Seawater Intrusion into Coastal Aquifers

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.

El Hamidi et al. [1] numerically simulate the effects of climate changes, over pumping, and sea-level rise on seawater intrusion in northern Morocco. They predict that a significant part of the aquifer will be salinized by 2040. Surface water, desalination plants, recycled water, and artificial aquifer recharge are suggested to improve the predicted results.

Characterization of density and salinity is crucial for understanding physical and chemical processes in aquatic systems. Traditionally, these parameters are measured indirectly in the field by an electrical conductivity sensor. However, in hypersaline brines, the salinity–conductivity relationship is not monotonic, and continuous monitoring was never conducted in the field. Based on vertical pressure differences, Mor et al. [2] developed a densitometer for monitoring density in hypersaline bodies. The densitometer was used in the Dead Sea coastal aquifer and an offshore buoy.

The width of the freshwater–saline water interface has been attributed in the past to tide fluctuations and aquifer stratification. Using a physical model, Ben-Zur et al. [3] show that haline convection and fingering created in a layered aquifer significantly magnified the width of the freshwater–saline water interface.

Cong-Thi et al. [4] use electrical resistivity tomography (ERT) to provide high-resolution information on salinity distribution in a river–coastal system in Vietnam. They show that the zone affected by seawater intrusion is much larger than previously expected based on shallow boreholes. Recent seawater intrusions into the river and uncontrolled exploitation of the aquifer are responsible for aquifer salinization.

Abdoulhalik et al. [5] study the relationship between toe length and height along the coastline using laboratory experiments and numerical models. They show that the logarithmic toe length at steady-state conditions could be expressed as a linear function of the head difference at the boundary. This relationship is important for the assessment of aquifer salinization with a rising sea level.

Anthropogenic effects on coastal aquifers are studied by Levy and Gvirtzman [6] at a location at the Dead Sea where the evaporation pond level is much higher than Dead Sea level. As a result, water leakage from the evaporation ponds to the Dead Sea is calculated to be tens of mcm/year, which further disturbs the natural flow.

Hasan et al. [7] perform vertical electrical sounding combined with geochemical and hydrogeological analysis to evaluate the characteristics of seawater intrusion in Pakistan. They show an intrusion in the order of tens of km landwards.

Etsias et al. [8] develop machine-learning techniques for automated image analysis of experimental procedures in sandbox investigations. They show that the experimental procedure is shortened by up to 50% of the time required without compromising the quality of the results.

Groundwater quality in the coastal aquifer of the Gaza Strip is evaluated by El Bab et al. [9]. Their research shows that water quality depends on the distance to the coastline and that the groundwater in a large area along the coastline is unsuitable for human consumption.

Pembe_Ali et al. [10] show that harvesting, storing, and partly infiltrating rainwater can be established to control runoff and improve groundwater recharge in Unguja Island, one of the Islands of Zanzibar, Tanzania. They claim that it can complement or even completely replace water. They also provide a novel framework for mitigating seawater intrusion and securing water supply for small islands.

Jeen et al. [11] review seawater intrusion studies in the western coastal aquifers of South Korea conducted over the past 20 years. They show that groundwater geochemistry is largely affected by mixing with seawater, cation exchange processes during seawater intrusion, artificial contamination, water–rock interactions, and redox processes. They suggest that more modeling, laboratory experiments, isotope sampling, and microbial communities monitoring should be conducted in these coastal aquifers.

This Special Issue highlights the fact that seawater intrusion has become the major constraint imposed on groundwater utilization. Climate changes may lead to groundwater overexploitation, increased desalination projects, and sea-level changes that could significantly affect coastal aquifers’ sustainability and seawater intrusions. Assessing these processes remains a major challenge, and future research is needed to further study the dynamics of coastal aquifers’ hydrology.