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Seawater Intrusion into Coastal Aquifers

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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Oceans and Coastal Zones".

Deadline for manuscript submissions: closed (10 May 2021) | Viewed by 44274

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Special Issue Editor

Dr. Eyal Shalev

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Guest Editor

Geological Survey of Israel, Jerusalem, Israel
Interests: groundwater hydrology, coastal aquifers, damage deformation, solute and heat transport

Special Issue Information

Dear Colleagues,

Coastal aquifers are one of the most important water resources in the world. In addition, the natural discharge of freshwater to the sea as submarine groundwater discharge (SGD) has an important role in the ecology of marine environments. The dynamics of seawater and freshwater within coastal aquifers are highly sensitive to disturbances, and their inappropriate management may lead to the deterioration of water quality. In many coastal aquifers, seawater intrusion has become the major constraint imposed on groundwater utilization. Groundwater exploitation and climate variations create dynamic conditions, which can significantly increase the intrusion of seawater into the aquifer and may result in the salinization of wells. They may also reduce SGD and affect the water budget of marine systems.

This Special Issue welcomes original research work dedicated to seawater intrusion and related subjects. Potential topics include but are not limited to the following:

Monitoring groundwater salinization
Numerical modeling of density-driven solute transport
Laboratory fresh–saline water interface experiments
Effects of pumping fresh and saline water onto the fresh–saline water interface and SGD

Dr. Eyal Shalev
Guest Editor

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Keywords

Coastal aquifers
Fresh–saline water interface
Seawater intrusion
Submarine groundwater discharge
Density-driven flow

Published Papers (12 papers)

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Editorial

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2 pages, 165 KiB

Open AccessEditorial

Seawater Intrusion into Coastal Aquifers

by Eyal Shalev

Water 2021, 13(19), 2719; https://doi.org/10.3390/w13192719 - 01 Oct 2021

Cited by 1 | Viewed by 1957

Abstract

This editorial presents a representative collection of 11 papers presented in the Special Issue on Seawater Intrusion into coastal aquifers. Coastal aquifers are one of the most important water resources in the world. In addition, the natural discharge of freshwater to the sea as submarine groundwater discharge (SGD) has an important role in the ecology of marine environments. The dynamics of seawater and freshwater within coastal aquifers are highly sensitive to disturbances, and their inappropriate management may lead to the deterioration of water quality. In many coastal aquifers, seawater intrusion has become the major constraint imposed on groundwater utilization. Groundwater exploitation and climate variations create dynamic conditions, which can significantly increase seawater intrusion into aquifers and may result in the salinization of wells. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

Research

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33 pages, 25563 KiB

Open AccessFeature PaperArticle

Numerical Modeling of Saltwater Intrusion in the Rmel-Oulad Ogbane Coastal Aquifer (Larache, Morocco) in the Climate Change and Sea-Level Rise Context (2040)

by Mohamed Jalal EL Hamidi, Abdelkader Larabi and Mohamed Faouzi

Water 2021, 13(16), 2167; https://doi.org/10.3390/w13162167 - 07 Aug 2021

Cited by 14 | Viewed by 4265

Abstract

Many coastal aquifers have experienced seawater intrusion (SWI) into fresh groundwater aquifers. The principal causes of SWI include over-pumping and events such as climate change (CC) and rising sea levels. In northern Morocco, the Rmel-Oulad Ogbane coastal aquifer (ROOCA) supplies high-quality groundwater for drinking water and agriculture. This favorable situation has led to increased pumping, resulting in environmental challenges such as dropping water table and SWI. Furthermore, the climate has resulted in less recharge, with an estimated annual precipitation of 602 mm and an average temperature of 18.5 °C. The goal of this study is to determine how CC, over-pumping, and sea-level rise (SLR) affect SWI. Computational groundwater and solute transport models are used to simulate the spatial and temporal evolution of hydraulic heads and groundwater solute concentrations. The calibration is based on steady and transient groundwater levels from 1962 to 2040. SWI simulations show that the NW sector of the coastal area would be polluted, with the toe reaching 5.2 km inland with a significant salinity (15–25 g/L). To protect the fresh water in the reservoir from SWI, enhanced groundwater development and management approaches for this aquifer are required, such as artificial recharge from surface water. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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15 pages, 4315 KiB

