*5.1. The Neogene-Quaternary Hydrostratigraphic Unit*

The Holocene sediments, mainly made up of clay and sandy clay five m in thickness, confine the underlying Late-Pleistocene aquifer. Locally, in palaeovalleys and/or sandy parts of the deposits, the aquifer is unconfined (wells # 38, 41, 43, 45, and 47). The depth of the wells is 3–10 m. The annual head fluctuation ranges from 0.15–0.48 m. In general, they exhibit upwards leaky conditions, which form swamps and marshes in many sites on the ground. At the Kalimbaki site, artesian wells (or once artesian) onto the ground surface, as well as manifestations of surface water, such as springs, lakes (e.g., Koumoundourou Lake), or marshes on the coastal area of Eleusis and Aspropyrgos Cities, could betoken part of groundwater discharge of either the Quaternary aquifers or the Triassic carbonate. Salinization of the water may have taken place due to (i) the dissolution of evaporite relics, (ii) evaporation of the irrigation water, and (iii) farms and domestic sewage [50]. It is also very likely that salinization took place during the Holocene transgression or/and due to the presence of connate water from the contemporary deposition of clay during the transgressions. Taking into account that (i) groundwater is not subject to hydraulic head fluctuations caused by the tides of the sea, (ii) there is an upwards leakage at many sites on the ground of the coastal area, (iii) the occurrence of Holocene clay, which probably continues below the sea bottom and prevents seawater directly intruding through the clay, and (iv) the hydraulic head is above sea level; it is considered that groundwater is not in hydraulic contact to the sea, and, finally, modern direct seawater intrusion in Holocene sediments is not possible. – –

Pleistocene sands, gravels, clay, or conglomerates are under confined conditions, as well. In this strata sequence, up to six aquifers are developed 2–10 m in thickness (Figure 5) at depths 20–140 m from the ground surface. Plio-Pleistocene marls, marly limestone, sandstone, clay, and conglomerates form a confined aquifer that reaches up to 90 m below the sea level. – –

**Figure 5.** Geological-Hydrogeological characteristics of the wells in the Thriassion Plain.

Pumping tests carried out in the wells # 54, 86, 128, 129, 131, 154, and 154P2 in the Pleistocene sediments were analyzed and evaluated for both confined and unconfined aquifers, using mainly unsteady-state flow methods [53–56] and recovery methods. Drawdown after 6–12 h of pumping was 0.86, 1.11, 5.24, 9.74, 4.66, 1.31, and 1.26 m, respectively. The Schafer equation [57], which provides an estimation of time *t*<sup>c</sup> at which the casing storage effect is negligible, was applied, as well. The analysis showed that the hydraulic characteristics of the wells were variable, depending on the sedimentation material that the aquifer was made up. Transmissivity *T* ranged from 3.5–275 m<sup>2</sup> /d (4 × 10 <sup>−</sup>5–3 ×10 <sup>−</sup><sup>3</sup> m<sup>2</sup> /s), storativity *S* ranged from 1.75 × 10 <sup>−</sup>3–8.9 × 10 −3 , and hydraulic conductivity *K* ranged from 0.4–25 m/d (4.6 × 10 <sup>−</sup>6–3 × 10 <sup>−</sup><sup>4</sup> m/s). The Pleistocene sediments are confined (dug wells 29, 31, 54, 58, 61, 74, 83, 128, 129, 131, 132, 134, 136, 140, 142, 175, and 176 and boreholes 75, 100, and 138) or semi-confined (12, 16, 20, 24, 44, 31, 54, 55, 62, 80, 86, 88, 109, 121, 165, 181, and 186), and, in some wells, unconfined aquifers are developed (dug wells 14, 27, 41, 43, 45, 47, 72, 98, 100 ′ , 102, 106, 154, and 187). The annual head fluctuation ranges between 1.43–2.40 m in the confined aquifer, 0.63–1.22 m in the semi-confined, and 0.36–0.58 m in the unconfined one. The head in the unconfined aquifer is between 1 and 34 masl and, in the confined/semi-confined system, is up to +9 masl. Figure 6 shows the hydraulic head in the Thriassion Plain aquifers in March 2014. An almost stagnant zone is developed under the coastal area, where the minor discharge occurs as an upwards leakage through the aquitards, forming wetlands or swamps on the ground surface. Groundwater discharge into the sea is negligible, as the seawater salinity and the temperature at the bottom of the Eleusis Gulf remained at 38.3‰ and 12.5 ◦C for many years [51,52]. It is very likely that the groundwater is prevented from progressing further beneath the seafloor due to the barrier of the Salamina Island and, therefore, moves very slowly eastwards. – – – −5– −3 −3– −3 – −6– −4 developed (dug wells 14, 27, 41, 43, 45, 47, 72, 98, 100′, 102, 106, 154, – – – at 38.3‰ and 12.5 °C for many years [51,52]. It is very likely that

