Hydrogen and oxygen stable isotopes in the shallow groundwater system in Kahramankazan Basin (Ankara, Turkiye)

Stable isotopes of hydrogen (deuterium-δD) and oxygen (oxygen-18-δ¹⁸O) are considered ideal tracers in addressing issues related to the research, development, and management of water resources. These isotopes circulate with groundwater within the hydrogeological system and are generally unaffected by physico-chemical processes. Consequently, they are widely used to determine the origin, recharge elevation, and mixing mechanisms of groundwater. In this study, in-situ physico-chemical parameters (electrical conductivity [EC], temperature, pH, dissolved oxygen [DO], and stable isotopic composition (δ¹⁸O and δD) of the shallow groundwater system in the Kahramankazan Basin (Ankara) were examined during dry (November 2021) and wet (April 2022) periods. Previous studies indicate that the shallow groundwater system in this area has been under stress for the past 40 years due to sand-gravel mining activities and groundwater over-abstraction. As a result, the thickness of the aquifer reportedly decreased from 15–45 m to 10–25 m over the years. In-situ analyses of the samples collected during the dry period showed that EC, pH and DO values varied from 600 to 6740 μS/cm, 7.18 to 7.92 and 2.51 to 7.15 mg/l, respectively. The isotopic compositions of groundwater during the dry period ranged from -8.1‰ to -9.5‰ for δ¹⁸O (mean: -8.82‰, n=9) and from -54.7‰ to -63.4‰ for δD (mean: -60‰). During the wet period, EC values slightly decreased, ranging from 563 μS/cm to 5070 μS/cm. Some samples deviated from the Local (Ankara) Meteoric Water Line (δD = 8δ¹⁸O + 11.54). For these samples, as expected, the effect of evaporation was greater during the dry period compared to the wet period. Recharge elevations, determined from the relationship between δ¹⁸O and elevation obtained in previous studies, were found to range from 820 m to 1274 m, consistent with the topography. This study revealed that along the flow path of the alluvial aquifer, there is a notable increase in EC values and isotopic enrichment of stable isotope values due to an increase in evaporation and the adverse impacts of urbanization.