Open AccessFeature PaperArticle

Hydrostatic Densitometer for Monitoring Density in Freshwater to Hypersaline Water Bodies

by Ziv Mor, Hallel Lutzky, Eyal Shalev and Nadav G. Lensky

Water 2021, 13(13), 1842; https://doi.org/10.3390/w13131842 - 01 Jul 2021

Cited by 2 | Viewed by 2784

Abstract

Density, temperature, salinity, and hydraulic head are physical scalars governing the dynamics of aquatic systems. In coastal aquifers, lakes, and oceans, salinity is measured with conductivity sensors, temperature is measured with thermistors, and density is calculated. However, in hypersaline brines, the salinity (and density) cannot be determined by conductivity measurements due to its high ionic strength. Here, we resolve density measurements using a hydrostatic densitometer as a function of an array of pressure sensors and hydrostatic relations. This system was tested in the laboratory and was applied in the Dead Sea and adjacent aquifer. In the field, we measured temporal variations of vertical profiles of density and temperature in two cases, where water density varied vertically from 1.0 × 10³ kg·m⁻³ to 1.24 × 10³ kg·m⁻³: (i) a borehole in the coastal aquifer, and (ii) an offshore buoy in a region with a diluted plume. The density profile in the borehole evolved with time, responding to the lowering of groundwater and lake levels; that in the lake demonstrated the dynamics of water-column stratification under the influence of freshwater discharge and atmospheric forcing. This method allowed, for the first time, continuous monitoring of density profiles in hypersaline bodies, and it captured the dynamics of density and temperature stratification. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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12 pages, 13871 KiB

Open AccessArticle

Haline Convection within a Fresh-Saline Water Interface in a Stratified Coastal Aquifer Induced by Tide

by Elad Ben-Zur, Haim Gvirtzman and Eyal Shalev

Water 2021, 13(13), 1780; https://doi.org/10.3390/w13131780 - 27 Jun 2021

Cited by 2 | Viewed by 2070

Abstract

Sea-tide effects on the fresh-saline water interface (FSI) in a stratified coastal aquifer are examined through laboratory experiments. The physical model, a two-dimensional rectangular flow tank, is filled with layered aquifers and aquitards. The aquifers serve as the main entrances/exits of water to/from the system through significant horizontal flows, creating unstable conditions of heavier saline water above lighter freshwater for short periods of time. Several processes create mixing; this instability results in haline convection, creating downward fingering, stable rising of horizontal saltwater front, and unstable upward fingerings of flushing freshwater. The time lag between the sea tide fluctuations and the emergence of adequate fresh- and saltwater is higher in a stratified system compared to a homogeneous system. Furthermore, longer tide cycles lead to the enlargement of the FSI’s toe horizontal movement range. The combination of tidal forcing with a layering aquifer structure leads to a wider FSI by creating a significant salt- and freshwater mixing inside each layer, vertical flows between the layers, and saltwater bodies at isolated areas. Haline convection within the FSI might be the reason for the wider fresh-saline interfaces that are found in field studies. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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24 pages, 6453 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Imaging the Structure and the Saltwater Intrusion Extent of the Luy River Coastal Aquifer (Binh Thuan, Vietnam) Using Electrical Resistivity Tomography

by Diep Cong-Thi, Linh Pham Dieu, Robin Thibaut, Marieke Paepen, Huu Hieu Ho, Frédéric Nguyen and Thomas Hermans

