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Coastal Aquifers: Seawater/Saltwater Intrusion

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A special issue of Water (ISSN 2073-4441). This special issue belongs to the section "Hydrogeology".

Deadline for manuscript submissions: closed (30 November 2022) | Viewed by 19568

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Prof. Dr. Longcang Shu

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Guest Editor

College of Hydrology and Water Resource, Hohai University, Nanjing, China
Interests: groundwater resources evaluation and management; groundwater reservoir; seawater intrusion

Prof. Dr. Fulin Li

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Guest Editor

Water Resources Research Institute of Shandong Province, Jinan, China
Interests: water resource; water environment; seawater intrusion monitoring and simulaition; hydrogeology; spring protection

Special Issue Information

Dear Colleagues,

Social and economic development have had an impact on the earth’s systems, and external factors such as climate change, the rise of sea level, and the overexploitation of groundwater have destroyed the dynamic balance between the saltwater/seawater and fresh groundwater in coastal areas, further aggravating the degree of seawater intrusion. This has greatly restricted the social and economic development of coastal areas.

This Special Issue is dedicated to “Coastal Aquifers: Seawater/Saltwater Intrusion” and seeks to capture the most up-to-date research and practices. We therefore call for original papers from researchers, practitioners, regulators, and decision-makers presenting their findings on seawater/saltwater intrusion from different perspectives.

The topics of interest but are not limited to: seawater/saltwater intrusion; the development and utilization of freshwater lens; the response of the saltwater–freshwater interface to rising sea levels; induced saltwater upconing in coastal aquifers by pumping; modeling aquifer storage and recovery systems in coastal zones; and modeling new managed aquifer recharge schemes in coastal zones.

Full research articles, reviews, as well as shorter commentaries/communications from practitioners are welcome.

Prof. Dr. Longcang Shu
Prof. Dr. Fulin Li
Guest Editors

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Keywords

saltwater intrusion
freshwater lens
saltwater–freshwater interface
saltwater upconing
modeling of coastal aquifers
management of coastal aquifers

Published Papers (9 papers)

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Research

20 pages, 3993 KiB

Open AccessArticle

Evaluation of Potential Seawater Intrusion in the Coastal Aquifers System of Benin and Effect of Countermeasures Considering Future Sea Level Rise

by Amos Agossou, Jeong-Seok Yang and Jae-Boem Lee

Water 2022, 14(24), 4001; https://0-doi-org.brum.beds.ac.uk/10.3390/w14244001 - 08 Dec 2022

Cited by 1 | Viewed by 1778

Abstract

In the present study, a three-dimensional SEAWAT model was developed to generally simulate the impact of climate change and anthropogenic activities on seawater intrusion (SWI) in the coastal region of Benin by the end of 2050. The model was calibrated and validated from 2015 to 2020, considering groundwater head and salt concentration measured in 30 wells. After calibration, a sensitivity analysis was performed with the model parameters (hydraulic conductivity, recharge, storage coefficient and boundary conditions). For the calibration, model computed and observed values displayed good correlation, approximatively 0.82 with a root mean square error (RMSE) of 0.97 m and 13.38 mg/L for groundwater head and salt concentration, respectively. The simulation results indicate that freshwater head had declined by 1.65 m from 2015 to 2020 (taking reference from the average groundwater head in 2015: 27.08 m), while the seawater intrusion area increased in the same period by an average of 1.92 km² (taking reference from the seawater intrusion area in 2015: 20.03 km²). The model is therefore used to predict groundwater level decline and seawater intrusion area increase by the end of 2050, considering the predicted sea level rise (SLR) and estimated groundwater pumping rate. Furthermore, the interface fresh groundwater–saltwater change was studied using the SHARP interface developed by USGS in 1990. The interface variation was found to be influenced by the distance from shoreline, sea level, groundwater level and geological formation hydraulic conductivity. Finally, the 3D model was used to simulate the effect of a managed aquifer recharge system on reducing SWI rate in the study region. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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19 pages, 3434 KiB

Open AccessArticle

Study on the Sources of Salinity of Groundwater in Holocene and Late Pleistocene Sediments Based on Hydrochemical and Isotopic Methods in Southern Laizhou Bay

by Yawen Chang, Xuequn Chen, Qinghua Guan, Chanjuan Tian, Dan Liu and Dandan Xu

Water 2022, 14(17), 2761; https://0-doi-org.brum.beds.ac.uk/10.3390/w14172761 - 05 Sep 2022

