, despite change in transmissivity. The results show that, during the intrusion phase, the transition zone remains relatively insensitive, regardless of where the boundary head change occurs (i.e., freshwater drop or sea‐level rise) or its magnitude. By contrast, a substantial widening of the transition zone was observed during the receding phase, with almost similar amplitude in the scenario involving a rise of the freshwater level compared with that caused by a drop of the saltwater level, provided that an equivalent absolute head change magnitude was used. This transition zone widening (occurring during saltwater retreat) was greater and extended over longer period in the low hydraulic conductivity aquifer, for both freshwater‐level rise and sea‐level drop scenarios. The concentration maps revealed that the widening mechanism was also enhanced by the presence of some freshwater sliding and into the wedge during saltwater retreat, which was thereafter sucked upward towards the interface because of density difference effects.

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“…Hydraulic conductivity varies by four to five orders of magnitude in the low-permeability zone (table 2), which likely results in the high anisotropy of the bulk hydraulic conductivity of the aquifer. Thick freshwater/saltwater interfaces reported in Abarca Cameo (2006), Abdoulhalik and Ahmed (2017), and Houben and others (2017) have been attributed to aquifer heterogeneity. The interbedded zones of silty very fine sand and fine sand below a depth of 107 ft, combined with the possible diversion of most freshwater flow through the upper permeable zone and small streams that receive groundwater discharge in the area, could result in a thick transition zone; however, further exploration of this hypothesis by using groundwater models was beyond the scope of this study.…”

Section: Implications Of Observations From Boring To Bedrock For Undementioning

confidence: 90%

Lithostratigraphic, geophysical, and hydrogeologic observations from a boring drilled to bedrock in glacial sediments near Nantucket Sound in East Falmouth, Massachusetts

Hull¹,

Johnson²,

Stone³

et al. 2019

Scientific Investigations Report

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How does layered heterogeneity affect the ability of subsurface dams to clean up coastal aquifers contaminated with seawater intrusion?

Cited by 53 publications

References 21 publications

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

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

Transience of seawater intrusion and retreat in response to incremental water‐level variations

Lithostratigraphic, geophysical, and hydrogeologic observations from a boring drilled to bedrock in glacial sediments near Nantucket Sound in East Falmouth, Massachusetts

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