Determination of the Origin of Dissolved Carbon Dioxide Using Hydrogeochemistry and Carbon Isotopic Modeling: A Case Study of Gariz Aquifer, Southwest of Yazd Province

High concentrations of carbon dioxide are present in the Gariz aquifer groundwater and this phenomenon has led to many environmental hazards. In order to determin the origin of dissolved carbon dioxide in Gariz aquifer, the groundwater has been sampled from wells (35 samples) in July and October 2019. Laboratory analysis included the concentrations of major ions and stable carbon isotope (δ13CTDIC) in groundwater samples. Also some physicochemical properties of water have been measured directly in the field. On the basis of chemistry of the ions, the majority of the water samples have a chloride-sulphate alkaline-earth to bicarbonate-alkaline-earth composition and bicarbonate concentrations of groundwater samples have increased due to the presence of carbonate minerals in the aquifer as well as the significant interaction of the aquifer with carbon dioxide-rich fluids. Hydrogeochemical evolution of the aquifer groundwaters has also significantly affected by the infiltration of saline water and mixing with CO2-rich fluids. Hence, for investigating the origin of carbon dioxide dissolved in the aquifer the theoretical δ13 values have been calculated in equilibrium with collected groundwater samples at respective sampling temperatures on the basis of the carbon isotopic balance and has been evaluated using the dissolved gases mixing model. The results highlighted two main CO2 components: 1) an inorganic deep sourced CO2 (13C-enriched) and 2) an organic CO2 (13C-depleted). Calculated pristine isotopic composition of C (-2‰ V-PDB) that interacted with the aquifer indicated the significant contribution from a deep source of carbon dioxide inorganic in origin and decarbonation in calc-silicate rocks.