The geological framework of Thrace, from the Bosphorus in the east to the Ergene River Basin in the west, is characterised by a Paleozoic basement of granite igneous with an extensive range of metamorphic rocks (gneiss, schist, metaquarzite, marble) up to the highest elevation (1031m) of the Istranja massif in the west. It is in this western zone in the vicinity of Vize where the best spring sources can be found today as in antiquity, when they were exploited extensively by the Byzantines to supply water to the city of Constantinople (Forchheimer and Strzygowski 1893).

Although much of the ground-water resource is related to the unconsolidated aquifers of the Ergene basin, the major spring discharge (associated with highest hydraulic potential of the region) derives from Eocene karst reservoirs. These are located mostly along the southern flank of the Istranja massif bordering the Ergene Plain, where the metamorphic basement outcrops. Similar conditions, at lower altitudes, can be observed towards the south-east in the Paleozoic basement in the Çatalca area between Terkos Lake (Black Sea) and Büyük Çekmece (Sea of Marmara). The igneous-metamorphic rocks of the Paleozoic basement generally have a low permeability and as a result make a limited contribution to the potential of the ground-water resources of the region.

On the basis of the geological and hydrogeological setting of Thrace, it is possible to hypothesise that the scarcity of springs with significant discharge during the dry season could explain the necessity to extend the catchment to the perennial sources of higher discharge in the distant area of Vize.

For a more detailed account of the hydrogeology of Thrace and a comparison with that of Rome, see Crow, Bono and Bayliss 2001.