(**a**)

**Figure 6.** *Cont*.

**Figure 6.** Potentiometric surface. (**a**) Triassic-confined and Triassic-unconfined Pleistocene aquifers and (**b**) a Holocene unconfined aquifer.

− μS/cm. Despite the increased drawdown by heavy pumping the last 50 years It was observed that confined aquifers turned into unconfined ones in the wells # 181 and 129 and 136, 62, and 165 under overpumping conditions, with a drawdown around 30 m and 20 m from the original static level, respectively. In the wells # 62, 131, 134, 136, 138, 140, and 154 located at the central part of the basin, the water level stands below the sea level during pumping without deteriorating the water quality; Cl <sup>−</sup> ranges between 1.2 and 8.1 mmol/L and EC between 810 and 1586 µS/cm. Despite the increased drawdown by heavy pumping the last 50 years, the static level is still above the sea level. Combined with the presence of good water quality, which occurs in the central part of the plain, as well as the limited contaminated with seawater coastal zone up to 2 to 3 km in width for at least 100 years, it is deduced that seawater cannot intrude inland further. It is very likely that the thick clay layers of the Pleistocene age extend under the sea bottom, and the multi-layered aquifer system occurs beneath the seafloor, as well; therefore, there is no vertical or another boundary of the multi-layered system with the seawater. Consequently, the Plio-Pleistocene aquifers are not in direct contact with the sea.

). The site altitude is 7.51 masl, and the well depth is 9.8 m (−2.29 mbsl). q' = T\*I = 200\*0.001 Trying to determine a salt-freshwater interface, in the first calculations, consider the well # 44 located 1220 m far from the shoreline tapping the Pleistocene confined aquifer with a thickness of 5 m (Figure 7). The site altitude is 7.51 masl, and the well depth is 9.8 m (−2.29 mbsl). The hydraulic gradient is 0.001, and the head is 2.5 masl. It is also assumed that T = 200 m<sup>2</sup> /d; thus, q' = T × i = 200 × 0.001 = 0.2 m<sup>2</sup> /d and k = 200/5 = 40 m/d. Then, the depth to the interface at well # 44 based on Equation (2) where no vertical flow occurs is z = (2×40×0.2×1220/40) 0.5 = 22 m. In this case, seawater could not intrude into well 44, since the depth to the interface exceeds the aquifer thickness; therefore, the seawater wedge is missing [2]. In addition, based on Equations (4) and (5), the depth to the interface at the shoreline is zo = 40×0.2/40 = 0.2 masl, and the width of the outflow face xo = 40×0.2/40 = 0.1 m. Sources of uncertainty to calculate the groundwater flux and the depth to the interface could be the hydraulic

(K, q', and i) and geometric (thickness and slope) characteristics of the aquifer. Homogeneity of the aquifer is also the source of uncertainty. hydraulic (K, q',

**Figure 7.** High salinity of groundwater in well Nr 44 could not be attributed to modern direct seawater intrusion, as the salt-freshwater interface is missing.

The hydrochemical data from such wells, as it is reported in the hydrochemical settings section, show that groundwater in the coastal area of about 2 to 3 km in width was found brackish during the drilling phase. This means that seawater intruded inland in a past geological time due to Pleistocene seawater fluctuations [17] or that it is entrapped (palaeo)seawater. Groundwater quality due to repeated irrigations with brackish water and the high evapotranspiration that is around 62% of the precipitation made the water quality worse, which led to high values of salinization.
