*5.4. Conceptual Model*

– – Stratigraphic and tectonic factors and Pleistocene sea level fluctuations strongly influenced the hydrogeological regime of the study area. The existence of Pleistocene clays and Pliocene marls has protected, in many places, the Pleistocene and part of the Triassic carbonate aquifers against seawater intrusion. A complex groundwater salinity distribution is encountered, the origin of which is mainly related to the geological history of the area that includes transgressions and regressions of the sea during Pleistocene. The deposition of thick layers of reddish clay in alternation with coarse materials during Pleistocene was crucial [59]. As a sequence, a multi-layered aquifer system was established where the higher-standing aquifer was exposed to seawater intrusion and the lower ones were protected against this phenomenon. During Upper Pleistocene (18 Ka BP), the subjacent marine strata was freshened up to depths of 100–120 m. The rapid increase of the sea level [41–43] resulted in seawater invading through the lowlands and the buried river valleys, as well as through fractured carbonate formations [60]. In this way, brackish water is encountered 8 to 9 km inland from the present shoreline through Mesozoic carbonates [29,37,49]. After Holocene clay deposition, seawater was entrapped due to the very low permeability of the clay, and the direct seawater intrusion stopped (Figure 9). Moreover, groundwater discharges on the ground surface as an upwards leakage through the preferential flow of clay deposition. Additionally, to the depth, marls protected the overlying aquifers from seawater, which intruded through the carbonate formations underlain marls. Fresh groundwater with fair quality, with EC values between 524 and 1481 µS/cm, is found at depths between 5 and 90 mbsl in the following wells: # 61, 74, 75, 100, 138, 140, 142, 154, 155, 161 ′ , 165, 175, and 176. groundwater with fair quality, with EC values between 524 and 1481 μS/cm, is found at depths ′

**Figure 9.** Schematical hydrogeological history 12 ka BP and present day. It is very likely that palaeo-seawater was entrapped in the Pleistocene sediments due to Holocene clay deposition.

Fresh groundwater is most likely to be found offshore under the seafloor of the Mediterranean Sea (Eleusis Gulf) (Figure 10). This is an important issue that could be investigated in detail. New ways of expanding the research in groundwater resources under the Eleusis Gulf seafloor could be sought. It is feasible due to the shallow waters of the Eleusis Gulf, which is 34 m in depth at the maximum (mean depth 18 m). In this way, a new perspective on water resource management could emerge. The wide Neogene basin around Attica, Euboea, and Peloponnese at least 15,000 km<sup>2</sup> in total extent presents the same hydrogeological conditions; the same conditions could prevail in many other coastal areas of Greece and even more around the world.

**Figure 10.** Hydrogeological conceptual model of the Thriassion Plain aquifers. Potential fresh groundwater under hydraulic gradient in submarine aquifers below the Eleusis Gulf is shown.
