Variations of stable oxygen and hydrogen isotope ratios in the cold and thermal springs of the Bazman volcanic area (in the southeast of Iran)

Abstract

The Bazman volcanic area, which comprises thermal and cold water springs with different natural characteristics, was selected as the case study of this research. The cold springs can be divided into western and eastern groups. In total, 16 water samples were collected from the thermal and cold water springs for hydrochemical and stable isotope analyses. In the study area, the groundwater temperature ranged from 28.1 °C (in the cold water) to 43.9 °C (in the hot springs). The electrical conductivity (EC) values of the groundwater samples were 1102–10,250 µS/cm. The highest EC values belonged to the thermal springs known to have brackish water in this area. The δ18O and δ2H values in the springs ranged from − 5.06 to 0.04 ‰ and − 24.1 to − 8.6 ‰, respectively. The relationship between δ2H and δ18O values and physicochemical characteristics was investigated. In the west of the area, the isotopic composition of the cold springs was close to that of the LMWL, while the eastern cold springs had an isotopic composition highly enriched in δ2H and δ18O. In contrast, the isotopic composition of the thermal springs was almost on that of the GMWL and their deuterium isotope was more depleted than that of cold springs. The difference between the isotopic composition of the thermal springs and the LWML can be the result of hot water–rock interaction and underground evaporation. However, magma was not effective on this difference. The results showed that the groundwater of the western cold springs moved with faster velocity through the long and deep macro-pores and preferential flow channels in carbonate and igneous rocks, while in the eastern springs, groundwater originated from fine fractures with a low depth in shale and sandstone rocks.