Unveiling sources and fate of sulfate in lake-groundwater system combined Bayesian isotope mixing model with radon mass balance model

The role of various natural and anthropogenic factors in controlling sulfate behavior in lake-groundwater system is not fully understood. In this study, a hybrid isotopic approach incorporating multi-isotopes (D/¹⁸O_H2O/³⁴S/¹⁸O_SO4/²²²Rn) and quantitative models was used to reveal and quantify the sources and fate of sulfate in two lake-groundwater systems (Daihai and Ulansuhai Lake basins) in Inner Mongolia, China. In the Daihai Lake basin, dissolved sulfate was mainly derived from evaporite dissolution (> 50 %) and sulfide oxidation (10 % ∼ 30 %; including mineralization and pyrite oxidation). In contrast, the main dissolved sulfate sources in the Ulansuhai Lake basin included evaporite dissolution (40 % ∼ 60 %) and sewage and manure (20 % ∼ 40 %) accompanied by the bacterial sulfate reduction process. Notably, the sulfate flux associated with lacustrine groundwater discharge in the Ulansuhai Lake basin (2.77 × 10⁸ g/L) was significantly greater than that in the Daihai Lake basin (1.63 × 10⁶ g/L). However, the contribution of groundwater-derived sulfate showed the opposite result. Differences in hydrogeological conditions and human activities in the two basins were the dominant factors controlling the variability in sulfate sources and fate. The approaches and findings of this study can enhance the ability to identify sulfate sources and fate in lake-groundwater systems in similar regions.