Water 2021, 13(13), 1743; https://doi.org/10.3390/w13131743 - 24 Jun 2021

Cited by 16 | Viewed by 3954

Abstract

With the growing population and the adverse effects of climate change, the pressure on coastal aquifers is increasing, leading to a larger risk of saltwater intrusion (SI). SI is often complex and difficult to characterize from well data only. In this context, electrical resistivity tomography (ERT) can provide high-resolution qualitative information on the lateral and vertical distribution of salinity. However, the quantitative interpretation of ERT remains difficult because of the uncertainty of petrophysical relationships, the limitations of inversion, and the heterogeneity of aquifers. In this contribution, we propose a methodology for the semiquantitative interpretation of ERT when colocated well data are not available. We first use existing wells to identify freshwater zones and characterize the resistivity response of clayey deposits. Then, we approximate the formation factor from water samples collected in the vicinity of ERT data to derive a resistivity threshold to interpret the saline boundary. We applied the methodology in the shallow aquifers of the Luy River in the Binh Thuan province, Vietnam, where water resources are under pressure due to agricultural, aquacultural, and industrial production. Twenty-one ERT profiles were collected and revealed a much larger intrusion zone, compared to the previous study. Saltwater is present in lowland areas of the left bank over almost the whole thickness of the aquifer, while the right bank is constituted of sand dunes that are filled with freshwater. At a larger distance from the sea, a complex distribution between fresh and saltwater is observed. Our methodology could be applied to other heterogeneous aquifers in the absence of a dense monitoring network. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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18 pages, 5425 KiB

Open AccessArticle

Towards a Correlation between Long-Term Seawater Intrusion Response and Water Level Fluctuations

by Antoifi Abdoulhalik, Ashraf A. Ahmed, Abdelrahman M. Abdelgawad and G. A. Hamill

Water 2021, 13(5), 719; https://doi.org/10.3390/w13050719 - 06 Mar 2021

Cited by 4 | Viewed by 2813

Abstract

Laboratory and numerical experiments were conducted to provide a quantitative steady-state analysis of the effect of incremental variations of water level on saltwater intrusion. The purpose was to seek mathematical correlations relating both the wedge toe length and the height along the coastline to the boundary head difference. The laboratory experiments were completed in a 2D sand tank where both freshwater and seawater levels were varied. The experiments were conducted for two bead sizes having different hydraulic conductivities. The numerical model SEAWAT was used to validate the results and then to perform sensitivity analysis. The experimental results show that at steady-state conditions, the logarithmic toe length could be expressed as a linear function of the boundary head difference. The linear relationship was recorded in both advancing and receding wedge phases. The linearity of the correlation was also well demonstrated with analytical solutions. Similar relationships were also derived in the scenarios where the sea level fluctuated while the freshwater boundary head was constant. The height of the saltwater wedge along the coastline was also found to be a linear function of the boundary head difference. The sensitivity analysis shows that the regression coefficients were sensitive to the hydraulic conductivity, the dispersivity, and the saltwater density, while the porosity and the rate of boundary head change induced negligible effects. The existence of a linear relationship between the logarithmic toe length and the boundary head difference was also well evidenced in a field-scale aquifer model for all the different hydrogeological aquifer conditions tested. This study is the first attempt in identifying the underlying correlation between the boundary water level variations and the main seawater intrusion (SWI) external metrics under controlled laboratory conditions, which is of great relevance from a water resources management perspective. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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18 pages, 5753 KiB

Open AccessArticle

Industry-Driven versus Natural Groundwater Flow Regime at the Dead Sea Coastal Aquifer

by Yehuda Levy and Haim Gvirtzman

Water 2021, 13(4), 498; https://doi.org/10.3390/w13040498 - 15 Feb 2021

Cited by 1 | Viewed by 2864

Abstract

The coexistence of nature and anthropogenic development requires continuous monitoring and research to address and respond to unforeseen threatening processes that occur with time. This is particularly relevant to the groundwater flow regime in the coastal aquifer adjacent to the Dead Sea, the level of which is dropping, and the industrial evaporation ponds, whose levels are rising. The increasing hydraulic gradient between the two water bodies has produced severe leakage through the pond embankments. To prevent this leakage, a vertical deep sealing wall was built along the embankment. In this study, the overall leakage is calculated by mass balance, and the subsurface leakage component is numerically simulated, based on the mass balance and hydrological observations. Some of the leakage discharges into surface canals and some at the Dead Sea. The leakage volume increased from 20 mcm/year in the 1980s to 100 mcm/year before the sealing wall was built (in 2012), and from 60 mcm/year once the wall was established to 80 mcm/year today. Using the calibrated model, the leakage volume is predicted to increase in the next few decades, mainly through the Ye’elim alluvial fan. Further research effort is needed to come up with new preventive measures. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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23 pages, 19983 KiB

Open AccessArticle

Geophysical Assessment of Seawater Intrusion into Coastal Aquifers of Bela Plain, Pakistan

by Muhammad Hasan, Yanjun Shang, Weijun Jin, Peng Shao, Xuetao Yi and Gulraiz Akhter

