**1. Introduction**

The sustainable management of coastal aquifer resources requires a good understanding of the relationship between salt and fresh water. Coastal aquifers have been extensively studied for more than a century by many researchers [1–18]. The common problem in coastal aquifers, which are hydraulically connected to the sea, is seawater intrusion mainly, but not exclusively, due to overpumping. Overpumping forces the salt-fresh water interface shift landward, resulting in the contamination of the fresh groundwater with seawater. This phenomenon is depended on (i) the geological-hydrogeological and hydraulic characteristics of the aquifer, (ii) human activities, and (iii) tidal effects and coastal and sea bottom conditions [5]. The hydrostatic equilibrium (Figure 1a) between salt and fresh water is described by the Ghyben-Herzberg principle, Equation (1):

$$\mathbf{z} = \frac{\rho\_{\mathbf{f}}}{\rho\_{\mathbf{s}} - \rho\_{\mathbf{f}}} \mathbf{h}\_{\mathbf{f}} \tag{1}$$

where z = is the depth to the salt-fresh water interface below sea level, h<sup>f</sup> = is the height of the fresh water above sea level, ρ<sup>s</sup> and ρ<sup>f</sup> = salt-water and fresh-water densities, respectively, and g = acceleration due to gravity.

ρ ρ

**Figure 1.** Dupuit-Ghyben-Herzberg model flow (**a**) in unconfined and (**b**) confined coastal aquifers, and the (**c**) actual groundwater discharge onto the sea floor; z = is the depth to the salt-fresh water interface below sea level, h<sup>f</sup> = is the height of the fresh water above sea level, xo= he width of the submarine zone through fresh groundwater discharge into the sea, and zo the depth of the interface below the coastline.

– z <sup>2</sup> = 2΄ ' In ideal conditions, the Ghyben-Herzberg principle states that the depth to the salt-fresh water interface *z* beneath sea level is approximately 40 times the height *h* of the fresh water above sea level. The application of the above principle is limited to conditions in which the two liquids are static, and it is valid under the occurrence of horizontal groundwater flow. It can be also applied in unconfined and confined aquifers (Figure 1b).

Δ*ρ* Based on Glover's analytical solution [ Based on the Dupuit–Forchheimer assumption that, in coastal aquifers, the equipotential lines are vertical (horizontal flow) in combination with the Ghyben-Herzberg principle, a one-dimensional flow can be used that yields the following expression for the x and z coordinates of the interface [2].

$$\mathbf{z}^2 = \frac{2\rho\_f q' \mathbf{x}}{\Delta \rho \mathbf{K}} \tag{2}$$

where *q*' = fresh groundwater outflow at the coastline per unit width, *K* = the hydraulic conductivity, and ∆ρ = the difference of the salt- and fresh-water densities.

Based on Glover's analytical solution [12], Cheng and Quazar [16] determined the interface depth as:

$$\mathbf{z}^2 = \frac{2\rho\_f q' \mathbf{x}}{\Delta \rho \mathbf{K}} + \left(\frac{\rho\_f q'}{\Delta \rho \mathbf{K}}\right)^2 \tag{3}$$

where *K* = the hydraulic conductivity.

The width xo (Figure 1c) of the submarine zone through fresh groundwater discharge into the sea can be obtained from Equation (3) by setting z equal to 0, which provides Equation (4), and the depth of the interface below the coastline zo by setting x equal to zero, which provides Equation (5).

$$\alpha\_0 = \frac{\rho\_f q'}{2\Delta\rho\mathcal{K}}\tag{4}$$

$$z\_0 = \frac{\rho\_f q'}{\Delta \rho \mathcal{K}} \tag{5}$$

In real field conditions, this interface does not occur; instead, a brackish transition zone exists where complex diffusion and mass transport theories are developed [19,20]. The overexploitation of coastal groundwater leads to both submarine groundwater discharge reduction, as well as an increase of seawater inflow and, consequently, an increase of the transition zone thickness [21].

Coastal aquifers, which are in hydraulic contact to the sea, are subject to fluctuations in the hydraulic head due to the tides [22]. The fluctuation parallels to the rise or fall of the tides after a time lag between the high tide and the peak of the groundwater level.

On the contrary, in confined aquifers extending below the sea floor without a sea front and which are separated from seawater by thick confining layers of very low permeability, fresh groundwater can be conserved against salinization. Identical geological-hydrogeological structures exist in many places all over the world, such as The Netherlands [1]; Eastern England [5]; Spain [20]; the Bay of Bengal; Bangladesh [23]; the USA [2,24]; and Estonia, Denmark, and France [25]. Thus, the boundary between the aquifer and seawater does not exist, or it migrates far from the coast. Therefore, the Ghyben-Herzberg seawater-fresh water interface does not occur near the coast. In these aquifers, fresh groundwater occurs beneath the sea floor fed from the onshore outcrops [1,2,4,24–27]. Offshore groundwater occurrence is a global phenomenon, but its direct observations are limited. The assessment of groundwater fluxes into the sea, or the submarine aquifers often need specific techniques and procedures, as well [28]. However, at many regions, onshore hydrogeological and hydrochemical data provide strong indirect evidence for the occurrence of fresh groundwater in submarine aquifers below the sea floor [25].

In this article, the functioning of the Thriassion Plain coastal aquifers was completely revised, and an attempt to evaluate the actual mechanism controlling the groundwater flow, the origin and distribution of saline water, and the existence of fresh groundwater in the submarine environment of the Eleusis Gulf in the East Mediterranean was made

#### **2. Study Area**

The Thriassion Plain is a coastal area of about 120 km<sup>2</sup> in extent (Figure 2) and lies from latitude 38.0 and 38.2◦ N and longitude 23.1 to 23.7◦ E. It is part of three hydrological basins of 475 km<sup>2</sup> in total extent. The plain is surrounded by the Mesozoic carbonate, which drains south into the Eleusis Gulf. A semiarid climate prevails in the area. The annual precipitation is around 380 mm/y, while the actual evapotranspiration is around 62% of the precipitation [29]. The mean groundwater temperature is 20.6◦ , which ranges between 17.5 and 22.8 ◦C [29]. The mean air temperature ranges between 9.2–29.9 ◦C in January and July, respectively. The mean sea surface temperature of the Eleusis Gulf ranges between 13 and 25 ◦C, while the mean sea temperature at the bottom of the Eleusis Gulf ranges between 12 and 13 ◦C. The Eleusis Gulf is a small and almost enclosed sea north of the Saronikos Gulf in the East Mediterranean Sea, which covers an area of 67 km<sup>2</sup> , having a maximum depth of 33 m. It is surrounded by the study area to the north and the Salamis Island to the south. Its connection to the Saronikos Gulf is through two shallow channels of 8 m in depth at the western end and 12 m in depth at the eastern end, respectively. This area has been degraded environmentally due to uncontrolled agricultural and industrial development during recent decades [30–33].

**Figure 2.** Study area.
