Hydrogeochemistry of Efteni Geothermal Area, Duzce- NW Turkey

Abstract

Efteni geothermal area is located on the western branch of North Anatolian Fault Zone (NAFZ). The last earthquakes on 17 August (Mw= 7.4) 1999 and 12 November 1999 (Mw =7.2), caused significant changes in geothermal manifestations especially in the north-western Turkey. A joint project has been started in 2003 by HU (Hacettepe University) -TUBITAK (The Scientific and Technical Research Council of Turkey). An observation network has been established by TUBITAK for monitoring possible parameters (T, EC, pH, Rn) to be used as precursors of earthquakes and one of the stations located on Efteni thermal spring, in the study area.
The basement rock is Precambrian metagranite and main lithologic units are lava and volcano sedimentary rocks (Eocene), clay and sandstone alternations (Pliocene) and alluvium (Quaternary). The geothermal reservoirs are consisted of joints and fracture zones in different lithologic units in the area.
Temperatures of the thermal springs range between 22-43?C, pH and EC values are in the range 6.0 - 10.0 and 300- 7000 ?S/cm, respectively. Ion characteristics of the geothermal springs generally are Na>Mg>Ca and HCO3>Cl >SO4. The estimated reservoir temperature by silica and cation geothermometers is in the range 50-110 ?C. Isotopic and chemical evaluations show that Efteni and Derdin thermal springs are associated with regional groundwater flow systems whereas Degirmen, Uyuz, and Yanangol springs are characterized by intermediate groundwater flow system in the area.