Water 2020, 12(12), 3408; https://doi.org/10.3390/w12123408 - 04 Dec 2020

Cited by 12 | Viewed by 2727

Abstract

Seawater intrusion is a major challenge in many coastal areas all around the world, mainly caused by over-exploitation of freshwater resources, climate change, and sea-level rise. Consequently, seawater intrusion reaches several kilometers inland, thus making the freshwater resources polluted and unsuitable for human use. Conventionally, the fresh-saline water interface is delineated by the number of laboratory tests obtained from boreholes. However, such tests suffer from efficiency in terms of data coverage, time, and cost. Hence, this work introduces Dar-Zarrouk (D-Z) parameters, namely transverse resistance (T_r), longitudinal conductance (S_c), and longitudinal resistivity (ρ_L) computed from non-invasive vertical electrical sounding (VES). Two-dimensional (2D) imaging of D-Z parameters provides a clear distinction of fresh-saline aquifers. Such techniques remove ambiguities in the resistivity interpretation caused by overlapping of fresh and saline aquifers during the process of suppression and equivalence. This study was carried out by 45 VES along five profiles in the coastal area of Bela Plain, Pakistan. D-Z parameters delineate fresh, brackish, and saline aquifers with a wide range of values such as freshwater with T_r > 2000 Ωm², S_c < 3 mho, and ρ_L > 20 Ωm; saline water with T_r < 1000 Ωm², S_c > 25 mho, and ρ_L < 5 Ωm; and brackish water with T_r between 1000–2000 Ωm², S_c from 3 to 25 mho, and ρ_L between 5–20 Ωm. The D-Z results were validated by the physicochemical analysis using 13 water samples and local hydrogeological setting. The obtained results propose that D-Z parameters can be used as a powerful tool to demarcate the fresh-saline aquifer interface with more confidence than other traditional techniques. This geophysical approach can reduce the expensive number of borehole tests, and hence contributes to the future planning and development of freshwater resources in the coastal areas. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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21 pages, 5515 KiB

Open AccessArticle

Optimizing Laboratory Investigations of Saline Intrusion by Incorporating Machine Learning Techniques

by Georgios Etsias, Gerard A. Hamill, Eric M. Benner, Jesús F. Águila, Mark C. McDonnell, Raymond Flynn and Ashraf A. Ahmed

Water 2020, 12(11), 2996; https://doi.org/10.3390/w12112996 - 26 Oct 2020

Cited by 9 | Viewed by 2925

Abstract

Deriving saltwater concentrations from the light intensity values of dyed saline solutions is a long-established image processing practice in laboratory scale investigations of saline intrusion. The current paper presents a novel methodology that employs the predictive ability of machine learning algorithms in order to determine saltwater concentration fields. The proposed approach consists of three distinct parts, image pre-processing, porous medium classification (glass bead structure recognition) and saltwater field generation (regression). It minimizes the need for aquifer-specific calibrations, significantly shortening the experimental procedure by up to 50% of the time required. A series of typical saline intrusion experiments were conducted in homogeneous and heterogeneous aquifers, consisting of glass beads of varying sizes, to recreate the necessary laboratory data. An innovative method of distinguishing and filtering out the common experimental error introduced by both backlighting and the optical irregularities of the glass bead medium was formulated. This enabled the acquisition of quality predictions by classical, easy-to-use machine learning techniques, such as feedforward Artificial Neural Networks, using a limited amount of training data, proving the applicability of the procedure. The new process was benchmarked against a traditional regression algorithm. A series of variables were utilized to quantify the variance between the results generated by the two procedures. No compromise was found to the quality of the derived concentration fields and it was established that the proposed image processing technique is robust when applied to homogeneous and heterogeneous domains alike, outperforming the classical approach in all test cases. Moreover, the method minimized the impact of experimental errors introduced by small movements of the camera and the presence air bubbles trapped in the porous medium. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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14 pages, 2585 KiB