Cited by 4 | Viewed by 1599

Abstract

A survey of the hydrochemistry and isotopes of the Quaternary aquifer on the southern coast of Laizhou Bay provides new insights into the hydrodynamic and geochemical relationships between freshwater, seawater, and brine at different depths in coastal sediments. This study used a combination of groundwater level analysis, hydrochemistry, and isotopic methods to study the chemical characteristics of groundwater and the origin of groundwater recharge and salinity. Because the sedimentary structure of the area and the formation background of saltwater were important factors controlling the distribution of groundwater, we analyzed the distribution of groundwater in Holocene and Late Pleistocene sediments. The variation of groundwater levels in the Holocene and Late Pleistocene sediments in the saline–freshwater transition zone over time showed that the Holocene and Late Pleistocene groundwater flow directions differed in the saltwater–freshwater transition zone. From south to north in the study area, the hydrochemical types of groundwater in the Holocene and Late Pleistocene sediments were as follows: HCO₃-Ca (freshwater), SO₄-Mg and HCO₃-Ca (brackish water), Cl-Na·Mg (saltwater), and Cl-Na (brine). The results of the hydrochemical and isotopic studies indicated that the saltwater in the Holocene and Late Pleistocene sediments and the brine in the Late Pleistocene sediments were the result of evaporation. The salinity of freshwater in the Holocene sediments was produced by rock weathering, while the salinity of freshwater in the Late Pleistocene sediments was not only derived from rock weathering, but was also affected by evaporation and precipitation. The salinity of brackish water in the Holocene and Late Pleistocene sediments was derived from evaporation and precipitation. Ultimately, the origin of groundwater recharge in the Holocene and Late Pleistocene sediments was atmospheric precipitation. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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13 pages, 1971 KiB

Open AccessArticle

Role of Reef-Flat Plate on the Hydrogeology of an Atoll Island: Example of Rangiroa

by Jean-Christophe Maréchal, Vivien Hakoun and Pauline Corbier

Water 2022, 14(17), 2695; https://0-doi-org.brum.beds.ac.uk/10.3390/w14172695 - 30 Aug 2022

Cited by 2 | Viewed by 1438

Abstract

On atoll islands, the fresh water lens (FWL) constitutes, for the island population, a very important fresh water resource for various usages. Its shape and thickness highly depend on the underground characteristics of several rock formations which constitute the underground of the island: upper Holocene sediments and lower Pleistocene limestone rocks separated by the Holocene Pleistocene Unconformity. In this study, several very simple investigation methods were applied on the Pacific atoll of Rangiroa in order to characterize the aquifer hydrodynamics and their impact on the FWL. A water budget of the aquifer was proposed, including the deep infiltration to the FWL. Pumping tests and tidal analysis demonstrated the confined character of the aquifer and its main hydrodynamic properties (storage and hydraulic conductivity). The role of the reef-flat plate on the hydrogeology of the atoll was highlighted. Its impermeable nature contributes to reduce the deep infiltration to the aquifer. It also contributes to the confined flow regime of the aquifer, inducing high and fast water level fluctuations due to tide forces, and consequently contributes to increase the thickness of the saline mixing zone. Both phenomena contribute to reduce the thickness of the FWL, which is only 2 m-thick in that atoll. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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17 pages, 44702 KiB

Open AccessArticle

An Integrated Approach for Investigating the Salinity Evolution in a Mediterranean Coastal Karst Aquifer

by Eleonora Frollini, Daniele Parrone, Stefano Ghergo, Rita Masciale, Giuseppe Passarella, Maddalena Pennisi, Matteo Salvadori and Elisabetta Preziosi

Water 2022, 14(11), 1725; https://0-doi-org.brum.beds.ac.uk/10.3390/w14111725 - 27 May 2022

