Geochemical evolution of the Thriassion Plain groundwaters, Attica, Greece

The geochemical evolution of groundwater in the coastal area of the Thriassion Plain, Attica, Greece, has been investigated using major and trace elements of chemical analyses. The groundwater of good quality (total dissolved solids TDS between 400 and 1100 mg/l) has been found in the Plio-Pleistocene sediments up to the depth of 90 m below sea level. Moreover, the existing contamination of groundwater in the Pleistocene-Holocene sediments possibly originates from the pumping of the old brackish water of the upper aquifers and the returns of the irrigation water as well as the possible occurrence of palaeo-seawater in the aquifers. A five-stage conceptual model is suggested for groundwater geochemical evolution from the main recharge area to the main discharge area. Limestone and dolomite dissolution in the recharge area leads to one end member of the Ca-Mg-HCO₃ type. These waters evolve to Ca-Mg-Cl-HCO₃ along their flow path possibly due to the dissolution of relics of evaporites, as well as mixing with seawater that is Na-Cl type, the other end member. Two more hydrogeochemical processes have also been identified; possibly gypsum, anhydrite, halite and fluorite dissolution; calcite, aragonite and dolomite precipitation.