Open AccessEditor’s ChoiceArticle

Evaluation of the Groundwater Quality Using the Water Quality Index and Geostatistical Analysis in the Dier al-Balah Governorate, Gaza Strip, Palestine

by Moustafa El Baba, Prabin Kayastha, Marijke Huysmans and Florimond De Smedt

Water 2020, 12(1), 262; https://doi.org/10.3390/w12010262 - 16 Jan 2020

Cited by 71 | Viewed by 7891

Abstract

Groundwater contamination is a major problem in the Gaza Strip. In this study we investigate the groundwater quality in the Dier al-Balah Governorate. Water samples were collected from 19 municipal wells in April 2009 and April 2014 and analyzed for physio-chemical parameters (pH, TDS, Ca²⁺, Mg²⁺, Na⁺, K⁺, Cl⁻, SO₄^2–, HCO₃⁻ and NO₃⁻). The aim of the research is to determine the groundwater quality and to produce groundwater quality maps using the water quality index (WQI) method and geostatistical analysis. The results show that all water samples are very saline due to the intrusion of Mediterranean seawater in the coastal aquifer. Differences in chemical composition between 2009 and 2014 indicate that about 1% more seawater was mixed with the groundwater in this period. The majority of the observed chemical parameters of all wells are well above the WHO water quality standards and all WQI values indicate that the water quality is problematic. The spatial variation of the WQI scores is modelled by a deterministic component expressing a linear dependence on the distance to the coastline and a stochastic residual described by an exponential variogram with a practical range of 3000 m. The mapping of the WQI scores and derived water quality classes is achieved through regression-kriging. The results indicate that the groundwater in a large area along the coastline is unsuitable for human consumption and comparison of the maps of 2009 and 2014 shows that this region further expanded by about 700 m inland in a period of 5 years. The results of this study are worrying, but they also contribute to a better understanding of the factors that determine the groundwater quality and can help authorities and stakeholders with sustainable development. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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Review

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27 pages, 2563 KiB

Open AccessEditor’s ChoiceReview

Review of Seawater Intrusion in Western Coastal Regions of South Korea

by Sung-Wook Jeen, Jiyoung Kang, Hyejung Jung and Jeonghoon Lee

Water 2021, 13(6), 761; https://doi.org/10.3390/w13060761 - 11 Mar 2021

Cited by 25 | Viewed by 4145

Abstract

Groundwater salinization in coastal aquifers because of seawater intrusion has raised serious concerns worldwide since it deteriorates the quality of drinking water and thereby threatens sustainable economic development. In particular, this problem has been a cause of growing concern in the western coastal regions of South Korea. In this paper, we review studies of seawater intrusion in western coastal regions of South Korea conducted over the past 20 years, particularly focusing on studies reported in international journals. We summarize the study locations, methods used, and major findings from individual and regional-scale studies. General methods used to identify and interpret seawater intrusion and subsequent geochemical processes are also presented. On the basis of insights gleaned from the previous studies, future research needs are discussed. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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Other

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19 pages, 2639 KiB

Open AccessConcept Paper

Application of the Kilimanjaro Concept in Reversing Seawater Intrusion and Securing Water Supply in Zanzibar, Tanzania

by Zuleikha Pembe-Ali, Tulinave Burton Mwamila, Mesia Lufingo, Willis Gwenzi, Janeth Marwa, Mwemezi J. Rwiza, Innocent Lugodisha, Qinwen Qi and Chicgoua Noubactep

Water 2021, 13(15), 2085; https://doi.org/10.3390/w13152085 - 30 Jul 2021

Cited by 6 | Viewed by 3131

Abstract

There is escalating salinity levels on small islands due to uncontrolled groundwater extraction. Conventionally, this challenge is addressed by adopting optimal groundwater pumping strategies. Currently, on Unguja Island (Zanzibar), urban freshwater is supplied by desalination, which is expensive and energy-intensive. Hence, desalinization cannot be afforded by rural communities. This study demonstrates that the innovative Kilimanjaro Concept (KC), based on rainwater harvesting (RWH) can remediate seawater intrusion in Unguja, while enabling a universal safe drinking water supply. The reasoning is rooted in the water balance of the whole island. It is shown that if rainwater is systematically harvested, quantitatively stored, and partly infiltrated, seawater intrusion will be reversed, and a universal safe drinking water supply will be secured. Water treatment with affordable technologies (e.g., filtration and adsorption) is suggested. The universality of KC and its suitability for small islands is demonstrated. Future research should focus on pilot testing of this concept on Unguja Island and other island nations. Full article

(This article belongs to the Special Issue Seawater Intrusion into Coastal Aquifers)

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