Etiology of Salinity and Water Origin, the Main Dilemma of Badab Sourt, a Unique Travertine Spring: R. Bagheri et al. Groundwater xx, no. x: xx-xx

: Badab Sourt travertine-depositing springs in the north of Iran, naturally create a unique surreal landscape containing a range of stepped travertine terraces, similarly found only in a few other places on earth. This site comprises of three travertine saline springs with different values of salinity and discharge (SP1 , SP2 , and SP3 ) and one non-travertine fresh karstic spring (SP4 ) within a distance of about 300 m. The etiology behind this salinity and the water origin are the main research's dilemma that were investigated using geological, hydrochemical, and stable isotopic techniques. Based on the topography and isotopic results, the carbonate formations in northern (Khoshyeilagh and Mobarak) and southern (Cretaceous limestone) parts of the springs potentially provide the initial hydraulic gradient for deep circulation of the water and CO2 . However, geological studies indicate that the hydraulic connectivity of the Cretaceous formation to the travertine springs is interrupted by impermeable geological formations. Based on the proposed conceptual hydrogeological model and mass balance calculations, the SP4 spring is locally recharged from the nearby karstic area of Khoshyeilagh formation through shallow, short and steep groundwater flow circulation that is completely different from the travertine springs. The travertine spring (SP1 ) is recharged from more distant areas having higher altitudes on Mobarak and Khoshyeilagh limestone and circulate more deeply before emerging on the surface. The SP2 and SP3 springs can derive from the mixing of the saline water (SP1 ) and fresh water (SP4 ). The dissolution of interlayers of halite in Shemshak formation is concluded as the main source of salinity. This is the first research article in detail to survey hydrogeology of the travertine springs in Iran.