*3.3. Hydrogeological-Hydrochemical Setting*

– The history of pumping started in 1900. Since then, brackish water has been found in the upper aquifers [45]. Some wells located west of the plain were connected by galleries, resulting in expanding the contamination of fresh water with brackish water. In order to study the origin of the saline water, the Institute for Geological and Underground Research monitored the tidal change impacts on the Thriassion aquifers [46]. Five automatic recording devices were placed more than a year at the study area. One device was placed in the Eleusis Gulf to monitor tidal changes. One device was placed at the Holocene sediments 1 km from the shoreline, while two of them were placed at the Pleistocene deposits 2 and 3 km far from the shore; the last one was placed in the Triassic carbonate 3.5 km from the shore north of Eleusis City. No influence of tidal changes was recorded on the Pleistocene-Holocene aquifers; instead, influence was observed in the borehole drilled in the Triassic aquifer. Groundwater response was 2 cm after a time lag (tlag) between the high tide and the peak of the groundwater level, tlag = 36 h. Tidal period was 24 h, and the tidal amplitude was 8 cm. The hydrogeological conceptual model that was proposed so far [30,46–48] suggested the existence of two unconfined aquifers in the plain: (i) the upper one, which occurs in the Pleistocene-Holocene deposits, and (ii) the lower one in the Mesozoic carbonate rocks, which both discharge at the shoreline of Aspropyrgos and Eleusis Cities.

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gradient 1‰. Transmissivity (T) in the Cretaceous aquife

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A mathematical model, based on the previous conceptual model, failed to describe the hydrogeological regime of the plain.

Saline water in the plain aquifers is limited to a zone of 2 to 3 km in width from the shoreline. The Na/Cl molar ratio is between 0.73–0.87, and the chlorides are between 12–205 mmol/L. In the western, eastern, and the northern parts of the study area where carbonate outcrops, saline groundwater was found 8 to 9 km inland, with total dissolved solids (TDS) between 1500–6000 mg/L and the chlorides between 15–90 mmol/L [29–33,46–49]. Groundwater flows SW with hydraulic gradient 1‰. Transmissivity (T) in the Cretaceous aquifer is around 5000 m<sup>2</sup> /d (0.5787 m<sup>2</sup> /s) [48]. During the last five decades, due to the heavy pumping in the Plio-Pleistocene aquifers, at the central part of the basin, the dynamic level was lowered below sea level; however, the static level fluctuated between 5 m below sea level and 3 m above sea level, and groundwater of good quality (TDS values between 400–1200 mg/L and chlorides between 1–9 mmol/L) was preserved for a long time, despite the overpumping conditions [40,46–50].

Studies on the Eleusis Gulf seawater [51,52] presented data on its temperature, salinity, dissolved oxygen (DO), and inorganic nutrients. Both the studies were in accordance with each other. It was also reported [51,52] that the temperatures of the water at 1, 10, 20, and 30 m in depth remain stable at 12.5 ◦C in January and fluctuate between 12.5 and 17 ◦C in May. Salinity fluctuates between 38.3 and 38.5‰ in January and 38.2–38.5‰ in May [51].
