The development of conceptual models for the functioning of geothermal systems using hydrogeological, hydrogeochemical and isotopic data: case studies from Rize (NE Turkey)

Abstract

The hydrogeological and hydrogeochemical properties and isotopic composition of hot and cold water springs in the Ilicakoy and Ayder geothermal areas were investigated and a conceptual model was developed to explain their function. Characteristics of hot groundwater from the Ilicakoy area were: temperature, 63 °C; EC, 6913 µS/cm; pH, 7.2, DO content, 0.75 mg/L; and water type, Na–Ca–HCO3. Characteristics of hot groundwater from the Ayder area were: temperature, 55 °C; EC, 255 µS/cm; pH, 9.2, DO content, 1.9 mg/L; and water type, Na–Ca–CO3–SO4. Ilicakoy has acidic pH values, while those of the Ayder area are alkaline. Carbon in hot water in both areas was derived from inorganic carbon in groundwater and from CO2 in pore spaces in the unsaturated zone. SO2 in hot water was derived from Cenozoic-aged gypsum. Low tritium and high Cl contents in hot waters indicate deep circulation, and Sr was passed from granitic rocks to waters by simple dissolution. High total alpha and total beta values were obtained in the hot waters from the Ilicakoy geothermal area. Ilicakoy and Ayder geothermal spring formation occurred by infiltration of precipitation through fractures in granitic bedrock from the Kackar granitoid, which forms a regionally significant geothermal reservoir. Water of meteoric origin is heated under by a locally steep geothermal gradient, and it rises through bedrock fractures and re-emerges at the ground surface. In the Ilicakoy and Ayder geothermal fields, which are low temperature geothermal systems, the reservoir rock temperature was 80 °C and 149 °C, respectively.