Hydrogeochemical characterization of a possible carbon sink from shallow saline-alkaline groundwater in the eastern Hetao Basin of Inner Mongolia in China

The question of how saline-alkaline groundwater can be used as a CO2 sink in arid saline-alkaline areas remains controversial. This study investigates the role of saline-alkaline groundwater as a CO2 sink using a mass balance method, Gibbs diagrams of the hydrochemistry, and carbon isotope (delta C-13(DIC)) measurements. Twenty-eight groundwater samples of varying electrical conductivity (EC; 1.52-52.34 mS cm(-1)) were collected at different depths (1-2 and 5-25 m) in the Hetao Basin of Inner Mongolia, China. The results showed that groundwater ions could be primarily concentrated from water-rock interactions and evaporation, and that there are two main groundwater types: a mixed Na center dot Ca center dot Mg-Cl center dot SO4 center dot HCO3 type and a Na-Cl type. The dissolved inorganic carbon (DIC) concentration in samples obtained from a depth of 1-2 m was less than that in samples from 5-25 m, and a downward migration trend of DIC in the groundwater was observed. The DIC concentration exhibited a significant positive correlation with pH (R-2 = 0.61, p < 0.05) and the saturation index of carbonates (R-2 = 0.93, p < 0.01). Groundwater with a higher pH contained a higher DIC concentration and could provide strong carbon sink potential. The delta C-13(DIC) values of the groundwater samples varied from -21.22 parts per thousand to -11.02 parts per thousand, indicating that DIC was derived from the dissolution equilibrium of pedogenic carbonates and atmospheric/soil CO2. The carbon sequestration of the shallow saline-alkaline groundwater in the Hetao Basin could reach 4.66 x 10(8) g C a(-1), which represents important potential of carbon sink in the biogeochemical cycle.