Cited by 9 | Viewed by 2228

Abstract

Coastal areas are characterized by considerable demographic pressure that generally leads to groundwater overexploitation. In the Mediterranean region, this situation is exacerbated by a recharge reduction enhanced by climate change. The consequence is water table drawdown that alters the freshwater/seawater interface facilitating seawater intrusion. However, the groundwater salinity may also be affected by other natural/anthropogenic sources. In this paper, water quality data gathered at 47 private and public wells in a coastal karst aquifer in Apulia (southern Italy), were interpreted by applying disparate methods to reveal the different sources of groundwater salinity. Chemical characterization, multivariate statistical analysis, and mixing calculations supplied the groundwater salinization degree. Characteristic ion ratios, strontium isotope (⁸⁷Sr/⁸⁶Sr), and pure mixing modelling identified the current seawater intrusion as a main salinity source, also highlighting the contribution of water–rock interaction to groundwater composition and excluding influence from Cretaceous paleo-seawater. Only the combined approach of all the methodologies allowed a clear identification of the main sources of salinization, excluding other less probable ones (e.g., paleo-seawater). The proposed approach enables effective investigation of processes governing salinity changes in coastal aquifers, to support more informed management. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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11 pages, 2089 KiB

Open AccessArticle

Using Freshwater Heads to Analyze Flow Directions in Saline Aquifers of the Pingtung Plain, Taiwan

by Yung-Chia Chiu, Chun-Hung Chen, Yun-Ta Cheng and Wenfu Chen

Water 2021, 13(24), 3491; https://0-doi-org.brum.beds.ac.uk/10.3390/w13243491 - 07 Dec 2021

Cited by 1 | Viewed by 2240

Abstract

The hydraulic head is the most important parameter for the study of groundwater. However, a head measured from observation wells containing groundwater of variable density should be corrected to a reference density (e.g., a freshwater head). Some previous case studies have used unknown density hydraulic heads for calibrating flow models. Errors arising from the use of observed hydraulic head data of unknown density are, therefore, likely one of the most overlooked issues in flow simulations of seawater intrusion. Here, we present a case study that uses the freshwater head, instead of the observed hydraulic head, to analyze the flow paths of saline groundwater in the coastal region of the Pingtung Plain, Taiwan. Out of a total of 134 observation wells within the Pingtung Plain, 19 wells have been determined to be saline, with Electric Conductivity (EC) values higher than 1500 μS/cm during 2012. The misuse of observed hydraulic heads causes misinterpretation of the flow direction of saline groundwater. For such saline aquifers, the determination of a freshwater head requires density information obtained from an observation well. Instead of the purging and sampling method, we recommend EC logging using a month interval. Our research indicates that EC values within an observation well within saline aquifers vary not only vertically but also by season. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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17 pages, 8302 KiB

Open AccessArticle

On the Understanding of the Hydrodynamics and the Causes of Saltwater Intrusion on Lagoon Tidal Springs

by Roger Pacheco-Castro, Paulo Salles, Cesar Canul-Macario and Alejandro Paladio-Hernandez

Water 2021, 13(23), 3431; https://0-doi-org.brum.beds.ac.uk/10.3390/w13233431 - 03 Dec 2021

Cited by 5 | Viewed by 2230

Abstract

Springs are common features on the Yucatán coast. They can discharge either under the sea (submarine) or inland in coastal lagoons and wetlands. Previous observations of a coastal lagoon located on the northern Yucatán Peninsula (La Carbonera) reported sea water intrusion on a spring that discharge on a coastal lagoon (lagoon tidal spring). The saltwater intrusion occurs when the tide is at its lower level, which is the opposite to what has been reported for submarine springs in the Yucatán Peninsula. In this study, the hydrodynamics of the spring is analyzed and the driving forces controlling the seawater intrusion are identified and discussed. Time series of water levels, salinity, and velocity measurements in the lagoon, the aquifer, and the spring are analyzed by means of tide component decomposition and cross-correlations analysis of the tide signals. Results show that the main driving forces causing the intrusion are the density differences and pressure head gradients, and the mechanisms influencing the driving forces driving those differences are the tides, the friction in the lagoon, and the confinement of the aquifer; other mechanisms are discussed to present a complete idea of the complexity of the interactions between the coastal aquifer, the coastal lagoons, and the sea. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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31 pages, 14943 KiB

Open AccessArticle

Laboratory and Numerical Study of Saltwater Upconing in Fractured Coastal Aquifers

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

Water 2021, 13(23), 3331; https://0-doi-org.brum.beds.ac.uk/10.3390/w13233331 - 24 Nov 2021

