Three geochemical surveys in ground waters were carried out in the Selianitika–Trapeza area, Gulf of Corinth, with the main purposes: (1) to define the geochemical patterns of the circulating ground waters; (2) to discriminate the role of active fault systems in affecting the ground water chemistry and hydrogeology, exploiting geochemical techniques tested in other areas; (3) to select sites for the continuous geochemical surveillance of the tectonic activity of the area. A total of 50 sites have been sampled, and numerous chemical and stable isotope analyses have been performed. We identify three geochemical active fault zones: (i) the Eliki–Nikolaikea area; (ii) the Selianitika–Rododafni–Nerazes area, and (iii) the Trapeza sector. The Nerazes well site and the Selianitika spa have been selected for exploiting a geochemical-hydrological monitoring surveillance of the tectonic activity.