Formation mechanism of high arsenic geothermal water in Gonghe basin, Northwest China

The occurrence of high arsenic (As) concentrations in groundwater within overlying aquifers, influenced by deep geothermal activities, has been reported globally. However, its genetic mechanisms remain inadequately understood. In this study, forty-one water samples were collected to analyze the major and trace chemical compositions of water, along with isotopic signatures (δ¹⁸O, δD, δ¹³C, δ¹⁴C, δ³²S_SO4). Results show that As concentrations in NGW (with an average 452 μg/L) are significantly higher than in QGW (with an average 36.0 μg/L). Additionally, hydrochemical type of QGW gradually evolves from Na-HCO₃ to Na-SO₄ and Na-Cl·SO₄ along the flow path. The δ¹⁸O and δD isotopic results suggest that local atmospheric precipitation is the primary source of QGW, while glacial meltwater or high altitude atmospheric precipitation serves as the main source of NGW. The high temperature environment contributes to an oxygen drift in the δ¹⁸O of NGW. Results of δ¹³C indicate that the main sources of inorganic carbon in geothermal water are likely from the dissolution of carbonates and decomposition of organic matter. According to δ¹⁴C data, the average apparent age of QGW is 18.8 ka, with relatively older ages found in the south and southeastern parts of the study area, whereas NGW has an average age of 29.1 ka. The continuous upwelling of deep geothermal energy raises the temperature of hot storage aquifer, promoting the release and migration of As. Additionally, silicate weathering and microbial sulfate reduction play significant roles in As enrichment in both QGW and NGW. Furthermore, As desorption from QGW and NGW is another factor contributing to the elevated As levels in the investigated region.