Cited by 7 | Viewed by 2102

Abstract

This study investigated the saltwater upconing mechanism in fractured coastal aquifers. Head-induced saline intrusion was initiated into three narrow sandbox aquifers containing individual horizontal discontinuities placed on different positions. Subsequently, using a peristaltic pump, freshwater was abstracted from the aquifers’ center, triggering saltwater upconing. Progressively larger pumping rates were applied until critical conditions, resulting in the wells’ salinization, were achieved. Advanced image analysis algorithms were utilized to recreate the saltwater concentration fields and quantify the extent of the saline wedges with a high accuracy. A numerical model was successfully employed to simulate the laboratory results and conduct a comprehensive sensitivity analysis, further expanding the findings of this investigation. The impact of the fractures’ length, permeability and position on the upconing mechanism was identified. It was established that the presence of high permeability discontinuities significantly affected aquifer hydrodynamics. The conclusions of this study could constitute a contribution towards the successful management of real-world fractured coastal aquifers. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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17 pages, 4218 KiB

Open AccessArticle

Impact of Island Urbanization on Freshwater Lenses: A Case Study on a Small Coral Island

by Zihan Ling, Longcang Shu, Ying Sun, Ran Wang and Yuxi Li

Water 2021, 13(22), 3272; https://0-doi-org.brum.beds.ac.uk/10.3390/w13223272 - 18 Nov 2021

Cited by 5 | Viewed by 2557

Abstract

Freshwater resources on small coral islands mainly exist in the form of freshwater lenses. The freshwater lens is highly vulnerable to salinization due to natural recharge variations and urbanization construction. However, it is unclear how a freshwater lens evolves under the influence of urbanization construction and which factors control its evolution. Based on the hydrogeological data of a small coral island in China, a corresponding 3D numerical model was established by the Visual MODFLOW software to investigate the formation and evolution of freshwater lenses under natural conditions. Thereby, the island reclamation scenario and impermeable surface scenario were set up and the changes in morphology and volume of the freshwater lens were analyzed. The results show the following: (1) After island reclamation and island building, the freshwater lens would reach a stable state after 25 years and the freshwater lens would also appear in the newly added part of the island with a thickness of 9.5 m, while the volume of the total freshwater lens would increase to 1.22 times that of the original island. (2) When the impermeable surface is built at different positions of the island, the reduction in the volume of the freshwater lens, in the order from large to small, is Scenario B (northeast side), Scenario A (southwest side) and Scenario C (central); with the increase in the impermeable surface area, the volume of the freshwater lens would gradually decrease and the volume of the freshwater lens would decrease by more than 50% with the impermeable surface exceeding 30% of the island area. The study has important implications for the conservation and rational development of subsurface freshwater resources on islands. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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15 pages, 2125 KiB

Open AccessArticle

Pumping Well Layout Scheme Design and Sensitivity Analysis of Total Critical Pumping Rates in Coral Island Based on Numerical Model

by Ran Wang, Longcang Shu, Yuxi Li and Portia Annabelle Opoku

Water 2021, 13(22), 3215; https://0-doi-org.brum.beds.ac.uk/10.3390/w13223215 - 12 Nov 2021

Cited by 2 | Viewed by 2207

Abstract

Groundwater on small coral islands exists in the form of freshwater lenses that serve as an important water resource for local inhabitants and ecosystems. These lenses are vulnerable to salinization due to groundwater abstraction and precipitation variation. Determination of the sustainable yield from freshwater lenses is challenging because the uncertainties of recharge and hydrogeological characteristics make it difficult to predict the lens response to long-term pumping. In this study, nine pumping well layout schemes along a line are designed using the orthogonal experimental design method, and an optimal well layout scheme is determined by multi-index range analysis and comprehensive balance analysis method. The total critical pumping rates of the freshwater lens corresponding to different schemes are calculated by numerical simulation, and the sensitivity of the total critical pumping rates to hydrogeological parameters is analyzed. The results show that the calculation of the total critical pumping rates needs to be combined with the specific well layout scheme with consideration to the length of well screens, the number of wells and the distance between wells. The difference in total critical pumping rates between different schemes can be up to three times. The uncertainty of hydrogeological parameters has a great impact on the total critical pumping rates. Within the range of a 30% reduction in parameters, α and K are the key risk factors of pumping; within the range of a 30% increase in parameters, α, n_e and K are the key risk factors; α-n_e combined changes had the greatest impact. The management of freshwater lenses and the assessment of sustainable yield will continue to be important tasks for coral islands in the future, and this study can help with the sustainable exploitation of island freshwater lenses. Full article

(This article belongs to the Special Issue Coastal Aquifers: Seawater/Saltwater Intrusion)

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