Hydrogeological investigation of the subsidence mechanism in Kozakli (Nevsehir) geothermal field

Kozaklı geothermal field (KGF) has the most important geothermal energy potential in central Anatolia. The primary aquifer of the Kozaklı geothermal waters (KGW) is the Premesozoic aged marbles and the secondary aquifer is Tertiary aged limestones. In KJA, geothermal waters (KGW) (45-96°C) are utilized for the heating of a total of 2900 residential houses, greenhouse and thermal tourism. A subsidence of ~ 10 m deep at ~ 40 m diameter occurred suddenly at the intersection of NW-SE and NW-SW faults in KGF on January 14, 2007. The purpose of this study is to examine the mechanism and development of this subsidence in the KJA. For this reason, geophysical, geotechnical, hydrological, hydrogeological, hydrogeochemistrical and isotopical methods have been applied to subsidence field (SF). Geophysical measurements in the subsidence field indicate that the upper part of the subsidence consists of fractured-cracked travertines and continues to the thermal hotels in the NW-SE direction. The cover unit (10-15 m) consists of silty gravel and low-plasticity silt overlying travertine with paleokarstic structure at the subsidence field. The hydrological data of KGF and its vicinity denote as high as sometimes 90% of the evapotranspiration at the basin. This suggests that the activity of hypogenic karstification is higher than the surface karstification in the carbonate rocks in the KGF. That is, the recharge of KGW which has Na-Ca-Cl-SO4/HCO3 water-type and over 35,000 years apparent age by 14C isotopes, extends to Holocene period. The mixing of deep and slow circulating KGS with cold waters during the ascent from deep to the surface has increased their corrosive feature (mixing corrosion). These corrosive waters led the progress of karstification in the KGF. In addition, the subsidence occurred due to over exploitation of the geothermal water (23,000 m3/day), lack of sufficient reinjection wells for the geothermal wastewater, and triggering of landscaping in the Kozaklı manucipal bath park area.