Hydrogeochemical processes and dissolved gases chemistry at the Gariz aquifer, Southwest of Yazd Province

The leakage of natural gases mainly carbon dioxide into the Gariz aquifer has increased the concentration of dissolved gases and decreased groundwater quality. Therefore, the water supply of the residents of this region in various needs such as agriculture, drink, domestic, and industry, has faced some problems. Groundwater wells have been sampled to evaluate the hydrogeochemical processes of gaseous aquifers and determine the nature of dissolved gases during two-stage in July and September of 2019. Laboratory analysis includes the concentration of major anions and cations and, the dissolved gases in groundwater samples, and some physicochemical properties of water have been measured directly in the field. On the basis of the chemistry of major ions, most of the samples have classified as calcium-bicarbonate (Ca-HCO3), sodium-chloride (Na-Cl), and calcium-chloride (Ca-Cl) types, although other mixing types are also recognized. The most significant processes that affect the chemical composition of the groundwater of the Gariz aquifer are water-rock interaction, leakage of CO2-rich fluids, infiltration of saline water through faults and fractures, and silicate dissolution. Bicarbonate ion has increased in the groundwater samples due to the interaction of CO2-rich fluids with groundwater aquifer. The dissolved gases composition also indicates that CO2 is the dominant gas phase in most samples, with up to 2100 times higher than atmospheric values. High concentrations of dissolved CO2 of more than 730 cubic centimeters per liter indicate the infiltration of CO2-rich fluids with deep origin into the aquifer. The ratio of dissolved O2/N2 in the aquifer is lower than air-saturated water (ASW), therefore geochemical processes responsible for dissolved oxygen consumption during redox reactions.