Sources and behaviors of dissolved sulfate in the Jinan karst spring catchment in northern China identified by using environmental stable isotopes and a Bayesian isotope-mixing model

In recent decades, surface water and groundwater in the Jinan spring catchment have become increasingly contaminated by sulfate due to the expansion of urbanization and the increasing intensity of anthropogenic activities. In this study, ten surface water samples and forty-three groundwater samples were collected in the catchment from June to September 2019, and their hydrochemistry and stable isotopes (δ34SSO4, δ18OSO4, δDH2O and δ18OH2O) combined with a Bayesian isotope mixing model were used to identify sources and transformations and to quantitatively evaluate the contribution of sulfate from multiple potential sources of pollution. The dual isotopes of sulfate and the isotope mixing model showed that sulfate in the surface water was mainly derived from evaporite dissolution (32.6%), sewage (18.8%), soil sulfate (15.4%) and sulfide oxidation (10.2%), while that in the groundwater was mainly derived from soil sulfate (32.4%) and sewage (30.6%). In groundwater, the proportions of sewage and evaporite dissolution increased from the indirect recharge area to the discharge area, while the contribution of soil sulfate and sulfide oxidation decreased. In regard to surface waters, the contribution proportion of evaporite dissolution to SO42− in June was 40.9%, which was significantly higher than 24.2% in September, but the proportion of sewage, soil sulfate and sulfide oxidation was lower in June (13.7, 12.9 and 9.5%, respectively) than in September (23.8, 17.8 and 10.8%, respectively). There was no significant correlation between the ratio of SO42−/Cl− and δ34SSO4 and δ18OSO4 in surface water and groundwater, indicating that bacterial sulfate reduction did not play a major role. This study can provide a reference for the development of effective pollution management strategies for water resources in karst areas of northern China. •The accuracy of sulfate tracing can be improved by combining multiple isotopes with land use and water chemistry.•A Bayesian isotope mixing model (Simmr) was employed to quantity SO42− source.•Major sulfate sources of surface water were evaporite dissolution, sewage, soil-derived sulfate and sulfide oxidation; while for groundwater were soil sulfate and sewage.•Sulfate pollution in karst groundwater and surface water might be the coupling result of natural input and anthropogenic activities.