Tracing sulfate origin and transformation in an area with multiple sources of pollution in northern China by using environmental isotopes and Bayesian isotope mixing model.

Sulfate (SO) contamination in groundwater and surface water is an environmental problem of widespread concern. In this study, we combined stable isotope analyses of SO (δS and δO) and water (δH and δO) with a Bayesian mixing model (SIAR), for the first time, to identify sources and transformation of SO in an area of northern China with multiple potential sources of pollution. The overall values of δS and δO-SO ranged from 1.3‰ to 16.3‰ and -3.8‰-8.8‰ in groundwater, and from -1.1‰ to 9.3‰ and 2.7‰-9.2‰ in surface waters, respectively. Analyses of SO isotopes and water chemistry indicated that SO in groundwater and surface water mainly originated from mixing of oxidation of sulfate, sewage, chemical fertilizers, dissolution of evaporite and precipitation. There was no significant correlation between δS and δO and SO concentration in groundwater, indicating that bacterial sulfate reduction did not affect the SO isotopic composition. SIAR model showed the main sources of SO in groundwater and surface water comprised oxidation of sulfide minerals and sewage. In groundwater, oxidation of sulfide minerals and sewage accounted for 37.5-44.5% and 35.5-42.7% of SO, respectively. In regard to surface waters, the contribution of oxidation of sulfide minerals to SO was higher in the wet season (31.8 ± 9.9%) than in the intermediate (22.4 ± 7.8%) and dry (20.9 ± 8.2%) seasons, but the contribution proportion of sewage was slightly lower in the wet season (19.9 ± 8.5%) than in the intermediate (23.8 ± 8.7%) and dry (24.2 ± 8.5%) seasons. This study indicates that it is necessary for local government to improve the treatment infrastructure for domestic sewage and optimize methods of agricultural fertilization and irrigation to prevent SO contamination of groundwater and surface water.