Geochemistry of thermal waters and its relation to the volcanism in the Kizilcahamam (Ankara) area, Turkey

Thermal waters issuing through the Tertiary aged volcanics in the Kizilcahamam (Ankara) area are all alkali-bicarbonate waters with temperatures ranging from 28°C to 86°C. The waters from the town center (MTA-1 and KHD-1 drilling wells, Big Bath and Small Bath) have the highest temperature and an intermediate TDS content, in comparison to the waters sampled from the localities outside the town center (Mineral Water Spring and Sey Bath). The Mineral Water Spring is characterized by the lowest temperature and the highest TDS content. The waters from the Sey Bath have an intermediate temperature and the lowest TDS content. The variations in the temperature and the chemical composition of the waters can be accounted for by a combination of processes including mixing between cold-shallow and hot-deep waters, boiling either before or after mixing, steam heating and conductive cooling. The chemical geothermometers, silica-enthalpy and enthalpy-chloride mixing models suggest a reservoir temperature of 124–190°C for the Kizilcahamam region, and a maximum of 71% deep, hot component for the thermal waters. Within the framework of water-rock interaction, the volume of volcanic rocks necessary to supply the annual water output has been estimated as 16.5 × 106 m3 a−1; the volume of the volcanic rocks necessary to supply the annual chemical output of the thermal waters has been estimated, on the basis of cation contents, as 16,785 m3 a−1. This suggests that, for the thermal waters to obtain their chemical composition, only 0.1 % of the volcanics is required to have undergone leaching. As this amount is rather low, the waters seem to have obtained their cation contents almost totally from the volcanics nearby. On the other hand, the bicarbonate enrichment in the Kizilcahamam waters points to the fact that groundwaters dissolve CO2 arising, probably, from a magmatic body solidifying at depth, which can also account for the heat source of the Kizilcahamam geothermal system.