Hydrogeochemical and isotopic monitoring of the Kestanbol geothermal field (Northwestern Turkey) and its relationship with seismic activity

Abstract

Kestanbol geothermal field is located in northwestern Turkey and is one of the highest temperature geothermal fields in the Biga Peninsula. In this study, one geothermal well, two geothermal springs, and two cold springs were monitored for one year in Kestanbol geothermal field to determine hydrogeochemical and isotopic characteristics. Additionally, any possible relationship between seismic activity and variations in the hydrochemistry of geothermal water was investigated. The Kestanbol geothermal field is controlled mainly by the right-lateral strike-slip Kaplıca fault with normal components. The distribution of the geothermal waters is roughly parallel to the fault. The temperature, electrical conductivity, salinity, and pH value of the geothermal waters were within the range of 59.5 to 74.6 °C, 30300 to 35700 μS/cm, 19.6 to 23.3‰ and 6.13 to 6.83, respectively. The temperature interval was from 11.2 to 25.4 °C for cold waters. The hydrochemical facies of the geothermal waters were Na-Cl type, and the cold waters were Ca-HCO3-Cl type. The high concentrations of As, Ba, Fe, Li, and Mn in geothermal waters were mainly derived from prolonged water-rock interactions under high-temperature conditions. The δ18O and δ2H contents of cold waters indicated meteoric origin. The geothermal waters were enriched in δ18O and δ2H and located on the mixing line between local groundwater and fossil seawater, indicating mixing processes. During our study period, 20 earthquakes with Mw 3.5 and above were recorded in the close surroundings of the Kestanbol geothermal field, and temporal variations in the physicochemical and chemical compositions of geothermal waters were observed. Concentrations Cl- of the geothermal waters exhibited decrease after the Tartışık-Ayvacık earthquake (Mw = 5.0), indicating more supplement of groundwater with shallow origin under the increase of tectonic stress.