The seasonal investigation of the chemical and isotopic composition of the Pamukkale thermal spring (Denizli)

This study aims to investigate the temporal variations in the discharge, chemical, and isotopic composition of Pamukkale thermal springs and the processes affecting these changes. For this purpose, 5 thermal water samples, 4 cold water samples and 1 rainwater sample from the Pamukkale site, and 1 thermal water sample from the Karahayıt site were collected. Chemical and isotopic analyses (delta oxygen-18, delta deuterium, tritium, and delta carbon-13) of the water samples were used to determine the chemical composition, origin, underground circulation time, mineral saturation, and reservoir temperature of the waters. The thermal spring reservoirs in the study area are composed of Neogene and Mesozoic limestones and Paleozoic Menderes Massif metamorphics. The impermeable Neogene units form the cover rock. Thermal springs have formed as a result of rainfall infiltration to deeper levels, heating by high geothermal gradient, and ascent to the surface via faults. The temperatures, electrical conductivities, and pH values of the thermal springs range from 27.7–60.6 degrees Celcius, 1950–3140 microsiemens/cm, and 5.82–7.00, respectively. The thermal waters are mostly of the calcium-magnesium-bicarbonate-sulfate type. Isotopic composition indicate that the thermal waters are of meteoric origin, with high-temperature waters having a recharge that predates 1952, and low-temperature waters reflecting a mixture of pre-1952 and recent recharge. The relationship between stable isotopic composition and elevations of cold waters suggests that the elevation of the thermal waters' recharge area ranges from 1190 to 1325 meters. According to delta carbon-13 values, the source of carbon dioxide in the thermal waters is primarily decomposition of the carbonate rocks. The discharge of Pamukkale thermal springs is controlled more by the deviation from the annual average precipitation than by instantaneous or seasonal rainfall. However, since 2015, the notable decrease in discharge is likely due to both natural and anthropogenic factors.